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Intensity history of the earth's magnetic field during the late quaternary as recorded by the sidements of the Blake/Bahama outer ridge, north Atlantic Ocean

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Intensity history of the earth's magnetic field during the late quaternary as recorded by the sidements of the Blake/Bahama outer ridge, north Atlantic Ocean

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Content INTENSITY HISTORY OF THE EARTH’S MAGNETIC FIELD DURING THE LATE QUATERNARY AS RECORDED BY THE SEDIMENTS OF THE BLAKE/BAHAMA OUTER RIDGE, NORTH ATLANTIC OCEAN by Martha Schwartz A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Geological Sciences) December, 1997 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 3110959 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 3110959 Copyright 2004 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90007 This dissertation, written by ttL........ under f/ie direction of h.&.'C..... Dissertation Committee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillm ent of requirements fo r the degree of DOCTOR OF PHILOSOPHY '"SfG raduate S tudies Date . DISSERTATION COMMITTEE Chairperson Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Dedication To My Mother Bella Rosenstreich Schwab 1911-1995 Quiet, Modest, Gentle But With an Iron Core Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgments I had hoped that I 'd never have to graduate, but now it's time to thank all the people who have helped to make the last few years quite wonderful. There's Beth Ambos, whose “won't you PLEASE at least let me call my friend, Steve Lund?" got me down here in the first place. And Steve, for being a princely advisor, friend, collaborator, coauthor, lunch partner, lab-fixer, father of a friend, provider of funds, data, and help. Not to mention his tolerance for my absent-mindedness and involvement in teaching and pet political projects. I've had an especially helpful committee. Doug Hammond has many times gotten hands-on involved in our chemistry work and papers. And it's always fun to work with Charlie Sammis - as TA, or in interesting lunchtime discussions of all sorts of interesting things. Thanks too to Dr. Cole for emerging from happy retirement to read this thesis. And I've gotten great support from others at USC, Rene and Cindy and Matt in the office, Donn Gorsline and Lawford Anderson, who helped extend our work to the second and third floors, Sally in the lab, Jack Worrell of CEMMA. I've especially enjoyed the opportunity and freedom to share our work with those helpful USC and CSUDH undergrads and the bright, hardworking (and well-trained) Torrance High students. I owe much more than the usual thanks to my darling husband, Rick. Not only the expected inconvenience and iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. loneliness of having a spouse in graduate school - no, Rick came down here for two summers on the Research Corporation grant and countless times for free to provide valuable bench-chemistry data and to supervise and help the high school students. He even gave a talk at AGU with us. Then, too, there's Jeff, Steve, and Danny, my sons. Danny claims there were times he had to “raised himself" because I was thinking of other things. I always appreciate the stimulating atmosphere in this department. Everyone here is doing something interesting, and the casual conversations in the hallways are often more entertaining to me than anything the entertainment industry can offer. Three hours on the TEM beats any movie I've ever seen. Even a couple of hours with the magnetometer and Bach on the stereo can be rejuvenating. Over these years I 've learned from and worked with too many student-friends to mention, from the undergraduates who worked in the lab to the train of international visitors and of course our top-notch graduate students. I intend to stay around, meet the new folks coming in, and trust that none of this magic will ever change. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents Chapter I .................................................. 1 Introduction ....................................... 1 Previous Work on Remanence Acquisition and Magnetic Parameters ..................... 7 Chapter I I ............................................... 11 M e t h o d s ............................................. 11 Magnetic Methods ............................. 11 Magnetic Mineral Separation ............. 12 XRF S t u d i e s ...............................13 Chemical Leaching Experiments .......... 13 Chapter I I I ............................................... 15 Environmental Factors as Complicating Influences in the Recovery of Quantitative Geomagnetic-Field Paleointensity Estimates from Sediments .... 15 A b s t r a c t ....................................... 15 Introduction ................................. 16 Blake Outer Ridge Relative Paleointensity Record ........................................... 17 Correlation With Other Relative Paleointensity R e c o r d s ................................... 24 D i s c u s s i o n ..................................... 28 Chapter I V ............................................... 30 Geomagnetic Field Intensity From 12-70 kybp as Recorded in the Sediments of the Blake/Bahama Outer Ridge, North Atlantic O c e a n ...........................30 A b s t r a c t ........................ 30 Introduction ................................. 32 B a c k g r o u n d ..................................... 34 The Cores, Sampling, and Sediment Magnetism . . 37 Normalization ................................. 42 Stages 2 - 4 .................................46 CH88-10P and CH88-11P ............... 46 C H 8 9 - 1 P...............................50 R e s u l t s......................................... 53 Correlation with the Normalizers ............. 55 Discussion of Normalizations ................. 56 Time Domain and Comparison with Other Records . 62 Conclusions..................................... 65 Chapter V ................................................. 66 Widespread Bacterial Magnetite in Surficial Deep-Sea Sediments of the Blake/Bahama Outer Ridge (N. Atlantic Ocean) and Its Lack of Importance as a Paleomagnetic Recorder ................... 66 v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A b s t r a c t ....................................... 66 Introduction ................................. 68 Sediment Magnetism ........................... 70 Electron Microscopy ........................... 74 Sediment Chemistry ........................... 77 P a l e o m a g n e t i s m.................................79 D i s c u s s i o n ..................................... 81 Chapter V I ............................................... 84 Early Sediment Diagenesis on the Blake/Bahama Outer Ridge, North Atlantic Ocean, and its Effects on Sediment Magnetism ........................... 84 A b s t r a c t ....................................... 86 Introduction ................................. 90 Deep-Sea Sediments of the Blake/Bahama Outer Ridge 90 Depositional Setting ..................... 90 Chronostratigraphy ........................... 91 Sediment Magnetisim ......................... 98 Bulk Sediment Magnetic Parameters . . . 98 Magnetic Mineralogy ................... 100 Interpretation of the Sediment Magnetism .................................... 103 Sediment Chemistry ......................... 105 X-Ray Fluorescence Measurements .... 105 Chemical Leaching Studies ............. 107 Interpretation of the Sediment Chemistry .................................... Ill Early Diagenesis and Sediment Natural Remanent Magnetization ......................... 118 Conclusions.................................... 119 Appendix 1: M e t h o d s ................................123 Chapter V I I .............................................. 127 Remanence Acquisition During Times of Rapid Geomagnetic Field Variability and Low Paleointensity: Grain Size Dependence of L o c k - i n ................... 127 A b s t r a c t ...................................... 127 Introduction ............................... 128 B a c k g r o u n d .................................... 130 Sedimentation on the Blake Bahama Outer Ridge .......................................... 132 Sediment Magnetic Characteristics .......... 133 Detailed Demagnetizations......... .......... 138 D i s c u s s i o n .................................... 152 The medium coercivity grains .......... 152 The hardest g r a i n s ....................... 154 Remanence acguisition during times of low paleointensity ......................... 161 Conclusions.................................... 162 vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter V I I I ..................................... 164 Relative Paleointensity in Marine Sediments Deposited During the Laschamp and Blake Geomagnetic Events: Assessment of Normalizing Techniques .... 164 A b s t r a c t ...............................164 Introduction ............................... 165 Core material and geological setting .... 167 M e t h o d s.................................175 Sediment Magnetic Properties ............. 177 Demagnetization Behavior ................... 179 NRM vs. ARM in Medium Coercivity Band .... 180 Comparison of Normalizers ................... 183 D i s c u s s i o n .............................188 Conclusions.............................192 Chapter I X ........................................ 196 General Conclusions ............................... 196 References........................................ 202 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Abstract The purpose of this work is threefold. One major goal is to calibrate, as well as possible, the methodology needed to extract accurate records of past geomagnetic field intensity from rapidly deposited sediments. A second goal is to examine the extent to which chemical changes subsequent to deposition may alter paleointensity estimates. The third is to present and evaluate a number of such paleointensity records for the last seventy thousand years of earth's history. This dissertation is a compendium of six manuscripts, which are in various stages of publication at this time. All of them deal with the general topic of paleomagnetic field intensity determinations from rapidly deposited sediments. In the broadest terms, the goal is to normalize the natural remanent magnetization (NRM) in the sediment by some measure of the varying amount of magnetic material variously available for alignment in the Earth's field. The problem is complex; the supply of magnetic minerals, their mineralogy, and their vulnerability to chemical alteration after deposition all vary in response to changing environmental conditions, and therefore with respect to time. All six papers are analyses of deep-sea sediment records from high sediment accumulation rate drift deposits of the Blake/Bahama Outer Ridge, in the North Atlantic Ocean. The first paper in the series (Chapter 3) describes a viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. record of paleointensity obtained from two cores, and demonstrates that, although this record agrees reasonably well with records obtained elsewhere, about 25% of the variability observed is still explainable by environmental contamination. Chapter 4 describes a method of comparing different normalizations, and replicate cores, within restricted intervals which exhibit minimal climatic variability, and then reconnecting the pieces by assuming continuous paleointensity trends across climate boundaries. The primary conclusion from this exercise is that the general placement of intensity highs and lows is quite well preserved even in moderately flawed cores, but that amplitude information, in terms of the “peakiness” of the records, is easily lost. The next pair of papers, Chapters 5 and 6, examine issues of chemical changes which occur in iron-bearing minerals after they are deposited in the sediment. These chapters tie the results of chemical analyses - x-ray fluorescence and iron leaching experiments, as well as electron microscopy of magnetic extracts, to the intensity of natural remanence and of the various commonly used normalizers. As part of this work, it was noted that biogenic magnetite is common in all Holocene carbonate-rich sediments in the study sites, and in some intervals at the base of 5180 stage 5 (last interglacial) sediments as well. ix Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. These “magnetosomes” contribute greatly to the intensity of anhysteretic magnetic remanence (ARM) but relatively little to the sediment NRM and the magnetic susceptibility, at least in these sediments. This makes the use of ARM normalization problematic in sediments which contain variable numbers of single domain magnetite crystals. The last two papers, Chapters 7 and 8, focus on general issues of remanence acquisition and paleointensity. They use two short core segments, each of which contains an excellent record of short duration, large-amplitude, swings in field direction (geomagnetic excursions). The two segments differed greatly in the amount of environmental change they contained. The principal technique here was to use coercive force (the intensity of alternating magnetic field strength necessary to demagnetize a grain) as a proxy for magnetic mineralogy and grain size. The important result of this work is the observation that medium-size grains (called pseudosingle domain) are the most reliable recorders of the field direction and intensity. Grains which demagnetize in the force range 30-70 mT lock in their remanence almost simultaneously and within a few cm of the sediment-water interface. The smallest, single-domain grains, recorded the field poorly, and in a noticeably stretched and smeared manner. The largest grains are magnetically soft and carry a ubiquitous present-day field overprint. x Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The “paleointensity problem" is not completely solved, but we are making progress. Broad patterns are reproducible and relative paleointensity is ready to take is place as an adjunct to other forms of paleomagnetic correlation. The short-duration data are good enough, in some instances, to be useful helping to understand some features of magnetic field variability, such as geomagnetic excursions. It is premature, however, to trust longer duration records to constrain detailed models of geomagnetic field mechanisms. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Tables Table Page Table 6-1: Characteristics of leaching solutions . . . .126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure List of Figures Page Figure 3-1: Site map for sediment cores cited herein . . 18 Figure 3-2: Sedimentological and rock magnetic data from Blake Outer Ridge core CH88-10P..................... 19 Figure 3-3: Relative paleointensity estimated from the ratio NRM2o/ARM2o ........................................ 21 Figure 3-4: A comparison of relative paleointensity records from the Blake Outer Ridge, Mediterranean Sea, and Lac du Bouchet, along with the ratio ARM/x (inverted) for core C H 8 8 - 1 0 P ....................................25 Figure 4-1: Site map for the Blake Bahama Outer Ridge . 38 Figure 4-2: Bulk sediment magnetism and percent carbonate for cores CH88-10P (a) and CH89-1P (b)............ 40 Figure 4-3: Representative demagnetization diagrams. . . 43 Figure 4-4: Representative coercivity spectra for sediment NRM, ARM and S I R M ................................44 Figure 4-5: Initial normalization of NRM in the three cores by ARM, susceptibility and S I R M ................. 45 Figure 4-6: NRM/ARM, NRM/SIRM, and NRM/x in intervals of CH88-10P and CH88-11P................................49 Figure 4-7: Examples of windowed normalizations in stages 2- 4 of core C H 8 9 - 1 P.................................51 Figure 4-8: Compilation of the final paleointensity time series for CH88-10P, CH88-11P, CH89-1P 57 Figure 4-9: Raw NRM data in the neighborhood of one interval of amplitude disagreement between CH88-10P and CH88-11P .......................................................59 Figure 4-10: Scatter plots of normalized intensity vs normalizers for core CH88-10P.................... 60 Figure 4-11: Time-depth curves for the three cores used in this s t u d y ........................................ 61 Figure 4-12: Averaged deposition rates for the three cores used in this s t u d y ................................63 Xlll Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4-13: Comparison of the paleointensity record from core CH88-10P and other published sediment records spanning the same time interval..................... 64 Figure 5-1: Map of the Blake/Bahama Outer Ridge showing the locations of 69 piston and gravity cores for which we have magnetic susceptibility (x) profiles ........ 69 Figure 5-2: Variations in magnetic susceptibility (x) along a transect of piston and gravity cores from the Blake/Bahama Outer Ridge..............................71 Figure 5-3: Sediment magnetism of the uppermost 5 m from piston core CH88-10P.................................73 Figure 5-4: Transmission electron photomicrographs of selected magnetite crystals observed between 30 and 70 cm in piston core CH88-10P...........................76 Figure 5-5: Transect of four piston cores along the axis of the Blake Outer Ridge showing the locations of solidphase Fe and Mn peaks (based on XRF measurements) within the uppermost 150 cm in each core............. 78 Figure 5-6: Scatter diagrams showing the relationships between A1 % (detrital flux indicator) versus carbonate (CaC03) % (left), NRM (center), and ARM (right) within the uppermost four meters of CH89-1P, CH88-11P, CH88- 10P, and GGC-24....................................... 80 Figure 6-1: Map of the Blake/Bahama Outer Ridge, western North Atlantic Ocean..................................87 Figure 6-2: Typical carbonate stratigraphy and magnetic susceptibility profiles for Late Quaternary deep-sea sediments of the Blake/Bahama Outer Ridge (BBOR). . 89 Figure 6-3: Magnetic susceptibility stratigraphy along the crest of the Blake/Bahama Outer Ridge................ 92 Figure 6-4: Sediment magnetism and carbonate stratigraphy of piston core CH88-10P.................................. 93 Figure 6-5a: (b-c on next two pages) Sediment magnetism of the uppermost several meters of (a) piston core CH88- 10P, (b) gravity core GGC-24, and (c) piston core CH89- 1P.................................................... 94 Figure 6-5b: See previous page for explanation........ 95 xiv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6-5c See previous page for explanation 96 Figure 6-6 (a-c): Photomicrographs of magnetite crystals interpreted to be bacterial magnetosomes. (d) Photomicrograph of an authigenic manganese micronodule formed near the Mn redox boundary in core CH88-10P. 101 Figure 6-7: A summary of the solid phase chemistry noted in four cores collected as a transect along the Blake/Bahama Outer Ridge ......................... 107 Figure 6-8: Results of chemical leaching experiments on gravity core G G C 2 4 .................................. 109 Figure 6-9: Correlation between various chemical leachates and magnetic susceptibility (on a carbonate free basis) within the surface sediments of core GGC24 .... 110 Figure 7-1: Site map showing the Blake Bahama suite of cores we have studied...................................... 134 Figure 7-2a (7-2b next page): Initial paleomagnetic results from the two intervals reported in this paper . . 135 Figure 7-2b: See previous page for explanation........... 136 Figure 7-3a (7-3b, next page): Bulk sediment magnetism for the two core s e g m e n t s ..............................139 Figure 7-3b: See previous page for explanation........... 140 Figure 7-4: Transmission electron micrographs of euhedral nanometer-range magnetite crystals in JPC .... 141 Figure 7-5(a and b): Results of vector differencing of the total vector at 10 mT intervals for single samples in JPC 1 4 .............................................. 142 Figure 7-5 (c and d): See previous page for explanation .143 Figure 7-6(a-d): Representative vector endpoint diagrams for selected samples from JPC 14........................ 144 Figure 7-6 (e-h): See previous page for explanation . . 145 Figure 7-7(a-d): Similar to figure 7-6, but showing only 30- 70 mT portion of the d i a g r a m s ..................... 149 Figure 7-7 (e-h): See previous page for explanation .. . 150 Figure 7-8a: Ratio of NRM100/NRM for both segments. . . 155 xv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 7-8b: See previous page for explanation ........ 156 Figure 7-9: Characteristic inclination and inclination measured after 100 mT for a) JPC 14 and b) JPC 22 157 Figure 8-1: Site map of the Blake/Bahama Outer Ridge, showing locations of cores (CH88-10P, CH88-11P, CH89-1P used in this study................................... 168 Figure 8-2a: Sediment magnetic characteristics of the two core segments........................................ 169 Figure 8-2b: (See previous page for explanation) . . . 170 Figure 8-3: 20 mT cleaned directions and initial paleointensity estimates (as NRM20/ARM20) for segments of core JPC14 (a) and JPC22 (b, next page). . . . 172 Figure 8-3b: (See previous page for explanation.) . . . 173 Figure 8-4: Comparison of demagnetization behavior of measured remanences, NRM, ARM, and SIRM for selected s a m p l e s ......................................... 174 Figure 8-5: Transmission electron micrographs of single domain, euhedral magnetite, interpreted as bacterial magnetosomes...................................... 176 Figure 8-6(a-d): Representative demagnetization behavior of selected samples of JPC 14 and JPC 2 2 .......... 181 Figure 8-6(e-h): See previous page for explanation. . . 182 Figure 8-7: Example of a scatter-plot of interval values of NRM vs. ARM when demagnetized at steps between 30 and 70 m T ............................................185 Figure 8-8a: NRM/ARM as calculated with only the 30-70 mT data, along with the "R" value for each point, in JPC 1 4 ................................................ 186 Figure 8-8b: Same as previous page, but for JPC 22. . . 189 Figure 8-9a: Comparison of 30-70 mT interval NRM/ARM with our initial 20 mT cleaned NRM/ARM, for JPC 14, and for JPC 22 (8-9b, next page........................... 189 Figure 8-9b: See previous page for explanation........ 190 Figure 8-10a: NRM for JPC 14 segment,(JPC 22, figure 8-10b, next page) normalized by ARM, SIRM, and x ........ 193 xvi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 8-lOa: NRM for JPC 14 segment,(JPC 22, figure 8-10b, next page) normalized by ARM, SIRM, and x .......... 193 Figure 8-10b: See previous page for explanation. . . . 194 xvii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter I Introduction The purpose of this research is three-fold. One major goal is to calibrate, as well as possible, the methodology needed to extract accurate records of past geomagnetic field intensity from rapidly deposited sediments. A second goal is to examine the extent that chemical changes subsequent to deposit may alter paleointensity estimates. The third is to present and evaluate a number of such paleointensity records for the last seventy thousand years of earth's history. The earth possesses a three dimensional vector magnetic field which is well-known to be both spatially and temporally complex. That the field direction changes rapidly (even on human time scales) has been known from direct measurements made at magnetic observatories for approximately the last 400 years. In addition, numerous polarity reversals, or complete flips in the (approximately) axial dipole component of the field, have occurred during geologic time, reported to us by the magnetic remanences set in rocks at the time of their formation. Indeed, these reversals have serendipitously provided the strong evidence instrumental in establishing the theory of plate tectonics, now the unifying paradigm in the earth sciences. Field reversals provide, in addition to this, a handy chronostratigraphic tool for investigators in several geological disciplines. Finer scale directional variation 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (paleosecular variation, PSV, defined as directional variability during times of stable polarity) is also becoming increasingly useful for purposes of dating and correlation. But despite the great utility of geomagnetic field variations, the mechanism by which small and even large changes occur is not understood. Underlying this lack of understanding is the scarcity of basic knowledge concerning the deep-earth core dynamo process generally believed to produce the field. It is now well known that the magnetic field changes continuously in magnitude, as it does in direction. Accurate records of this waxing and waning, would, if they existed, provide (literally) an extra dimension to constrain theoretical models of the core dynamo process. They would also provide useful additional chronostratigraphic tools. Indeed, the few existing continuous paleointensity records are already being so used (Peck et al.,1996). Yet another motivation for desiring accurate field intensity records is that the spallation (and thus input) rates of a number of cosmogenic elements are modulated by the geomagnetic field. These (e.g. 14C, 10Be, 26A1) are increasingly employed for a variety of geological purposes, including absolute agedating of relatively young earth materials. Quantitative paleointensity records would provide important calibration points for these data. Reliable information about the past intensity of the 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reliable information about the past intensity of the geomagnetic field is much scarcer than data concerning its directional variability. This is due to the fact that, both conceptually and in practice, it is much easier to extract a good estimate of the magnetic direction remanent in a rock than it is to deconvolve the strength of the causative ambient field from the sample intensity. The earliest, and to date the only absolute, measurements of past field intensity, have come from analyses of thermoremanences (TRM) in lavas and in such archeological artifacts as baked clay and hearths. In these materials the remanences are acguired as the materials cool from above their Curie temperatures in the presence of ambient field. The magnetization of the material is known to be (?***) directly proportional to the strength of the field in which it formed. It is unnecessary to know the proportionality constant for any given material because the sample can be demagnetized and given a new laboratory TRM in a precisely known field. The paleointensity may then be TRM. t.*H„ tv calculated from a simple proportion: H. =--- — "lo”e”)— ^ (r~ nl) TRM([ab) where H is of course in units of magnetic field. There are some instrinsic problems associated with absolute intensity measurements, most of them due to changes in magnetic mineralogy and/or grain size introduced during the laboratory heating and cooling process, (e.g. Dunlop) But 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. for uncomplicated samples, and with careful work, the method works well. The utility of absolute paleointensity is limited, in any case, by the spotty spatial and temporal occurrence of lava flows and baked prehistoric artifacts. Absolute intensity measurements are moreover laborious to produce and subject to inaccuracies in age control. For more or less continuous and high resolution records of paleointensity it is necessary to turn to rapidly deposited marine and lacustrine sediments. Such sediment paleointensity estimates, which are strictly relative, are normally recovered by the application of some concentration-dependent normalizing parameter to the sample's natural remanent magnetization (NRM). This is done to remove the effect of varying concentrations of magnetic minerals among samples. The most commonly used normalizers are low field magnetic susceptibility (x), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM), usually given in a field sufficient to saturate the sample moment (SIRM). One necessary assumption made is that the sample NRM is proportional to the magnetic field extant at the time of deposition. This is in general true (Tauxe, 1993), but the complete physical mechanism by which any sediment records the ambient magnetic field is poorly understood. Its efficiency (the proportionality constant between the 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magnetic field strength and the magnetization of the sediment) is almost certainly a function at least of magnetic mineralogy and grain size, and must differ from site to site. At best, however, only a small percentage of grains align to produce the sample remanence. A second important requirement is that the magnitude of the normalizing quantity fairly estimate the concentration of magnetic material which had been available for alignment to produce the NRM. An ongoing difficulty concerns the choice of an appropriate normalizer for a given sediment core. The underlying problem stems from the fact that the measured intensity of each of the normalizers, like NRM, depends on such variables as magnetic mineralogy and grain size as well as concentration, and often in a nonlinear and somewhat unpredictable manner. These extraneous factors are subject to environmentally-driven changes throughout the depositional history of any core. For this reason it is difficult to be certain that the normalizer chosen fairly represents the magnetic grain fraction which actually carries the NRM in any sediment mix. And, as the sediment mix changes downcore, subtle, but possibly large, biases are bound to be introduced into the normalized record. It is unlikely that any sediment core of sufficient length and deposition rate will be taken which is completely free in downcore changes in magnetic mineralogy and grain size. The focus of the paleointensity community has 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. therefore been to hammer out a set of reasonable conditions for sediment cores to be considered for paleointensity studies, as well as criteria for the choice of an acceptable normalizing parameter. The earliest work (Levi and Banerjee, 1976) strongly suggested that a remanence (rather than susceptibility) be used and that the remanence chosen ought to be one carried by the same grains contributing to the NRM, as tested by comparison of the alternating field (AF) coercivity of the NRM and of the prospective normalizers. A later important study (King et al., 1983) suggested limits on the variability within a given core: the remanent material should be magnetite or a similar composition, the (magnetic) particle size should be in the 1-15 micron range, and the magnetic concentration should vary no more than a factor of 20-30. Since that time the restrictions have been made a bit more stringent. A recent review paper by Tauxe (1993) summarizes the current consensus on this issue nicely. Related to the problem of depositional changes is the distinct possibility that the magnetic character of a sediment may be, subsequent to deposition, altered by the process of early diagenesis. Magnetite, which is a mixed valence iron oxide, Fe304 (Fe0»Fe203) is a somewhat chemically active species which can be altered and even dissolved postdepositionally. An important mechanism for magnetite destruction is bacterially mediated reductive 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. diagenesis. When free oxygen is no longer available for the metabolism of organic carbon within the sediment, bacteria will utilize a thermodynamically determined sequence of alternate electron acceptors, among them Fe+3, S04"2. Under conditions of sulfate reduction HS~ will be formed and will combine with sources of iron to form pyrite. (Leslie et al.,1990) In the absence of a sufficient input of more reactive iron, magnetite will be destroyed through this reaction. In addition, magnetite, and other remanent materials, can also be grown within the sediment column by a number of authigenic and biogenic pathways. A variety of iron bearing minerals may serve as reactants (Burdige, 1993). It is thus possible for remanence to be destroyed or chemically overprinted. Any alterations in magnetic concentration, mineralogy, or grain size which postdate the acquisition of NRM can seriously compromise paleointensity estimates. Previous Work on Remanence Acquisition and Magnetic Parameters The net magnetic moment produced during or after deposition of a sediment containing previously magnetized grains is known as a DRM, an acronym for "detrital" (or sometimes "depositional") remanent magnetization. Most workers refer to a magnetization which is set at the time of 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. deposition as a DRM and one which occurs somewhat later in the unconsolidated sediment as a pDRM, or post-depositional remanence. Such post-depositional remanences are possible because bioturbating organisms open new pore spaces within the sediment and allow realignment of grains which may have been initially poorly oriented at the sediment-water interface (Irving and Major, 1964). This process introduces a time lag in the sediment paleomagnetic record, but it is also beneficial to it in that it is believed to correct inclination shallowing in many sediments, and in general to improve the alignment process. (Verusub, 1979, for a review of shallowing research.) The grain size dependence of DRM/pDRM acquisition has been investigated in laboratory studies (e.g. Dunlop, 1981, figure 1-1) . Here, grains in the 2-10 micron size range produced the strongest remanence (best alignment) while both smaller and larger grains were less effective. Presumably, the smallest grains fail in alignment because thermal agitation (Brownian motion) is able to overcome the aligning magnetic torque. (Stacy, 1972) Alignment of the larger grains is inhibited instead by inertia and viscous drag (Collinson, 1965); such grains may also fail in pDRM because of insufficient pore space within the sediment. Magnetic susceptibility, x» is defined as x=M/H where H is a (usually small) applied magnetic field and M is the magnetization which is induced in the material in the 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. presence of H. Susceptibility is easy to measure and nondestructive to the sample. But it has two disadvantages as a normalizer for sediment DRM /pDRM . The first is that, since it is measured in the presence of a field, it "sees" all iron containing minerals including those which are purely paramagnetic or for other reasons have no contribution to the natural remanence. The second is that the grain size dependence of susceptibility is quite different from the sediment NRM . Volume susceptibility, in general, increases slightly with grain size. (King et al, 1982). ARM acquisition has been studied in the laboratory by a number of workers. Figure (l-2a), a compilation from King et.al. (1983) illustrates the grain size dependence of ARM. ARM acquisition decreases with increasing grain size. The dependence is nonlinear, and appears to be assymptotic, growing large, perhaps exponentially, for submicron grains and approaching zero for large ones. I have not seen direct studies on the grain size dependence of IR M acquisition. However, a closer look at figure l-2b, ARM normalized by SIRM, indicates that SIRM acquisition must decrease even faster than ARM at the largest grain sizes, but less steeply for smaller ones. The body of this dissertation consists of six papers, now at various stages in the publication cycle. A ll are based on measurements of several piston and gravity cores recovered from the Blake/Bahama Outer Ridge and Bermuda Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rise, in the western North Atlantic Ocean. Chapter 3 is a short discussion of the lingering environmental noise present in better sedimentary paleointensity records. Chapter 4 is a longer paper presenting a detailed comparison of normalizing techniques applied to three sediment cores. Chapters 5 and 6 discuss the problems of early sediment diagenesis, the associated growth of bacterial magnetites, and their effect (or lack of effect) on the sediment natural remanence. The next two chapters are concerned with the results of very detailed alternating field demagnetizations of sediment core segments which record incidences are rapid geomagnetic field variability known as the Laschamp Excursion and the Blake Event. Chapter 7 examines issues of (NRM) remanence acquisition in these sediments, deposited during times of especially rapid geomagnetic change, and chapter 7 with the problem of normalizations for paleointensity in light of the distinctly different sediment lithologies recording the two events. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter II Methods Magnetic Methods All measurements were made on eight cubic centimeter paleomagnetic samples. The general sequence was to first measure initial NRM on our SCT cryogenic magnetometer. At least two pilot samples per meter of each core were measured during stepwise three-axis alternating field demagnetization to 100 mT, the limit of out Schoenstet AF demagnetizer. ARMs were imparted to samples by the application and gradual removal of 100 mT alternating field in the presence of a constant (DC) 500 mT field aligned with the sample Mx" (north) direction. All susceptibilities were measured on samples in a Bartington bridge at the low frequency setting and in SI units on 8 cc samples. IRM was imparted to samples by brief exposure to large DC magnetic fields created by the electromagnet associated with a EE&G PARC vibrating sample magnetometer. Saturation values were imparted at 1.25 T applied field, and "backfield" IRM was likewise given at -.IT and -.3T relative to the SIRM direction. IRM measurements were made on a Molspin portable spinner magnetometer. The "S" ratio was calculated as IRM(_.3T)/SIRM or, if specified, IRM(_.1T)/SIRM and "H" as (SIRM +IRM,_.3T))/2, or (SIRM +IRM,..3T))/2 when specified. pIRM is defined as IRM(.3T)-IRM(.1T). 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Magnetic Mineral Separation Magnetic separates were recovered from paleomagnetic (2x2x1.8 cm) sediment cubes at 22 horizons in three cores (arrows, Figure 7). Each sample was air dried and then immersed in 50 ml of a 10% Calgon-water solution. Calgon helped significantly to de-flocculate the clayey sediment fraction. The sediment was further dispersed by shaking capped 50-ml beakers and occasional sonification. After each sample contained no noticeable clasts op nodules, it was shaken up and then a 2-cm rare-earth magnet wrapped in parafilm was suspended in the beaker. After almost all of the sediment had settled to the beaker bottom, the magnet was removed and carefully rinsed to remove any non-magnetic material that had adhered to it. Then the magnet was removed from the parafilm and magnetic material that remained on the parafilm was washed into a small petri dish. The magnetic material was then re-suspended in water and pipetted onto one or two carbon-coated copper TEM grids. Occasionally, this process was repeated a second time to produce as many as four grids per sample. A classification of 'abundant', 'common', or 'rare/absent' magnetosomes was based on magnetosome abundance relative to background 'detrital' magnetic material, on all grids, many grains were examined using energy-dispersive X-ray analysis to determine proportion of Fe, Ti, 0, S, Si, and Mn. 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. XRF Studies Sediment major and trace element chemistry was determined using an automated, wavelength-dispersive, Rigaku 3070 X-ray fluorescence spectrometer. Sample preparation methods were modified from (Harvey et al., 1973; Norrish and Hutton, 1969). Samples were powdered in a tungsten-carbide grinding mill and oven-dried for >24 hrs before weighing. 4 grams of sample were homogenized with cellulose binder and pressed into pellets for trace element analyses. 0.8 grams were fused with lithium tetraborate-lithium carbonate flux containing La203 as an absorber and cast into glass discs for major element analysis. The mass lost on ignition was determined gravimetrically using USGS, NBS, and SSC calibration standards. Major (trace) elements are typically accurate to <1-3% (<l-9%) of amount present. Chemical Leaching Experiments Chemical leaching experiments were carried out using the methods of Canfield (1988). Separate 0.1 gr samples of dried/weighed sediment were digested at room temperature for differing periods in 40-50 ml of various leaching solutions (Table l, in chapter VI) in order to solublilize specific iron minerals. The leaching solutions were subsequently centrifuged, filtered and diluted before analysis by atomic 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. absorption spectroscopy using Fe standards prepared in a similar matrix. The precision of replicate leaches was 5-10% of the iron percentage measured. Recipes are as follows: (1) A buffered solution (pH 4.8) was prepared by dissolving 58.8 gr of sodium citrate (Na3C6H507'2H20) and 20.0 ml glacial acetic acid (HC2H302) per liter of solution. For leaching, 2.5 gr sodium dithionite (Na2S204) was added to 50 ml of this solution. (2) A solution 0.2 M in oxalate was prepared by dissolving 16.1 gr ammonium oxalate ((NH4)2C204) in 570 ml H20 and adding 430 ml of 0.2 M oxalic acid (18.0 gr H2C204 per liter) to adjust the to pH 2.00. (3) A 1 M solution of hydroxylamine HC1 (NH20H'HCL) was prepared by dissolving 70 gr in one liter of 25% V/V acetic acid. Leaching time was 56 hours. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter III Environmental Factors as Complicating Influences in the Recovery of Quantitative Geomagnetic-Field Paleointensity Estimates from Sediments Abstract We have recently recovered replicate records of the Earth's magnetic-field (relative) paleointensity for 12- 71,000 years BP from marine sediments of the western North Atlantic Ocean. Our records are remarkably similar to two other recently published relative paleointensity records from marine and lacustrine sediments in Europe. This suggests to us that, even over this wide region, sediments may serve as reasonable, correlatable recorders of fluctuations in the strength of the Earth's magnetic field. If this is true, then similar records from around the globe could eventually provide valuable information about the deep-Earth processes which create the field. We note, however, that our intensity record is significantly correlated with the down-core ratio of magnetic susceptibility (x) to anhysteretic remanent magnetism (ARM). This ratio is a measure of the relative grain size of magnetite (the primary magnetic mineral in these sediments), and as such is a sediment magnetic property which is 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. controlled only by the local depositional environment (and indirectly by global climate). We are concerned that the European records may be similarly biased by climatic or other environmental factors, possibly synchronous with ours. We caution that extreme care must be taken to understand and remove any such sediment magnetic influences from sediment relative paleointensity records before they are used as quantitative estimators of the past intensity of the Earth's magnetic field. Introduction The variability of the Earth's magnetic field during prehistoric time can only be measured indirectly by studying the natural remanent magnetization (NRM) trapped in sediments and rocks as they form. Sediments have been used routinely to recover records of paleomagnetic field secular variation (PSV) (e.g., Lund, 1996; Turner and Thompson, 1981), but such sediment paleomagnetic studies commonly only recover the vector pattern of PSV. This is due to the fact that records of paleomagnetic field intensity variation are somewhat more difficult to recover as part of normal sediment paleomagnetic studies (Tauxe, 1993). Even so, there has recently been a strong increase in the number of paleointensity records derived from sediments (e.g., Tauxe and Valet, 1989; Trie et al., 1992; Thouveny et al., 1993; Weeks et al., 1995), and these records have been used to 16 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. place important new constraints on the pattern of PSV over the last 100,000 years. We have recently recovered a vector PSV record from deep-sea sediments of the Blake Outer Ridge (western North Atlantic Ocean) which span the last 100,000 years (Lund et al., 1989; Lund, 1993). We report here our first attempt to recover a paleointensity record from these sediments. We note that, while our final composite record is comparable to other published records cited above, it is flawed by problems associated with environmental influences on the sediment magnetism. We think that such environmental effects are more common than is normally reported, and that future paleointensity studies must give more careful consideration to these complications before they can be used as true quantitative indicators of paleointensity variation. Blake Outer Ridge Relative Paleointensity Record We have recently recovered two long (15-20 m) piston cores (CH88-10P and CH88-11P separated by «150 km) from the Blake Outer Ridge, a region of anomalously high sedimentation rates (>20 cm/kyr; Johnson et al., 1988; Haskell et al., 1991) in the western North Atlantic Ocean (Figure 3-1). Sedimentology studies of carbonate content (Figure 3-2) and grain size distribution, as well as chronostratigraphic studies using oxygen isotopes have been 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 0 M D 8 4 6 2 9 L A C ' D U B O U C H 3 0 C H 8 8 - 1 1 P D E D 8 7 0 7 D E D 8 7 0 8 KET8251 C H 8 8 - 1 O P 70°W 3 0 ° W5 0 ° W 1 0 ° W 1 0°E Figure 3-1: Site map for sediment cores cited herein that estimate the Earth's magnetic-field intensity variation for the last 70,00 years. 18 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60r Stage 2 1 Stage 3 Stage 4 Stage 5 1 :r l%% C A R B O N A T E , t j K ' I I I ! : 1 N R M 1 A A l )JL [*'wifVAY\ ■ I I l i t ***** 5 10 D E P T H (M) Figure 3-2: Sedimentological (Haskell et al.,1991) and rock magnetic data from Blake Outer Ridge core CH88-10P. SPECMAP 5180 stage boundaries are shown for clarity. Rock magnetic data from 5180 stages 2-4 were used to reconstruct an estimate of the magnetic field paleointensity between 12- 71,000 years BP. NRM - natural remanent magnetization, ARM - anhysteretic remanent magnetization, SIRM - saturation isothermal remanence, CHI - magnetic field susceptibility. 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. carried out on these and related cores (Johnson et al., 1988; Haskell et al., 1991). They indicate that both of the cores extend back to fi180 stage 5 (ca. 100,000 years BP). We have also carried out detailed paleomagnetic (Lund, 1993) and sediment magnetic studies on these cores in order to better understand the pattern of prehistoric magnetic field variation recorded within them. We have measured the natural (NRM), anhysteretic (ARM), and saturation isothermal (SIRM, acquired at 1.25 T) remanent magnetizations and susceptibility (x) in both of the cores. Detailed alternating-field (a.f.) demagnetization (eight steps up to 100 mT) of the various remanences in selected samples (every 50 cm in each core) indicate that the NRMs are well-behaved, exhibiting univectorial decay to the origin, and that the NRM a.f. coercivity is more similar to that of ARM than SIRM. On the basis of these observations, all remaining samples in each core received measurements of NRM, ARM, SIRM, andxr with a.f. demagnetization of the remanences at 20 and 60 mT. The stratigraphic variations of the various sediment magnetic parameters within core CH88-10P are shown in Figure 3-2. Almost identical variations were also noted in core CH88-11P. Several previous sediment magnetic studies (Levi and Banerjee, 1976; Banerjee et al., 1981; King et al., 1983; see also Tauxe (1993) for a recent review) have established 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■I1 i i i — i— t t - j — i— i— i— r - r — i— i— i— i— |— i— n — m — i— i— i— p - |— r Stage 2 Stage 3 Stage 4 ? C H 8 8 - 1 O P £ 0.4 C H 8 8 - 1 1 C O M P O S I T E 5 10 D E P T H (M) Figure 3-3{ Relative paleointensity estimated from the ratio NRM20/ARM20 (NRM and ARM after alternating-field demagnetization at 20 mT) for cores CH88-10P, CH88-11P, and their stacked average (10-cm averaging interval). 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. both a general method for recovering relative paleointensity estimates from sediments, and a set of criteria for evaluating the guality of that paleointensity record. The relative paleointensity method, first formulated by Levi and Banerjee (1976), is based on the assumption that the NRM intensity in sediments is due to both the amount of magnetic material in the sediment, which is determined by local environmental conditions, and the degree to which individual magnetic minerals are aligned during deposition, a process controlled primarily (?!) by the intensity of the Earth's magnetic field. The amount of magnetic material at each sampling horizon can be normalized by dividing the NRM by whichever artificial remanence (ARM or SIRM) best mimics the NRM a.f. coercivity spectrum. In our study, the ARM coercivity spectra always matched the NRM coercivity spectra much more closely than did the SIRM spectra; therefore, we divided the NRMs by ARMs (after a.f. demagnetization of both at 20 mT, N R M 2o / A R M 2o ) to recover two replicate, but independent, relative paleointensity estimates (Figure 3-3). In addition, the sediment magnetic data were examined in light of well-defined magnetic criteria (Levi and Banerjee, 1976; Banerjee et al., 1981; King et al., 1983) which determine when a sediment interval is suitable for paleointensity studies. These criteria are: 1) the magnetic fraction must be magnetite or a similar composition, 2) its particle size should be in the range 1-15 microns (pseudo22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. single domain), and 3) its concentration should vary by no more than a factor of 20-30. The grain size of the magnetic fraction, which is often estimated from the ratio ARM/x (King et al., 1982; 1983), appears to us to be particularly crucial, for the NRM and ARM are acquired by quite different remanence acquisition processes and the efficiency of each process is highly, (and non-linearly) dependent on grain size. On the basis of these criteria, we excluded the Holocene (fi180 Stage 1) portions of both cores from our paleointensity study for the magnetic grain size is very small (based on ARM/x) and the magnetite concentration is diluted by high carbonate content (Figure 3-2). Similarly, we excluded the y180 Stage 5 portions of both cores from consideration, for the magnetic grain size is relatively coarse (based on ARM/x) and the magnetite concentration is likewise diluted by carbonate. Within y180 Stages 2-4, the mean grain-size of the sediment ranges from 3 to 6 microns and ARM/x varies by less than a factor of three (comparable to results from Lac du Bouchet (Thouveny, 1987; Thouveny et al., 1993) and the Mediterranean Sea (Trie et al., 1992) which are discussed below). Our relative paleointensity analysis is thereby restricted to 12-71,000 years BP (5180 Stages 2-4). The results of ARM normalization for both cores are shown in Figure 3-3. Results from these cores, when 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. adjusted for differing sedimentation rates, are remarkably similar. The two sites, separated by ca. 150 km, display identical patterns of variability even for features only a few thousand years in duration. Figure 3-3 also shows a final composite record derived from stacking the two NRM/ARM relative paleointensity estimates and averaging the stacked record within 10 cm intervals. We also tested SIRM normalization methods for both of these cores and obtained relative paleointensity records that were not significantly different from those shown in Figure 3-3. We consider this similarity in the two normalization methods to provide corroborating evidence for the overall quality of the paleointensity records. Correlation With other Relative Paleointensity Records Figure 3-4 shows our relative paleointensity record plotted on the same time axis with similar published relative paleointensity records from Lac du Bouchet (France) (Thouveny, 1987; Thouveny et al., 1993) and the Mediterranean Sea (Trie et al., 1992). The chronologies for the Blake Outer Ridge and Mediterranean Sea records are based on oxygen-isotope stratigraphy, while that for the Lac du Bouchet lacustrine record is based on radiocarbon dating and pollen stratigraphy. Although these records are by no 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. l a c DU BOUCHET MEDITERRANEAN SEA Lu « 0.2 BLAKE OUTER RIDGE ARM/CHI 0 10 20 30 40 50 60 70 80 YEARS BEFORE PRESENT (x1Q3) Figure 3-4: A comparison of relative paleointensity records from the Blake Outer Ridge, Mediterranean Sea, and Lac du Bouchet, along with the ratio ARM/x (inverted) for core CH88-10P. Although agreement among the three records is by no means identical, their similarities are readily apparent, especially when dating difficulties are taken into account, remarkably good, there is also a significant correlation with the rock magnetic record. For example, dashed lines indicate the correlation between two intensity lows and ARM/x peaks. 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Of particular note is the low intensity interval seen in all of the records near 40,000 years BP (arrows in Figure 3-4). This low has been independently associated in all three locations with the Laschamp excursion, a time of unusual magnetic direction and intensity first seen in European lava flows (Bonhommet and Babkine, 1967; Roperch et al., 1988). The three records also show a distinctive intensity low near 60,000 years BP, a time of anomalously high 10Be concentration (arguably linked to low dipole magnetic field intensity) in the Vostok Antarctic ice core (Mazaud et al., 1991). Despite the strong correlation among the records both in long-term trends and in some higher frequency features as well, we are still not convinced that we have isolated a clean paleointensity signal. Some environmental effects may still be present, which have not been completely removed by ARM normalization. We have plotted the variation in ARM/x within core CH88-10P to illustrate this possible complication. ARM/xis a rock magnetic parameter which (in magnetite) is purely a function of the sediment magnetic grain size; large grains have low values relative to finer particles. The squared coherence (see Tauxe (1993) for a review) between ARM/x (paleoenvironment indicator) and N R M 2o / A R M 2 o (paleointensity indicator) from core CH88-10P reaches peak values of 0.47, which suggests that about 25% of the low-frequency paleointensity variation can be 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. explained by grain size (ARM/x) changes. Unfortunately, sediment magnetic techniques currently available cannot uniquely determine how magnetic grain size variations will affect relative paleointensity estimates in sediments. Qualitatively, however, we see marked similarity both in specific intensity lows in the Blake Outer Ridge record which match peaks in ARM/x (dashed lines in Figure 3-4), and in some broader paleointensity and ARM/x trends. At the same time, we might argue that the effects of varying magnetic grain size are encouragingly subtle, based on the broad correlation between the three paleointensity records. We must also note, however, that the Lac du Bouchet and Mediterranean Sea paleointensity records also have factor of three variability in ARM/x# and the Mediterranean Sea record has several matches between ARM/x and paleointensity peaks/troughs. Another recent paleointensity study of deep-sea sediments from the northeast Atlantic Ocean (Weeks et al., 1995) also illustrates some of the environmental complications that arise from variable magnetic mineral grain size. In their study, they calculated paleointensity estimates only from intervals where ARM/x was within a factor of ±2.5 from the average. They also used both ARM and SIRM as normalizers and noted that the two methods were similar in most intervals. Where they differed (especially 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15-25,000 years B.P.), ARM normalization bettered matched the Lac du Bouchet record (Figure 3-4) while SIRM better matched the Mediterranean Sea record (Figure 3-4). Our ARM and SIRM normalizations both better match the Lac du Bouchet record in this interval. These differences strongly suggest that environmental factors can still have a significant influence on sediment relative paleointensity records using current normalization methods. Discussion At this time, there is no unique or consensus methodology available to quantitatively evaluate to what degree environmentally-caused magnetic grain-size variations bias sediment relative paleointensity records. However, we believe that such biases are present in our paleointensity record and are likely to occur in the other two paleointensity records as well. For this reason, we caution that extreme care must be taken to understand and remove any such sediment magnetic influences from sediment relative paleointensity records before they are used as quantitative estimators of the past intensity of the Earth's magnetic field. More sediment paleointensity records and better relative paleointensity methods are needed before we can quantitatively assess whether magnetic field intensity variations are dominated by either dipolar (global) or 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magnetic field intensity variations are completely responsible for 14C or 10Be variations (Thouveny et al., 1993; Mazaud et al., 1991). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter IV Geomagnetic Field Intensity From 12-70 kybp as Recorded in the Sediments of the Blake/Bahama Outer Ridge, North Atlantic Ocean Abstract This paper reports refinements made to our recently published (Schwartz et al., 1996) record of magnetic-field paleointensity as recovered from three piston cores from the western North Atlantic Ocean. In it we compare commonly used normalizations (by magnetic susceptibility, x, anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization, SIRM) but here "locally" normalized within short time windows which display uniform sediment magnetic characteristics. We find that most features of our records are preserved regardless of normalizer choice, but we now choose SIRM as the least environmentally biased normalizer for this material. Our records preserve and agree upon a number of very high freguency paleointensity features; lacking evidence to the contrary we conclude that these are real geomagnetic signals of at least regional extent. We also identify a number of differences between our records which must be artifacts of remanence acquisition or of the coring and sampling process. We note that when such artifacts occur they are most often seen as baseline or high frequency mismatches in the absolute amplitude of events; 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the timing and placement of relative highs and lows is in general preserved. 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION Direct measurements of the Earth's magnetic field directions have been accumulating for several centuries, collected first in European observatories (McElhinny,1973). Even from such short historical records it is clear that the field is not static but rather changes continuously. Paleomagnetists, over the last several decades, have taken advantage of the natural magnetic remanences recorded in rocks to stretch knowledge of the geomagnetic field variability (paleosecular variation, PSV) backward in time (Lund, 1993). This line of study has provided the magnetic polarity stratigraphy which helped to establish the theory of plate tectonics and which has become a standard tool for dating and correlating geological materials. (Cox et al., 1968) Records of PSV have, as well, become useful in correlating sedimentary sequences for paleoclimate and other studies (e.g. Creer et al. 1990). Despite the great dating utility of paleomagnetic field variation, there is still much to learn about the dynamo mechanism it stems from. It is now well known that the Earth's magnetic field changes continuously in intensity as well as in direction. But since it is easier to produce high quality directional paleomagnetic data than it is to arrive at reliable 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. estimates of past field intensity, many paleomagnetic studies have reported directional data only. Reliable measurements of past intensity fluctuations should therefore yield valuable information in the quest toward a finer understanding the of the core dynamo process. For this reason there has been considerable interest in measuring both absolute values of past field intensity and relative intensity changes, (e.g. Mejia et al.1996, Garnier et al.1996, Lehman et al., 1996) Absolute values of field intensity require the analysis of materials such as lavas and baked archeological artifacts which contain thermally acquired remanences. In such materials the process by which the remanences were acquired in nature is relatively well understood; moreover, this process can be approximately duplicated in the laboratory. Such materials are, however, notoriously spotty in their spatial and temporal coverage, their measurement is laborious, and they are often difficult to date precisely. (Tauxe, 1993) Relative intensities can be estimated by normalizing detrital remanences in rapidly deposited sediments. In sediments the natural remanence process is more poorly understood, so we simply argue it is not time-varying and normalize NRM by some measure of the amount of magnetic material present. Such studies have the potential of offering continuous, reliable, spatially variable, albeit 33 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. offering continuous, reliable, spatially variable, albeit relative, records of past fluctuations in geomagnetic field intensity. This potential can be realized if inherent problems are overcome. This paper presents a detailed record of normalized NRM intensity for 12-70 kybp from three cores recovered from the western North Atlantic Ocean. We discuss the normalization process critically and in detail, in terms of the replicability of these records and in light of the current criteria for relative paleointensity determination. (Levi and Banerjee, 1976; King et al. 1982, Tauxe, 1993) Our principal effort will be to compare normalizations by different methods within each of our three cores and between the cores. We assume that geomagnetic field intensity was essentially uniform within the region from which we took these cores; any differences between our records or between normalization methods must be environmental in origin or otherwise explainable as artifacts. BACKGROUND Intensity information is normally recovered from sedimentary samples by the application of some normalizing parameter to the sample NRM (Tauxe, 1993). This is intended to remove the effects of varying concentrations of magnetic minerals downcore. A number of concentration-dependent 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magnetic parameters have been commonly employed in NRM normalization: anhysteretic remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM) and magnetic susceptibility (x)• Difficulties arise because these quantities depend on a variety of properties other than simple concentration, principally magnetic mineralogy and grain size. Since changes in mineralogy and grain size can be driven by climatic variability, sediment paleointensity records may be contaminated by systematic noise of paleoenvironmental origin, introduced by the normalization process. It is unlikely that any sedimentary environment will ever be identified which is completely free of time dependent depositional changes. Therefore, research has focused on establishing reasonable criteria intended to place limits on the acceptable range of mineralogic and grain size variability. Early contributions (Levi and Banerjee, 1976) focused on the importance of choosing a normalizer in the same stability range as the grains actually contributing to the NRM and (King et al., 1983) on the magnetic mineralogy (magnetite or similar), range of grain size (l-15jLim), and concentration variability (no more than a factor of 20 to 30). Subsequently the requirements have been tightened. For example, Tauxe (1993) suggests that concentration be allowed to vary no more than an order of magnitude, that various methods of normalization be tried 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and compared, and that paleointensity time series should be checked to see that they are not coherent with the normalizing parameter. Several research groups have been actively pursuing sediment relative paleointensity records. A recent paper by Lehman et al. (1996) summarizes and compares higher resolution records produced by normalization of the sediment natural remanent magnetization (NRM) from a number of sedimentary environments. The regions represented are the Azores (new work in that paper), the North Atlantic, (Weeks et al., 1995), the Mediterranean (Trie et al., 1992), the Somali Basin (Meynadier et al., 1992), the Ontang-Java Plateau (Tauxe and Shackleton, 1994), the West Caroline Basin (Yamazaki and Ioka, 1994), and Lac du Bouchet, France (Thouveny et al., 1990). Among the sedimentary records there exist common features, such as a broad intensity low between 25-45 kybp and a similar high centered at around 50 kybp (figure 4-13). These must represent substantial real variations in field intensity at least on a wide regional scale. Occasionally the records also show noticeable agreement of higher frequency features. Yet there are substantial differences between the records, particularly in the absolute amplitudes and timing of features. Fundamental questions therefore remain. Are the differences observed principally due to the well-known difficulties surrounding the relative 36 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. paleointensity experiment - sediment smearing (time integration of the signal within a single sample), subtle environmentally driven changes in magnetic (and matrix) mineralogy and grain size, imprecise dating, or inappropriate choices of normalizing parameter? Which of the observed differences are instead due to the spatial inhomogeneity of the earth's magnetic field - nondipole features which may be regionally important and possibly time-transgressive? Such site specific variations, if they are truly indicative of inhomogeneities in the spatial magnetic field over time, can provide valuable insight into the core dynamo process. The likelihood of such differences being instead due to small sedimentological effects can best be examined by careful comparison of several records from the same region. THE CORES, SAMPLING, AND SEDIMENT MAGNETISM We report relative paleointensity results from three piston cores recovered from the Blake/Bahama Outer Ridge, North Atlantic Ocean (figure 4-1). The cores, designated CH88-10P, CH88-11P, and CH89-1P, were collected in 1988-89 by Lund and Tom Johnson, then at Duke University. The cores are 15-20 meters long, and were dated and correlated at Duke by means of 5180/160 and carbonate stratigraphy, as well as 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32°N Hatteras O o Blake o :H89-1P Outer Abyssal Plain GGC31 GGC32 ® T GGC30 :H 88-11P Blake 4000 4 5 0 0 ^ dge •a P 8 \G G C 2 9 V \ X 30°N Plateau C H 88-10P ^ Bahama^ GGC23 \* \ GGC21#jf>C22 Outer Ridge ,. . JPC14 28°N 76*W 74°W Figure 4-1: Site map for the Blake Bahama Outer Ridge showing locations (circled) of the cores (CH88-10P, CH88- 11P, CH89-1P) reported in this chapter. 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. by paleomagnetic secular variation and magnetic susceptibility. The BBOR is a sediment drift, with deposition rates which are high relative to those usually seen in the deep ocean - on the order of tens of centimeters per thousand years (Haskell et al., 1991). Core CH88-10P has the highest deposition rate of our three, reaching isotope stage 5 (70 kybp) at sixteen meters depth. Core CH88-11P reaches the same boundary at about 14.5 meters and CH89-1P at seven meters. The sediments are hemipelagic mixtures of silt and clay with carbonate dilution ranging from around 5% in glacial stage material to 40-50% in interglacial sediments (Haskell, 1991). We took single (standard 8 cc) paleomagnetic samples at high resolution from all three cores. CH88-10P and CH89-1P were sampled at 2.5 cm spacing and CH88-11P at 5 cm intervals. Measurements were made of magnetic susceptibility ( X ) / natural remanent magnetization (NRM) as collected and at least two levels of alternating field (AF) demagnetization, anhysteretic remanent magnetization (ARM) with similar demagnetizations, as well as saturation isothermal remanent magnetization (SIRM) (Figure 4-2). Detailed pilot AF demagnetizations of all remanences were done on at least two samples per meter in all three cores. Figure 4-3 shows some representative Zijderveld plots of 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a . 40 Z 0 0 100 10 l/l £ 1 1 0 0 500 1000 1500 0*pth in Sadimanu (cm) Depth in Sediments (cm) Figure 4-2: Bulk sediment magnetism and percent carbonate (from Haskell et al., 1991) for cores CH88-10P (a) and CH89- l P ’(b). (CH88-11P is almost identical to CH88-10P and is not shown.) Final paleointensity determinations are restricted to data from 12-70 kyr (SPECMAP 5180 stage 2-4). 40 Vo Carbonate ■ H i — r V (- ,ARM/X ISIRM 71 s 1 8q Stagey 2-4 b. i.i .01 I o PV» > i o l - 0.1 looo too 10 Carbonate S,90Stigta2'4 J I I I L 200 400 600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. selected samples. Alternating field demagnetizations of NRM and ARM are typically very similar in these cores (figure 4- 4). For this reason we have previously (Chapter 3) reported ARM normalizations, but will comment more about that practice later in this paper. Energy dispersive analysis done in the course of electron microscopy of magnetic separates has indicated that magnetic grains contain only iron, oxygen and varying amounts of titanium. Based on this observation and the AF demagnetization behavior of our samples we interpret the principal magnetic carrier to be a ferrimagnetic iron oxide, probably titanomagnetite, but we do not rule out some degree of maghematization. Measured magnetic susceptibilty and anhysteretic remanence have very different dependences on magnetic grain size. Therefore, the variability of the ratio of ARM/x is commonly used as a rough estimate of grain size inhomogeneity within a sediment core. The ratio is fairly flat within 5180 stages 2-4 in all three cores, although it varies somewhat more in CH89-1P. (Sediment magnetic data, figure 4-2.) In the Holocene sediment in all of these cores this ratio is anomalously high compared to values seen lower in the cores. We attribute this to the presence of abundant bacterial magnetosomes (single domain magnetite on the order of 50 nm average dimension) regionally common in these recent carbonate-rich sediments (Schwartz et al., in press). In contrast, ARM/x, along with the absolute intensity of all 41 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magnetic concentration indicators, is distinctly low in the stage 5 sediments in all three cores, although this effect is less pronounced in CH88-10P. We attribute these observations to varying amounts of magnetic mineral dissolution. The interglacial sediments therefore contrast too strongly with the material deposited during oxygen isotope stages 2-4, or 12-70 kybp, and in keeping with accepted criteria we will exclude them from the present analysis. Within this restricted range, grain size in CH88- 10P and CH88-11P is reasonably uniform. Core CH89-1P, which has a lower sedimentation rate than the other two, also exhibits relatively greater magnetic grain size variation, even with the interglacial material excluded. This core is therefore less than ideal for paleointensity normalization. It is nonetheless instructive to attempt it, to see how good a record it can provide, compared to its better-behaved neighbors. NORMALIZATION We normalized each core segment by division of cleaned (20 mT AF) NRMs by each of the three concentration dependent quantities, ARM, SIRM, and x» The resultant depth series were then further normalized by division by their own means. This gave us series with comparable numerical values and 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a ° q £ r 0 z 11c o* a _ x • 3 o 1 0 a. q r 0 z 1iC O » oa £ o (/) o a.D£ r o z_ M I ? c * <££ 1299.0 3 o in i 0 OS t o a. q £tr0 - Z o 1 i Ic « 0 Q oi 1 • o I/) 0 0 1 0 2 -Eost Figure 4-3: Representative demagnetization diagrams. These are from core CH88-10P. Note the good agreement between samples from the same interval and the straight line decay to the origin after 20 mT. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o “ 5 o o -5 o “J o ~3 o 100 0 500 cm O 100 0 1000 cm O 100. 0 1(00 cm O o 100 0 600 cm O 100 0 1200 cm 100 0 1700 cm o 100 800 cm O 100 1400 cm O 100 1800 cm O mT 100 0 mT 100 0 mT 100 Figure 4-4: Representative coercivity spectra for sediment NRM, ARM and SIRM. In nearly all cases, ARM demagnetization behavior more closely mimicked NRM than did SIRM. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CH88-1 OP CH88-1 1 P CH89-1P 200 400 600 D e p t h i n S e d i m e n t s ( c m ) Figure 4-5: Initial normalization of NRM in the three cores by ARM (circles), susceptibility (diamonds) and SIRM (crosses). For most intervals in CH88-10P and CH88-11P the agreement between cores and between normalization methods is generally good. In all three cores (with the exception of Holocene material in CH89-1P) the normalizations agree well with repect to the overall patterns of variability and the placements of relative intensity highs and low. Where they differ, the mismatch is seen as a disagreement in the amplitude of peaks and troughs. 45 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. identical means of one. It was then simple to superpose the various normalizations in a single plot and visually examine them for consistency. Figure 4-5 shows the results of these normalizations for the three cores, including the Holocene and late stage 5 material. An interesting observation is that, although within each paleoclimatic "regime" we may see a reasonable pattern of possible paleomagnetic field variability, the effect of crossing into another stage is a steep offset of the baseline - a "DC" shift in the record. Stages 2-4 CH88-10P and CH88-11P CH88-10P has the highest deposition rate and therefore the best resolution of these three cores. It also has the least grain size variation, and is relatively free of chemical alterations. All three normalizations are in good agreement, sharing overall trends and most high frequency features. There are two intervals in which the three normalizers track somewhat less perfectly. One of these is the interval between six and nine meters in which the ratio ARM/x is also somewhat enhanced; the other, from nine to eleven meters, has a slightly depressed ARM/x- We culled each of these intervals from the depth series to examine 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. separately (figure 4-6). For this purpose we renormalized the short intervals using their individual (internal) windowed means. This allowed us to examine the shorter term intensity variations within segments with uniform properties, unskewed by possibly extreme values from outside intervals. Within these shorter windows we see that again the three curves have markedly similar general shapes; they differ only in the amplitudes of their variability. We note that where the three curves diverge ARM is the outlier, while SIRM and susceptibility normalizations agree almost perfectly. We have long suspected that ARM variability in our cores in controlled by very fine grained magnetite which contributes poorly to the sediment NRM. (Chapter V) In these cores, we see from the bulk sediment magnetic data that overall trends in NRM, SIRM and x are similar, but that ARM has its own unique pattern of variability. That is, the ARM values are dominated, or at least contaminated, by a component which has much less influence on the other concentration-dependent quantities, including NRM. We therefore conclude that, despite its better coercivity match, ARM is not the best choice of normalizer for this core. We opt therefore to use SIRM instead, rather than susceptibility, because it is a remanence (as per Levi and Banerjee, 1976), and because susceptibility would have produced a nearly identical record. 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Core CH88-11P has a slightly lower deposition rate than CH88-10P; grain size variability, while still within acceptable limits, may be subtly larger. Again we see apparent general agreement in relative paleointensity, independent of our choice of normalizer. This general agreement breaks down somewhat in several short intervals, most strikingly in the interval between 12.5 and 14 meters, (figure 4-7) Here we see large and probably unreasonable swings in the amplitude of apparent paleointensity - but only in the ARM normalized curve. It is interesting to note, though, that even in segments where the sediment magnetic variability is a bit too large, the errors introduced are in the relative amplitudes of individual peaks and in the amplitude "offset" between differing core segments. There is no guestion as to the relative placement of higher and lower values. The difference (of ARM values relative to others) is nonetheless significant. It carries with it an increase in relative between-peak amplitude as well as a longer wavelength offset. The combination of these amounts to an approximate doubling of apparent relative changes. As we did in CH88-10P, we looked at internally averaged short windows, each of which was approximately uniform in magnetic characteristics. Figure 4-6 shows one example. When locally averaged, the biggest differences between the normalizers often evaporated. And within these intervals, ARM again proved to be the occasionally nonconforming 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.5 900 9SO 1000 I0 S0 1100 z 0.3 0 550500 700 750 aoo 850 900 O.S 1250 1300 13 SO 1400 Figure 4-6: NRM/ARM(circles) NRM/SIRM(crosses) and NRM/x (diamonds)for (a)the interval from six to nine meters in core CH88-10P. For this plot the three were normalized by their average value in this window only. In this interval, x and SIRM normalizations agree markedly, while ARM is somewhat different. ARM in this interval is probably responding to a slight extra flux of very fine material which is somehow not reflected in the sediment NRM. (b) the interval from nine to eleven meters in CH88-10P. Again x and SIRM normalizations agree but ARM does not. This must reflect a subtle shift to coarser magnetic grain size in this limited interval, (c) twelve and one half to fourteen meters in core CH88-11P. Restricting renormalization to this more uniform interval produces better between method agreement than seen in the whole core measurements (figure 5). In particular, the very high values of NRM/ARM seen in the whole-core normalizations are absent. These are apparently an artifact produced by extreme values of ARM from outside the interval. 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. parameter. With good agreement between SIRM and X/ we again chose SIRM as the preferred parameter for normalization. CH89-1P CH89-1P provides less than satisfactory material for relative paleointensity normalization. Its deposition rate is on the order of half that of the other two cores, or about 10 cm/kyr overall. Worse, overlapping of the three normalization methods shows poor agreement over most of the time interval in question. And worst of all, both ARM and susceptibility normalizations extend the amplitude of variability considerably past the more reasonable range established by CH88-10P (fig 4-2). Even so, we note that all three normalizations agree internally as to the number and location of intensity peaks and toughs. Their serious disagreement is again in the amplitude of intensity changes. Of interest to us, too, is the observation that the pattern of the disagreement between normalizers within the single core changes abruptly at clearly visible sediment magnetic boundaries. To first order, we see that the three normalizing methods are in decent agreement, and within reasonable bounds, between 1.5 and 3 meters. From 3 to 4.2 meters the three are hugely different, with the susceptibility curve significantly high and the ARM curve lower. From 4.2 meters to about 6.5 meters SIRM and x are in 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o.s 0.5 150 200 . 250 300 290 295 300 305 310 315 320 32S 330 0.5 0.8 0.6 320 330 340 ISO MO 370 380 370 380 390 400 410 Figure 4-7: Examples of windowed normalizations for overlapping intervals from isotope stages 2-4 in core CH89- 1P.(Similar to intervals shown as figure 6 for core CH88-10P and CH88-11P, except that in that case the windows were separated for examination only.) This core exhibits much greater variability in sediment magnetic characteristics than do the other two. Nevertheless, there is still fair to excellent local agreement between normalization methods within relatively uniform intervals. In an attempt to reconstruct a relative paleointensity record from this flawed material, we used the locally normalized (to the interval means) NRM/SIRM values from each of the separate intervals, and tied them together by the algebraic addition of small constants to each segment - for continuity across the segment boundaries. 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reasonable agreement, but ARM is moderately higher. We wondered if part of the expected paleointensity signal might still be present in this flawed record. The agreement among the three normalizers as to the location of peaks and troughs was encouraging. We also noted the presence of a few of the shorter duration features we had seen in the other two better-behaved cores. We therefore mentally divided the core along apparent sediment magnetic boundaries into several overlapping shorter intervals, 1.5-3 meters, 2.9-3.3 meters, 3.2-3.8 meters, 3.7-4.15 meters, 4.04-4.6 meters, and 4.5-6.6 meters (fig 4-8). We renormalized each of these intervals separately with its own internal mean value, (as described above for CH88-10P and IIP, where, however, we isolated windows for examination only.) Within these restricted, more uniform, intervals the three normalizations are in better internal agreement; the mismatches in amplitude are lessened by this process but clearly not completely removed. The choice of a "best" normalizer was less compelling in this core than in the others. We again chose SIRM, but here only in the interest of consistency. To reconstruct a continuous record from the overlapping small pieces we assumed the youngest SIRMnormalized segment to be "correct" and tied the rest of the strands to it by the algebraic addition of small appropriate constants. The result of this operation can be seen in figure 4-8. 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RESULTS Figure 4-8 shows our best estimate of relative paleointensity as recorded by the three cores, when correlated by paleosecular variation (PSV) and carbonate stratigraphy to a common (depth in CH88-10P) scale, and each smoothed with a three point running average. For reference, the intensity low at nine meters corresponds to the Laschamp Excursion, for which we have excellent directional data in two other cores. (Lund et al., in review.) To the first order, cores CH88-10P and CH88-11P agree very well in the number and timing of large-scale paleointensity peaks and troughs. For the most part the two cores are also remarkably similar in their fine-scale detail. For example, the broad peak between six and eight meters has three smaller subpeaks in both records. And we see a few such high frequency features preserved even in the much lower quality reconstructed record from CH89-1P. These "wiggles" then, represent rapid variations which are replicable and coherent over a fairly wide region. We assume that these represent real geomagnetic field fluctuations, although we cannot positively rule out regionally coherent environmental effects which are too subtle for us to see in this data. We observe a few small differences between the two higher quality records as well. These differences manifest themselves again principally as disagreements in the 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. amplitudes of individual peaks. A good example of this is the high intensity peak slightly deeper than ten meters (figure 4-9) in CH88-10P. It reaches nearly twice the coreaverage value in CH88-11P, where it represents the highest paleointensity in the record, but it is only about 1.5 times the average in CH88-10P. We see nearly the opposite mismatch between the two records at fourteen meters, where the highest value in CH88-10P is comparatively muted in CH88- 11P. There are several other examples of this relative amplitude stretching and shrinking. Such disparities in "peakiness" cannot be due to geomagnetic field differences; they must be artifacts of some sort, most likely of subtle environmental or mechanical origin. Examination of the raw data around the ten meter peak (fig 4-10) reveals a somewhat noisy NRM in CH88-10P relative to CH88-11P. Perhaps, in this instance, a physical disturbance, either in situ or during the coring process, has disturbed the grain alignment. But other explanations are possible here and at the other peakheight anomalies. The two cores vary in deposition rate patterns in a slightly out of phase manner; it is possible that the magnetic properties of the sediments vary similarly, but very subtly. Our data do not allow us to evaluate this possibility. It is also possible that some differences are related to differential amounts of smearing and perhaps even aliasing of the signal between the cores. It is not always obvious which record is "correct" in a 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. particular interval. Only the future recovery and careful analysis of new records from the region can completely sort this question out. The process of internal interval normalization of the flawed core CH89-1P was successful at removing the big amplitude-offset artifacts caused by the core's large sediment magnetic changes. The process also necessarily removed real geomagnetic amplitude information on the same scale. Comparison with the other two cores confirms that this loss was most severe at long periods, but the relative heights of smaller peaks were affected as well. The distortion manifests itself both as a loss of the low frequency signal and as stretching of some smaller peaks and compression of others. Nevertheless, we believe we have recovered much of the high frequency signal even in this core. An overlapping plot of the three records (figure 4-8) shows that the overall pattern of intensity variation is recorded similarly in all three cores. CORRELATION WITH THE NORMALIZERS It is important to examine normalized intensity records to see if they bear any resemblance to the normalizing parameter. For this paper we choose to present such comparisons in the time domain rather than as coherence spectra (as in Tauxe and Wu, 1990) because this simplifies 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. comparison of the different normalizing methods. For core CH88-10P, in the interval of isotope states 2-4, we have normalized each of the sediment magnetic series (x, ARM, and SIRM) by division with its own mean. Thus, each normalizer (as well as each normalized intensity series) was given an artificial mean of one. We then plotted each intensity series as a scatter plot against its normalizer, and calculated the best-fit regression line. For data prepared this way, a perfect correlation would produce a straight line at negative forty-five degrees (slope = -1) going through the origin. For CH88-10P the correlations are small (figure 4-10). Susceptibility vs NRM/x produced the weakest correlation - R2 =.0625 with slope= -0.526. SIRM did only slightly worse, R2 = .0747 and slope = -.536, but ARM showed a still small but more significant correlation: R2=.2561 and a steeper slope, -.815. This is another piece of evidence that ARM may not be the best choice of normalizer in this sediment system. We interpret the slightly better performance of susceptibility over SIRM to be due to the contributions of nonremanent iron-bearing minerals (such as silicates) to bulk susceptibility. DISCUSSION OF NORMALIZATIONS We have recovered two high quality and one "almost passable" records of relative paleointensity for 12-70kybp 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 CH88-1 OP 2 n c 1 CO0)2 CH88-1 1P c o 05Q. 4-* J5 (V cc CH89-1P 2 0 2 4 6 8 10 12 14 16 Depth in Core CH88-1 OP (m) Figure 4-8: Compilation of the final (SIRM normalized) paleointensity time series; between core correlations were done by consideration of paleomagnetic directional data and carbonate stratigraphy. The two better records are remarkably similar, especially in the number and placement of relative paleointensity highs and lows; even the (restored, see figure 4-7) flawed CH89-1P record contains most of the major features. Between core discrepancies are seen primarily as mismatches in the "peakiness" of the records. Rather than stack these records we report our results from CH88-10P as our preferred paleointensity record. The bottom curve is a compilation of the three above; CH88-10P a solid line, CH88-11P open circles, and CH89-1P, squares. 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. from the BBOR. We see strong agreement among these records, making us optimistic that we are seeing real geomagnetic field variability. Where we do see large and small disagreements among our records, either between normalization methods or between cores, the differences nearly almost manifest themselves as mismatches in peak amplitude. These disparities have to be artifacts of some part of the recording or recovery process - either during NRM acquisition itself within the sediment, or of coring or normalization. It is likely that each has a different cause, some of which we have speculated on above. In any case, it seems to be a bit too easy to lose amplitude information, even when the general patterns of paleointensity seem to be faithfully preserved. Peak amplitude errors should cause relatively minor problems in paleointensity studies produced for the purpose of intercore correlation and relative dating. Such errors are much more significant as we attempt to use the records to constrain theoretical models of geomagnetic field behavior. In this case, we want to be more certain that we have achieved truly quantitative estimates of intensity variation. One example of such theoretical use is the recent controversial "saw-tooth” patterns reported by Valet and Meyandier (1993). Perhaps it is necessary to remain openminded in interpretation of such patterns until several more replicate records are obtained and carefully analyzed. 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.5 CH 88-1 OP 2 1.5 0.5 CM! O 0 800 850 900 950 1000 1050 1100 o CM 2.5 CH88-11P 2 1.5 1 0.5 0 700 750 800 850 900 950 1000 Depth in Sediments (cm) Figure 4-9: Raw NRM data in the neighborhood of one interval of amplitude disagreement between CH88-10P and CH88-11P. In this interval the NRM signal in CH88-10P is somewhat noisy. Perhaps here there was some physical disturbance, either in situ or during coring, which has disturbed the NRM. Reasons for other between-core discrepancies must be somewhat different but equally subtle. 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a. I 5 ) 2 i ; 3 3.5 l S3 I N R M /A R M 2 25 15 iyi 0.5 0 9 0.5 1 1.5 2 N R M /SIRM * >•» 0.5 0 o as i is t NRM/g Figure 4-10: Scatter plots of normalized intensity vs its normalizer for (a) NRM/ARM, (b) NRM/SIRM, and (c) NRM/*, for core CH88-10P. In these plots, perfect correlation would be seen in a regression line of slope*-1 and R2*1.00. NRM/ARM shows small but significant correlation with ARM; slope*- 0.81 and R2=.26. For SIRM and x the correlation is negligible. The marginally better performance (lack of correlation) with x is possibly due to the contribution of nonremanent iron-bearing minerals, such as silicates, to the saidple susceptibility. 60 4‘«0.Q74«SS Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. AGE (kyears) 140 120 C H 8 9 - 1 P 00 8 0 C H 8 8 - 1 1 P C H 8 8 - 1 O P 6 0 4 0 20 0 5 0 0 1000 1 5 0 0 2000 Depth in Sediments (cm) Figure 4-11: Time-depth curves for the three cores used in this study. Dating was accomplished with carbonate and y10O stratigraphy and a limited number of 14C dates. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TIME DOMAIN AND COMPARISON WITH OTHER RECORDS Cores from the North Atlantic Ocean are routinely dated by oxygen isotope and carbonate stratigraphy. PSV records give us an added between-core correlation tool with significantly better resolution. For these cores, we have also tied the relative stratigraphy to a number of radiocarbon dates (S. Lund, personal communication). Figure 4-11 displays the core chronology as time/depth curves, and figure 4-12 presents average deposition derived from them. Absolute dating allows comparison of these records with sediment records recently published. Figure 4-13 (adapted from Lehman et al., 1996 - expanded for this time interval and with the record from CH88-10P added) is a compilation of these studies for the time period reported in this paper. There are encouraging similarities between our BBOR record and the previous work. For example, we see the same general (but "bumpy") upward trend in intensity between about 25 to 12 kyr, in all but the Somali Basin record. All of the records agree that the field was quite low at around 40 kyr and that it was high at around 50 kyr. Other time intervals are more equivocal. One such example is the interval from about 30-25 kyr, during which it is difficult to trace a consistent pattern from one record to the next. For that time window our record most resembles the record from Lac Du Bouchet (Thouveny, 1993). It is difficult to 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 I i * i I r Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 © to DC c o jo if < E *- O c — © E TJ© CO 3 £0 40 30 20 " 10 - C H 8 8 - 1 0 P C H 8 8 - 1 1 P C H 8 9 - 1 P r * - 20 40 60 Age (kybp) 80 100 Figure 4-12: Averaged deposition rates for the three cores used in this study. Deposition rate is slowest in CH89-1P. As a general pattern, rates are highest during glacial stages and lowest during warm intervals. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This study VADM ( 1022am2 ) 4 - - l A z o r e s | * 6 - S f L Relative intensity Weeks et al, 1995 VADM (1 022AM2 ) 6 4 Meynadier et al, 1992 Mediterranean North Atlantic Somali Basin Relative Intensity VADM ® _ _ (1 0 22AM2 ) 4 - - Thouveny et al, 1990 Ontang-Javs Plateau West Caroline Basin >a . Lac Du I Bouchet Lehman et al, 1995 u VADM 4 (1 0 22AM2 ) I Trie et al, 1992 6 VADM 4 ( i o 22am2) Tauxe and Shackleton, 1994 Yamasaki and loka, 1994 VADM (1 022AM2 ) 25 50 AGE (kyr) (Figure adapted from Lehman et al, 1995) Figure 4-13: Comparison of paleointensity records from Core CH88-10P with other published sediment records as summarized in Lehman et al. (1995). (Original references are indicated on the figure.) Some features, such as a broad intensity low between 30 and 40 kyr, and high values ca. 50 kyr, are common to all or most of these records, even accounting for dating difficulties. The records agree less well in other time windows, e.g. 25-30 kyr. It is not clear if these differences are of sediment magnetic origin or if they are regional features of the paleomagnetic field. 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. speculate on the reason(s?) for this disagreement - regional field sources or regional/local environmental contamination? CONCLUSIONS We have recovered and compared records of relative paleomagnetic field intensity from three sediment cores from the Blake/Bahama Outer Ridge. The cores agree remarkably well on many short and long period features. These likely represent true paleointensity variations on at least a regional scale, although we cannot absolutely rule out the presence of environmental contamination which is also regional in scale. Comparison with other records from other environments makes us hopeful that the former is true. We acknowledge the presence of amplitude-related artifacts in our data, and suggest that these may be impossible to completely remove. We therefore urge caution in the use of relative paleointensity as quantitative field measurements for theoretical dynamo studies. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter V Widespread Bacterial Magnetite in Surficial Deep-Sea Sediments of the Blake/Bahama Outer Ridge (M. Atlantic Ocean) and Its Lack of Importance as a Paleomagnetic Recorder Abstract Magnetic susceptibility(x) profiles from 27 deep-sea sediment cores collected from the Blake/Bahama Outer Ridge (N. Atlantic Ocean) show a zone of anomalously high % in their surface sedimtents extending over more than 10,000 square kilometers. Sediment magnetism, solid-phase chemistry, and transmission electron microscopy suggest that this zone is related to the presence of magnetite-producing bacteria living above the current Mn+4/Mn+2 and Fe+3/Fe+2 redox boundaries in the sediment column. Crystal morphology of <0.1/um euhedral magnetite crystals, which are interpreted to be bacterial magnetosomes, suggests that several different types of bacteria may be present. Magnetosomes are rare to absent below the redox boundaries due to continued migration of the living bacterial population to follow ambient redox boundaries and/or previous environmental conditions which did not favor their growth. Detailed paleomagnetic measure66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. merits of several cores suggest that the bacterial magnetite is poorly aligned and does not contribute significantly to the sediment natural remanent magnetization (NRM) which records the Earth's paleomagnetic field behavior. These observations raise fundamental questions about when and how bacterial magnetic material can ever be an effective contributor to a sediment NRM. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction Bacteria play a critical role in the decomposition of organic matter within sediments. They mediate a variety of biochemical reactions which use successively 02, Mn+4, NOj1, Fe+3, SO, 2/ and C02 as terminal electron acceptors in organic matter oxidation (Froelich et al., 1979 and Bender et al., 1984). A number of these bacteria also grow fine-grained (<0.lpm) crystals (magnetosomes) of magnetic material, especially magnetite (Fe304), as part of these redox processes (Blakemore,1984, and Frankel and Blakemore, 1990). In fact, magnetite-producing bacteria are inferred to be the primary contributor of magnetic material to the sediment natural remanent magnetization (NRM) within siliceous (Yamazaki et al., 1991)and carbonate (McNeill, 1990) deepsea sediments. In such sediments, very little detrital magnetic material is present to account for the observed NRM. However, in most sediment magnetic studies, the association between the NRM and biogenic magnetite is simply assumed based on (1) presence of magnetosomes and/or (2) similar magnetic characteristics of bacterial magnetite and the bulk sediment (McNeill, 1990, Vali et al., 1989, and Stolz et al., 1986). Except for (Karlin et al., 1987), remarkably few studies of natural sediments have identified the stratigraphic/geochemical conditions wherein magnetic bacteria 68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32°N — 30°N — 28°N Hatteras Blake o O C H 89-1P J *J P C 6 \ m GGC31 CH88-1 IP Outer Abyssal Plain GGC32 GGC30 Blake 4000 4So„ Ridgex#<xv \GGC29 v v S kl\ CH88-1 OP Plateau -GGC24 J P C 1 3 ^ - J P C 1 2 \ Bahama^ ggc23 \*\ GGC21 ^ Outer Ridge JPC14 78°W 76°W 72 W JPC18 Figure 5-1: Map of the Blake/Bahama Outer Ridge showing the locations of 69 piston and gravity cores for which we have magnetic susceptibility (%) profiles. Only selected cores are labeled. Cores noted by solid circles contain a surface zone of high % associated with high carbonate percentages. See text for further discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reside and evaluated whether magnetosomes actually align with an ambient magnetic field to produce an NRM. Here we present a study of deep sea sediments which contain bacterial magnetite. Geochemical and sediment magnetic results are used to characterize the depositional and geochemical conditions which give rise to bacterial magnetite and its sediment distribution. Paleomagnetic data suggest that the bacterial magnetite is not a significant contributor to NRM in these sediments. This raises fundamental questions of when and how bacterial magnetite may be a significant contributor to a sediment NRM. Sediment Magnetism Sediments within 69 piston and gravity cores [10] collected from the Blake/Bahama Outer Ridge, N. Atlantic Ocean (Figure 5-1), are all well-sorted hemipelagic clayey-silts with mean grain size of a5 pm. They contain 5-50% calcium carbonate (CaC03) and « l - 2 % organic carbon (Haskell, 1991) . Organic carbon and carbonate decrease away from the coastline and toward deeper water. Magnetic susceptibility (%) records from 27 of these cores (solid dots, Figure 5-1) display a 50 to 150 cm-thick surface zone of relatively high % values (Figure 5-2) associated with high ("50%) carbonate content of Holocene age (61B0 Stage 1). By contrast, the underlying late Pleistocene (51S0 Stage 2) sediments have low ("5-10%) carbonate content 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MAGNETIC SUSCEPTIBILITY (x10“» SI) J,0.. .2,0 JPC6 GGC31 GGC32 CH88-11P CH88-10P GGC28 GGC29 BLAKE OUTER RIDGE GGC24 JPC13 B GGC23 GGC21 JPC18 A BAHAMA OUTER RIDGE C Figure 5-2: Variations in magnetic susceptibility (x) along a transect (A-B-C; Figure 5-1) of piston and gravity cores from the Blake/Bahama Outer Ridge. Note the surface zone of high x values within all of these cores. We associate the high x zone with a complex pattern of sub-oxic early sediment diagenesis and abundant biogenic magnetite. We infer that the Holocene/Pleistocene boundary occurs near the base of the high % zone in all cores based on detailed studies of selected cores (see text for further discussion). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and x- If 1 primarily indicates detrital magnetic material in sediments, the more detrital-rich sediments should have higher xvalues than carbonate-rich sediments. The opposite pattern is observed. Within an area of more than 10,000 km2, relatively high % occurs in surficial carbonate-rich sediments . To better understand this association, we carried out sediment magnetic, geochemical, and electron microscopy studies on four cores (CH89-1P, CH88-11P, CH88-10P, GGC24; Figure 5-1). Sediment magnetic results for CH88-10P are shown in Figure 5-3. These results are typical of all four cores. The 8"0 stratigraphy (Haskell et al., 1991)of CH8 8- 10P indicates that the Holocene/Pleistocene boundary lies at the high/low carbonate transition near 95 cm. Laboratoryinduced anhysteretic (ARM) and saturation isothermal (SIRM) remanences show that there are anomalously high concentrations of remanent magnetic material in the surface sediments. This is consistent with high % values, but x°an also reflect iron-bearing paramagnetic minerals, like clays, that do not have an NRM. High values (-0.95) of 'S' (a ratio of IRMs acquired in different applied fields (King and Channel, 1991)) in this interval indicate that ferrimagnetic minerals (magnetite) are the primary source of the anomalous remanence. Other sediment magnetic indicators, ARM/xandpIRM (IRM acquired between 0.1-0.3T), suggest that the magnetite 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. m <M m O —IM <o eO IM -0 SEDIMENT OEPTH (m ) Figure 5-3: Sediment magnetism of the uppermost 5 m from piston core CH88-10P. Shown from left to right: aluminum (Al) %, Fe/Al ratio, Mn/Al ratio, CaCO} %, NRM, 'S', pIRM, saturation isothermal remanent magnetization (SIRM), ARM/x, ARM, and magnetic susceptibility (x). The dashed lines indicate, in sequence from top to bottom, the boundaries between (1) 02 and Mn+4 reduction, (2) NOj'1 and Fet3 reduction, and (3) high and low carbonate (810o Stages 1 and 2). (All magnetizations xlO'2 A/m.) 73 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. is fine grained (single-domain size range; <0.1nm)even though the bulk-sediment mean grain size is -5^m. The anomalously high ARM and SIRM intensities and dominance of fine-grained magnetite in the surficial carbonate-rich sediments is typical of all four cores and could be caused by growth of bacterial magnetite within the surface sediments. Electron Microscopy Scanning and transmission electron microscopy were used to identify magnetic minerals and search for magnetosomes in cores CH88-10P, CH89-1P, and GGC-24. Electron microscopy of magnetic separates (see Chapter 2 for magnetic separation methods) revealed abundant <0 .lnm-20pin irregularly-shaped iron-oxide grains with variable amounts of titanium and <0 . lpm euhedral crystals of pure iron oxide (titanium-free). The first are interpreted to be detrital magnetite grains while the second are interpreted to be magnetite magnetosomes. The magnetosomes are usually cubo-octahedral, pseudo-hexagonal, or teardrop-shaped (Figure 5-4). We also observed one pure iron-oxide crystal in the shape of an equilateral triangle (Figure 5-4), which we interpret to be a magnetosome (Petersen et al., 1986). Assuming uniformity in our sample preparations (see chapter 2 ), we can qualitatively discern when magnetosome 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. concentrations are 'abundant' versus 'common' or 'rare/absent '. Magnetosomes are common to abundant within the highcarbonate surface interval of high ARM, SIRM, and xithey become rare to absent as ARM and SIRM values decrease, and are apparently absent in the low-carbonate sediment. In all three studied cores, high values of ARM, ARM/x, and the 'S' parameter best reflect the presence of magnetosomes. The pattern of magnetosome occurrence is illustrated in Figure 5-5. By contrast, the TEM and SEM Transect of four piston cores along the axis of the Blake Outer Ridge (see Figure 5- 1 for core locations) showing the locations of solid-phase Fe and Mn peaks (based on XRF measurements) within the uppermost 150 cm in each core. Also shown are the high/low CaC03 boundaries (Stage 1/2 boundary) in each core and the locations of sediment intervals with common to abundant bacterial magnetosomes (based on TEM observations at sediment depths indicated by arrows). Sediment coloration was also noted irregularly in the fresh sediment and is indicated by the following letters: r = subtle red hue, ry = subtle red yellow hue, gr = subtle green hue. studies indicate that detrital magnetic grains are abundant throughout the sediment column. 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5-4: Transmission electron photomicrographs of selected magnetite crystals observed between 30 and 70 cm in piston core CH88-10P (see also Figure 5-5). Magnification in a) and b) is 175,000 times, and in c) and d) 105,000. Four distinctive crystal morphotypes have been observed: cubooctahedral, pseudo-hexagonal, teardrop, triangular. The first three morphotypes are common; the fourth type (Figure 5-4c) is rare. Note also that the teardrops are sometimes well-formed (Figure 5-4c) and at other times quite irregular (Figure 5-4b). 76 Sediment Chemistry Magnetite-producing bacteria normally grow under suboxic to mildly anoxic geochemical conditions (Blakemore and Heggie, 1982 and Frankel and Blakemore, 1990). In order to better characterize the sediment geochemical conditions associated with bacterial magnetite, solid-phase sediment chemistry was determined at 10 to 20 horizons in all four cores using x-ray fluorescence(XRF; see Chapter 2 for methods) . Figure 5-3 displays the concentrations of redox sensitive metals (Mn and Fe) in core CH88-10P. It is apparent that there are 'spikes' in the concentrations of Mn and Fe (normalized to Al, an indicator of detrital flux) at depths of 65 cm and 80 cm respectively, just above the base of the high-carbonate interval. Such spikes have been noted elsewhere in deep-sea sediments (Burdige, 1993 and Lyle, 1983) 16] and are indicators of early diagenesis. Mn/Al and Fe/Al spikes are also present in the upper meter of CH89-1P, CH8 8- 11P, and GGC-24. The Mn spike marks the boundary above which free oxygen occurs and below which Mn+4 is reduced to soluble Mn+2. The oxygen loss and Mn+4 reduction are both related to the bacterially-mediated breakdown of organic matter. The Mn spike develops as soluble Mn+2 diffuses upward to the freeoxygen zone where it is re-oxidized and precipitated as Mn02 (Burdige, 1993). The Fe spike marks a similar boundary above which N O 3 " 1 is the electron acceptor in organic matter 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. iNW 0 E u Q . uj Q UJ 50 2 100 o UJ co 150 CH89-1 P CH88-11P ( 3 1 8 0 m (3 34 0 m water depth) water depth) — 5 0 k m ------- ; r x . peak ■125 km CH88-10P (3820 m water depth) Mn peak Fe peak base of high CaC03 interval (Stage 1/2 boundary) 40 kmGGC24 (4250 m SE water depth) --- n I < « TEM sample bacterial magnetosomes abundant bacterial magnetosomes common bacterial magnetosomes rare/absent Figure 5-5; Transect of four piston cores along the axis of the Blake Outer Ridge (see Figure 5-1 for core locations) showing the locations of solid-phase Fe and Mn peaks (based on XRF measurements) within the uppermost 150 cm in each core. Also shown are the high/low CaC03 boundaries (Stage 1/2 boundary) in each core and the locations of sediment intervals with common to abundant bacterial magnetosomes (based on TEM observations at sediment depths indicated by arrows). Sediment coloration was also noted irregularly in the fresh sediment and is indicated by the following letters: r = subtle red hue, ry = subtle red yellow hue, gr = subtle green hue. 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where Fe+2 diffuses upward, re-oxidizes, and precipitates ferric oxyhydroxides. Both spikes migrate upward in the sediment column as sedimentation continues and mark the current redox boundaries (Burdige, 1993 and Lyle, 1983).The locations of Mn and Fe spikes in all four of our studied cores is summarized in Figure 5-5. Bacterial magnetite is common in both sub-oxic and anoxic sediments related to the Mn+4, NO3'1, and Fe+3 phases of organic matter breakdown. Magnetosomes become rare to absent below the Fe metals spike. Their absence is likely due to movement of the living bacteria to follow the ambient redox boundaries and/or local Pleistocene conditions (lower organic flux) which did not favor their growth. Paleomagnetism Given the magnetosome abundance in the Holocene sediments and the common view that they should contribute significantly to a sediment NRM, we also considered the pattern of NRM variability in our cores. The NRM variation is always quite different from that of %, ARM, and SIRM (Figure 5-3) . NRM intensity is always low in the carbonate-rich (“50%) surface sediment and increases by a factor of 2 lower in the cores as carbonate drops to 5-10%. The detrital fraction (indicated aluminum concentration, Al%) increases by a factor of 2 in the same interval due to decreased carbonate 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18 16 • C H 88-1 OP O C H 8 9 -1 P □ GGC-24 A C H 8 8 -1 1P $ 1 2 14 2 3 < 12 10 - N*. As *D O 0 AB^Ch s R -0 .9 4 5 10 ° s sX 20 30 CARBONATE t t s o S - o 40 50 R -0 .6 3 4 0.5 1 1.5 2 NRM 0 * 0^ R-0.878 20 ARM Figure 5-6: Scatter diagrams showing the relationships between Al % (detrital flux indicator) versus carbonate (CaCO3) % (left), NRM (center), and ARM (right) within the uppermost four meters of CH89-1P, CH88-11P, CH88-10P, and GGC-24. Note the linear relationship for Al % vs. NRM (center), and the inverse relationship for Al % vs. carbonate % or ARM (left, right). The correlation coefficients for least-squares line fits are shown at the bottom of each plot. The lower correlation coefficient for Al % vs. NRM is due to the fact that NRM intensity is also strongly related to variations in ambient magnetic field intensity. (See Figure 5-1 for core locations. See Figure 5-5 for sediment chemistry and location of bacterial magnetite zones.) 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dilution. Figure 5-6 illustrates the relationships between NRM, ARM, carbonate %, and Al% in all four cores. NRM increases linearly with the detrital fraction (Al%), while carbonate % and ARM decrease. The scatter in NRM vs. Al% (Figure 5-6) is due to the fact that NRM intensity also varies with changes in magnetic field intensity (Chapters 3 and 4). The strong correlation between the NRM and detrital fraction and inverse correlation to ARM indicates that the NRM is due primarily to detrital magnetite and is not strong ly related to biogenic magnetite which is best estimated from ARM intensities (Figures 5-3, 5-6). Thus, sediment magnetic evidence for abundant biogenic magnetite does not necessitate that the biogenic magnetite contribute significantly to the sediment NRM. Discussion Magnetosomes which reside within living bacteria should not contribute to a sediment NRM since any instantaneous magnetosome alignment will be countered by continuing bacterial motion. Only after bacteria lyse (breakdown) do individual magnetosomes or magnetosome chains become available for alignment and contribution to a sediment NRM. There may be two reasons, however, why such magnetosomes are not effective contributors to the NRM in our study. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The first possibility is that most magnetosomes reside in living bacteria which follow the ambient redox boundaries. In this case, the proportion of magnetosomes which are dead and could contribute to the NRM is very small compared to the NRM due to detrital magnetite. This possibility has profound implications for the pattern of NRM that might be ultimately associated with magnetosomes, assuming they align with an ambient magnetic field after death. The living population is arguably spread over 50-80 cm of sediment thickness and may randomly see the death and lysis of individual bacteria over this whole region. If individual magnetosomes or magnetosome chains align with the ambient magnetic field at the time of death/lysis, then the NRM at any one depth in the sediment column will contain contributions from many different times (equal to the time required to deposit 50-80 cm of sediment) and the age of the average NRM direction will lag behind the actual time of sediment deposition by 25-40 cm. The first effect will produce an NRM that is quite smeared and incapable of recording high-resolution variations in the Earth's magnetic field behavior, while the second effect will significantly bias the sediment's 'NRM' age with respect to its 'deposition' age. Also, magnetosome lock-in need not occur directly at the time of lysis but could occur some time thereafter and deeper in the sediment. This would lead to an NRM that is even more strongly smeared and biased in apparent age. 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The second possibility, which cannot be rejected by our results, is that loose magnetosomes are simply too small to effectively overcome thermal agitation (Brownian motion) within excess pore space (Stacy, 1972), and do not efficiently align with the ambient magnetic field. If this is true, then detrital magnetite (>lfim)in our sediments will more efficiently align to produce an NRM and swamp (in intensity) any limited bacterial magnetite (<0.1 (im) NRM component. If either of these possibilities are true, then it is not clear whether bacterial magnetite can ever be an important or effective contributor to a sediment NRM in any deep-sea setting. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER VI EARLY SEDIMENT DIAGENESIS ON THE BLAKE/BAHAMA OUTER RIDGE, NORTH ATLANTIC OCEAN, AND ITS EFFECTS ON SEDIMENT MAGNETISM Abstract Sediment magnetic and geochemical studies of a suite of deep-sea sediment cores from the Blake/Bahama Outer Ridge (BBOR, North Atlantic Ocean) have identified two current redox boundaries in surficial (Holocene) carbonate-rich sediments over much of the BBOR. The upper Mn+4/Mn+2 redox boundary is associated with a spike in the concentration of solid-phase Mn (as Mn02); the lower Fe+3/Fe+2 redox boundary is associated with a spike in the concentration of solidphase Fe (as goethite, aFeOOH). Over much of the BBOR, high sediment magnetic intensities occur in surficial carbonaterich sediments associated with these redox boundaries and lower intensities occur in deeper (late Pleistocene) carbonate-poor sediments. This relationship is opposite to that expected if sediment magnetism simply reflects the clastic (non-carbonate) sediment fraction. The surficial, high sediment-magnetic intensities are due primarily to two factors: (1 ) magnetic mineral authigenesis associated with early diagenesis and (2 ) the presence of abundant <0.1 /um magnetite crystals interpreted to be bacterial magnetosomes. Magnetosomes are almost absent in the late Pleistocene low84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. carbonate sediments due, most likely, to local Pleistocene environmental conditions (high clastic flux, low organic flux) which did not favor their growth. The sediment natural remanent magnetization (NRM) is strongly correlated with the sediment clastic fraction and is relatively unaffected by early diagenesis and the presence of abundant bacterial magnetite. If this is typical, bacterial magnetite may be more abundant in nature, but less important to sediment paleomagnetic records, than previously thought. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION Paleomagnetic studies of deep-sea sediments have made important contributions to our understanding of the Earth's prehistoric magnetic field behavior. Such studies have provided high-resolution records of paleomagnetic field reversal transitions (e.g., Clement and Kent, 1986; Clement, 1992) and both directional (e.g., Lund, 1993; Lund and Keigwin, 1994) and intensity (Trie et al., 1992; Meynadier et al., 1992) records of normal paleomagnetic field secular variation (PSV). Paleomagnetic and sediment magnetic studies of deep-sea sediments have also made important contributions to paleoceanography. Sediment magnetic records have provided regional-scale lithostratigraphic correlations and important evidence for local paleoenvironmental changes related to global paleoclimate (e.g., Bloemendal and DeMenocal, 1989; Tarduno et al., 1991). Directional and paleointensity PSV records are also beginning to be used as high-resolution chronostratigraphic tools for regional and global-scale correlation (e.g., Meynadier et al., 1992; Lund, 1993). A number of previous studies have noted, however, that sediment magnetic records can be subtly or severely altered by early chemical diagenesis. Early diagenesis may cause the addition of magnetic material at selected horizons due to 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32°N 30°N — 28^ Hatteras Blake o OCH89-1P JPC6\ GGC31 GGC32 GGC30 T CH88' 11P \ Outer Abyssal Plain Blake 4000 4500xRid9e^ v \GGC29 CH88-10P Plateau D ^GGC24 JPC13^ JPC12 B aham aV G G C 23 \# \ GGC21 \ Outer Ridge JPC14 78°W 76°W 72.°W 70°W JPC18 Figure 6-1: Map of the Blake/Bahama Outer Ridge, western North Atlantic Ocean. Circles indicate the locations of long (15-25 m) piston and (3-5 m) gravity cores which we have studied. Core sites noted by solid (open) circles have anomalously high (low) values of magnetic susceptibility during the Holocene. The line ABC denotes a transect of cores which we have studied in more detail to delineate the cause of the magnetic susceptibility highs. (See also Figure 6-6 .) 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. direct chemical precipitation at redox boundaries or through growth of bacterial magnetic material (e.g., Karlin et al., 1987). Alternatively, it may lead to selective dissolution of magnetic material under iron or sulfate reducing conditions (e.g., Karlin and Levi, 1985; Canfield, 1987; Leslie et al., 1990). Recently, a few studies (e.g., Karlin, 1990; Tarduno, 1994) have also begun to note that patterns of early diagenesis may undergo significant time-dependent changes due to varying local paleoenvironmental factors, and these time-dependent variations can also profoundly affect sediment magnetism. We are currently studying a suite of 69 deep-sea piston and gravity cores from the Blake/Bahama Outer Ridge, western North Atlantic Ocean (Figure 6-1). These sediments have unusually high accumulation rates, typically greater than 20 cm/ky, which make them ideal candidates for the recovery of high-resolution records of paleomagnetic field behavior. We have, in fact, recovered both directional (Lund, 1993) and relative paleointensity records (Schwartz et al., 1996) of PSV from some of these cores. We have noted in these studies, however, that early sediment diagenesis can have a complicating influence on portions of our records. Moreover, there is clear indication that these diagenetic effects are time-dependent due to temporal variations in local environmental conditions. In this paper, we summarize sediment 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carbonate Percentage o 20 40 MAGNETIC SUSCEPTIBILITY STAGE I CH88-1 OP CH88-1 OP STAGE 2 JPC12 JI'L I -I C (2.21) tf(2 2 3 ) 1(3-13) 1(3-3) 0 (3.3 1) STAGE4 » r 5.1) (5.2) S Figure 6-2: Typical carbonate stratigraphy and magnetic susceptibility profiles for Late Quaternary deep-sea sediments of the Blake/Bahama Outer Ridge (BBOR). The carbonate stratigraphy has been tied to 5180 stratigraphies in two cores (Johnson et al., 1988; Haskell et al., 1991) and can be used as a proxy for the 51S0 record. Selected SPECMAP 5180 features have been labeled on that basis. Magnetic susceptibility fluctuations, along with other time-stratigraphic features not shown, can be correlated between cores spanning more than 200 km along the BBOR axis. 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magnetic and geochemical evidence for the current pattern of early sediment diagenesis on the Blake/Bahama Outer Ridge; in a separate paper, we will characterize the time-dependent aspects of the sediment diagenesis. DEEP-SEA SEDIMENTS OF THE BLAKE/BAHAMA OUTER RIDGE Depositional Setting The Blake/Bahama Outer Ridge (BBOR) is a complex submarine topographic high which extends southeastward from the North American continental margin more than 700 km into the western North Atlantic Ocean (Figure 6-1). The BBOR forms a natural barrier to the Western Boundary Undercurrent (WBUC- ), which is a relatively fast contour current flowing southward along the western boundary of the North Atlantic Ocean (Heezen et al., 1966; Amos et al., 1971; Hogg, 1983). As the WBUC flows over and around the BBOR at depths of 2500-5000 m (Haskell, 1991), it slows down, creating a sedimentary drift deposit with bulk accumulation rates of 10-60 cm/kyr (Johnson et al., 1988; Keigwin and Jones, 1989; Haskell et al., 1991). Sedimentary analyses (Johnson et al., 1988; Haskell et al., 1991; Haskell, 1991) document that the BBOR sediments are typically hemipelagic clayey silts, dark gray to green in color, with mean grain sizes near 3-6 /xm. The sediments 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. also contain 5% to 60% calcium carbonate and 0.5% to 2% organic carbon. Carbonate percentage is high during interglacial conditions and low during glacial conditions; most of the carbonate variability is due to dilution by clastic detritus carried by the WBUC, but in some time intervals and locations on the BBOR it may also be affected by chemical dissolution below the CCD. The organic content varies systematically with highest values occurring near the coastline and lower values occurring at more distal (and deeper) sites. Chronostratigraphy The chronostratigraphy of late Quaternary deep-sea sediments from the western North Atlantic Ocean, including the BBOR, has been described routinely on the basis of variations in both foraminiferal y180 and calcium carbonate (CaC03) percentage (e.g., Johnson et al., 1988; Keigwin and Jones, 1989). Carbonate percentage tends to be high during interglacial times and low during glacial times throughout this region and serves as a good proxy for SPECMAP y180 variability (Figure 6-2). We have also recovered high-resolution directional PSV records (e.g., Lund, 1993) from three of the piston cores (CH88-10P, CH88-11P, and CH89-1P in Figure 1) and they further verify the time-stratigraphic nature of the carbonate stratigraphy. Comparison of our directional PSV records and magnetic susceptibility profiles within several piston cores also 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. =5 U o <=>; "f ©- 'N/ v - © 2 u © 3 °< 2 < m _ . < w a s © X © H* Q. U J CM K Q. < 2 UJ CO ©1 Figure 6-3: Magnetic susceptibility stratigraphy along the crest of the Blake/Bahama Outer Ridge. A transect of selected cores (see Figure 6-1 for site locations) illustrates the broad region in which high values of magnetic susceptibility occur. 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 CAR3CNATE ' ■ 1 1 ■ 11 ■ ■ ■ ■ t ■ i ■ 0 5 10 15 20 OEPTH (M) Figure 6-4: Sediment magnetism and carbonate stratigraphy of piston core CH88-10P. The sediment magnetic components (NRM, ARM, SIRM, CHI) are defined and discussed in the text. Note that magnetic susceptibility (CHI), SIRM, and ARM are all high during interglacial stage 1 (Holocene) but low during interglacial stage 5. The carbonate stratigraphy (see also Figure 6-2) was used to delineate the oxygen-isotope stage boundaries. 93 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SEOIMENT DEPTH (m) Figure 6-5a: (b-c on next two pages) Sediment magnetism of the uppermost several meters of (a) piston core CH88-10P, (b) gravity core GGC-24, and (c) piston core CH89-1P. The Sediment magnetic components are defined and discussed in the text. Also shown are aluminum (Al as Al203) and normalized manganese (Mn/Al as Mn02/Al203) and iron (Fe/Al as Fe203/A1203) solid-phase metal concentrations within the bulk sediment. The spikes in metals concentrations are attributed to dissolution of metals below redox boundaries and upward diffusion to horizons where they are oxidized and re-precipitated. Dashed lines indicate depth of Mn spike, Fe spike, and base of high carbonate sediment. In (c), the sulfur measurements suggest that the iron peak at 130 cm is due to localized iron sulfide formation (see Figure 6- 7). All chemical ratios are ratios of oxide %. All magnetizations are in units xlO'2 A/m. 94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T3 U 00 in in ro Q£ 00 C \ J LO gs a o C\J CO <0 CO 50 100 150 SEDIMENT DEPTH (cm) -0 Figure 6-5b: See previous page for explanation. 95 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. uf; eg CO CO CtL a' co <M OS (VI in u. <M O 200 250 3o8 Depth (cm) 50 100 150 Sediment T Figure 6-5c: See previous page for explanation. 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. indicates that magnetic susceptibility variations are synchronous across the BBOR (Figure 6-2) and can be used as another time-stratigraphic proxy under certain conditions discussed below. Commonly, magnetic susceptibility is a proxy indicator of the magnetic material within the sediment clastic fraction and it might be expected to vary inversely with carbonate fraction. It is apparent in Figure 6-2, however, that such is not the case on the BBOR. For example, the magnetic susceptibility profiles in many cores (sites indicated by solid circles in Figure 6-1) show a strong peak in susceptibility within Stage 1 (Holocene) at a time of very high carbonate (Figure 6-2). This pattern is illustrated by a transect of magnetic susceptibility records down the axis of the BBOR in Figure 6-3. We consider this pattern to be anomalous, in part, because the magnetic susceptibilities of comparable interglacial, high-carbonate sediments within Stage 5 are always relatively low (Figure 6-2). We have previously proposed that this interval of high magnetic susceptibility is associated with magnetite-producing bacteria (Schwartz et al., 1993) and is a product of early chemical diagenesis. The sediment-magnetic and geochemical characteristics of this zone of early sediment diagenesis are described in detail below. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SEDIMENT MAGNETISM Bulk Sediment Magnetic Parameters Detailed sediment magnetic studies were conducted on four cores (CH88-10P, CH88-11P, CH89-1P, GGC24) situated along the magnetic susceptibility transect (Figures 6-1,6-3) in order to better understand the processes that have produced high magnetic susceptibility values in carbonate-rich Holocene sediments. The sediment magnetic results from core CH88-10P, summarized in Figure 6-4, are typical of the pattern over much of the BBOR. More detailed sediment magnetic records for the uppermost 5 m of cores CH88-10P and GGC24 are displayed in Figure 6-5. Measurements of laboratory-induced magnetic remanences, both anhysteretic (ARM) and saturation isothermal (SIRM) remanent magnetizations, indicate that there are high concentrations of magnetic material in the surface sediments just as was noted by magnetic susceptibility (x) measurements. ARM, SIRM, and CHI all increase by about 50% in the uppermost 50 cm or so of the sediment column, exhibit one to three peaks in intensity, then decrease to less than their surface values at depths of 100-150 cm and remain fairly constant for the next two to four meters. High sediment magnetic intensities correlate with high carbonate content 98 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Stage 1), the transitions to lower intensities correlate with the transition from high to low carbonate (Stage % boundary), and intervals of lower, more constant intensities correlate with low carbonate sediments of Stage 2. The initial rise in ARM, SIRM, and CHI values below the sediment/water interface is natural and not an artifact of higher water content nearest the sediment/water interface. (All cores were sampled one to three years after core recovery and all surface samples have similar densities.) Other sediment magnetic parameters associated with the carbonate-rich surface sediments include high values of (a) the 'S' parameter (-IRM_o.3T/SIRM; King and Channell, 1991), (b) ARM/x, (x) pIRM (IRMq.3j-IRMo.it) , and (d) both ARM and SIRM remanence coercivities. Measurements of freguency-dependence of magnetic susceptibility (xfd/ labeled 'fd' in Figure 5b) on core GGC-24 also show a peak in Xfd associated with the primary peak in x itself at 25 cm, but not with a lower peak in CHI at 55 cm. The deeper, carbonate-poor sediments have significantly lower values of 'S', ARM/x, and both ARM and SIRM coercivity. The deeper sediments do, however, have significantly higher values of the *H' parameter (IRM_0.3T+SIRM)/2; King and Channell, 1991) than do the carbonate-rich surface sediments. In some cores (sites indicated by open circles in Figure 6-1), we have detected a substantially different pattern 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of sediment magnetism in the surface sediments. Core CH89-1P typifies this group, and its sediment magnetic characteristics within the uppermost 3 m are shown in Figure 6-5c. CH89-1P has the same high values of ARM, ARM/x, and 'S' noted in the other cores. However, it has very low values of CHI, SIRM, and pIRM in the carbonate-rich sediments and relatively higher values in the older carbonate—poor sediment. The intensities of CHI, ARM, ARM/x, SIRM, and pIRM in the deeper, carbonate-poor sediments are, however, quite similar to values noted above in CH88-10P and GGC-24 for sediments of comparable age and lithology. Magnetic Mineralogy Optical, scanning (SEM), and transmission (TEM) electron microscopy were carried out on numerous magnetic separates (see Chapter II for methodology) recovered from several cores. In all samples, there is common to abundant black magnetic material having a detrital appearance (angular to subhedral grain shapes) and grain sizes of <0.1/xm to >10/im; this is similar to the grain-size distribution of the overall clastic sediment. X-ray diffraction studies and transmission electron (TEM) microscopy indicate that much of the black magnetic material is magnetite with small, variable amounts of titanium (Ti). We have previously noted (Schwartz et al., 1993) that 100 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6-6 (a-c): Photomicrographs of magnetite crystals interpreted to be bacterial magnetosomes. Magnification of a. and c. is 175,000x, of b., 105,000. (d) Photomicrograph of an authigenic manganese micronodule formed near the Mn redox boundary in core CH88-10P. Magnification 4000x. 101 TEM observations of magnetic separates from the carbonaterich surface sediments also contain abundant euhedral crystals of co.ljum pure (Ti-free) magnetite which we attribute to bacterial origin. TEM photomicrographs of bacterial magnetite grains, termed magnetosomes (e.g., Blakemore, 1982; Chang and Kirschvink, 1989), are shown in Figure 6-6. These pictures illustrate that the euhedral magnetite crystals are of several shapes: cubo-octahedral, tear-drops, pseudo-hexagonal, and triangular. All of these forms have been noted previously in other studies of biogenic magnetite within deep-sea sediments (e.g., Petersen et al., 1986). The variety of crystal shapes suggests that there may be as many as four different types of magnetite-producing bacteria on the BBOR. Assuming uniformity in our magnetic separate preparations (see Chapter II), we can qualitatively discern when magnetosome concentrations are 'abundant' versus 'common' or 'rare/absent'. TEM observations (Figure 6-7) suggest that the magnetosomes are present only in the high-carbonate surface sediment, while clastic magnetite is abundant in both high and low-carbonate sediments. Sediment color was also observed at more than 50 horizons in four cores (Figure 6-7) when sediments were dispersed in water as part of bulk sediment grain-size studies. It was noticed that the sediments in the uppermost 30 cm routinely have a pale yellow-red tint while sediments below that often have a pale red tint. We have also noted 102 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. pale green color in dispersed sediment lower (>200 cm) in cores CH88-10P and CH89-1P and in one isolated interval of Holocene sediments in core CH89-1P (Figure 6-7). Interpretation of the Sediment Magnetism The observed abundance of clastic and biogenic magnetite grains in the surface sediments (Figure 6-7) and high values of the 'S' parameter (>0.95; Figure 6-5) both suggest that magnetite is, indeed, the predominant magnetic mineral in the high-carbonate surface sediments. However, biogenic magnetite seems to be common/abundant only in the carbonaterich sediments and essentially non-existent in the lower carbonate-poor sediments (Figure 6-7). Clastic magnetite, on the other hand, may double in quantity in the lower carbonate-poor sediments as overall clastic content doubles due to reduced carbonate dilution (CaC03 content reduces from about 50% to less than 10% in Figure 6-5). In this scenario, high values of ARM/CHI and ARM, which always occur with high values of 'S' (Figure 6-5) and common/abundant observed concentrations of biogenic magnetite (Figure 7) and drop precipitously in the carbonate-poor sediments, may be the best indicators of bacterial magnetite content in the surface sediments. High values of ARM/CHI, in this case, do not indicate a uniform shift to finer overall grain size of all magnetite grains (e.g., King et al., 103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1982), just the presence of a component of very fine-grained (single-domain) bacterial magnetite with much higher ARM intensities. An alternative scenario is that the sediment provenance of the carbonate-rich (Holocene) sediments is significantly different from the carbonate-poor (Pleistocene) sediments and that clastic magnetite concentrations are significantly higher in the surface sediments. Previous sedimentology studies (e.g., Haskell, 1991) and solid-phase sediment chemistry (see below), however, argue that there were no dramatic changes in clastic sediment sources over the last 100,000 years, or so. The largest changes are probably related to increased values of 'H' in selected small intervals of the carbonate-poor sediment (e.g., Figure 6-5c) due to variable additions of clastic hematite caused by North American glacial activity (e.g., Haskell, 1991). Sediment coloration (Figure 6-7) also suggests that there is some change in iron minerals with sediment depth that is not clearly discernible using x-ray diffraction or SEM/TEM studies. Red-yellow to red colors in the uppermost sediment suggest that iron oxyhydroxides (e.g., ferrihydrite, amorphous FeOOH; lepidocrosite, gFeOOH; goethite, aFeOOH) may be common in the uppermost sediments. Some hematite (Fe203) may also be present, but relatively low values of 'H' indicate it is not abundant. Pale green colors in dispersed sediment lower in cores CH88-10P and CH89- 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IP and in one isolated interval of Holocene sediments in core CH89-1P suggest the presence of Fe+2 attributable to more reducing geochemical conditions (Lyle, 1983). High values of pIRM, SIRM, and CHI occur with red-yellow to red sediment coloration and abundant biogenic magnetite in cores CH88-10P and GGC-24 (Figures 6-5, 6-7) but low values occur with abundant biogenic magnetite in core CH89— IP (Figure 6- 7). On this basis, we suggest that pIRM, SIRM, and CHI may be more sensitive to goethite/hematite/clastic (multi-domain) magnetite concentration than to biogenic magnetite concentration. SEDIMENT CHEMISTRY X-Ray Fluorescence Measurements Solid-phase chemistry of the surface sediments was determined using x-ray fluorescence (XRF) and chemical leaching (see Chapter II for XRF and leaching methods) in order to further characterize the chemical state of these cores and search for evidence of early diagenesis. XRF defines the elemental composition of bulk sediment while chemical leaching estimates the concentration of specific minerals. Figure 6-5 summarizes the concentrations of aluminum, iron, and manganese within cores CH88-10P, GGC-24, and CH89-1P. Al is the best elemental indicator of clastic flux in bulk sedi105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ment because of the common presence of aluminosilicates in clastic detritus. Silicon (Si) is also a good clastic flux indicator, but it can be biased by biogenic silica contributions. The observed variations in Al concentrations within all studied cores are negatively correlated to variations in carbonate %. This strongly suggests that Al is a good indicator of clastic flux that is being variably diluted by carbonate . In all three cores (as well as CH88-11P) there are systematic spikes in the concentration of manganese (Mn) and iron (Fe) relative to aluminum (Al) near the sediment/water interface. Although sampling resolution is limited in some cores, a peak in Mn/Al systematically lies about 10-30 cm above a peak in Fe/Al. These metals spikes occur at slightly different depths in different cores as summarized in Figure 6-7. We have also noted one additional Fe spike at 110 cm in core CH89-1P (Figures 6-5, 6-7) that is assumed to be pyrite because it is associated with a corresponding peak in solidphase sulfur (S). Chemical Leaching Studies Leaching experiments were carried out on core GGC-24 in order to extract iron and better evaluate the relationship between the Fe metal spikes, iron mineralogy, and sediment magnetism. Chemical leaching methods of Canfield (1988; 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N W 50 £ U r i—0. UJQ Z 100 o LU <Sl 150 CH39-1P CHS 3-1 IP v.3130 .-n i 3 3-10 m .va’ er Jsotn) *a te r ieotrt) — 50 km — ^ _ Mn peak CJ123 km C H 3 3 -1 OP G G C 24 i 3320 m .■»230 m 5: *a:sr ^*o;n) -vacer -eocni 'r y j Fe peak base of high CaC03 interval (Stage 1/2 boundary) < » TEM sample bacterial magnetosomes abundant bacterial magnetosomes common bacterial magnetosomes rare/absent Figure 6-7: A summary of the solid phase chemistry noted in four cores collected as a transect along the Blake/Bahama Outer Ridge (see also Figure 6-1). The positions of Mn/Al, Fe/Al, and S/Al spikes are noted and correlated based, in part, on sediment magnetic characteristics of the cores (e.g., Figures 6-3 and 6-7). Subtle colors noted when the sediments are wet or in solution are labeled for comparison (r=red, y=yellow, gr=green). Also noted by arrows are intervals in three cores where TEM studies clearly document the presence of magnetite magnetosomes (<0.1 micron euhedral crystals) grown by bacteria. In CH89-1P there is a maximum in reduced sulfur that tracks the stage 1/2 boundary. We believe this is created by formation of iron sulfides at a boundary between iron-rich glacial sediments that have pore waters containing excess Fe+2 and low-iron interglacial sediments where pore waters contain excess S'2. This core shows evidence for some magnetite dissolution between the two iron peaks. The other three cores do not exhibit a sulfur peak and do not show evidence for dissolution at such shallow depths. 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1989) (see also Chapter II) were used which involved selectively dissolving bulk sediment in three separate chemical leaching solutions containing dithionite, hydroxylamine hydrochloride (HAHCL), or oxalate. Each leaching solution should selectively dissolve particular iron minerals, and a combination of different leaches applied to individual sediment horizons should provide quantitative estimates of the various iron phases present (Canfield, 1988; 1989). Table 6- 1 lists the characteristics of the three leaches and the behavior of iron minerals. The results of our leaching experiments are shown in Figures 6-8 and 6-9. As might be expected from XRF measurements of total iron and the bulk sediment magnetic results, the concentrations of iron (normalized to Al) removed by all three leaches are higher in the carbonate-rich surface sediments. These leaches measure easily reduced iron oxides but should dissolve little from iron silicates, the phase containing most iron. Consequently the relative amplitude of the depth dependence of easily reducible iron is greater than for the XRF data. Based on SIRM measurements and arguments similar to those presented by Leslie et al. (1990), magnetite represents less than 0.05% of the Fe%, and its distribution does not significantly influence the leaching data in Figure 6-8. One unexpected observation is that the amount of iron extracted by HAHCL is almost double that extracted by oxalate, although both should leach similar 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mn Fe dithiom te leach — 35 resi 91 20 VL. 4-> re c o SIk. HAHCL leach re u 91 “■ 0.5 $ HX a LU oxalate leach 0 50 100 GGC24 SEDIMENT DEPTH (cm) 150 IS) k/1 o y—* - (TS I” CQ -J 4> CD 0) w 1- UO. LU 4) o IS) m 3 c IS) o X) u CO t— o LU z o < z Figure 6-8: Results of chemical leaching experiments on gravity core GGC24 (Figures 6-1, 6-7). The percentage of Fe extracted by three different leaching agents, dithionite (circles), hydroxylamine hydrochloride (HAHCL, diamonds), and oxalate (sguares) are plotted on a carbonate free basis versus depth in GGC24. Each of these leaches is sensitive to a different group of Fe-bearing oxide minerals (Table 1). The magnetic susceptibility (thick line) is also plotted on a carbonate-free basis for comparison. Also shown are two intervals noted by spikes in the concentration of Mn and Fe as determined from x-ray fluorescence (see also Figure 6-7). See text for further discussion. 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Com parative Leach Efficiencies Leaching vs. Susceptibility .45 y • -0.17 ♦ 0.56* R- 0.95 * .35 wu e aHO 0.S 0.6 0.7 0.8 0.9 1 * * 9 eo le Q I.S y • 0.44 ♦ 0.032* R« 0.5 •• .5 0 5 10 IS 20 25 HAHC1 Leach Fe (wt. %) Leaching vs. Susceptibility .45 i ,s - y - 0.13 + 0.01lxR-0.65 t 3 •• o <• .25 xO 0 S 10 IS 20 25 Susceptibility (10'5 SI) Leaching vs. Susceptibility .9 • .8 .7 .6 - y- 0.48 * 0.022*8-0.87 .5 0 s 10 IS 20 2S Susceptibility (10 s SI) Susceptibility (10'* SI) Figure 6-9: Correlation between various chemical leachates and magnetic susceptibility (on a carbonate free basis) within the surface sediments of core GGC24 (see also Figure 6-8). Note the strong correlation between the oxalate and HAHCL leachates, and good to moderate correlations between each of the leachates and magnetic susceptibility. 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. phases. We are uncertain of the cause of this difference, but these two leaches are highly correlated (Figure 6-9) despite their absolute difference. The pattern of both leaches indicates that ferrihydrite (amorphous FeOOH) and lepidocrosite (crystalline SFeOOH) are more concentrated in the upper sediments. Dithionite alone appears to extract a large amount of Fe from 50 to 60 cm. A comparison of potential Fe phases which might be dissolved using Table 6-1 suggests that this peak may be due to extraction of more goethite (aFeOOHt and/or hematite (Fe203) through this zone. None of the leaches removed any anomalous amount of Fe from intervals where we have noted bacterial magnetite, but the leaching techniques lack sensitivity to observe any magnetite variations. The presence of similar and stable levels of all the leaches within the lov-carbonate Pleistocene sediments of Stage 2 suggests that some amount of iron oxyhydroxides may still be present in these sediments although their proportion to the overall clastic fraction is significantly lower than in the Holocene sediments. Interpretation of the Sediment Chemistry XRF major and trace element concentrations (data not shown), with the exception of the Fe and Mn spikes, generally follow the concentrations of aluminum and silicon. This 111 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. suggests that bulk clastic sediment composition (and provenance?) has not changed markedly over the time interval of our sediment cores. This also suggests that the surface sediments reflect, to good approximation, the percentage of Fe (per unit of clastic material) that has reached the BBOR over the last 20 ka or more. Total Fe concentrations (normalized to Al, Figure 6-5) above the primary Fe spike (Figure 6-7) are reasonably constant and probably reflect a mixed iron mineral assemblage that includes clastic magnetite (Fe304), ferrihydrite (amorphous FeOOH), lepidocrosite (SFeOOH) and/or goethite (aFeOOH), relatively minor amounts of hematite (Fe203), and a variety of iron-bearing non-ferromagnetic silicates. Variations in several sediment magnetic indicators noted above, on the other hand, suggest significant changes in concentrations of various remanent and paramagnetic iron minerals. However, the lack of a large change in the total Fe/Al ratio suggests that the sediment magnetic variations are mostly due to movement of iron between different chemical phases as part of early diagenesis. Mn and Fe metal spikes have been noted elsewhere in surficial deep-sea sediments (Lyle, 1983; Burdige, 1993) and are interpreted to indicate current redox boundaries associated with early chemical diagenesis near the sediment/water interface. In our cores, we interpret the Mn spike to mark the current Mn+4/Mn+2 redox boundary above which oxygen (02) is the primary terminal electron acceptor utilized during 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. microbial oxidation of organic matter, and below which oxygen is markedly depleted or absent and solid phase Mn+4 acts as a terminal electron acceptor and is reduced to soluble Mn+2 (Shaw et al., 1990; Reimers et al., 1992). The reduced Mn+2 is soluble and diffuses upward to the redox boundary where it is re-oxidized and precipitates as Mn02 (Froelich et al., 1979; Aller, 1980; Burdige and Geiskes, 1983). As sedimentation continues, the Mn spike will migrate upward with the current Mn+4/Mn+2 redox boundary. Optical and scanning electron microscopy (SEM) of bulk sediment smears in CH88-10P and GGC-24 revealed the common presence of black nodules <10/z to >100 (jl in diameter within about 10 cm of the Mn spikes in both cores. Figure 6-6 shows an SEM photomicrograph of one nodule from 67.5 cm in CH88- 10P which consists primarily of manganese and oxygen. These are interpreted to be manganese micronodules which form at the Mn+4/Mn+2 redox boundary and are the cause of the Mn/Al spikes in all our cores. The Fe spike is interpreted to indicate the Fe+3/Fe+2 redox boundary below which solid-phase Fe+3 serves as a terminal electron acceptor and is reduced to Fe+2 as a result of microbial activities (Lyle, 1983; Wilson et al, 1986). Fe+2, which is soluble in the absence of S"2, is released during iron reduction and diffuses upward to the zone of nitrate reduction where it is re-oxidized according to one of two postulated reactions: 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10Fe2+ + 2N03" + 14H20 =► N2 + lOaFeOOH + 18H+ Mn02 + 2Fe2+ + 2H20 => Mn2+ + 2aFeOOH +2H+ As with manganese, this internal iron cycling should produce a zone enriched in solid phase iron that will migrate upward with the current Fe+3/Fe+2 redox boundary as sedimentation continues. The dithionite leach in GGC-24 isolated a narrow zone containing high concentrations of either hematite or goethite associated with the Fe spike. From these postulated reactions, we would argue that the Fe phase is goethite (aFeOOH). Lower Fe oxyhydroxide concentrations (per unit clastic material) below the Fe spikes, based on leaching results from GGC-24, reflect selective dissolution of easily oxidizable ferric Fe phases (Fe oxyhydroxides and hematite) as part of early Fe diagenesis near the Fe spike. While pore water data were not collected from these cores, measurements of nitrate and oxygen profiles at comparable water depths to the north of our study area (Hales et al., 1994) reveal penetrations of these oxidants to depths consistent with our interpretation of the Mn and Fe spikes as redox boundaries. A second iron spike located in core CH89-1P is associated with a sulfur spike and may reflect localized precipitation of iron sulfides at the base of the Holocene highcarbonate sediments at this site. This is further supported by the green sediment color observations noted earlier at this horizon. (It is not clear whether a second Fe spike is 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. also present in CH88-10P and GGC24 and simply condensed into the small space between the upper Fe spike and the high-carbonate boundary, or not present at all.) The presence of significant hematite in late Pleistocene sediments from all of our cores (based on 'H' concentrations) as well as the continued late-Pleistocene presence of Fe oxyhydroxides in GGC-24 (and likely all of our cores) both suggest: that pore waters did not become significantly enriched in H2S (a product of sulfate reduction) in the Stage 2 sediments of any of our cores. However, the presence of pyrite framboids at greater depths in most of our cores does indicate that sulfate reduction is occurring locally. The limited organic content and large Fe supply are controlling factors which permit the continuing presence of fine-grained iron oxyhydroxides and hematite to co-exist with local sulfate reduction (e.g., Canfield and Berner, 1987; Canfield, 1989; Leslie et al., 1990) Total iron concentrations (normalized to Al, Figure 5b) above the primary Fe spike (Figure 6-7) are reasonably constant in the surficial interval where ARM, CHI, and SIRM are all increasing significantly with depth (Figure 6-5). However, all three Fe chemical leaches either remain constant or show a slight decrease (Figure 6-8) in easily leachable Fe oxyhydroxide concentration while ARM, SIRM, and CHI are rising. This increase in sediment magnetism can be best attributed to an increase in fine-grained magnetite, presum115 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ably of biogenic origin. One interpretation is that Fe oxyhydroxides are being scavenged by bacteria as they produce biogenic magnetite. The peak in ARM above the Mn redox boundary may then indicate the center of a zone of living magnetite-producing bacteria. The peak in CHI cannot be due to true single-domain bacterial magnetite, but it might be due to superparamagnetic dissimilatory bacterial magnetite. The coincidence of peaks in CHI and CHIfd above the Mn spike in GGC24 are consistent with this interpretation, for high values of CHIfd can be caused by high concentrations of veryfine superparamagnetic magnetic minerals (biogenic magnetite, in this case). ARM, ARM/X/ and 'S' all start to diminish below the Mn peak, but the most significant intensity drops occur below the Fe spike. The patterns of intensity loss are fairly complex due, in part, to the addition of authigenic magnetic minerals (especially goethite) related to the Fe-reduction front as noted by the leaching experiments. A modest peak in the 'H' parameter around the Fe spike in most cores supports the idea of localized goethite (and perhaps hematite) authigenic mineral growth. Petermann and Bleil (1993) were the first to observe the distribution of living magnetite-producing bacteria in a deep-sea setting (African coastal margin, South Atlantic Ocean) similar to ours based on several box-core transects (<200m to >4000m water depths). They noted that the bacteria 116 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. live in a sediment zone less than 10 cm thick located within, but mostly below, the zone of nitrate reduction. But, they had no evidence for what controls the lower boundary of their life zone. Their results are broadly consistent with our observations, although our assemblages seem to live at much deeper water depths (2500-4500 m) than do theirs (<3000 m ) . In our cores, the zone of nitrate reduction should occur above the Mn spikes, and we do routinely see bacterial magnetite in that region. The fact that we see bacterial magnetite almost up to the sediment/water interface may be due to bioturbation (uppermost 5-15 cm) mixing the magnetosomes upward toward the surface. One last question is why do the magnetosomes disappear in the lower Holocene sediment on the BB0R? Within most of our cores (typified by CH88-10P and GGC-24), the base of the bacterial zone matches the base of the Fe-reduction zone and base of the high-carbonate sediment. This could be interpreted to indicate either that many fewer magnetite-producing bacteria grew in the low-carbonate Pleistocene sediment (presumably due to lower organic content), or that magnetosomes are effectively dissolved by Fe reduction. The results from CH89-1P indicate that the latter is not the case, however, for bacterial magnetite exists down to the second Fe spike and their disappearance there is probably related to dissolution during sulfate reduction in the early Holocene high-carbonate sediment. It thus seems likely that 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magnetite-producing bacteria were not important in the clastic-rich, organic-poor Pleistocene sediments. EARLY DIAGENESIS AND SEDIMENT NATURAL REMANENT MAGNETIZATION A final topic to consider in this study is the relationship between the sediment natural remanent magnetization (NRM) and early diagenesis. We note that the pattern of NRM intensity variation in all of our studied cores is quite different from that of the magnetic susceptibility, ARM, and SIRM (Figures 6-5). NRM values are always low in the carbonate-rich (50%) surface sediment and increase by a factor of two lower in the cores as carbonate drops to 5-10%; the sediment clastic fraction increases by a factor of two in the same interval. This pattern suggests that the NRM is controlled by factors closely related to the clastic fraction and not closely related to any biogenically-derived magnetite. This is consistent with the suggestion that NRM resides mostly in clastic magnetic material (predominantly magnetite) and that biogenic magnetite is somehow not well aligned in the surface sediments. The bacterial magnetite may not be an effective contributor to the NRM for three reasons. First, the bacterial zone we have identified may be primarily the habitation zone of a living population. The proportion of magnetosomes that 118 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. are loose and available for alignment with the NRM may be relatively small. If the conditions at the surface were steady-state, then eventually there might be a larger number of magnetosomes to contribute to the NRM. But in our study, it appears that the magnetosomes disappear below the habitation zone due to time-dependent diagenetic conditions which did not previously support their growth in this region. A second possibility is that bacterial magnetosomes are simply too small to overcome thermal agitation (Brownian motion) within excess pore space (Stacey, 1972), and thus do not effectively align with the ambient magnetic field. If this is true, then bacterial magnetosomes may be abundant in nature, but relatively unimportant as paleomagnetic recorders. A third possibility that we assess in a separate paper is that bacterial magnetosomes, because of their small size, may be especially susceptible to electrostatic interactions with clay particles. This may seriously limit their ability to acquire a stable and un-biased NRM. CONCLUSIONS We have carried out sediment magnetic and geochemical studies on a suite of 69 piston and gravity cores from the Blake/Bahama Outer Ridge (BBOR, western North Atlantic Ocean) in order to assess the importance of early sediment diagenesis and its effects on sediment magnetism. Over most 119 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the BBOR, high sediment magnetic values occur in surficial (Holocene) carbonate-rich sediments and lower values occur in deeper (late Pleistocene) carbonate-poor sediments. This relationship is opposite to that expected if sediment magnetism simply reflects the clastic sediment fraction and the bulk sediment is variably diluted by carbonate. The observed sediment magnetic variability is due to three factors: early sediment diagenesis/authigenesis, the presence of bacterially produced magnetite (magnetosomes), and limited change in sediment provenance. Solid-phase chemistry has identified two metals spikes in several of the BBOR cores. The upper Mn spike, which occurs typically at <5-40 cm depth, is interpreted to mark the current Mn redox boundary above which oxygen (02) in the sediment pore water is reduced during microbial oxidation of organic matter, and below which oxygen is markedly depleted or absent and solid phase Mn+4 is reduced to soluble Mn+2 by the same general process. The Mn spike develops by diffusion of the soluble Mn+2 upward to the redox boundary where it is reprecipitated as Mn02, forming micro-manganese nodules. In all of our studied cores, an Fe spike is located below the Mn spike, typically at 5-60 cm depth. The Fe spike represents the current Fe redox boundary below which solid-phase Mn+4 is depleted and solid-phase Fe+3 is reduced to soluble Fe+2 during microbial oxidation of organic material. The Fe spike develops as soluble Fe+2 diffuses upward and is 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reprecipitated as ferrihydrite (amorphous FeOOH), lepidocrosite (5FeOOH), and goethite (aFeOOH). This process can be seen in several of the sediment magnetic parameters and is also substantiated by chemical leaching experiments. In one core, we also noted a second, deeper, spike in both Fe/Al and S/Al which we interpret to indicate a sulfide reduction front. The Holocene carbonate-rich sediments also have abundant <0.1 jum magnetite crystals which are interpreted to be bacterial magnetosomes. The bacteria appear to live near, but above, the Mn and Fe redox boundaries, and may play a significant role in the oxidative decomposition of organic matter in the surface sediments. The bacterial magnetite is the primary source of the unusually strong ARM values noted in the surface sediments. Magnetosomes are essentially absent in the late Pleistocene low-carbonate sediments. It is most likely that this is due to different local environmental conditions (high clastic flux, low carbonate flux) with lower organic content which did not favor growth of the bacterial colonies. The sediment NRM is strongly correlated with the sediment clastic fraction and is relatively unaffected by early diagenesis and presence of abundant bacterial magnetite. The lack of correlation between the sediment NRM and the zone of abundant bacterial magnetite may indicate that the bacterial magnetite grains (1) reside primarily in living bacteria and 121 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. are unavailable for remanence acquisition, (2) are simply too small to overcome thermal agitation and align effectively with the earth's magnetic field under the conditions found at our sites or (3) are electrostatically coupled to clay particles which limits their ability to acquire an NRM. If this is typical, bacterial magnetite may be more abundant in nature, but less important to the paleomagnetic record, than previously thought. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 1: Methods Magnetic Mineral Separation: Magnetic separates were recovered from paleomagnetic (2x2x1.8 cm) sediment cubes at 22 horizons in three cores (arrows, Figure 7). Each sample was air dried and then immersed in 50 ml of a 10% Calgonwater solution. Calgon helped significantly to de-flocculate the clayey sediment fraction. The sediment was further dispersed by shaking capped 50-ml beakers and occasional sonification. After each sample contained no noticeable clasts or nodules, it was shaken up and then a 2-cm rareearth magnet wrapped in parafilm was suspended in the beaker. After almost all of the sediment had settled to the beaker bottom, the magnet was removed and carefully rinsed to remove any non-magnetic material that had adhered to it. Then the magnet was removed from the parafilm and magnetic material that remained on the parafilm was washed into a small petrie dish. The magnetic material was then re-suspended in water and pipetted onto one or two carbon-coated copper TEM grids. Occasionally, this process was repeated a second time to produce as many as four grids per sample. A classification of 'abundant', 'common', or 'rare/absent' magnetosomes was based on magnetosome abundance relative to background 'detrital' magnetic material. On all grids, many 123 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. grains were examined using energy-dispersive X-ray analysis to determine proportion of Fe, Ti, 0, S, Si, and Mn. XRF Studies: Sediment major and trace element chemistry was determined using an automated, wavelength-dispersive, Rigaku 3070 X-ray fluorescence spectrometer. Sample preparation methods were modified from (Harvey et al., 1973; Norrish and Hutton, 1969) . Samples were powdered in a tungsten-carbide grinding mill and oven-dried for >24 hrs before weighing. “4 grams of sample were homogenized with cellulose binder and pressed into pellets for trace element analyses. "0.8 grams were fused with lithium tetraboratelithium carbonate flux containing La203 as an absorber and cast into glass discs for major element analysis. The mass lost on ignition was determined gravimetrically using USGS, NBS, and SSC calibration standards. Major (trace) elements are typically accurate to <1-3% (<1—9%) of amount present. Chemical Leaching Experiments: Chemical leaching experiments were carried out using the methods of Canfield (1988). Separate "0.1 gr samples of dried/weighed sediment were digested at room temperature for differing periods in "40-50 ml of various leaching solutions (Table 1) in order to solublilize specific iron minerals. The leaching solutions were subsequently centrifuged, filtered and diluted before analysis by atomic absorption spectroscopy using Fe standards prepared in a similar matrix. The precision of replicate leaches was 5-10% of the iron percentage measured. 124 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Recipes are as follows: (1) A buffered solution (pH 4.8) was prepared by dissolving 58.8 gr of sodium citrate (Na3C6H507'2H20) and 20.0 ml glacial acetic acid (HC2H302) per liter of solution. For leaching, 2.5 gr sodium dithionite (Na2S204) was added to 50 ml of this solution. (2) A solution 0.2 M in oxalate was prepared by dissolving 16.1 gr ammonium oxalate ((NH4)2C204) in 570 ml H20 and adding 430 ml of 0.2 M oxalic acid (18.0 gr H2C204per liter) to adjust the to pH 2.00. (3) A I M solution of hydroxylamine HCL (NH2OHHCL) was prepared by dissolving 70 gr in one liter of 25% V/V acetic acid. Leaching time was 56 hours. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 6-1: Characteristics of Leaching Solutions (From Can field, 1988). Leach Solution Composition Leaching Time(hr) Extracted Minerals Untouched M inerals Dithioni te 0.29 M sodium dithionite 0.35 M acetic acid 0.20 M sodium citrate 1 ferrihydrite lepidocrosite hematite goethite magnetite Oxalate (pH=2) 0.11 M ammonium oxalate 0.09M oxalic acid 2 ferrihydrite lepidocrosite magnetiteb hematite goethite HAHCL 1 M hydroxylamine hydro-chloride in 25% (v/v) acetic acid 56 ferrihydrite lepidocrosite goethite magnetite hematite a) silicates including nontronite, biotite, chlorite, garnet, amphibole, and glauconite were also tested. All leaches solubilized less than 10% of the iron in each of these silicates with the exception of dithionite, which dissolved 25% of the notronite. Any amorphous FeS that was originally present and oxidized during storage should be extracted. b) approximately 40% of the magnetite was dissolved. 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter VII Remanence Acquisition During Times of Rapid Geomagnetic Field Variability and Low Paleointensity: Grain size Dependence of Lock-in ABSTRACT Two short magnetite bearing marine sediment core segments, deposited during times of anomalously fast, high amplitude geomagnetic secular variation, have been demagnetized in detail. Using coercivity as a proxy for magnetic grain size, we found that "mid-coercivity" grains (those demagnetizing between 30 and 70 mT alternating field strength and presumably of detrital origin) completed the lock-in process within 10 cm or less below the last episode of mixing. It was also found that the hardest, and therefore smallest grains, achieved lock-in over a longer total time period, ranging from possibly immediately upon deposition to much later than larger grains. These biogenic and small detrital grains thus provide a relatively smeared version of the paleomagnetic variability recorded in the sediments. Morever, the hardest grain fraction was found to be poorly aligned and quantitatively unimportant to the overall sediment natural magnetization. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION Interpretation of paleomagnetic records obtained from sediments depends critically on understanding of the processes by which those sediments acquired their remanences. Those processes are incompletely understood despite the thirty years of thought and experimentation which has taken place since Collinson (1965) set out the first theoretical model of the behavior of magnetic grains falling through a column of quiet water. This theory predicts that magnetized grains should align perfectly while still within the water column; this was later confirmed by experiment. (Yoshida and Katsura, 1985) It is well known, that only a small percentage of grains remained well-aligned once they hit bottom and are randomized by a number of physical processes. Many sedimentary sequences nonetheless provide apparently good paleomagnetic records. A landmark study (Irving and Major, 1963) demonstrated that it is possible for magnetic grains to improve their alignment postdepositionally - by movement within the excess pore space which is present before the sediment has been completely consolidated. This realignment process (postdepositional remanent magnetization, or pDRM) is enhanced by mixing, probably aided by the action of benthic organisms (e.g. Kent, 1973). As a sediment is gradually buried beyond the reach of bioturbating organisms and coincidentally compacted and dewatered, the pore 128 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. spaces effectively disappear and the grains are forced to remain in their existing orientations. When the magnetic grains have finally been completely immobilized, the remanence is said to be locked in. What is the role of magnetic grain size on the pDRM process? If the effective pore spaces were to become gradually smaller during the burial of a sediment, one would expect that the largest grains would be locked first (or perhaps never oriented in the first place) with gradual lock-in over time of smaller and smaller grains. And in as much as some tiny pore spaces may never completely close, there might also be a lower size limit beyond which the tiniest grains would also never achieve permanent orientations. Another issue is the extent to which the intensity of the aligning ambient magnetic field influences the efficiency of grain alignment both in the water column as well as after the grains become part of the sediment pile. For typical values of geomagnetic field intensity, the aligning function is believed to be linear with field strength,(the principal assumption required to recover relative paleointensity estimates from sediments) but there has been some suggestion recently that the linearity might break down in conditions of very low paleointensity. (Quiddelleur et al., 1995). 129 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BACKGROUND The details and timing of DRM/pDRM acquisition in a particular sedimentary environment are difficult to evaluate. Many laboratory studies have addressed these issues(Verosub, 1977, for a review); some of the better of these (Lovlie, 1973, Tucker, 1979) have involved abruptly changing the intensity or direction of the laboratory field at various times during the deposition or consolidation of the sediment. This strategy allows measurement of the timing and fidelity of grain alignment within the artificial sediment. These experiments are instructive, but because of the impossibility of duplicating natural conditions in the laboratory, they are necessarily inconclusive. This paper presents the results of a study of the lock-in process in natural marine sediments which were deposited and consolidated during a time in which nature herself abruptly changed the direction of the ambient magnetic field several times. A major goal of the study is to examine the ability of various sized grains to respond to these abrupt changes in field direction. To this end we present results of detailed alternating field demagnetizations of segments of two deep-sea sediment cores which contain records of paleomagnetic excursions. The study suggested itself to us because of the highly unusual (for this region) multicomponent behavior noted on pilot 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. demagnetizations of a few core samples. Samples which exhibit this complex behavior share a single property; they have characteristic paleomagnetic directions which are excursionary, or else they come from a very few centimeters beneath samples which are excursionary. Within these narrow intervals the rates of change of paleomagnetic directions and intensity observed are extraordinarily rapid. We believe that these unique samples offer an excellent, and serendipitous, opportunity to observe grain-size dependence of permanent magnetic grain alignment (final lock-in.) Alternating field demagnetization ought to be a good proxy for magnetic grain size in that grain coercivity removes contributions in order of decreasing grain size. If indeed, grains also acquire final magnetic directions in a size-dependent manner, with the largest grains locking first, then the coercivity should mimic the lock-in patterns. We note, however, that when the rate of directional change of the aligning magnetic field is slow relative to the rate of remanence acquisition, all magnetic grains in the sediment should show similar directions. This is indeed what commonly happens, and in such cases we would find all coercivity fractions pointing in much the same direction, i.e., the sample should demagnetize cleanly to the origin. This ideal condition is met for most of the samples we have recovered from the region of this study, which gives us a standard with which to compare the more unusual ones. Only 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. when the field direction has changed rapidly relative to the lock-in rate should we, observe a gradual lock-in of variously coercive grains within a single sample. In this anomalous case early-locking grains show younger directions than those which lock-in later. In this case, higher force demagnetizations must show a bias, which would be observed as a turning toward younger directions with increasing demagnetizing force. Moreover, the maximum depth below a fast change in field direction at which such a grain size effect is observed can provide an estimate of the total lock-in depth of the sediment. SEDIMENTATION ON THE BLAKE BAHAMA OUTER RIDGE The sediment cores sampled for this study come from the sediment drifts of the Bahama Outer Ridge, western North Atlantic Ocean (fig 7-1). The sediments are well-sorted silts to clays with carbonate content varying from about 5- 50%. (Haskell,1991) They are, in general, well mixed. Mean physical grain size is on the order of 4-7 /xm. Sediment accumulation rates on the Blake-Bahama Outer Ridge (BBOR) are high compared to the deep sea in general. They vary with location and paleoceanographic conditions, but average 10-70 cm/kyr, in contrast to millimeter per thousand year rates seen elsewhere in the open ocean. (Keigwin and Jones, 1989) 132 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SEDIMENT MAGNETIC CHARACTERISTICS Samples for this study come from short segments of two giant piston cores, JPC 14 and JPC 22. These cores were part of a large suite collected by Lund and Lloyd Keigwin (WHOI) aboard the research vessel Knorr during a cruise in the North Atlantic in November 1993. The JPC 14 segment contains a high resolution record of the Laschamp Excursion (Bonhommet and Babkine, 1967; ca. 40,000 years bp) and the JPC 22 segment has a similar record of the Blake Event(Smith and Foster, 1969, ca 100,000 years bp). Figure 7-2 presents the directional and initial relative paleointensity results for these segments. Although the cores are not azimuthly oriented, the single segments used for this study have internally consistent relative declinations. The methodology used to obtain paleointensity estimates will be reported in another paper. We have previously done extensive sediment magnetic, geochemical, and electron microscopy studies on a suite of sediment cores from around the BBOR, and find that detrital magnetite in the size range 1 -10 nm is the usual carrier of the NRM. We have, as well, observed short intervals containing admixtures of various amounts of detrital hematite, as well as occasional additions of authigenic and biogenic magnetite and/or other iron oxides. Bulk sediment magnetic measurements for the two core segments reported on in this paper are shown in figure 7-3. The two intervals were clearly 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32°N o £ Blake 0 Blake 30°N Plateau 28°N Outer 4000' OCH89-1P n#JPC6\ \ rrr^«*GGC31\ GGC30^CH88- 11P 78°W 76°W 4S00.Rid9e • v y 28 \G G C 2 9 lV ^ CH88-10I^V \ JPC13^^ BahamaVGGC23 GGC2lApC)^ 5t 74°W 14 Outer Ride Hatteras Abyssal Plain ^GGC24 JPC12s 72°W 70°W Figure 7-1: Site map showing the Blake Bahama suite of cores we have studied. JPC 14 and JPC 22 are from the Bahama Outer Ridge, and the southern edge of this map. 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 120 z z j -40 -80 Z o H •< Z 13 <y z 60 40 -20 -40 > 33 z Cd z o Cd -1 «< a. •60 -80 0.3 0.2 Cd > < Cd ex 0.1 1SS0 1570 1590 1610 1630 1650 1670 1690 1710 S E D I M E N T D E P T H ( C M ) Figure 7-2a (7-2b next page): Initial paleomagnetic results from the two intervals reported in this paper. Directions are 20 mT cleaned; for methodology used in estimating relative paleointensity, see chapter 8. 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Relative Paleointensity Inclination D eclination 180 .06 .03 080 S ediment D epth (c m ) Figure 7-2b: See previous page for explanation. 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. deposited during times very different oceanographic conditions, and their differing magnetic properties reflect this. The glacial-age, clastic-rich (<10% carbonate) material which records the Laschamp interval in JPC14 is uniform in character, containing only minimal downcore changes in magnetic mineralogy, concentration, or grain size. The Blake Event (JPC 22), in contrast, occurred contemporaneously with the rapid climatic and oceanographic change associated with the last glacial termination, at the base of 5180 stage 5. The sediments in that interval are gradually transitional between carbonate-rich interglacial muds and the much more detritus-dominated glacial-stage material. By far the most striking and unusual magnetic feature in the JPC 22 Blake record is the order of magnitude step increase observed in ARM (anhysteretic remanent magnetization), and about half as large increases in magnetic susceptibility (x) * and IRM (isothermal remanent magnetization,) all of which occur at 2090 cm core depth. Magnetic extracts from disaggregated material from this "high ARM" interval, when examined by transmission electron microscopy, were seen to contain abundant euhedral magnetite grains in the 40-50 nm size range. These exhibit several distinctive crystal morphologies, including pseudohexagonal prisms, cubes and "tear-drops" (figure 7-4). We believe these to be bacterial magnetosomes, and interpret them to be the cause of the abrupt increase in ARM. Experience with similar material in the surface 137 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sediments (Chapter 6) leads us to suspect that the increases in susceptibility and SIRM are also related to processes of bacterial magnetite formation and early chemical diagenesis. DETAILED DEMAGNETIZATIONS In order to examine the dependence of physical lock-in on magnetic grain size alone, we carried out detailed (10 mT steps to 100 mT) alternating field (AF) demagnetizations on a large suite of samples - 43 samples from in and around the Laschamp excursion from JPC 14 (sub-sampled at 2.5 cm spacing) and 49 samples similarly selected from around the Blake Event in JPC 22 (sub-sampled at 1.25 cm spacing.) We calculated vector differences for all demagnetization steps. Figure 7-5 is a stratigraphic two component plot (inclination and relative declination) of the natural magnetizations measured for each coercivity interval in both core segments. Figure 7-6 shows standard Zjiderveldt plots (demagnetization diagrams) for a variety of samples. It is important to note that in samples deposited during times of normal polarity and relatively slow secular variation the demagnetization goes fairly cleanly to the origin. In most of our samples, we could easily distinguish three distinct coercivity-dependent directions. All of the samples carry a soft component which is removable by 10 mT in most cases, although it requires 20-30 mT for complete 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. X X 00 ARM/Chi E N < 3000 COI O 2000 xw E 20 < COI NRM o X 1550 1650 1750 Depth in Sediments (cm.) Figure 7-3a (7-3b, next page): Bulk sediment magnetism for the two core segments. Values for the JPC 14 are quite uniform throughout; the JPC 22 segment is marked by a large step function increase in all concentration-dependent parameters except NRM which occurs at exactly 20.9 meters core depth. The single point low in all values at 21.16 meters is caused by a very narrow (< 2 cm)zone of magnetic dissolution. 139 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12 10 Susceptibility 6 CO 50 x CO 10 ARM 5 ARM/Chi 2000 SIRM * 1000 10 CO NRM 0 2080 2120 2150 D e pt h in Sediments (cm.) Figure 7-3b: See previous page for explanation. 140 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 7-4: Transmission electron micrographs of single domain, euhedral magnetite, interpreted as bacterial magnetosomes. A variety of morphologies are present. Magnetization is a) 62500x, b) and c) 105000x, c) and d) 135000x. 141 OGr NRMo !• Al 0-20 too! I A30-40 1001 A40-50 = 100 A50-60 100 A60-70 100 A70-80 -100 100 1-100 A90-100 NRMioo •100 1600 1650 SEDIMENT DEPTH (cm) 17001550 NRMo A 0 - 1 0 40F 40 •4 0 4 0* A30-40 -4 0 40 A40-50 ■40 = 40 -4 0 40 A70-80 -4 0 40 A80-90 -4 0 A90-100 ' “ • 4 0 4 0* NRMioo -40 -8 0 1550 1600 SEDIMENT DEPTH (cm) 1650 DEPTH 1700 Figure 7-5(a and b): Results of vector differencing of the total vector at 10 mT intervals for single samples in JPC 14, displayed as inclination and declination for each interval. Directional signal is remarkably stable for coercivity interval fro about 20-70 mT. Note the strong present day overprint in the 0-10 mT band, (c and d, next page) As above for the JPC 22 segment. 142 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INCLINATION INCLINATION INCLINATION 80 E, N R M 80 A 1 0 - 2 0 80 A 20-30 -80 80 Hz' A 30-40 -80 80 -8 0 80 -8 0 80 A 60-7 -8 0 A 70-80 80 -8 0 A 8 0 -9 0 80 -8 0 80 A 90-1 00 -8 0 2080 2120 2150 180 -1 80 180 -1 80 180 P -1 80 A 2 0 -3 0 -180 180 A 3 0 -4 0 -1 80 180 A40-50 P -1 8 0 A 50-60 LU -1 8 0 180 A 60-70 -1 8 0 180 -1 80 A 70-80 180 < -1 8 0 □ 180 a 0 ° -1 8 0 180 A 80-90 A 90-100 -1 80 2080 2150 2120 SEDIMENT DEPTH (cm) SEDIMENT DEPTH (cm) Figure 7-5 (c and d): See previous page for explanation. 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1550A 3 r. o 2 C 5 o Q D O to o m 0 I -0 .5 0 0.5 West- -East a D ^ c o 0 2 1 m o c ? o Q. m sz o D o O 00 0.15 - 0.1 -0.05 0 West------ East 1637.5A C. o m o o o -0.1 -0 .0 5 0 d . 1690A o CN 0 1 o I -0 .4 0.2 0 West------ East West------ East Figure 7-6(a-d): Representative vector endpoint diagrams for selected samples from JPC 14. Samples 1550 and 1690 are taken from intervals of normal secular variation; 1635 and 1637.5 were deposited during the time of rapid geomagnetic directional change. 144 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 8 2 .5A f . 2 0 9 2 .5 A ci <N —0.4 — 0.2 0 0.2 V) - 0.2 0 0.2 West- -East West- -East 2097.5A h. 2126.25A a. <N o o o c° ? n i -0 .4 - 0.2 0 0.2 Q. 3 r 0 z 1 I c i o Q s£ 3 o (/) 0.04 - 0.02 0 West------------- East West- -East 1637.5A 1690A a.3 xf 10 o v_O 2 3 o cn o -0 .0 4 0.02 0 0.02 o JC 3 o in 0.15 0.1 0.05 0 West------------- East West--------------East Figure 7-7(a-d): Similar to figure 7-6, but showing only 30 70 mT portion of the diagrams. Even in this more stable interval, it was possible to see some directional movement during demagnetization, but only in samples taken from within 10 cm of those exhibiting excursionary directions. 1560 and 1690 are from intervals of normal secular variation; 1630 and 1637.5 are from intervals of rapid directional change. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. e. 2118.75A f * 2121.25A Q_ <NO o o CO (/■I -0.04 - 0.02 0 0.02 -0.04 - 0.02 0 0.02 West-------------East West--------------East 2126.25A 2081.25A CN Q_ in o CM o in - 0.2 0 0.2 -0 .0 5 0 0.05 West-------------East West--------------East Figure 7-7(e-h):Similar to 7-7 (a-d), above, but for JPC 22. 2081.5 and 2126.5 are from intervals of normal secular variation; 2118.75 and 2121.25 are from intervals of rapid directional change. 150 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. remanence which remains after 100 mT demagnetization. (See figure 7-5.) But the grains making up the finest fraction were apparently poor quantitative contributors to the sediment NRM. This is most clearly seen in the JPC 22 Blake record, where an order of magnitude step function in ARM, which occurs at 2090 cm, marks the upper boundary of a zone containing abundant magnetosomes (described above). This step-function increase in fine-grained magnetic material concentration is also easily seen in magnetic susceptibility and in SIRM as well as in ARM. Yet it is not apparent that this abundant fine fraction significantly enhances the intensity of the NRM, which shows no obvious parallel increase at this horizon (fig 7-3). Its presence as a component of the NRM is noticed only as a slight relative increase in the percentage of NRM remaining at 100 mT demagnetization. We illustrate this in figure 7-8; the ratio of NRM remaining at 100 mT over the initial NRM remains at or below .04 throughout the cores but jumps steeply within the region of maximum ARM. One other important observation can be made of the (noisy) remanence carried by the hardest grains. A comparison of the characteristic remanences with the 100 mT demagnetized versions (Figure 7-9) shows that the smallest grains produce a record of field variability which is considerably smeared and stretched. As one example from the JPC 14 record, the uppermost appearance of a negative inclination is 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. seen at 1625 cm depth, but only in the NRM remaining after 100 mT demagnetization. We may discount this as noise if we choose to do so, although previous experience with material from the BBOR suggests this is unlikely. In the sample at 1630 cm there are negative inclinations, but only at demagnetization steps greater than 70 mT. Not until 1637.5 cm is the sample unambiguously reversed. A similar pattern is seen below the excursion. The deepest occurrence of a Laschamprelated steep (<-50°) negative inclination in the characteristic remanence occurs at 1647.5 cm. There are scattered instances of steep negative inclination in the harder fractions, however, even as far down as 1675 cm core depth. A similar pattern can be observed in the JPC22 Blake record. DISCUSSION The detailed demagnetizations can be characterized by patterns of discrete behavior in soft, medium and hard coercivity bands within each sample. The softest portion we consider to have carried a viscous overprint and we will ignore it. We will consider the medium and hardest fractions separately below. The medium coercivity grains With the exception of a few samples which came from within about 5 cm below big directional jumps, our samples demagnetized cleanly within the restricted 30 - 70 mT coercivity band. In no case did any sample showed any "mid152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. range" contamination by any direction more than ten cm distant in the core. Only when the grains had "seen" extremely rapid, and high amplitude, changes in field direction was there any sign of signal smearing based on delayed lock-in of variously-sized grains, and these effects were small and subtle. Lock-in of grains in this "medium" size range was therefore nearly simultaneous and was apparently complete within a very short depth beneath the last episode of mixing. Smearing based on integration of directions over the lock-in time of the samples was therefore slight. The almost simultaneous lock-in within the midcoercivity range (as opposed to gradual lock-in involving all but perhaps the smallest and largest grains) was surprising and difficult to explain. We have no way to precisely quantify the range of actual physical grain sizes which demagnetize within this coercivity band, but based on previous work with magnetic separates from the BBOR we assume these grains to be detrital magnetite of approximately 1-10 /urn size. Perhaps the coercivity of grains within this range has a strong dependence on size-independent factors such as grain shape and lattice defects. An entirely different possibility is that the sediment contains a significant population of grains of similar physical sizes but which contain domains which are variably coercive. Such a grain would behave physically as a single unit but demagnetize gradually in a uniform direction. Yet a third 153 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. speculation is that perhaps what we see as a characteristic remanence is the outcome of a single last burst of bioturbation of this well-mixed material. In this case the time available for grains to reorient would have been small, and the size of pore spaces available to those grains also limited. This would imply that the range of grains sizes which demagnetizes over this wide band is quite narrow. The hardest grains The behavior of the magnetically harder fractions, those grains which were still remanent after 70-100 mT demagnetization, was very different. These data are somewhat noisy, which is unsurprising because of the small intensity of the remanence remaining at these levels. Yet they record the two excursions tolerably well. One surprising feature of the signal carried by the smallest grains is that it significantly stretches and smears the pattern of field variability. As we have shown, the unusual directions associated with the Laschamp and Blake events occur both higher and lower in the cores within the hardest fractions relative to the characteristic remanences. We see this effect clearly in these two core segments and also in several other cores which we are presently studying. The delayed orientation of some of the hardest grains is probably due to these tiny grains' ability to rotate freely in excess pore space within the sediment for a considerable time after larger grains have been immobilized by 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 1550 1600 1650 1700 1750 Depth in Sediments (cm) Figure 7-8a: Ratio of NRM100/NRM for JPC 14 segment. (JPC 22 segment shown at same scale, figure 7-8b, next page.) The addition of a large concentration of single domain magnetite (as magnetosomes) is seen only as a slight elevation of this ratio in the "high ARM" portion of JPC 22. 155 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.14 12 0.1 0.08 0.06 0.04 0.02 0 2080 209 0 210 0 211 0 212 0 2130 214 0 Depth in Sediments (cm) Figure 7-8b: See previous page for explanation. 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -40 JPC 14 1550 1600 16S0 1700 1750 80 40 0 -40 JPC 22 2080 2 0 9 0 2100 2110 2120 2130 2140 Depth in Sediments (cm) Figure 7-9: Characteristic inclination (closed symbols) and inclination measured after 100 mT (open symbols) for a) JPC 14 and b) JPC 22. The hardest fraction still shows the general pattern of directional variability, but in a noticeably stretched and smeared fashion. 157 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sediment consolidation and dewatering. Finding a plausible mechanism for the early orientation of other hard grains is more problematical. One possible explanation is that electrostatic coupling of small magnetic grains to clay particles may occur at the time of deposition. Laboratory experiments (Sun and Kodama, 1992) offer convincing evidence that such attachment can take place. The large resulting composite particle would require a considerable pore space to realign, and would therefore mechanically resemble a much larger magnetic grain but still be magnetically hard, magnetically hard but would have to lock-in early. Alternatively, a single domain magnetite crystal residing as an inclusion in a larger nonmagnetic grain would likewise be mechanically large and have a relatively small magnetic moment. We do not have data to distinguish between these possibilities. While it is clear that a few of the hardest grains lock in throughout the remanence acquisition process, including both earlier and later than larger grains, we suggest that the majority of single domain particles in this sediment may be unable to align at all. In the Blake interval of JPC 22, and in similar intervals in other BBOR cores, large observed increases in single-domain magnetic concentration (indicated by, ARM, IRM, and ARM/x) are essentially invisible in the NRM records. In addition to this general observation, we have the NRM demagnetization behavior of the two records focused on in this paper. Although these two 158 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. records contrast markedly in the amount of single domain material they contain, their NRMs demagnetize almost identically. The only NRM clue to the increased abundance of the finest fraction of JPC22 compared with JPC14 is a slight increase in the relative intensity of remanence remaining after 100 mT, coincident with the interval of maximum ARM in JPC22. (figure 9) We argue that despite the obvious abundance of magnetosomes in the JPC 22 record, the observed NRM is carried largely by small multidomain detrital magnetite grains, just as it is in the non-magnetosome bearing JPC14 and indeed in most of our BBOR material. We can only speculate on the reasons for the nonalignment of the smallest grains, especially those of bacterial origin. Some possibilities: 1) Brownian motion (Stacey,1972) is never overcome in some of the smallest grains. 2) When magnetic bacteria cells die and disintegrate they may liberate a relatively large number of strongly magnetized crystals within their immediate microenvironment, causing grain interactions to become locally important. We have seen many instances, under TEM magnification, (figure 4) of side-by-side pairs, and also of clumps of magnetosomes. Grains in these configurations would likely be self-canceling, and possess only small net magnetic moments. If configurations similar to ones seen in magnetic separates had existed as well (at least locally) in the intact sedi159 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ment, these clumped grains would be poor contributors to the NRM. 3) The bacteria may have lived and died below the mixing zone in the sediment. In surface sediments which have magnetic properties very similar to the JPC22 interval (Schwartz et al., 1997) we have identified populations of magnetic bacteria which are likely living and growing at a meter or more depth in the sediments. The process by which magnetite crystals grown by those populations might eventually work loose and orient themselves to produce a locked remanence is problematical. If, and when, one of these cells dies and its magnetosomes become free and available for alignment, the crystals may find themselves considerably below the zone of bioturbation. A few such grains must, by chance, become loose in a conveniently large pore space and these will easily align. Our data show that a few do. What might happen to the rest of them? In a sense, this question is similar to asking "what is the role of bioturbation in the pDRM process?” If the effect of mixing is only to open porosity in the sediment, it is hard to imagine sediments so compacted as to close the majority of the .1 mm or less spaces such tiny grains would require. On the other hand, the grains may also require bioturbation to jar them loose from some previous, metastable geometry and give them another opportunity to orient themselves. Perhaps many of the magnetosomes in our sediment simply grew at too great a 160 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. depth to be significantly affected by bioturbation and therefore the pDRM process, or indeed by any phenomenon with the ability to produce a magnetic remanence. 4) The magnetosomes recovered from twenty meters depth in JPC 22 may have been contained in still-living, motile, bacteria. This possibility seems farfetched but cannot be absolutely ruled out. Because our BBOR cores contain sufficient nonbiological magnetite we are satisfied that they have provided good records of paleomagnetic field variability. Nevertheless, we would now be more careful in interpreting records from other materials in which extremely fine magnetite were seen to be the most important remanence carrier. And more seriously, this result calls into question the suitability of using any of the usual normalizers (ARM, IRM, or x) to obtain estimates of relative paleointensity whenever the presence of significant concentrations of ultra-fine magnetite is known or suspected. We will discuss this problem more completely in another paper. Remanence acquisition during times of low paleointensity There has been some recent discussion about the ability of sedimentary grains in general to align during low intensity times (Quidelleur et al., 1995). Both of these core segments were deposited during times of undeniably low 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. paleomagnetic field intensity. They (excluding the finest fraction) were nonetheless apparently capable of good quality recording of high frequency, high amplitude, secular variation features. There exists the possibility that intensity of magnetization is nonlinear with field at the weakest field strengths. If this is so, we would have an exaggerated picture of the relative field intensity prevailing during the lowest intensity intervals. We have no way to evaluate this possibility from the current data. CONCLUSIONS The purpose of this study was to examine the natural remanence acquisition behavior of hemipelagic sediments by making use of the extra information provided by the extraordinarily rapid magnetic field variability which occurred coincident with their deposition. This variability was accompanied by low paleointensity. We find that 1) Detrital grains were able to align effectively throughout this time interval and provide good paleomagnetic field recording . 2) The smallest (single domain) grains were quantitatively unimportant contributors to the remanence even where they were clearly abundant, and their small contribution stretched and smeared the paleomagnetic signal appreciably. 162 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3) Remanence lock-in of the small multidomain grains was complete within ten centimeters of the bottom of the bioturbation zone. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter VIII Relative Paleointensity in Marine sediments Deposited During the Laschamp and Blake Geomagnetic Events: Assessment of Normalizing Techniques Abstract We have tested commonly used NRM normalization techniques on short segments of two sediment cores recovered from the Blake/Bahama Outer Ridge, North Atlantic Ocean. Each of the segments records a complete paleomagnetic excursion; each therefore contains a large range of magnetic field paleointensity. The two segments, however, contrast markedly in their level of sediment magnetic variability. For the segment with uncomplicated sediment magnetic characteristics, which we believe contains a purely detrital remanence, normalizations by ARM, x t SIRM and coercivityinterval ARM yield nearly identical results. For the second segment, which contains a large localized concentration of single domain magnetite (bacterial magnetosomes,) normalizations by the different parameters match less well. For such material, ARM is extremely sensitive to the concentration of magnetosomes, as well as a poor paleomagnetic recorder, and is probably the least satisfactory normalizer. 164 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction Complete understanding of the earth's magnetic field behavior necessarily requires temporal knowledge of all three components of the field vector, that is, intensity information as well as inclination and declination. Directional information has always been much easier to recover from sediments than has intensity. The most accurate records of past field strength have traditionally come instead from materials carrying thermal remanences, such as lavas and fired archeological artifacts. But fired clays and lava flows are notoriously spotty in both their spatial and temporal coverage. For more or less continuous records of magnetic field behavior it is necessary to tease records of relative paleointensity out of lacustrine and marine sediment cores. The intensity of sediment natural remanent magnetization (NRM) is believed to be a linear function of the strength of the aligning field, but sample intensity is also dependent on the concentration of magnetic material in the sediment (Review by Tauxe, 1993). To compensate for changes in magnetic mineral supply with depth, we normalize the NRM by some parameter dependent on magnetic concentration. The most frequently used normalizers are anhysteretic remanent magnetization (ARM), magnetic susceptibility (x), and saturation isothermal magnetization (SIRM.) 165 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Problems arise because the intensity of NRM, and of each of these normalizers, depends also on properties independent of concentration, most importantly on the magnetic mineralogy and grain size of the sediment. (King et.al., 1982). These difficulties would obviously disappear if the magnetic properties in a core were uniform with depth. No sedimentary environment has yet been identified which has proven completely free of environmentally-driven changes in sediment magnetic characteristics. The goal has therefore been to keep the variation in magnetic concentration and properties to within "acceptable" limits and to choose the best normalizer for each particular sediment mix. Generally accepted criteria, both pertaining to the choice of material suitable for paleointensity work and for the proper choice of a normalizer in a particular sediment, stem from early work of Levi and Banerjee (1976) and King et.al. (1983) These have been strengthened somewhat over the years, and are summarized and brought up to date in a review paper by Tauxe. (1993) Yet in spite of the relatively strict criteria for magnetic mineralogy, concentration, and grain size, relative paleointensity records may still contain subtle biases of environmental origin, and they are subtly sensitive to the choice of a normalizer. (Chapters 3 and 4) In spite of the difficulties, records of relative paleointensity have now been produced from a variety of quaternary sediments, (summarized in Lehman et al.,1996) and 166 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. these look broadly similar over a wide region, with several peaks and troughs evident at roughly the same times. It is, of course, impossible to verify or calibrate any of these results in the absence of reliable synchronous absolute intensity measurements. Nevertheless, the approximate consistency of the records is encouraging. In this paper we use data from two short contrasting sediment core segments to directly illustrate the effect which changing magnetic properties and the choice of normalizer can have on estimating paleointensity. Core material and geological setting The sediments used for this study are short portions of piston cores collected from the sediment drift environment of the Blake/Bahama Outer Ridge (BBOR), North Atlantic Ocean, (figure 8-1) The cores consist of hemipelagic silts and clays with carbonate content varying between about 5 (glacial stages) and 50 (interglacial stages) percent. (Haskell, 1991) The BBOR provides an excellent sedimentary environment for paleomagnetic studies, because its relatively rapid sediment accumulation rates and low to moderate organic carbon supply provide cores with decently uniform magnetic characteristics. Because the sediment is 167 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32°N ■ o Blake 0 o o Blake 30°N 28°N Plateau \ OCH89-1P Outer _# jpC6\ \ ^ ^ • • GGC31 \ GGC32 • % CH8a ,, p 4000' GGC30 8 4500^dgeX v 28 \GGC29y^ C H 8 8 -1 0 IN i Hatteras Abyssal Plain \ JPC13^- ^GGC24 JPC 12s BahamaV'GGC23 GGC21, \*\ 78°W tf^PC2'2 Outer Ridge/^ ( ^ 76°W 74°W C 72°W 70°W Figure 8-1: Site map showing the Blake/Bahama Outer Ridge suite of cores we have studied. JPC 14 and JPC 22, from the Bahama Outer Ridge, are discussed in this chapter. 168 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GO 20 0 5 s < COI o EN < COI o 3000 2000 20 : ARM/Chi 1550 1650 D e p t h in Sediments (cm.) 1750 Figure 8-2a: Sediment magnetic characteristics of the two core segments. JPC 14 (a) has uniform magnetic properties. In JPC 22 (b, next page), and important boundary occurs at 2090 cm., below which all concentration-dependent quantities (except NRM) increase markedly. 169 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Susceptibility x ARM ARM/Chi CO 2000 SIRM * 1000 CO NRM D e p t h in Sediments (cm.) Figure 8-2b: (See previous page for explanation) 170 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reworked, the supply of the most reactive organic carbon is probably low compared with shelf material with similar deposition rates. Even here, however, our cores contain short intervals, usually less than one meter, with unusual (for this setting) sediment magnetic characteristics, indicative of early diagenetic reactions. We see occasional partially dissolved short intervals, as well as some which are enhanced with authigenic iron phases as well as single domain magnetite of bacterial origin. (Chapter 4 and 5). These anomalous intervals are nearly always associated with high carbonate values, and must reflect temporary conditions of higher than normal organic carbon supply. We believe that one of the segments used for this study (the one from JPC 14) represents a purely detrital, monotonous system, while the other (from JPC 22) is subtly overprinted by an appreciable amount of bacterial magnetite and perhaps by the presence of some authigenic iron minerals and some minor dissolution as well (figure 8-2). Each of the core segments contains a complete record of a geomagnetic anomaly, the Laschamp Excursion in core JPC 14 and the Blake Event in JPC 22. Figure 8-3 presents directional data and our initial ARM-normalized paleointensity estimates. (The reason for the absence of relative intensity data above 2090 cm core depth will be explained in this paper.) Although both are short records, each contains, because of the unusual geomagnetic behavior at the time of 171 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 120 z < z 4 0 *1 2 ■40 -80 z 9 •< Z 3 9 z 60 40 20 •20 -40 •60 •80 0.3 a. 0.2 0.1 1550 1570 1590 1610 1630 1650 1670 1690 1710 S e d i m e n t d e p t h (c m ) Figure 8-3a: 20 mT cleaned directions and initial paleointensity estimates (as NRM20/ARM20) for segments of core JPC14 and JPC22 (8-3b, next page.) 172 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Relative Paleointensity Inclination D eclination 180 r\j .06 .03 080 S ediment D epth (c m ) Figure 8-3b: See previous page for explanation. 173 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o ~3 o o ~3 o o o 100 0 o 100 0 1000 ca O 100. 1600 ca O O 100 0 600 ca O 100 0 1200 ca O 100 0 1700 ca o 100 800 cm O 100 1400 cm O 100 mT 100 0 ml 100 1800 ca O aT 100 Figure 8-4: Comparison of demagnetization behavior of measured remanences, NRM (solid circles), ARM (open circles), and SIRM (open triangles) for selected samples. Usually, ARM coercivity mimicked NRM behavior better than did SIRM. 174 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. its deposition, an enormous range of paleointensity. Both of these records are, of course, subject to signal smearing, caused by the time integration of wildly divergent directions within certain samples. But because of high sediment accumulation rates, on the order of tens of centimeters per thousand years, and our tight sample spacing, this effect must be relatively small. The quality of the high-frequency directional results from the segments supports this assumption. METHODS JPC 14 and JPC 22 are part of a large suite of piston and gravity cores collected by Lund and Keigwin from the research vessel Knorr during November 1993. The cores were dated and correlated by a combination of oxygen isotope, carbonate, and magnetic susceptibility stratigraphy as well as paleomagnetic secular variation. The JPC 14 segment was deposited at a rate of about 28 cm/kyr and sampled at 2.5 cm spacing; the JPC 22 segment was deposited more slowly, about 7-10 cm/kyr, but sampled at 1.25 cm intervals. Both segments therefore provide roughly century-scale resolution. Nearly all of the samples from these two important intervals were subjected to stepwise alternating field demagnetization by 10 mT steps from 10 to 100 mT. The remanence acquisition implications of the detailed demagnetizations are reported 175 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 8-5: Transmission electron micrographs of single domain, euhedral magnetite, interpreted as bacterial magnetosomes. A variety of morphologies are present. Magnetization is a) is 62500X, b) 86000X, c) 105000X, and d) 135000X. 176 in a separate paper (Chapter 7). After NRM demagnetization, A R M S (anhysteretic remanent magnetizations) were given to all samples, by application of a peak alternating field of 100 mT and DC bias field of 500 /zT. These were measured and demagnetized, in similar fashion to the NRM treatment. We then imparted and measured SIRM (saturated isothermal remanent magnetizations at 1.25 T) followed by backfield IRMs given at -.1 and -.3 T. Selected samples were also given a second SIRM which we demagnetized at the same steps as NRM and ARM. Figure 8-2 shows X/ ARM, ARM/x, SIRM, and NRM for both segments plotted on similar scales. A few samples from JPC 22 were also disaggregated for transmission electron microscopy of magnetic separates. Several other selected JPC 22 samples were finely ground and made into glass wafers for x-ray fluorescence (XRF) major element analysis. SEDIMENT MAGNETIC PROPERTIES For most of these samples, and for this region as a whole, ARM is the laboratory remanence which best matches the NRM and is therefore the best choice of a normalizer according to the criteria of Levi and Banerjee (1976). Figure 8-4 shows a comparison of coercivity spectra for several representative samples. There are considerable differences,however, between the magnetic properties of the 177 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. JPC 14 (5180 stage 3) and JPC 22 (early stage 5/late stage 6) sediments. In the glacial stage JPC 14 record (fig 2a), X/ ARM, and SIRM signals are highly correlated with each other, and each varies minimally within the segment. All three of these parameters are sensitive to grain size and mineralogy; their good correlation indicates that differences in grain size are small and that concentration is being effectively measured. The ratio of ARM/x is nearly flat, supporting this observation. The pattern of NRM intensity, however, in JPC 14, is quite different from the other concentration dependent parameters. For NRM, the raw intensity varies over a full order of magnitude, and drops to lowest values in samples representing the excursion. The sediment magnetic properties are considerably more complicated and problematical within the short JPC 22 interval. (figure 8-2b) All concentration-dependent parameters are low in the uppermost portion of the segment. We believe this is due to dilution of the magnetic fraction by stage 5 carbonate and perhaps also minor dissolution of the magnetic fraction. At exactly 2090 cm core depth, x, ARM, and SIRM show large step-function increases. Susceptibility and SIRM approximately double, while ARM abruptly rises a full order of magnitude. We believe that most of the huge increase in ARM is due to the large local concentration of bacterial magnetosomes, which has a sharp upper boundary. Figure 8-5 178 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. is a TEM micrograph of euhedral single domain magnetite separated from material within the high ARM interval. But NRM is relatively low within the segment, and rises fairly smoothly downcore. It appears to respond not at all to the sharp change in magnetic concentration we observe we see in X, ARM, and SIRM. Raw NRM intensity appears instead to be controlled by the relative importance of the clastic fraction in the matrix, as seen from its similarity to the pattern of aluminum content in the core, as measured by x-ray fluorescence (figure 8-2b). DEMAGNETIZATION BEHAVIOR A few samples showed unusually complicated demagnetization behavior, due to the rapid change in geomagnetic field direction and low field paleointensity which was occurring at the time of their deposition. For many samples this was observed as both viscous overprints in the normal direction, along with some early as well delayed directions recorded by the hardest magnetic fraction. We discuss this phenomenon more fully in another paper (Chapter 7) but show a few representative Zijderfeldt diagrams here as illustration, (figure 8-4). In some of the weakest samples the viscous overprint was nearly as large as, and nearly antipodal to, the characteristic remanence of the sample. In these samples, NRM intensity increased during early demagnetization steps. 179 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In nearly every sample from both segments, however, a characteristic remanence direction was identifiable from 30 mT (and usually at lower demagnetization steps) and this direction was maintained cleanly until at least 70 mT, at which point noise and the unusual late-locking behavior of the hardest grains (see chapter 7) obscured the signal. Within this medium-coercivity band, the samples were virtually univectorial and indistinguishable in behavior from the pseudosingle domain magnetite-dominated material common to most of our cores from the BBOR. NRM V S . ARM IN MEDIUM-COERCIVITY BAND In light of the above, we plotted, separately for each completely demagnetized sample, scatter graphs of NRM intensity removed at each 30-70 mT step versus ARM removed within the same coercivity intervals (figure 8-7 is an example). We computer-fitted a line to these NRM vs. ARM data and used the slopes so obtained as the NRM/ARM paleointensity estimate for each sample. Figure 8 shows the (mean-normalized) paleointensity estimates obtained in this manner, along with the computer-generated "R" (goodness of fit) values for each horizon. in figure 8-9 we contrast the relative paleointensity values obtained by this method with our original 20 mT cleaned version of NRM/ARM. In the simpler case (JPC 14) the 180 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1550A Cl 3 0 2 1 I C S o o 3 o o ir> o l -0 .5 0 0.5 West- -East 1635A a D c o I I in o o c $ o Q j£ 3 o tn -0.15 -0.1 West----- -0.05 -East 16J7.5A C . o m o o o -0 .1 -0 .0 5 0 West------------- East d . 1690A CM o -0 .4 - 0.2 0 West------------- East Figure 8-6(a-d): Representative demagnetization behavior of selected samples of JPC 14 and JPC 22 (e-h, next page). In each case, the first and last examples represent samples away from the geomagnetic excursions; the second and third are from times of rapidly changing field direction. 181 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. e. 2 0 8 2 .5A 2 0 9 2 .5A a D c o c 5 o Q 3 O cn o Csl 6 i -0 .4 - 0.2 0 0.2 Cl ID _c o c % o a 3 o (/) (N o o CM 6 l - 0.2 0 0.2 West- ■East West- -East 2097.5A h. 2126.25A 0 2 1 o c S o o sz 3 o in - 0.2 - o . t 0 0.1 a 3 sr.+-< 0 2 1 c O a 3 O in o m 6 I -0 .5 0 0.5 West- -East West------ East Figure 8-6(e-h): See previous page for explanation. 182 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. agreement between ARM normalizing methods is nearly perfect, indicating that consideration of the larger coercivity band produced no appreciable improvement over simple 20 mT cleaned NRM/ARM. And even in the complicated case, JPC 22, the more tedious interval normalization changed the overall picture very little. COMPARISON OF NORMALIZERS We next tried normalizing the 20 mT cleaned NRM with the other common parameters, SIRM and x* For purposes of comparison, we normalized each of these again by dividing each depth series by its own mean. Figure 8-10 displays the results of these comparisons. In JPC 14, the three curves overlap nearly perfectly. This is consistent with our earlier observation of the strong covariance of all non-field dependent concentration indicators, Xi ARM, and SIRM. In the magnetosome-laden and chemically-challenged JPC 22 record, the three curves still show some similarity. That is, all three show relative highs and low within the same intervals. But the differences between them are apparent as well. The susceptibility and SIRM normalized curves are almost identical in the interval between 2090 cm and the bottom of the segment. The ARM normalized curve is similar in shape, but somewhat lower within this interval. This lowering is due, of course, to the presence of the large 183 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. concentration of single domain biogenic magnetite, to which the ARM is extraordinarily and nonlinearly sensitive. All three curves suggest some increase in apparent field intensity above 2090 cm. The magnitude, and real existence, of this increase depends critically upon which normalizer, if any, one chooses to believe. 2090 cm, of course, marks the very sudden and severe change in the magnetic characteristics we have noted in this core. The abrupt decrease in relative paleointensity seen directly below 2090 cm may consequently be, to some extent, a function of the core's great enhancement in biogenic magnetite, and perhaps other (authigenic?) magnetic phases present below this horizon. Neither the biogenic magnetite, nor any other possible authigenetically enhanced phase, appears to contribute significantly to the sediment NRM. Therefore, although we see no reason to doubt that magnetic field intensity must have recovered significantly during the time interval immediately after the excursion, the sediment magnetic complications make it impossible for us to use this data to pin down the precise timing and magnitude of that increase. Certainly, the order of magnitude change in the ARM normalized curve, which comes exactly at the sharp boundary in sediment magnetism, must be an artifact. The SIRM and x normalized curves can also be at least partially explained by the sudden change in magnetic properties in the core, and are suspect 184 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.15 0.1 2 CE Z 5 cc 0.05 0 01 2 3 4 5 6 7 8 Raw ARM Figure 8-7: Example of a scatter-plot of interval values of NRM vs. ARM when demagnetized at steps between 30 and 70 mT. The slope of the best-fit line was taken as the ARM normalized paleointensity for each sample. The "R" value is a goodness of fit estimator. 185 J P C 2 2 Sample 2 0 9 7 3 0 - 7 0 m T Range - - - y = 0 . 0 1 1 1 6 1 + 0 . 0 1 6 5 2 2 x R = 0 . 9 9 5 4 9 ’ - / . - / - / / ; / ] - / - / ■ / / • ; . / - / ■ / / . ] . / - / - / > 1 ! / . / • / > ■ 1 1 -I— 1— L. i— L 1-1 1 1 1 l - L L I 1 1-1-1 1 1 1 I.I..I 1 1 1 1 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.5 0.4 0.8 0.3 0.6 0.2 0.4 0.1 0.2 0 1550 1600 1650 1700 1750 D epth in Sedim ents (cm ) Figure 8-8a: NRM/ARM as calculated with only the 30-70 mT data(closed circles), along with the "R" goodness of fit value (open circles) for each point, in JPC 14. For comparison, figure 8-8b, JPC 22, next page. 186 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.25 0.2 0.05 0 2080 2090 210 0 2110 2120 2130 D epth in Sedim ents (cm ) 0.8 0.6 0.4 0.2 0 2140 Figure 8-8b: Same treatment as previous page, but for JPC 22. 187 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. as well. Although some part of the intensity recovery must be real, it will require similarly detailed study of replicate records to sort out the possibilities. We suggest, however, that the gradual decrease in relative intensity from 2140 cm upward into the excursion interval must approximately reflect a real decrease in the geomagnetic field leading up to the Blake Event. The magnetic properties are somewhat more uniform throughout that localized interval. We cannot sort out, however, the extent to which more subtle sediment magnetic complications may obscure the finer details of that decrease. DISCUSSION The normalization of a sediment NRM record by some parameter dependent on magnetic concentration can provide a fair to excellent relative paleointensity record. In ideal cases, with uniform material, such as our short segment of JPC 14, we find that the choice of a normalizer is immaterial in that all normalizers produce virtually identical estimates. Of course, sediment records alone cannot give us absolute intensity records. Even in our excellent JPC 14 core, we cannot be sure that, at the extremely small field intensities which existed during the excursion, the acquisition of the NRM was still linear with field strength. It should be noted that the paleointensity decreases, 188 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.5 0.4 0.3 0.2 0.1 0 — 1550 1600 1700 1750 1650 DEPTH IN SEDIMENTS (cm) Figure 8-9a: Comparison of 30-70 mT interval NRM/ARM (filled circles)with our initial 20 mT cleaned NRM/ARM (open circles) , for JPC 14. For JPC 22, see figure 8-9b, next page. 189 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 6 5 4 3 2 1 0 2080 2090 2100 2110 2120 2130 2140 2150 DEPTH IN SEDIMENTS (cm) Figure 8-9b: Same as previous page, but here each time series has been normalized by division with its own mean. 190 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. related to both excursion, begin earlier in the record than the excursionary directions, and persist longer. Other material from the same region, which may have been deposited at much slower rates, may therefore smear the directional record of the excursions but preserve the paleointensity lows. Indeed, we see this pattern in our BBOR core CH88-10P, which records the intensity low around the Laschamp event but contains now excursionary directions. If this pattern is common, then paleointensity lows may provide additional finescale correlation tie points between sediment paleomagnetic records in which the directional PSV is smeared. The less perfect JPC 22 Blake data still gives us a general picture of field intensity variation through the excursion. We see a large drop-off in intensity before the excursionary directions, a small-scale recovery at a time of steeply negative inclination, and a recovery of field strength some time after the directions return to normal. These observations are probably indicative of real field behavior and we are presently considering their theoretical implications. But sediment magnetic complications, which occur here with large amplitude and very high frequency, obscure the record of intensity increase after the excursion; we cannot state with any certitude exactly when it occurred, how quickly, or with what magnitude. We would be very uncomfortable drawing detailed geomagnetic inferences from this record. We would expect the same difficulties in 191 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. any sediment core which contained intermittent concentrations of bacterial magnetite. One important observation is, if varying concentrations of bacterial magnetite are present in a core, ARM may be the worst choice for a normalizing parameter. This is true even if ARM coercivity mimics NRM well in the material as a whole. CONCLUSIONS We used two short core segments, each containing a wide range of sample intensity, in order to compare the results obtained from several common NRM normalizing procedures. We found that the paleointensity estimate we obtained from our JPC 14 segment, which has monotonous sediment magnetic properties, was robust and impervious to our choice of normalizing technique. In contrast, our results from the JPC 22 section, which contains a short interval rich in bacterial magnetite, were less definitive. Although we are satisfied that we have recovered the general pattern of paleointensity, the details are less clear. We suggest that ARM normalization be avoided in core material which contains short intervals rich in single domain magnetite, even if such cores seem otherwise acceptable. In such cases, SIRM or susceptibility may be better, but still imperfect, choices. Such material may be suitable for rough paleointensity estimates, probably sufficient for such tasks as regional, or potentially even 192 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.8 0.6 0.4 0.2 1550 1600 1750 1650 1700 DEPTH IN SEDIMENTS (cm) Figure 8-lOa: NRM for JPC 14 segment, (JPC 22, figure 8-10b, next page) normalized by ARM (open circles), SIRM (diamonds) , and x (closed circles). Each series is further normalized by its own mean for ease of comparison. 193 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.5 2.5 0.5 2080 2090 2100 2110 2120 2130 2140 2150 DEPTH IN SEDIMENTS (cm) Figure 8-10b: See previous page for explanation. 194 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. global, correlation of records. It is probably wisest, however, to avoid the use of any core which may contain inter- * mittent bacterial magnetite for theoretical studies of magnetic field intensity behavior. 195 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter IX General Conclusions The unifying goal of the series of papers comprising this thesis is to better characterize the ability of various magnetic grains deposited in sediment to accurately record past geomagnetic field intensity, and to consider criteria for extracting this information from them. The underlying problem is that, even in simple titanomagnetite systems, the ability of a grain to align in the earth's field is a function not only of the field intensity existing at the time, but of such extraneous factors as the grain's size and magnetization. The intensity of magnetization measured for any sediment sample also depends critically on the concentration of magnetic grains present in the sample, and whether these grains were present and available for alignment at the time the sediment was deposited. A variety of laboratory remanences, as well as magnetic susceptibility, are available and commonly used to assess the magnetic concentration of sediment samples. All of these “normalizers” are more or less efficiently acquired depending on such variables as magnetic grain size and mineralogy. The grain size dependencies appear to be the most important complication, and the grain size dependency of each normalizer is different from the others, and all are different from the grain size dependency of the sediment natural remanence. If the magnetic grain size within a sediment core is 196 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. uniform enough, however, it is possible to obtain a reasonably good record of paleointensity. The best test of uniformity currently consists of normalizing the sediment natural remanence separately by each of the available normalizers, then renormalizing each with respect to its own mean, for comparison. These should agree closely, and should, moreover, agree with replicate cores from the same general region. Whenever possible, records should be compared with others from a broader region. This is even more useful when the cores used for comparison come from a different sedimentological regime. The BBOR material, particularly cores CH88-10P and CH88-11P, yields apparently reliable paleointensity information for the time span of 12,000-70,000. Where records differ, either between alternate normalizations or between cores, the “difference of opinion” manifests itself most often as a mismatch in the relative amplitude of peaks and troughs. Only under particularly bad conditions is the number and placement of intensity highs and lows in question. For this reason, I suggest that careful paleointensity normalization is reliable enough for assisting in regional correlations between sites, but should be used with caution as input data for field modeling or inversion studies. Mismatches between normalizing methods and among cores become more serious when sharp environmental(climate change) boundaries are crossed. In the BBOR material, the transition 197 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. between glacial and interglacial stages is marked by sharp lithologic boundaries, seen most easily in the percentage of carbonate present (5% during glacial stages and up to 50% during interglacial stages.) This is compounded by an apparent change in the reactive iron/organic input as stage boundaries are crossed. This has implications for the survival of magnetite in the sediment. Early diagenetic changes, involving the dissolution of iron minerals, the precipitation of iron-containing metals “spikes” and the growth of magnetite in bacteria, can alter the magnetic signal postdepositionally. In the deeper water BBOR cores, there are moderate but persuasive diagenetic changes in the iron mineralogy. This does not seem to result in any loss of magnetite within the glacial-stage sediments. These presumably received enough reactive iron as input to protect all of the somewhat less reactive iron in titanomagnetite (or titanomaghemite.) The iron so liberated apparently diffused upward in the sediment and deposited as migrating metal spikes of goethite/hematite now seen at the base of the various carbonate peaks in the cores. Reductive diagenetic changes, however, are considerable in all of our Holocene material. In one class of cores, characterized by CH88-10P and GGC 24, the (Holocene) surface sediments are rich in several iron phases, especially such poorly crystallized phases such as lepidocrocite and ferrihydrite. There is also some goethite/hematite (?) en198 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. richment within the lowermost Holocene. The Holocene material is also rich in magnetosomes - bacterially grown . 1/um euhedral magnetite crystals. These phases, with the possible exception of hematite, occur in reduced amounts, if at all, within the underlying Pleistocene material. Those sediments are therefore depleted in relatively reactive phases, and it is reasonable to assume that iron from lower levels has diffused upward and fed diagenetic processes and magnetic bacterial growth in the carbonate-rich surface sediments. In the stage 1-4 material, aside from the growth of magnetosomes within the carbonate, we can see no clear evidence of loss or gain of magnetite. The sediment natural remanence correlates decently with the percentage of clastic material, strongly suggesting that the paleomagnetic signal is carried almost exclusively by lithogenic titanomagnetite grains. The interglacial stage sediments received significantly less clastic detritus, hence less reactive iron, and presumably more organic carbon. Here, the demand for readily reduced-iron compounds was greater during interglacials than in the glacial sediments. The surface magnetic material, which represents the input function for at least this interglacial, consists of a mix of easily reacted iron, probably as ferrihydrite and lepidocrocite, and detrital magnetite grains. This mixture alters significantly within a few tens of centimeters depth. In CH89-1P the easily reacted fraction 199 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. is gone within about five cm of the sediment/water interface. In the other cores, which come from greater water depth and have somewhat higher depositional rates (and therefore greater reactive iron input) these minerals persist to greater depths, but they are gone by the bottom of the Holocene section. In addition, iron which has diffused upward has deposited as goethite/hematite at approximately the climate-change boundary. All of the diagenetic changes have implications for paleointensity. Our data all suggest that the paleomagnetic (NRM) signal is carried principally by magnetite of detrital rather than diagenetic origin. Except in some obvious badly dissolved intervals, this detrital magnetite seems to be intact. But the normalizers all react differently to the presence of the various phases. ARM is extraordinarily sensitive to small concentrations of magnetosomes, susceptibility to large magnetite grains and some of the reactive phases, and SIRM to smaller magnetite grains and the reactive phases. Although the natural remanence in this mix may be unharmed, all of the potential normalizers are differently biased by the mineralogic change. It may be impossible to tie intensity records together across such profound sedimentological boundaries in the absence of independent information, such as absolute paleointensity measurements. Nevertheless, short duration features may still be recognizable. 200 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Core intervals containing rapid geomagnetic field changes (field excursion) allowed us to examine the remanence acquisition process in detail. Using coercivity as a grain-size proxy, we performed a series of very detailed demagnetizations across intervals containing the Laschamp Excursion and the Blake Event. We found a ubiquitous present-day overprint in the softest coercivity fraction. The fraction which demagnetized between 30 and 70 mT was nearly identical over that range, suggesting that all such grains achieved lock-in simultaneously. This was true even in intervals of very low paleointensity. The hardest grains provided a very small percentage of the total remanence, and those which did align presented a significantly smeared and stretched picture of the field directional variability. 201 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. References Aller, R. 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Abstract (if available)

Abstract The purpose of this work is threefold. One major goal is to calibrate, as well as possible, the methodology needed to extract accurate records of past geomagnetic field intensity from rapidly deposited sediments. A second goal is to examine the extent to which chemical changes subsequent to deposition may alter paleointensity estimates. The third is to present and evaluate a number of such paleointensity records for the last seventy thousand years of earth's history. ❧ This dissertation is a compendium of six manuscripts, which are in various stages of publication at this time. All of them deal with the general topic of paleomagnetic field intensity determinations from rapidly deposited sediments. In the broadest terms, the goal is to normalize the natural remanent magnetization (NRM) in the sediment by some measure of the varying amount of magnetic material variously available for alignment in the Earth's field. The problem is complex

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Asset Metadata

Creator Schwartz, Martha (author)

Core Title Intensity history of the earth's magnetic field during the late quaternary as recorded by the sidements of the Blake/Bahama outer ridge, north Atlantic Ocean

School Graduate School

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publication Date 12/09/2020

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Lund, Steve P. (committee chair), Cole, Robert K. (committee member), Hammond, Douglas E. (committee member), Sammis, Charles G. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-411899

Unique identifier UC11666559

Identifier 3110959.pdf (filename),usctheses-c89-411899 (legacy record id)

Legacy Identifier etd-Schwartz

Dmrecord 411899

Document Type Dissertation

Rights Schwartz, Martha

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA