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Stromatolites in the ancient and modern: new methods for solving old problems

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Stromatolites in the ancient and modern: new methods for solving old problems

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Content STROMATOLITES IN THE ANCIENT AND MODERN:
NEW METHODS FOR SOLVING OLD PROBLEMS
BY
Victoria A. Petryshyn
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
EARTH SCIENCES
August 2013
Copyright 2013 Victoria A. Petryshyn

DEDICATION
I dedicate this to my parents, siblings, grandparents, great-grandparents, other family (wheth-
er actually related or not), and all the numerous people who have worked so hard so that I can
have it so easy.
I appreciate every last bit of it.
Table of Contents
Chapter 1: Stromatolites in the Ancient and Modern: Not so Analogous Analogues .............7
Abstract ....................................................................................................................................................7
Introduction .............................................................................................................................................8
Modern and Ancient Stromatolites: Not so analogous analogues .....................................................10
Modern Microbialites ..........................................................................................................................10
Proterozoic stromatlites .......................................................................................................................11
A new analogue: Walker Lake stromatolites .......................................................................................13
Outstanding problems with stromatolites ...........................................................................................13
Rate .....................................................................................................................................................13
Biogenicity ..........................................................................................................................................14
New Biosignature – Magnetic Susceptibility .......................................................................................15
Chapter 1 Conclusions ..........................................................................................................................16
Chapter 2: Stromatolite Lamination Frequency, Walker Lake, Nevada: Implications for
Stromatolites as Biosignatures ....................................................................................................18
Abstract ..................................................................................................................................................18
Introduction ...........................................................................................................................................18
Walker Lake ...........................................................................................................................................19
Carbonate budget of Walker Lake .......................................................................................................20
Walker Lake Stromatolites ...................................................................................................................20
Location ...............................................................................................................................................20
Petrography .........................................................................................................................................21
Previous Work .....................................................................................................................................21
Methods ..................................................................................................................................................23
Results………………………………………………………………………………………………….24
Growth Rate………………………………………………………………………………………….24
Lamination Rate…………………………………………………………………………………..…24
Discussion ...............................................................................................................................................25
Reliability of 14C date ........................................................................................................................25
Multi-year lamination period………………………………………………………………………...27
Significance………………..………………………………………………………………………...28
Chapter 2 Conclusions ..........................................................................................................................31
Chapter 3: Analysis of Growth Direction of Domed Stromatlites from Walker Lake, Western
Nevada ...........................................................................................................................................33
Abstract ..................................................................................................................................................33
Introduction ...........................................................................................................................................34
Walker Lake stromatolites ...................................................................................................................36
Solar radiation, predicted growth directions, and predicted thicknesses…………………………….37
Methods ..................................................................................................................................................38
Results………………………………………………………………………………………………….39
Discussion ...............................................................................................................................................41
Growth Direction ................................................................................................................................41
Thickness .............................................................................................................................................42
Biogenicity of Walker Lake stromatolites ...........................................................................................43
Significance of Walker Lake stromatolites ..........................................................................................43
Chapter 3 Conclusions ..........................................................................................................................44
Chapter 4: Magnetic Susceptibility as a Biosignature ..............................................................46
Abstract ..................................................................................................................................................46
Introduction ...........................................................................................................................................47
Magnetic Susceptibility .........................................................................................................................48
Advantages of magnetic susceptibility as a biosignature ....................................................................49
Proof of Concept – Tahitian Microbialites ..........................................................................................50
Methods- Laboratory Experiments .....................................................................................................51
Abiotic carbonate precipitation ...........................................................................................................51
Biofilm experiments ............................................................................................................................52
Magnetic susceptibility measurement procedure ...............................................................................52
Avoiding potential biases ....................................................................................................................53
Experimental Results ............................................................................................................................54
Abiotic carbonate precipitation ...........................................................................................................54
Biofilm experiments ............................................................................................................................54
Discussion of Experimental Results .....................................................................................................55
Natural Samples .....................................................................................................................................56
Location and Descriptions ...................................................................................................................56
Methods- Natural Samples ...................................................................................................................60
Avoiding potential biases ....................................................................................................................61
Results – Natural Samples ...................................................................................................................61
Known biotic sample – Yellowstone hot spring stromatolite .............................................................61
Known abiotic sample – Hydrothermal vein stromatolite ..................................................................61
Unknown sample #1 – Likely biotic stromatolite: Johnnie Formation, Death Valley .......................62
Unknown sample #2 – Likely biotic stromatolite: Green River Formation, Wyoming ......................62
Unknown sample #3 – Biotic/abiotic mix: Green River Formation, Wyoming ..................................63
Unknown sample #4 – Likely abiotic stromatolite: Furnace Creek Formation, Death Valley ..........63
Unknown sample #5 – Likely abiotic stromatolite: Walker Lake, Nevada .........................................64
Discussion – Natural Samples ...............................................................................................................64
Chapter 4 Conclusions ..........................................................................................................................66
References .....................................................................................................................................67
Figure Captions ............................................................................................................................79
Chapter 1 Figure Captions ...................................................................................................................79
Chapter 2 Figure Captions ...................................................................................................................81
Chapter 3 Figure Captions ...................................................................................................................84
Chapter 4 Figure Captions ...................................................................................................................86
Figures – Chapter 1 ......................................................................................................................90
Figure 1.1 ................................................................................................................................................90
Figure 1.2 ................................................................................................................................................91
Figure 1.3 ................................................................................................................................................92
Figure 1.4 ................................................................................................................................................93
Figure 1.5 ................................................................................................................................................94
Figure 1.6 ................................................................................................................................................95
Figure 1.7 ................................................................................................................................................96
Figure 1.8 ................................................................................................................................................97
Figure 1.9 ................................................................................................................................................98
Figure 1.10 ..............................................................................................................................................99
Figure 1.11 ............................................................................................................................................100
Figure 1.12 ............................................................................................................................................101
Figure 1.13 ............................................................................................................................................102
Figures – Chapter 2 ....................................................................................................................103
Figure 2.1 ..............................................................................................................................................103
Figure 2.2 ..............................................................................................................................................104
Figure 2.3 ..............................................................................................................................................105
Figure 2.4 ..............................................................................................................................................106
Figure 2.5 ..............................................................................................................................................107
Figure 2.6 ..............................................................................................................................................108
Figure 2.7 ..............................................................................................................................................109
Figure 2.8 ..............................................................................................................................................110
Figure 2.9 .............................................................................................................................................. 111
Figure 2.10 ............................................................................................................................................112
Figure 2.11 ............................................................................................................................................113
Figure 2.12 ............................................................................................................................................114
Figure 2.13 ............................................................................................................................................115
Figure 2.14 ............................................................................................................................................116
Figure 2.15 ............................................................................................................................................117
Figure 2.16 ............................................................................................................................................118
Figure 2.17 ............................................................................................................................................119
Figure 2.18 ............................................................................................................................................120
Figure 2.19 ............................................................................................................................................121
Figure 2.20 ............................................................................................................................................122
Figure 2.21 ............................................................................................................................................123
Figures – Chapter 3 ....................................................................................................................124
Figure 3.1 ..............................................................................................................................................124
Figure 3.2 ..............................................................................................................................................125
Figure 3.3 ..............................................................................................................................................126
Figure 3.4 ..............................................................................................................................................127
Figure 3.5 ..............................................................................................................................................128
Figure 3.6 ..............................................................................................................................................129
Figure 3.7 ..............................................................................................................................................130
Figure 3.8 ..............................................................................................................................................131
Figure 3.9 ..............................................................................................................................................132
Figure 3.10 ............................................................................................................................................133
Figure 3.11 ............................................................................................................................................134
Figures – Chapter 4 ...................................................................................................................135
Figure 4.1 ..............................................................................................................................................135
Figure 4.2 ..............................................................................................................................................136
Figure 4.3 ..............................................................................................................................................137
Figure 4.4 ..............................................................................................................................................138
Figure 4.5 ..............................................................................................................................................139
Figure 4.6 ..............................................................................................................................................140
Figure 4.7 ..............................................................................................................................................141
Figure 4.8 ..............................................................................................................................................142
Figure 4.9 ..............................................................................................................................................143
Figure 4.10 ............................................................................................................................................144
Figure 4.11 ............................................................................................................................................145
Figure 4.12 ............................................................................................................................................146
Figure 4.13 ............................................................................................................................................147
Figure 4.14 ............................................................................................................................................148
Figure 4.15 ............................................................................................................................................149
Figure 4.16 ............................................................................................................................................150
Figure 4.17 ............................................................................................................................................151
Figure 4.18 ............................................................................................................................................152
Figure 4.19 ............................................................................................................................................153
Figure 4.20 ............................................................................................................................................154
Figure 4.21 ............................................................................................................................................155
Figure 4.22 ............................................................................................................................................156
Figure 4.23 ............................................................................................................................................157
Figure 4.24 ............................................................................................................................................158
Figure 4.25 ............................................................................................................................................159
Table List ....................................................................................................................................160
Tables – Chapter 2 .....................................................................................................................161
Table 2.1 ................................................................................................................................................161
Table 2.2 ................................................................................................................................................162
Table 2.3 ................................................................................................................................................163
Table 2.4 ................................................................................................................................................164
Tables – Chapter 3 .....................................................................................................................165
Table 3.1 ................................................................................................................................................165
Table 3.2 ................................................................................................................................................166
Tables – Chapter 4 .....................................................................................................................175
Table 4.1 ................................................................................................................................................175
Table 4.2 ................................................................................................................................................176
Table 4.3 ................................................................................................................................................178
7
CHAPTER 1: STROMATOLITES IN THE ANCIENT AND MODERN: NOT SO
ANALOGOUS ANALOGUES.
CHAPTER 1 ABSTRACT
Stromatolites are commonly defined as laminated organo-sedimentary structures built by the
trapping and binding and/or precipitation of minerals via microbial processes. They are thought to
represent evidence of some of the oldest life on Earth, and are targets for geobiologic and astrobiologic
studies. Stromatolites first appear in the Archean, and rise in form diversity through the Proterozoic.
The occurrences of stromatolites decline drastically in the Phanerozoic, coincident with the rise of
metazoans. Despite their high profile in the geobiologic community , the processes that control the
different aspects of stromatolite morphology (i.e. form, growth rate, texture, and lamina formation)
are poorly understood. Modern marine stromatolites, are known to form by the trapping and binding
activity of microbes in close association with diatoms and algae. However, the texture formed by this
trapping and binding action is coarse. Laminations found in these microbialites are millimeter-scale,
if laminated at all, and commonly discontinuous. Conversely, Archean/Proterozoic stromatolites,
which are typically composed of much finer grained material, display sub-millimeter scale lamination.
This difference in texture, as well as the close association that modern marine microbialites share with
eukaryotes, make them less than ideal analogues for Precambrian stromatolites. Modern lacustrine
carbonate accretions are found in such places as Pavilion Lake in British Columbia, and Fayetteville
Green Lake in New York. However, these build-ups are also weakly to non-laminated. While interest-
ing on their own, these lacustrine structures fare no better than modern marine microbialites as textural
analogues for finely laminated Precambrian stromatolites. Compounding this problem is the fact that a
combination of abiotic mechanisms (e.g. the fallout of suspended sediment and surface normal abiotic
precipitation) can reproduce geometries similar, if not identical to, ancient stromatolites.
Stromatolites have been studied in detail since their first description in 1908, however there
remains a lack of definitive data describing the processes (biotic and abiotic) and rates involved in their
formation. If stromatolites are going to be held up as proof of the earliest life on Earth (and perhaps as
8
proof of life elsewhere), some of the unknowns and confusion involved in their study must be re-
solved.
INTRODUCTION
Simply stating that stromatolites are laminated sedimentary structures of debated biogenicity
that span most of geologic history would be cause for controversy. When first described by Kalkowsky
(1908), they were assumed to be biologic in origin, specifically “or ganogenic, laminated calcareous
rock structures, the origin of which is clearly related to microscopic life, which in itself must not be
fossilized”. This definition served to both state unambiguously that stromatolites were biogenic in
nature, and that direct paleontological evidence of microbes in them could never be found. Little has
changed about the common accepted definition of stromatolites since that time, though the most sited
definition would likely be Walter’s (1976) assertion that “Stromatolites are organosedimentary struc -
tures produced by sediment trapping, binding, and/or precipitation as a result of the growth and meta-
bolic activity of micro-organisms, principally cyanophytes”. While widely accepted, these definitions
are problematic, as they assume a biologic origin without evidence. Semikatov et al., (1979) alterna-
tively, proposed a more descriptive definition for stromatolites, calling them ‘laminated sedimentary
structures accretionary away from a point or surface’. Despite the arguments over terms, context, and
evidence, stromatolite biogenicity is often taken as fact, instead of a working hypothesis (Grotzinger
and Knoll, 1999).
The record of stromatolites on Earth is extensive. Generally, stromatolites are known to have
first appeared in the Archean (roughly 3.5 Ga, Walter et al., 1980); diversified throughout the Paleo and
Mesoproterozoic (Awramik and Sprinkle, 1999), eventually comprising a major component in Precam-
brian carbonate platforms (e.g. Awramik, 1992); and declined in the Neoproterozoic, coincident with
the rise of metazoans (Figure 1.1) (Awramik, 1971; 1992; Semikhatov and Raaben, 1993; Awramik
and Sprinkle 1999). Despite this long history, much of what is conventionally assumed about stro-
matolites is derived from studying modern forms. The first stromatolites studied in the modern oc -
curred in freshwater environments, and were formed by the precipitation of carbonate around algal or
cyanobacterial filaments (Hoffman, 1973). This led to the assumption that ancient stromatolites were
9
formed in this manner, by in situ precipitation of minerals in freshwater or brackish environments (e.g.,
Bradley, 1929; Walcott, 1914). Stromatolites were then discovered in coastal marine environments in
the Bahamas (Black, 1933), where they were described as ‘algal sediments’; and in Shark Bay, West-
ern Australia (Logan, 1961), where they occur in an intertidal, hypersaline setting. These stromatolites
seemed to form predominantly by the microbial action of trapping and binding loose sediment, which
would then be cemented into layers. Grotzinger and Knoll (1999) noted that this caused a shift in
thinking about how ancient stromatolites form, from a precipitation model to a ‘trapping and binding’
model. This shift, the authors noted, was not caused by any new observations or data from the ancient
samples. Instead, it stemmed from a new perception of what the most appropriate modern analogues
were. In fact, most ancient stromatolites contain neither remains of filaments, nor fabrics associated
with trapping and binding (Grotzinger 1986; Grotzinger and Read, 1983; Sumner and Grotzinger 1996;
Sumner 1997). Regardless, since all apparent modern analogues were at least partially influenced by
microbial processes, the mechanisms responsible for stromatolite morphogenesis were still considered
biotic in nature.
The most prominent feature of stromatolites is their lamination, and the morphology of a stro-
matolite is a function of how laminae evolve over time (Grotzinger and Knoll, 1999) (often a puta-
tive microbialte that lacks lamination, and instead has a clotted microstructure, will be defined as a
‘thrombolite’). Much has been assumed about how lamina form, with most studies hypothesizing that
stromatolite lamination represented daily, seasonal, or yearly frequency, with the most commonly cited
mechanism (daily) tied to the diurnal rhythm of photosynthetic microbial mats (e.g. Hoffman, 1973;
Walter, 1976; Vanyo and Awramik, 1985). This assumption of daily lamination was even used to re-
construct the length of a year in the Proterozoic (Vanyo and Awramik 1982; 1985). Once again, how-
ever, we are confronted with the fact that the modern does not closely resemble the ancient (discussed
below). Lamination in modern stromatolites is coarse and often influenced by eukaryotes (e.g., algae
and diatoms) (Awramik and Riding, 1998), making them less-suitable analogues for many Precambrian
forms, which are typically composed of much finer-grained material; display finer, sub-millimeter
scale lamination; and depending on age, formed prior to the advent of most benthic eukaryotes (Grotz-
10
inger and Knoll, 1999; Awramik and Grey, 2005). Compounding the textural differences is the lack of
direct measurements of stromatolite growth and lamination rates. Only a few studies have addressed
this directly (they are discussed below), and both have focused on modern forms. A mountain of work
has been preformed on stromatolites since they were first described over 100 years ago, but much of it
leans on assumption. The unfortunate fact is that while many aspects of stromatolites have been stud-
ied, very little is actually known.
MODERN AND ANCIENT STROMATOLITES; NOT SO ANALOGOUS ANALOUGES
If ancient stromatolites were made by the same processes that produce modern stromatolites
(the trapping and binding of loose sediment by microbial mats, and precipitation of carbonate driven
by heterotrophic decomposition of bacterial sheaths), then ancient forms should physically resemble
modern forms. However, as it was noted by Awramik and Grey (2005), “none of these or any of the
other occurrences of actively forming stromatolites serves as an all-purpose model to understand the
morphogenesis of stromatolites in the pre-Phanerozoic”.
Modern Microbialites
Microbialites are “organosedimentary deposits that have accreted as a result of a benthic
microbial community trapping and binding detrital sediment and/or forming the locus of mineral
precipitation” (Burne and Moore, 1987). These structures have influenced some of the core tenets of
conventional stromatolite morphogenesis theories. Modern microbialtes have been found in a variety
of environments, from hypersaline marine to freshwater systems.
Marine Microbialtes: The first ‘modern’ stromatolites that were similar in size and shape to ancient
forms were discovered around the hypersaline Hamelin Pool in Shark Bay, Western Australia in the
1960s (Logan, 1961). These structures form by the precipitation of aragonite crystals in the spaces be-
tween trapped and bound sediment grains (Logan, 1961) though the structures reach several meters in
height, lamination is on the cm-scale (Figure 1.2). Stromatolites have also been discovered in the Ba-
hamas, and described extensively (Figure 1.3); Dravis, 1983; Dill et al., 1986; Reid and Brown, 1991;
Ried et al., 1995). Laminaiton in these build ups is coarse, and clearly microbial process. Several
11
different types of bacteria, as well as eukaryotes are involved in lithifying and stabilizing the structure
(Reid et al., 2000; Visscher et al., 1998).
Lacustrine Microbialites: Modern microbialites are also known from several modern lacustrine set-
tings, such as Pavillion Lake in British Columbia (Figure 1.4; Laval et al., 2000); Green Lake in New
York (Figure 1.5; Thompson et al., 1990); Lake Clifton in Western Australia (Figure 1.6; Moore and
Burne, 1994; Konishi et al., 2001; Wacey et al., 2010); and Lake Tanganyika in the East African Rift
system (Figure 1.7; Cohen et al., 1997). It is common for these lakes to be alkaline (pH > 7), which is
favorable for calcium carbonate precipitation. These microbialtes range from very coarsely laminated
to clotted and thrombolytic. The structures can reach several meters in height, and are also clearly
influenced by microbial processes. Cyanobacterial mats trap and bind sediment, and the heterotrophic
degradation of their exopolymeric substances (EPS) leads to in situ carbonate precipitation (Laval et
al., 2000; Thompson et al., 1990; Konishi et al., 2001). While it is clear from these examples that bac-
teria heavily influence modern ‘stromatolites’, but it cannot be avoided that these modern forms bear
little textural resemblance to their Precambrian counterparts.
If lamination is the most prominent feature of stromatolites, it is fitting then that lamination is
the area where modern stromatolites differ most noticeably from ancient samples. Archean/Proterozoic
stromatolites, which are typically composed of much finer grained material, display sub-millimeter
scale lamination (e.g., Awramik and Riding, 1998; Grotzinger and Knoll, 1999; Awramik and Grey,
2005). In the past 30 years, it has been observed that ancient stromatolites are, for the most part, made
up of the crystal fabrics that suggest in situ precipitation, even when seen through the eyes of diagen-
esis (Grotzinger 1986; Grotzinger and Read, 1983; Sumner and Grotzinger 1996; Sumner 1997).
Proterozoic Stromatolites
Figures 1.8- 1.13 show the microstructure of several well-known Proterozoic stromatolites.
The stromatolites of the Bitter Springs Formation, found in the Northern Territory of central Australia
are roughly 850 Ma (Figure 1.8-1.9; Vanyo and Awramik, 1982; Williams et al., 2007; Cloud and Se-
mikhatov, 1969). Lamination in these stomatolilites is on the sub-mm scale (50-150 μm thick), which
12
contrasts with the coarse lamination of modern microbialites. While diagenesis has altered the miner-
alogy of the sample, the lamination remains intact. No filamentous sheaths or apparent trapped and
bound grains are contained within the columns, giving them a macro and microstructure that is unlike
those of modern microbialites.
Stromatolites of the Deep Spring Formation in Nevada are also depicted (Figure 1.10-1.11).
This Ediacaran-age formation represents mixed siliciclastic-carbonate deposition on a shallow marine
ramp (Cloud and Nelson, 1966). Stromatolites from the Deep Spring Formation are domed, laminated,
and on the order of centimeters high in hand sample. Lamination is poorly defined in thin section due
to diagenesis (Figure 1.11), but is on the order of ~100 μm. The columns are made up of interlock-
ing carbonate crystals, with no body fossils, or evidence of trappend and bound detrital grains (Figure
1.11).
As a contrast to these, a known biotic stromatolite from the Paleoproterozoic (1.8 Ga) Gunflint
Formation (Ontario, Canada) is depicted in Figures 1.12 and 1.13. Laminations in the stromatolites
(on the scale of ~50 μm; Figure 1.13c) are defined by dark, fine-grained, or ganic-rich layers that are ir-
regular and thicken over crests (Figure 1.13). When magnified, laminae are found to contain significant
amounts of filamentous microfossils (Figure 1.13c; Cloud 1965; Barghoorn and Tyler, 1965; Schopf
et al., 1965). While clearly formed at least in the presence of biology, these stromatolites also do not
texturally resemble most modern stromatolites.
It seems certain from the work discussed above that modern stromatolites are made under the
influence of microbial activity. It is equally certain, however, that modern stromatolites bear at best
only a fleeting resemblance to ancient forms. Is it wise, then, to claim that in the field of stromatology ,
the present is still the key to the past? In order to follow such an actuallistic approach, a new, better
analogue is needed for ancient stromatolites.
13
A NEW ANALOGUE – WALKER LAKE STROMATOLITES
The stromatolites of Walker Lake, an alkaline (pH ~9.4) sodium bicarbonate lake in Western
Nevada (the subjects of Chapters 2 and 3), exhibit a finely laminated microstructure made up of in -
terlocking calcium carbonate crystals. The domed, columnar stromatolites constitute a much better
textural match to Precambrian forms than do their modern marine counterparts (see Figures 2.7-2.14).
The relatively young, undeformed nature of these stromatolites, and the well-constrained history of the
lake they accreted in, allow for the kinds of testing not possible in Precambrian forms.
14
C age recon-
structions of the stromatolites allow for the calculation of both lamination and growth rates (Chapter
2), while studies of how and where the stromatolites formed answer some questions about the need for
photosynthesis in creating a domed structure (Chapter 3).
OUTSTANDING PROBLEMS WITH STROMATOLITES
The lack of a suitable modern analogue, and the amount of diagenesis that most ancient stroma-
tolites have been subjected to has led to a notable lack of information regarding some key features of
stromatolites; namely the rate at which lamination forms, and the over-arching question of whether or
not stromatolites are biologic structures.
Rate
Much about the rate of lamina formation is assumed. Since stromatolites are presumed to be
microbial structures, lamination is presumed to form on a microbial timescale. As previously men-
tioned, most studies hypothesize that stromatolite lamination represents daily, seasonal, or at most,
yearly frequency, with the most commonly cited mechanism (daily) tied to the diurnal rhythm of
photosynthetic microbial mats (e.g. Hoffman, 1973; Walter, 1976; Vanyo and Awramik, 1985). This
assumption (applied to the Bitter Springs stromatolites) was even used to calculate the length of a year
in the Neoproterozoic (Vanyo and Awramik, 1982)
14
Despite how pervasive the assumption of short-timescale lamination is, very few studies have
directly measured it. Studies of baterial photosynthesis, sulfate reduction, and respiration rates (Viss-
cher et al., 1998) found that this lamination process occurred at a rate of weeks to months. Lamination
in these forms is not daily, but is within the acceptable timescale for microbial processes. The growth
rate of the Shark Bay stromatolites was also reconstructed (Chivas et al., 1990). Carbon-14 dating of
the lithified structures revealed a growth rate that was roughly ≤ 0.4 mm/yr . These rates are about 250
times slower than what would be expected from lamination counts of the structures alone, assuming
the lamina were formed daily (which, given the crude nature of the lamination, would be surprising).
This was a first indication that lamination may not be tied strictly to microbial timescales, even in
microbial structures. Chapter 2 of this study uses a suitable modern analogue for Precambrian stroma-
tolites to shed light on the issue of lamination rate.
The Question of Biogenicity
The weakness of modern forms as analogues is not the only reason to doubt the blanket as-
sumptions made of Precambrian stromatolites. Compounding the lack of data involving rates of
processes, many researchers have also found ways in which stromatolite-like structures can be made, if
not in the total absence of life, at least not under the direct influence of microbes.
Models have shown that a combination of abiotic mechanisms can reproduce geometries simi-
lar, if not identical to, ancient stromatolites (Grotzinger and Rothman, 1996; Grotzonger and Knoll,
1999; Verrecchia, 1996; Dupraz et al., 2006). Grotzinger and Rothman (1996) showed that domed,
laminated, columnar stromatolites could be produced by four mechanisms: Surface-normal chemical
precipitation; Fallout and diffusive rearrangement of suspended sediment; and uncorrelated random
noise. While the authors note that bacteria were likely present on the surfaces of all but the most an-
cient forms, they do not have to be directly involved in making the structure, or defining its morpholo -
gy. In addition to numerial modeling of abitoic stromatolite growth, studies have shown that ‘abiotic’
stromatolites do exist. McLoughlin et al. (2008) showed that the simple spattering of paint can create
structures that domed, columnar, laminated, and generally indistinguishable from natural stromatolites
at some scales. Questions have also been raised about the biogenicity of the most ancient samples
15
(Buick et al., 1981; Lowe, 1994; Braiser et al., 2006). It has also been recognized that there are
‘chemical’ stromatolites, dominated by in situ mineral precipitation that are characterized by crystal
fan fabrics and isopachous laminae (laminae that have highly regular thickness, Pope and Grotzinger,
2000; Pope et al., 2000).
All in all, a complicated story has emerged from studying stromatolites. The forms are as-
sumed to be biogenic, even though there is often no direct evidence to support the assumption. Mod-
ern ‘stromatolites’ are demonstrably microbial, though they at best only vaguely resemble ancient
forms. Clouding the picture further are the studies that prove the existence of purely abiotic stroma-
tolites. Adding to all this is the fact that any ancient form has been through millions, if not billions,
of years of diagenesis, which will most likely burn away any concrete evidence of life in the structure
(body fossils, stable isotope signature, etc). In order to get a handle on the way Precambrian stromato-
lites form, a new, appropriate modern analogue is needed. In order to prove the biogenicity of ancient
samples, new biosignatures need to be developed.
NEW BIOSIGNATURE – MAGETIC SUSCEPTIBILITY
How do we tell the abiotic, laminated sedimentary structures from the bona fide microbialites?
Microscopic investigation can at times provide some insight, but as shown above, many putative
microbialites in the rock record have been subjected to post-depositional alteration, rendering most
putative microbialites ultimately ambiguous with respect to their biogenicity. Most microbialites are
composed of carbonate minerals, so carbon isotopes are commonly cited as a potential biosignature.
However, isotope ratios can also be problematic; microbial phototrophic CO
2
fixation drives the sur -
rounding carbon δ
13
C positive, and microbial sulfate reduction drives it negative, making carbonate-
carbon isotopes an uncertain biomarker. As previously noted, organic matter is rarely preserved in
such structures, and actual microbial fossils are even more rare and usually require special circum-
stances for preservation (e.g., the early silicification of the Gunflint stromatolites). Chapter 4 proposes
a new biosignature, based on the inherent stickiness of microbial biofilms (as compared to abiotic
systems) coupled with new advances in the ability to measure minute concentrations of magnetic
minerals: magnetic susceptibility as a biosignature. Ultimately, I am optimistic that this biosignature
16
can be used to gain information about the presence or absence of a biofilm that itself is not preserved
in ancient samples.
CHAPTER 1 CONCLUSIONS
Stromatolites, first described in 1908, have arisen to celebrity status in the geobiologic and
astrobiologic community. Their first appearance roughly 3.5 billion years ago makes them candidates
for the first life on Earth, as well as a possible fossil that may be found on another planet. Despite the
volumes of work that has been done on stromatolites, very little is concretely known about processes
that control lamination, growth direction, and texture. Many assumptions have been made about the
rate at which laminations form, but very little data exists to corroborate them. It is conventionally
thought that microbes must be involved in the construction of stromatolites, and thus lamination must
take place on a microbial timescale. Lamination should be either daily, reflecting the diurnal cyclicity
of photosynthetic bacteria; seasonal, reflecting the differences in nutrient availability caused by spring
runoff, summer blooms, and fall die-offs and winter hiatuses; or at most, yearly. The data that is avail-
able gives rates of lamina formation that ranges from weeks to months (in Bahamian stromatolites) to
multiple years (Shark Bay, Australia). These forms, however, are not morphologically similar to an-
cient stromatolites. Compounding all of these problems is the fact that many abiotic forms have been
found that, at macro and microscales, are indistinguishable from their biotic counterparts. These facts
force us to ask: What do we really know about stromatolites?
Given the high status of stromatolites in geobiology, their candidacy for the first fossils on
Earth, and the likelihood of possibly encountering forms similar to these on other planets, it is clear
that more information needs to be gathered. A first step would be to find a true modern analogue for
Precambrian stromatolites: one that is finely laminated, domed/columnar , and undeformed enough to
be studied in ways impossible in ancient forms. I am optimistic that the stromatolites located at Walk-
er Lake in western Nevada can be used as such. Chapters 2 and 3 are dedicated to the study of these
stromatolites, and the implications they hold for the fossil record. Secondly, a way to definitively test
the biogenicity of stromatolites, modern and ancient, is needed. Chapter 4 outlines a new biosignature
that can be used to test for the presence of a biofilm at the time of formation of a stromatolite. This
17
biosignature (magnetic susceptibility) does not rely on body fossils, textures, isotopes, or of the other
traditional methods that have so often led to ambiguous results. It is my hope that the sum total of this
study sheds light on the processes that form stromatolites, and the rates at which these processes occur.
18
CHAPTER 2: STROMATOLITE LAMINATION FREQUENCY, WALKER LAKE,
NEV ADA; IMPLICATIONS FOR STROMATOLITES AS BIOSIGNATURES.
CHAPTER 2 ABSTRACT
Lamination in stromatolites is commonly interpreted to record the periodic response of a mi-
crobial community to daily, seasonal, or perhaps yearly environmental forcing, but the inability to date
ancient stromatolites precludes an understanding of the formation processes, and ultimately, stromato-
lite biogenicity. Here, high resolution
14
C dating of Holocene stromatolites from Walker Lake, Nevada
is used to construct a record of lamination rate over the course of accretion. Laminations formed at a
rate of 5.6 ± 1.6 yrs/lamination at the base of the structure, 2.8 ± 1.9 yrs/lamination and 1.6 ± 0.9 yrs/
lamination in the middle, and 4.5± 0.8 yrs/lamination at the top. Thus, much of the stromatolite grew
in response to a forcing with a longer periodicity than a typical microbial community. The 4-6 year
periodicity indicates that lamination formation is likely more closely related to local climate factors
(e.g., ENSO) for these stromatolites than microbial metabolism. While we cannot preclude microbial
involvement in the Walker Lake stromatolites, the time scale of accretion is more strongly linked to
climatic forcing than microbial life cycle, exceeding the typical daily, seasonal, or yearly rate classi-
cally assumed for most ancient stromatolites. These results show that generalizations regarding the
influence of microbial mats on stromatolite formation and the use of stromatolites as biosignatures (on
ancient Earth or elsewhere) need careful consideration.
INTRODUCTION
Stromatolites, laminated structures commonly thought to form via microbial processes, con-
stitute some of the oldest putative fossils on Earth, first appearing in the Archean (~3.5 Ga), rising in
form diversity and abundance through the Proterozoic (2.5 Ga – 550 Ma), and declining drastically in
the Phanerozoic (<540 Ma) (Walter, 1976; Awramik and Sprinkle, 1999; Grotzinger and Knoll, 1999).
Despite their status as a quintessential geobiologic structure (Grotzinger and Knoll, 1999), the process-
es that control stromatolite growth rate, laminae formation, and texture are poorly understood. Process
and rate of stromatolite morphogenesis are difficult to study in ancient forms, as the conditions under
19
which they formed become more difficult to discern with increasing age, thus the biogenicity of many
of these ancient forms is in question (Grotzinger and Knoll, 1999; Awramik and Grey, 2005). As stated
in Chapter 1, modern marine stromatolites, such as those forming in the Bahamas today,make less-
than-suitable analogues for many Precambrian forms (Awramik and Grey, 2005). The stromatolites
described in this study originate from Walker Lake, a closed basin alkaline lake in western Nevada that
is a remnant of the Pleistocene Lake Lahontan complex.
WALKER LAKE
Walker Lake is a closed basin, sodium carbonate lake, with an approximate area of 100 km
2

and a maximum depth of 28 meters. The lake is located in western Nevada, just east of the California-
Nevada border (Figure 2.1a). Currently, the lake is bordered on the western side by the Mesozoic-age
Wassuk Range. In the Pleistocene, Walker Lake was part of the giant pluvial Lake Lohantan, however,
Walker Lake has the highest sill in Lohantan system, and thus has been separated from the rest of the
Lohantan Basin for approximately the last 12,000 years. The lake has undergone cycles of expansion
and contraction due to climate and anthropogenic use of Walker River water (Benson et al., 1991; Yuan
et al., 2004). Since the 1900’s, the water level has fallen 50 m as a result of the diversion of the Walker
River for agricultural purposes. The total present dissolved solid content is 17 g/L and the pH is 9.4.
Surface sediments within the lake are fine grained, composed of ~50% silts and clays and up to 50%
CaCO
3
, with monohydrocalcite (CaCO
3 ˙
H
2
O) as the dominate form of carbonate in the sediments. The
lake currently stratifies annually between June and November (Cooper and Koch, 1984), during which
anoxic conditions are present below 20 m (Figure 2.2). The increase in salinity that accompanied the
decrease in lake water level has caused an increase in the duration of bottom water anoxia, which has
impacted both the lake fish and the surface nutrient budget (Benson et al., 1991; Beutal, 2001; Beutal
et al., 2001).
20
Carbonate Budget of Walker Lake
Walker Lake is a calcium-limited system with respect to calcium carbonate precipitation.
While it is replete with carbonate, the overall concentration of calcium is less than 0.2 mmol (Figure
2.2). The main source of calcium to Walker Lake is the Walker River, which, for the time period 1940-
1987, had an average discharge of 0.38 km
3
/yr (Benson et al., 1991). Given the calcium concentration
of river water (~0.2 mM), and the assumption that all input calcium will be converted to CaCO
3
, this
river input would produce 2.5x10
5
mol CaCO
3
per day. Sedimentation rates in Walker Lake have been
calculated to be between 1.5 and 3.0 mm/yr (Benson et al., 1991). If a sedimentation rate of 1.5 mm/
yr is assumed, along with a 15 weight % carbonate ratio (both low estimates), a carbonate burial rate
of 1.3x10
6
mol CaCO
3
per day is required. This burial rate requires an order of magnitude more CaCO
3

than is currently being supplied to the system by the Walker River. This mismatch is either a result of
a non-uniform accumulation of carbonate on the lake floor, or, more likely, there are other sources of
calcium to the lake (e.g., groundwater, discussed below).
WALKER LAKE STROMATOLITES
Location
Stromatolites are well known from the Pleistocene Lahontan system and from Holocene
Walker Lake (Osborne et al., 1982; Newton and Grossman, 1988). The stromatolite horizon of inter-
est extends from the current shoreline to approximately three meters above present lake level on the
southwest corner of Walker Lake (Figure 2.1b). Curiously, the stromatolites are not located near the
mouth of the Walker River (the main source of calcium input into Walker Lake), but at the base of a
large, porous and permeable alluvial fan (Figure 2.1a). Here, cobbles and boulders are encrusted with
4-5 cm thick, branching calcium carbonate stromatolites (Osborne et al., 1982) (Figure 2.1c).
21
Petrography
The internal microstructure of the stromatolites reveals three distinct portions: a lower (~20-25 mm
thick) well-laminated portion that closely resembles the morphology of many Precambrian stromato-
lites (Grotzinger and Knoll, 1999; Awramik and Grey, 2005)), a middle (~19 mm thick) weakly to non-
laminated portion, and a non-laminated outer portion (~10 mm thick) (Figure 2.3). In thin section, the
laminations are observed composed of light-dark couplets of microspar (Figures 2.4; 2.5). No discern-
able fossil cells can be found in the structure, and there is no evidence of trapped and bound detrital
material (Figures 2.4-2.10). The stromatolite is nearly all calcite, with very few grains of detrial silici-
clastic material at all. At higher magnification, one can see that the very tiny calcite crystals are inter -
locking (Figures 2.5; 2.8; 2.10). There appears to be no difference between light and dark laminations,
except for the amount of pore space/density of the calcite layers (this was also noted by Osborne et al.,
1982) (Figures 2.4-2.8). There are no observable dissolution surfaces in the finely laminated portion,
indicating that the structure accreted in a regular fashion throughout this section. The non-laminated
outer portion of the structure is made up of the same crystal sizes and minerals as the laminated por-
tions, but lacks regular variations in pore space, thus giving it a non-laminated appearance (Figures
2.9; 2.10). According to the classification scheme devised by Riding (2008), this kind of texture would
be considered somewhere between a “hybrid” and “abiogenic sparry crust”, meaning that, if one where
to only consider the microstructure of the stromatolites, their biogenicity would be ambiguous.
Previous Work on Walker Lake Stromatolites
Chronology and Stable Isotopes: A number of papers have been written on Walker Lake, most dealing
with the lake history and its connection to the rest of the Lahontan Basin. Osborne et al. (1982) first
described the stromatolites as modern and discussed the algal and cyanobacterial communities living
on them. Newton and Grossman (1988) later provided two
14
C dates for the stromatolites—which they
termed “laminated encrusting tufa”—both yielding ages around 2100 + 90 ybp, demonstrating that
the stromatolites were Holocene in age, and not modern. Additionally, δ
13
C and δ
18
O were measured
along a transect through the samples. The δ
13
C in the stromatolites is consistent around +3‰, varying
22
only by tenths of a mil. The δ
13
C of Walker Lake water was +2.9‰ at the time of this study. Varia-
tions in δ
18
O were also small. The laminated portion of the structure interpreted as having been formed
by ‘the to and fro movement of water over the clasts in a warm shoreline pool’, analogous to the ‘cave
popcorn’ found in limestone cavern pools. Thus, the variations in δ
18
O were attributed to temperature
fluctuations at the surface of Walker Lake, where the “tufas” were assumed to have grown.
Clumped Isotope Paleothermometer: Recent work by Katherine Huntington and John Eiler at the
California Institute of Technology examined the ‘carbonate clumped isotope thermometer’ of the
stromatolites. The clumped isotope temperature proxy is based on the number of heavy (
13
C
18
O
16
O)
isotope ‘clumps’ found in CO
2
produced by the dissolution of carbonate in acid (Ghosh et al., 2006;
2007). There is an inverse correlation between the proportion of heavy clumps and temperature at the
time of carbonate formation. By using this proxy, it is possible to discern the temperature of the fluid
that the stromatolites formed from, and thus whether the stromatolites formed at the surface, or deep in
the lake. The finely laminated portion of a Walker Lake stromatolite yielded a clumped isotope paleo-
temperature of 9.4°C, indicating that it formed in relatively deep water.
Carbonate Associated Sulfate: Berelson et al. (2009) studied the amount of trace sulfate bound in the
carbonate structures in order to reconstruct lake level fluctuations. As a conservative ion in Walker
Lake, the sulfate concentration tracks lake volume. As lake level decreases, the amount of sulfate in
the lake, and thus the amount of sulfate bound into the carbonate crystal lattice, increases. If lake level
increases, sulfate levels decrease. Berelson et al. (2009) found that, based on the trace sulfate concen-
trations in the stromatolite, the laminated portion of the stromatolite grew during lake level fall, and
started growing in a relatively deep lake ~3300 ybp. At this time, several lake level reconstructions
(e.g., Yuan et al., 2004; Adams, 2007) agree that lake level was much higher. These reconstructions
suggest that the stromatolites were formed at roughly 40 meters water depth and agree well with the
previously discussed clumped isotope paleo-temperature.
23
METHODS
Dates were obtained from four different stromatolites at similar distances measured from the
apex of the outer boundary (Tables 2.1; 2.2). Three of the stromatolites were collected from an in situ
boulder found ~three meters above current lake level. A fourth stromatolite, found dislodged next to
the in situ samples and identical in appearance to the others, was also dated. One hundred mg of car-
bonate material was obtained by micro-drilling 1 mm holes along stromatolite laminae.
14
C analyses
were run at the Keck Carbon Facility at the University of California, Irvine. Sample ages were 2σ cali-
brated to account for changing levels of atmospheric
14
C using the INTCAL04 database, and assume a
300 yr reservoir effect for CO
2
in Walker Lake (Benson et al., 1995; Broecker and Walton, 1959; Yuan
et al., 2006) (the reliability of
14
C dates is discussed below). All dates are reported as Calibrated Years
Before Present (ybp, years before 1950 via standard
14
C conventions) (Stuiver et al., 1998, Table 2.2)).
In contrast to most of the previous studies, each stromatolite was sequentially sampled from bottom to
top in order to build a radiometric age dated framework from which to determine the rate of lamina-
tion formation. The results of the age vs. distance montage (Figure 2.11) demonstrate the coherence
in growth between the various stromatolites. In order to count laminations, a large photo-mosaic was
assembled from forty photomicrographs of a thin section of a single radiometrically-dated stromatolite
(WL_6), and annotated. The thin section preserved the drill holes from radiometric dating so that the
rate of lamination formation could be calculated. Using the dates, the number of laminations, and the
distance between sets of drill holes, the average stromatolite growth rates (Figure 2.11) and lamina-
tion periods (Figure 2.12; Table 2.3) were determined. Errors were calculated using the maximum and
minimum possible ages resulting from the 2σ calibration (Table 2.3). The fine, sub-millimeter scale
lamination of the stromatolite means that several individual laminations (as many as 16) are included
in the span of one drill hole (Figure. 2.12a; Table 2.3). The
14
C date is assumed to come from the lami-
nation located in the middle of the sets of drill holes. While it is unlikely that the
14
C is not an average
age from all the material contained within the drill hole, a ‘worst-case scenario’ error was constructed
by counting the maximum and minimum number of laminations included between a set of drill holes
(from the bottom of the lowermost set of hole to the top of the upper ones for the maximum, and from
24
the top of the lowermost set to the bottom of the uppermost for the minimum) (Figure 2.12c; Table
2.3),
RESULTS
Growth Rate
All samples show the same general age distribution (Figuure 2.11), and results are consistent
with age dates from previous studies (Newton and Grossman, 1988). These dates indicate that stro-
matolite growth initiated ~3300 ybp when the lake was 30-60 m higher than the stromatolite horizon.
Stromatolite growth initiated in a relatively deep lake, such that disconformities in accretion because
of wave activity is avoided for these samples. On average, stromatolite growth rate slowed from 0.390
to 0.005 mm/yr as they grew, when measured from the axial zone of growth (the thickest portion of the
stromatolite). Interestingly, the growth rates drop an order of magnitude within the finely laminated
portions (from 0.390 mm/yr to 0.066 mm/yr) with no observable changes in external morphology.
However, as stromatolites are complex three-dimensional structures, the bulk growth rate may change
depending on what region of the stromatolite was sampled; thus, the stromatolites all have slightly dif-
ferent initial growth rates, though all show the same trend. Therefore, it is more instructive to examine
lamination frequency, which does not vary with position in the stromatolite, versus bulk growth rate.
Lamination Rate
Counts of single laminations of stromatolite WL_6 reveal an average rate of 0.18 laminations/
yr for the bottom most part of the sampled stromatolite (zone I on Figure 2.12b; 3307.5 to 3000 ybp),
or a period of one lamination every 5.58 ± 1.6 yrs (Figure 2.12b). Laminar thickness appears to be in-
ternally consistent (about 100 μm) for zone I. The laminations themselves are smooth, evenly spaced,
and have sharp divisions between light and dark layers. The growth rate for this stromatolite starts out
at 0.117 mm/yr, which is slower than the group average of 0.390. This is likely because of the posi-
tions from which the first set of samples were taken (higher in the stromatolite than in other samples).
For the second set of dates (zone II, 3000 to 2823 ybp), overall structure grew more slowly (0.030 mm/
yr specifically), while lamina formation actually increased to a rate of 0.36 laminations/yr , or one lami-
25
nation every 2.81 ± 1.9 yrs. Lamina in zone II are thinner (averaging 71 μm) than in the lower portion,
though internally consistent, and retain the sharp, smooth contacts of the lowermost portion. The third
set of dates (zone III, 2823 to 2671.5 ybp) represent a further slowing of overall growth rate (0.024
mm/yr for this portion of WL_6) and further increase of lamination rate, to 0.63 laminations/yr, or one
lamination every 1.58 ± 0.9 yrs. Laminations in zone III thin further (averaging 59 μm), but the transi-
tions between light and dark laminations are still clearly defined. In the youngest segment (zone IV ,
2671.5 to 2274.5 ybp), lamination frequency decreases to a rate of 0.22 laminations/yr, or one lamina-
tion every 4.46 ± 0.8 yrs, while growth rate continues to decline (0.006 mm/yr for this stromatolite at
during these times) (Figures 2.11, 2.12b). Laminations thicken in zone IV (averaging 77 μm), though
not to the size of zone I. Note that the apparent break in continuity in zone IV (Figures 2.6, 2.12a) is
an artifact of the thin sectioning process and did not affect the analysis. Above zone IV , the overall
structure changes from finely to weakly laminated, and lamination period cannot be resolved (Figures
2.7; 2.8; 2.12a).
DISCUSSION
Reliability of
14
C Dates
In previous studies, two main sources of error have been discussed in conjunction with the
14
C
dating of Lahontan carbonates: the reservoir effect (variations in the initial
14
C/C ratio of the lake),
and the open system effect (addition of modern carbon after precipitation) (Yuan et al., 2004; Benson,
1993).
Currently, the reservoir effect of carbon in Walker Lake is reported to be 300 years (Benson
et al., 1995; Broecker and Walton, 1959; Yuan et al., 2006). However, lake level fluctuated signifi -
cantly during stromatolite formation. Since it is impossible to know the exact amount of carbon dis-
solved in a paleolake, the extent of the reservoir effect during past times is speculative, but some basic
constraints can be placed on the change in reservoir effect relative to lake level variation. In a larger
lake, the reservoir effect would be more significant, which would make the stromatolites appear older
than they are. In order to test the consistency of our results with a varying reservoir effect, we offset
26
the dates as a function of presumed lake level, where a local high-stand occurred ca 3400 ybp (Ad-
ams, 2007), a low-stand at 2400 ypb (Yuan et al., 2004) (close to present day lake level), followed by
a rising lake (Yuan et al., 2004)(Table 2.4). Using these constraints we made a linear interpolation of
reservoir effect. The oldest date (3307.5 Calibrated ybp) is adjusted a further 267 years to account
for a larger lake volume. As lake level falls, the magnitude of the additional offset decreases (Figure
2.12b;c, Table 2.4). In this case, the periods would change to one lamination every 3.78 ± 1.6 yrs for
zone I, decreasing to one lamination every 2.27 ± 1.9 yrs in zone II, to 0.89 ± 0.9 yrs zone III, and
back up to one lamination every 4.46± 0.8 yrs in zone IV (Figure 2.12c). While these numbers do not
precisely match the first set, they do not vary widely enough to impact the findings of this study , that
the lamination period for much of the stromatolite exceeds the daily, seasonal, or yearly lamination
interval commonly inferred for ancient stromatolites.
When considering a porous carbonate rock, we must take into account the fact that carbon may
have been incorporated into the structure after precipitation, altering the apparent
14
C ages (the open
system effect). However, in this case, several lines of evidence can be used to show that the dates have
not been influenced by the addition of modern (or radio dead) carbon. Petrographic analysis revealed
the stromatolites are composed of interlocking microspar, and while there is pore space, possibly
indicating some dissolution, there is no evidence for second generations of carbonate precipitation
(Figures 2.4-2.10). In other Lahontan carbonates, such as those at Pyramid Lake, secondary carbon-
ate precipitates are found as visible inclusions within the porous tufa (Benson et al., 1995). No such
inclusions are found in the Walker Lake stromatolites studied here. The consistency of the dates also
argues against the alteration of the apparent
14
C ages. Four separate stromatolites (three in situ and
one loose block) were dated, and all showed the same trend, with no date out of sequence. The same
position in different samples gave similar dates. Consistency in sample ages has been used in previous
studies in the Walker Lake subbasin to indicate the maintenance of a closed system after deposition
(Benson, 1993; Yuan et al., 2004). Further evidence of the validity of the
14
C dates can also be found
in the study of carbonate associated sulfate (CAS) record presented by Berelson et al, 2009. As the
stromatolite accreted, trace amounts of sulfate were bound into the calcium carbonate crystal lattice.
27
Because Walker Lake is a closed system, the total amount of sulfate depends on the volume of the
lake. If lake level is high, sulfate concentrations will be low, and vice versa. The amount of CAS in
the Walker Lake stromatolites is initially relatively low, when the dates suggest the lake to be at a high
stand. CAS rises steadily though the midsection of the stromatolite, reaching a peak that coincides
with the date of the major lake low stand. Given the petrographic evidence, the consistency of the
14
C
ages (within this study and when compared to the previously reported dates), and the reported CAS
evidence (Berelson et al., 2009), we conclude that there is no significant af fect on the apparent
14
C ages
due to the incorporation of modern or radio dead carbon into the stromatolites.
Multi-year Lamination Period
To our knowledge, the Walker Lake stromatolites are currently the best-dated stromatolites in
the world, and the only fully lithified stromatolite on which growth rate and lamination frequency have
been measured at such a fine scale. Previous studies hypothesized that stromatolite lamination repre -
sented daily, seasonal, or yearly frequency, with the most commonly cited mechanism (daily) tied to
the diurnal rhythm of photosynthetic microbial mats. Strikingly, the Walker Lake stromatolites do not
fit the classic models for stromatolite lamination formation. The average lamination period in all zones
is likely far greater than the diurnal response of a microbial mat for the constant reservoir case, and
always greater than a year for zones I and IV regardless the potential errors/reservoir effect. Conven-
tional stromatolite morphogenesis theory—constructed in the absence of high-resolution age control
that we now possess—would not have predicted such a complicated history or such a long duration
between laminations. While it cannot be ruled out that microbial mats were present and may have
somehow affected stromatolite growth, it is clear that the daily or even seasonal activities of putative
microbial mats did not control lamination formation for the majority of stromatolite growth for this
particular system. The stromatolites grew in response to some forcing beyond the typical time scale of
microbial metabolism.
28
Significance
What might control the formation of lamination in Walker Lake stromatolites if not daily or
annual cycles of microbial mats? It is striking that the periodicity in zones I, II, and IV (~2-7 years) is
similar to the local wet/dry climate cycles in the region, thought to reflect the El Niño Southern Oscil -
lation (ENSO) climate cycle. Previous studies have noted that ENSO events can affect climate in this
region (Redmond and Koch, 1991), although stream discharge in the Great Basin does not always
correlate with the Southern Oscillation Index (Yuan et al., 2006).

Such non-annual lamination has also
been reported in sediments from the Santa Monica Basin (Christensen et al., 1993), which is also af-
fected by ENSO cycles. Certainly the prospect of a long term, high-resolution record of climate oscil-
lations is intriguing and deserves further study.
As an alkaline lake, carbonate precipitation in Walker Lake primarily varies with Ca-delivery
(e.g., runoff and groundwater input) and thus responds to climate. The photosynthetic activity of puta-
tive microbial mats, often cited to increase local pH via CO
2
uptake and foster carbonate precipitation
(e.g., Reid et al., 2000) would have little effect in the highly alkaline system. Zone I, II and IV formed
when the lake was larger and reveal longer lamination periods with thicker laminations, whereas zone
III formed when the lake was smaller and displays yearly but thinner laminations. Interestingly, the
position of the stromatolites on a large, porous and permeable alluvial fan/delta would enhance subsur-
face, calcium-rich fluid flow to the loci of stromatolite formation. In this way , the lamination frequency
would be controlled by the delivery of Ca
2+
to the lake system, which is in turn controlled by climate
features, in contrast to a morphogenetic model driven primarily by microbial processes. In a larger
lake, it may take a larger forcing function to drive carbonate precipitation, but a large lake may have
the potential to generate thick carbonate layers (e.g., zone I), especially if carbonate precipitation is fo-
cused on a limited spatial area, such as the alluvial fan upon which the stromatolites grew. We hypoth-
esize that during especially wet years (e.g., El Nino years), significantly more Ca
2+
will be delivered to
the lake, potentially triggering lamination formation in the stromatolites, represented by thicker lami-
nations given the considerable carbonate-precipitating capacity of a larger lake. During a lowstand,
represented by the middle laminated portion, a lesser input of Ca
2+
may drive stromatolite lamination,
29
but the reduced carbonate-precipitating capacity of a smaller lake would produce thinner laminations
as seen in zone III (smaller, but more regular rainfall may have contributed to yearly, but thinner, lami-
nations). Additionally, in a smaller lake, the carbonate system will be affected more by annual overturn
and stronger seasonality of temperature.
Interestingly, because of the clumped isotope paleotemperature, and the oxygen isotope record
provided by Newton and Grossman (1988), the theory that the position of the stromatolites on a large
alluvial fan and subsequent ground water input had an effect on the growth and laminae formation can
be tested. Conventionally, he δ
18
O of the carbonate, and the δ
18
O of the water are used to back calcu-
late the temperature of formation. In lakes, the equation:
from Friedman and O’Neil (1997) is used; where α is the isotope fractionation factor of the heavy and
light isotopes in the solid (carbonate) and liquid (water) phases:
In the case of the Walker Lake stromatolites, it was assumed that since the δ
18
O
c
of the stromatolites
varied so little, the carbonate had to have precipitated in equilibrium with lake water, and the δ
18
O
w

had not changed significantly in the last few thousand years (Newton and Grossman, 1988). Given
this information, Newton and Grossman (1988) derived paleotemperatures of formation ranging from
28 to 34 °C, which was consistent with their interpretation of the stromatolites as shoreline tufa depos-
its.
Since it is now clear from the evidence that the stromatolites are not shoreline deposits, we
can instead use the measured clumped isotope paleotemperature of 9.4 °C and the δ
18
O
c
of the finely
laminated portion (from Newton and Grossman,1988) to calculate the δ
18
O of the water of formation.
Use of the Friedman and O’Neil (1977) equation in this instance yields a δ
18
O
w
of -7.97 to -6.75 ‰
(Standard Mean Ocean Water, SMOW). Currently, Walker River water ranges from -15.8 to -10.8 ‰
SMOW, while Walker Lake water ranges from 0 to +2.0 ‰ SMOW (Lopes and Allander, 2009; Yuan,
30
2006).
It could be inferred from this calculation that the stromatolites are forming as Walker River water
mixes with Walker Lake water. However, if the stromatolites were forming as a result of calcium
coming in from the Walker River, they would most likely be located at the mouth of the river. Instead,
the stromatolites are located on the opposite side of the lake, at the base of a porous and permeable
alluvial fan (which would deliver significant amounts of groundwater into Walker Lake)(Figure 2.1a).
Groundwater in the area is also light with respect to δ
18
O
w
(-16 to -9 ‰ SMOW) (Lopes and Allander,
2009), making it plausible that the stromatolites are forming as calcium rich groundwater comes in
through the alluvial fan, mixes with lake water, and provides the appropriate geochemical setting for
the precipitation of these structures.
Currently, it is estimated that, yearly, the total amount of groundwater delivery to all of Walker
Lake is 11,000 acre-ft per year (or 1.36 x 10
10
L/yr) (Thomas, 1995). This is a trivial amount of water
compared to the discharge of the Walker River (1 x 10
9
L/day) (Benson et al., 1991), but it can useful
for the purpose of estimating the extra calcium delivery to the area where the stromatolites accreted.
The area that the stomatolites are found in is a relatively small stretch of beach, measuring about 1
x 10
-6
km
3
. This area, given a lake calcium concentration of 0.2 mM, would contain 2.5 x 10
5
mmol
Ca
2+
, meaning that it is under-saturated (Ω = 0.35) at the current salinity (17 ppt) and temperature
(25 °C). If less than one tenth (600 acre-ft, or 7.4 x 10
8
L) of groundwater were to seep into the area
the stromatolites were accreting through the alluvial fan, the total calcium content of the area would
increase (assuming total mixing), depending on the calcium content of the groundwater. If the ground-
water has the same calcium content as the lake (it would likely be much higher, but this can be used as
an end-member low estimate), the total calcium concentration would increase to 6.06 x 10
5
mmol, or
6.06 x 10
-4
M. This alone would cause a slight oversaturation of the area with respect to calcium car-
bonate (Ω = 1.07). If it is assumed that groundwater has a calcium content that is more like river water
(1.0 mM, a more feasible number) (Becker et al., 2001; Benson and Leach, 1979), an additional 2.42
x 10
6
mmol of Ca
2+
would be added, resulting in a saturation index of 4.3. In any case, the addition of
groundwater to the area of stromatolite growth would make calcium carbonate much more likely to
31
precipitate.
For the Walker Lake stromatolites, it is likely that local climate forcing, hydrology, and geo-
chemistry had more to do with stromatolite formation than the presence of putative microbial mats.
Although we do not suggest that Walker Lake is a direct match for the Precambrian ocean, it is inter-
esting to note that some hypothesize Archean/Paleoproterozoic oceans were highly alkaline (Grotz-
inger, 1990; Kempe and Degens, 1985) and/or carbonate precipitation may have been calcium-limited
(Groztinger and Kasting, 1993).
CHAPTER 2 CONCLUSIONS
Several interesting findings have emerged from the high-resolution
14
C dating of Walker Lake
stromatolites. The fact that the lower and uppermost portions shows a 2-7 year trend suggests that local
ENSO cycles could be the driving factor in lamination formation, and that these structures may con-
stitute a high resolution climate record for the region. The Walker Lake stromatolites, which closely
resemble the ancient forms thought to be evidence of early life on Earth, do not conform to some of the
most widely assumed characteristics of stromatolites. The growth rate of the stromatolites is not con-
sistent; it slows dramatically over the course of accretion, though this change in rate is not reflected in
the macroscopic morphology. The lamination frequency/period is not fixed throughout the structure;
the rates changed significantly over the course of stromatolite formation, and like growth rate, are not
necessarily accompanied by any obvious changes in external morphology. While it has been generally
assumed that stromatolite lamination represent the record of the daily, seasonal, or at most, yearly re-
sponses of a microbial community, this study shows that such assumptions could be problematic, as the
frequency of lamination for much of the stromatolite is longer than the daily, seasonal, or even yearly
cycles characteristic of typical microbial mats.
While it is clear that some stromatolites are built in response to a microbial forcing (e.g., Awra-
mik and Grey, 2005; Reid and Browne, 1991), our study demonstrates that other stromatolites may not
record microbial processes. Caution must be used when interpreting the meaning of stromatolites and
stromatolitic lamination, especially with respect to ancient Earth or extraterrestrial locale (e.g., Mars).
32
Without additional information, the rate of formation of lamina in a stromatolite cannot be taken as a
daily, yearly, seasonally or even a regular process, and the involvement of microbes may or may not
represent a significant contribution to stromatolite morphogenesis depending on the system.
33
CHAPTER 3: ANALYSIS OF GROWTH DIRECTIONS OF DOMED STROMATOLITES
FROM WALKER LAKE, WESTERN NEV ADA.
CHAPTER 3 ABSTRACT
Typically, stromatolites have a domed morphology that is commonly taken as an indication of micro-
bial phototrophism and phototaxis (growth or movement towards incident light). Despite the fact that
stromatolites may represent evidence of some of the oldest life on Earth, much about the processes
that control stromatolite form remains enigmatic. The stromatolites of Walker Lake can be found on all
sides of boulders situated on the southwestern shore. Because of their positions, they are an ideal test
case for commonly assumed hypothesis that stromatolites grow upwards toward incident light. It was
predicted that the columns on the steeply dipping sides of the boulder would bend upwards towards the
light during growth if phototropism is a significant influence on stromatolite morphogenesis. Angle of
growth measurements on >300 stromatolites demonstrate that the stromatolites grew nearly normal to
their growth surface, regardless of the inclination of their growth surface. No significant dif ferences
in the distribution of growth angles between north, south, east or west facing samples were observed,
and stromatolite lamina thickness did not systematically vary with position on the boulder. The lack of
a phototropic response does not rule out a biological origin for the Walker Lake structures, but it does
suggest that phototropic growth was not a dominant factor controlling stromatolite morphogenesis in
these stromatolites, and that column formation cannot be uniquely attributed as a phototropic response
in stromatolites. It is interesting to note that the morphology of the stromatolites on the top of the boul-
der are identical to stromatolites on the steep sides. Stromatolite morphogenetic models that predict
branching typically require a vertically-directed sedimentary component, a feature that would have
likely affected the stromatolites on the tops of the boulders, but not the sides, suggesting that other fac-
tors may be important in stromatolite morphogenesis.
34
INTRODUCTION
Stromatolite laminations are thought to be the record of the daily, yearly, or seasonal responses
of the growing microbial community to some environmental forcing (Riding and Awramik, 2000),
and the domed growth pattern of many laminated carbonate accretions is commonly taken as evidence
of the photosynthetic response of microorganisms (e.g., Semikhatov et al., 1979; Walter et al., 1980;
Awramik and Vanyo, 1986; Vanyo and Awramik, 1985; Schopf, 1999). This photosynthetic response
includes both phototaxis, or the movement of bacteria towards light, and phototrophism, the differen-
tial growth of microbes depending on where in the growing structure they are located (with a prefer-
ence for photosynthesizing bacteria that are receiving more light). In the conventional stromatolite
growth model, microbes (most commonly phototrophic cyanobacteria) on the areas of the growing
structure that receive more light (commonly the top), will outcompete those on the areas that receive
less light (commonly the sides), often leading to a domed morphology over time (Walter, 1976; Vanyo
and Awramik, 1985). Additionally, the gas bubbles produced by oxic and anoxic photosynthetic com-
munities commonly ‘lift’ the mats, enhancing topographical relief, aiding in the formation of vertical
structures such as cones, tubes, and towers, and causing the contortion of laminae that is seen in many
ancient stromatolites (Bosak et al., 2009; 2010).
As stated in previous chapters, microbial activity is not necessarily a prerequisite for the pro-
duction of a stromatolite with a domed, laminated growth pattern. Grotzinger and Rothman (1996)
used the Kadar-Parisi-Zhang (KPZ) equation to deduce that certain stromatolite forms could be pro-
duced abiotically using four parameters: 1) the fallout of suspended sediment; 2) diffusive smoothing
of the settled sediment in accordance with physical sorting laws (such as down slope movement due
to gravity); 3) surface-normal growth; and 4) uncorrelated random noise (Grotzinger and Rothman,
1996; Batchelor et al., 2004). The KPZ model predicts the formation of domal stromatolites with
time (Grotzinger and Knoll, 1999), though it does not tend to produce branching and columnar forms
(Batchelor et al., 2000; Dupraz et al, 2006). A Diffusion Limited Aggregation (DLA) model, in which
particles (ions, nutrients, etc.) arrive at a site of deposition through Brownian motion (e.g., diffusion)
until they make contact with an aggregate (Witten and Sander, 1983; Verrecchia, 1996; Grotzinger and
35
Knoll, 1999), creates dendritic patterns, but not necessarily stromatolite-like morphologies. A com-
bination of the DLA model and episodic sedimentation (Chan and Grotzinger, unpublished observa-
tions cited in Grotzinger and Knoll, 1999) does produce the columnar branching forms reminiscent of
ancient branching stromatolites. Thus, stromatolite-like morphologies can be produced without the
influence of microbial communities when the unit of growth is abiotic mineral precipitation, or with
the influence of microbial communities when the increment of growth is a mat. Dupraz et al (2006)
combined the extrinsic environmental factors governed by the DLA model with intrinsic factors, mod-
eling microbial mats as “light-driven-engines” to produce the Diffusion Limited Aggregation-Cellular
Automata (DLA-CA) model, which can produce a range of stromatolite morphologies. Like the find -
ings of Chan and Grotzinger (in Grotzinger and Knoll, 1999), the DLA-CA model produced branching
columns when a strong “motion index” was specified, where particles follow a ballistic trajectory over
Brownian motion, and/or the inclusion of episodic sedimentation into the model.
The ability to assess whether or not a carbonate accretion has been produced via microbial
influence is essential to answer questions pertaining to the first appearance of life on Earth. Thus, find-
ing a way to distinguish biotic carbonate accretions from their abiotic counterparts based on morphol-
ogy and texture would be of great value in the search for evidence of extraterrestrial life. Therefore,
it is a priority in the fields of geobiology and astrobiology to find modern analogues for Precambrian
stromatolites, and to understand the processes (biotic and abiotic) that mediate their overall morphol-
ogy. Given that many if not most ancient stromatolites lack evidence for direct microbial involvement
(e.g., microbial fossils, grains trapped beyond the angle of repose, etc., cf. Grotzinger and Knoll, 1999)
the widespread assumption of column formation as a phototropic response is oft cited but rarely dem-
onstrated. Here, stromatolites from Walker Lake, Nevada, are examined in order to test the assumption
of column formation as a phototropic response. The small branching digitate columns found in Walker
Lake constitute an excellent test case, as they formed on the sides and tops of meter-tall boulders; the
angle of growth versus the angle of the substrate from which they grew can be used to test the pho-
totropic hypothesis, where structures formed in association with a phototropic response would bend
upwards towards the light. More subtle phototropism, such as preferential growth or thickness towards
36
incident light, can also be tested.
Walker Lake Stromatolites
Today, at least four discrete horizons of carbonate accretions, previously described as “dense”
tufas (Benson, 1994) are found associated with Walker Lake. While each horizon contains carbonate
forms with different micro and macrostructures, all horizons display the predominantly precipitated
fabrics that resemble Precambrian forms, rather than the coarser “trapping and binding” fabrics associ-
ated with modern marine stromatolites.
As mentioned in Chapter 2, the stromatolite horizon of interest to this study is confined to a
narrow location on the southwestern corner of the lake, where a large alluvial fan enters the lake (Fig-
ure 2.1-2.4). The stromatolites began accreting around 3300 ybp, and grew as lake level fluctuated.
Currently, the decreasing lake volume due to anthropogenic use of the Walker River is causing the
lake to warm, promoting the growth of algae. These factors limit the depth of light penetration to the
uppermost 10 m of the lake (Horne, 1994; Cooper and Koch, 1984). Recently, the mean depth of the
euphotic zone was found to be 8.2 m (Cooper and Koch 1984). Without knowing the clarity of Walker
Lake in the latest Holocene, it is unclear what amount of light could have penetrated to 40 m.
The Walker Lake stromatolites form the focus of this study because their morphology most
closely matches that of domed Precambrian samples (Figure 3.1). Additionally, the relatively young
ages and undeformed nature of these accretions allows for analyses not possible in ancient forms.
Much of the remaining calcium carbonate accretions in the Walker Lake area are significantly older,
dated to be >18,000 yr (Newton and Grossman, 1988).
Given the preliminary information, it is clear that these intriguing structures merit further study.
The positions and growth patterns of the Walker Lake stromatolites, make it possible to investigate any
phototrophic influence in the macrostructure of these forms. If the domed morphology of the stroma-
tolites is a record of the response of phototrophic organisms to the availability of light, it should be
represented at a number of scales.
37
Solar Radiation, Predicted Growth Directions, and Predicted Thicknesses.
Stromatolites of Walker Lake are located at 38°N latitude, therefore given the tilt of the Earth’s
axis and the position of the sun, the incoming solar radiation should be predominantly from the south.
The daily east-west passage of the sun should be averaged out, so the net growth would occur in the
north-south plane (Vanyo and Awramik, 1985). However, as noted previously, the stromatolites were
not growing in shallow surface waters: the incoming solar radiation would be diffused through 40 m of
water before it filtered down to the stromatolite accretion horizon. This diffuse light would only per-
sist so long as the water depth remained high. The results of Chapter 2 show that this is not the case;
lake level falls over the course of stromatolite accretion. Therefore, if the morphology of the stromato-
lites is a phototrophic response, it is expected that the columns will have grown inclined toward the di-
rection of incoming solar radiation (the south). If the morphology of the stromatolites was not formed
under the direct influence of photosynthesizing bacteria, no such trend will be seen.
If a phototrophic or phototactic response is present, stromatolites growing off the steeply
angled sides of the boulders are predicted to show an inclination to grow at angles up towards the light,
rather than perpendicular to the growth surface. Stromatolites on the tops of the boulders should grow
up towards the incident light, and slightly towards the south. No stromatolites would be predicted to
grow in the ‘down’ direction, away from incident light, or strongly toward the north (Figure 3.2a).
Alternatively, construction of the stromatolite via a non-phototrophic community, or even abiotic pro-
cesses, would display mainly surface-normal growth. Surface normal growth is likely to develop due
to the diffusive transport of Ca
2+
(recall from Chapter 2 the very high CO
3
2-
concentrations in Walker
Lake— the concentration of calcium is the factor that limits carbonate precipitation), nutrients and
other metabolically important compounds (i.e., sulfate for sulfate reducing bacteria) from lake water to
the growing stromatolite. Therefore, non-phototrophic growth will trend directly away from the boul-
ders (at a surface normal trajectory) toward the source of necessary ions and compounds (Figure 3.2b).
Furthermore, stromatolites built under the influence of a phototrophic community will show
trends in thickness due to the availability of light. In this case, the thickest stromatolites should be
found on the sides of the boulders that face the south, at the lowest angles of inclination (Figure 3.3a).
38
Thinnest samples should be found on north facing samples at high angles of inclination (Figure 3.3a).
Stromatolites in the east-west plane would show a more symmetrical distribution, with the thickest
stromatolites at the shallowest angles, and the thinnest at the steepest angles (Figure 3.3a). A non-pho-
totrophic community would have thickness may be slightly variable due to the availability of chemi-
cal species, but would not be strongly skewed to being thicker on the top, sides, or toward the south.
(Figure 3.3b). A summary of hypothesized results and their interpretations is listed in Table 3.1
METHODS
In order to test whether or not the influence of a phototrophic community is present in the
morphology of these stromatolites, a study of growth direction was undertaken by sampling stroma-
tolites that were encrusting all exposed sides of four in situ boulders located on the southwest side of
Walker Lake in the spring of 2010 (Figure 2.4b;). Samples were taken from the north, south, east, and
west sides of the boulders, both from the steep (>75 degree) sides and from the near-horizontal top. In
a few cases, the forms were growing on the underside of boulder lips as well (essentially beyond 90
degrees/over-hung). Stromatolites were transported back to the University of Southern California, cut
perpendicular to the growth surface, polished and scanned at high resolution. Growth directions were
determined by measuring the angle between the original surface of the boulder and the crest of the
dome on 305 individual stromatolites (Figure 3.2c).
In this system, 90 degrees represents surface normal growth, not the absolute angle of growth.
Deviations from 90 degrees would indicate growth in the up or down directions, where <90 degrees
represents growth in the “up” direction and >90 degrees represents growth in the “down” direction
(Figure 3.2c). If a sample was determined to be growing in the “Northeast” direction, it is only includ-
ed once, in the “North facing” group to avoid reporting the same sample twice. In the case of samples
taken from the top of stromatolites, where growth in the ‘up’ direction is equal to 90 degrees, anything
deviating from 90 is taken to be growth in a direction away from incident light.
In addition to binning the data by the cardinal direction in which the samples faced, a separate
analysis was performed in which the samples were grouped according to the angle of inclination of the
39
growth surface. Here, the inclination of samples growing off the top of boulders is 0 degrees and the
inclination of samples growing directly off the side of a boulder is 90 degrees. In this analysis, it is ex-
pected that a phototrophic community growing off of a surface that is inclined vertically (90 degrees)
would display a stronger tendency towards growth in the ‘up’ direction than a community growing
off of a 50 degree surface, and that those samples growing on the top of the boulders (at 0 degrees
inclination) would be the only group to display mainly surface-normal growth, Bulk sample analyses
were performed both with and without the top samples included to be sure that their measurements did
not pull the whole sample towards a surface-normal interpretation. In order to assess whether or not
samples on the sides of the boulders were growing as robustly as those on the top, the overall thickness
of the finely laminated portions were also measured when possible. In samples where it was apparent
that some of the finely laminated portion had cracked off the bottom, measurements were not included.
Due to the fragile nature of the samples, the resulting data set of thickness measurements is much
smaller than that of the growth directions.
RESULTS
In total, 305 samples were measured (measurements listed in Table 3.2). Overall, the angles
of growth off the initial surface range from 54 to 120 degrees, with an average growth angle of 82.1
± 11.7 degrees (Figure 3.4). Sixty percent of the samples fell within 10 degrees of surface normal.
Groups of binned samples all had the same general distribution, regardless of the cardinal direction
they faced or the initial inclination of the surface off which they accreted (Figures 3.5 and 3.6, respec-
tively).
Of the 305 samples, only 199 could have the thickness of their finely laminated portion ac -
curately measured (measurements listed in Table 3.2). The mean of these samples is 1.41 ± 0.54 cm,
with a range of 0.56 cm to 3.32 cm (Figure 3.7).
East facing samples (30 in total) range from 62 to 97 degrees, with an average of 80.1±8.5
degrees. Twenty-one of these samples could have their thickness measured, and those averaged 1.2
± 0.31 cm. West facing samples (22 in total) range from 56 to 110 degrees, with an average of 75.8 ±
40
12.9 degrees. Eighteen of these samples had reliable thickness measurements, averaging 1.4 ± 0.36
cm. South facing samples (135 in total) range from 54 to 120 degrees, with an average of 82.1 ± 12.6
degrees. Eighty-two of these samples were included in thickness measurements, averaging 1.28 ±
0.47 cm. North facing samples (65 in total) range from 55 to 100 degrees, with an average of 80.3±9.6
degrees. Fifty-one of these samples yielded thickness measurements, with a mean of 1.44 ± 0.60 cm.
Fifty-three samples in total were collected from the tops of boulders. Of these, only six measured a 90
degree, surface-normal direction of growth. Nineteen samples were greater than 90 degrees, and 28
were less than 90 degrees. The measurements spanned from 54 to 120 degrees. Since samples from
the top of the structure were measured in a slightly different way, any sample measuring other than sur-
face normal indicates non-vertical growth. In this case, 87 % of the samples were non-vertical. Only
about half (27) of the samples from the tops of boulders could be included in the thickness measure-
ment. These samples have thicknesses that range from 0.75 to 2.8 cm, with a mean of 1.97 ± 0.53 cm
(Figures 3.5; 3.7).
Samples were binned into four groups based on the angle of inclination of the substrate on
which the stromatolites grew (Figures 3.6 and 3.9). Samples originated from 0 (the top of the boulders,
thus the same as the previous analysis), 50, 80, and 90 degrees. The samples that initiated their growth
at an inclination of 90 degrees (91 of them in total) had a mean growth direction of 82 ±13.2 degrees.
Sixty-three of these samples had reliable thickness measurements, averaging 1.4+ 0.49 cm, and rang-
ing from 0.77 to 3.32 cm. Samples beginning at an inclination of 80 degrees (110 total) had an average
growth angle of 77.2 ±10.8 degrees. Seventy-three of these samples yielded thickness results, averag-
ing 1.29 ± 0.56 cm, ranging from 0.56 to 2.54 cm. Samples that began at an inclination of 50 degrees
(51 total) had a mean growth angle of 86 ± 6.3 degrees. Thirty-six of these samples have thicknesses
associated with them, with an average of 1.33 ± 0.33 cm, and a range of 0.85 to 2.06 cm. Of the 90
samples that were growing off of a surface that was approximately vertical (90 degrees inclination),
50 samples were found to grow within 10 degrees of surface normal. Of the 110 samples that accreted
off of an initial surface that was inclined around 80 degrees, 45 were found to be within 10 degrees of
surface normal. Of the 50 samples that were growing at an initial inclination of 50 degrees, 43 were
41
found to be within 10 degrees of surface normal. The samples taken from the top of the boulders were
the only ones with an initial inclination of 0 degrees (53 samples, 87% of which grew away from inci-
dent light, 27 of which had measured thicknesses).
DISCUSSION
Growth Direction
Of the 305 measured samples, the majority (210 samples – 69%) fall within the standard
deviation of surface normal, regardless of the initial surface inclination or cardinal direction, which is
not the outcome predicted from a phototropic response. While there are samples that do grow upwards
at angles less than 90 degrees (the extreme being 54 degrees), and thus towards incident light, there
are nearly an equal number growing at angles higher than 90 degrees (up to 120 degrees), away from
incident light. Figure 9 shows the range of angles measured according to their position on the boulder,
demonstrating that a strong phototropic response is not apparent in the data. Perhaps the stromatolites
nucleated on the vertical sides of the boulders are the most informative in this study: they would be
predicted to show the strongest phototropic response, yet their growth directions are nearly surface
normal.
Examined critically, the mean growth direction of the collective dataset is in fact closer to 84
degrees versus 90 degrees, so there is a small, ~6 degree bias towards upward growth. On the one
hand, the small upward bias could be seen as an indicator of a minor phototropic response. A closer
investigation of the data and the boulders themselves offers an alternative hypothesis. On one of the
measured boulders, the south side was in direct contact with the underlying sediment, and the north
side displayed an overhanging area where additional stromatolite growth was observed. Samples from
the north side, which were not in direct contact with the sediment, average 79.5 degrees, while samples
taken from the south side and the west side, which were in direct contact with the sediment, average
72.3 and 75.8 degrees, respectively (that is, slightly more skewed upwards). Samples at 90 degrees
inclination on other boulders with overhangs average 80.1 degrees and 86.6 degrees. Thus, the small
upward deviation may result from a lack of accommodation space at the base of the boulder, where a
42
growing column was in contact with the surrounding sediment. The lack of downward accommoda-
tion space would force the accreting stromatolite to initially grow slightly upwards away from the
sediment, and preclude growth in the downward direction. The fact that the angle of growth is much
closer to surface normal than towards the incident light, compounded with the observation that many
columns grow down away from incident light indicates that a strong phototrophic response was not
directly involved in the formation of these stromatolites.
The data was analyzed both with the samples from the tops of the boulders included and ex-
cluded (since all the samples on the top would be surface normal regardless of whether or not a pho-
totrophic community was influencing their structure) to ensure that this sample set did not bias the
overall data. However, even when the top samples are excluded, the same distribution is observed
(Figure 3.5). In fact, the top samples show some of the highest variability of all sides and inclinations,
which is not predicted from a simple phototrophic model.
Thickness
Thickness data are much less straightforward to interpret than growth direction data. Stroma-
tolites are complex, three-dimensional structures, and the thickness of the finely laminated portion
will vary depending on where the column was cut; thus, comparing a slab through one stromatolite
to another may not appropriate. Overall, growth direction is not subject to this flaw in the laminated
portion of the stromatolite, where the laminations guide the determination of growth direction. Further-
more, the top of the boulders have gone through much more intense weathering than those on the sides,
ensuring that only the most robust columns survived fully intact, which inherently biases that data set
toward thicker, more robust columns that would survive weathering. Thus, while the thickness data is
interesting, it may not be as informative as the growth direction data. Even so, the overall thicknesses
of the top samples are not drastically different from those on the sides (Figure 3.9). The data from
the top samples fall well within the ranges of samples measured from other inclinations. Overall, the
thickest columns (3-3.31 cm) are found not on the top of the boulder, but on the steeply angled (near-
vertical) sides, suggesting that position on the boulder was not a controlling factor in determining the
thickness of the columns. Furthermore, it was predicted that because of the angle of incoming solar
43
radiation, samples that face the south would be the thickest, and those facing the north would be the
thinnest. This is not the case. Samples that face the south have some of the thinnest columns, aver-
aging 1.28 ± 0.47 cm (Figure 3.10; 3.11). Thickness data are far more spread out and variable than
growth direction, showing no clear clustering toward a specific column height, and perhaps indicating
that accommodation space may be playing a role in how the stromatolites accreted.
Biogenicity of the Walker Lake Stromatolites
When the results of the thickness and growth direction analyses are compared with the hypothetical
results presented previously, it is clear that growth towards incident light (phototropism) was not the
driving force behind generating the columnar morphology of the stromatolites of Walker Lake. The
lack of a phototropic response does not rule out biological involvement with respect to stromatolite
formation, it simply demonstrates that column formation in this case was not driven by the phototropic
response of phototrophic organisms. Thus, the tacit assumption that stromatolite doming is necessarily
a universal phototropic response is incorrect. However, this does not rule out the impact of other, non-
phototrophic microbial communities in the creation of the structure. Bailey et al. (2009) noted the abil-
ity of chemotrophic mats to both trap and bind sediment and aid in the production authigenic carbonate
minerals. While it is, of course, not possible that these stromatolites formed in the complete absence
of any biology (biofilms are ubiquitous in most lacustrine environments on Earth), the hallmarks of
biogenic influence, such as the presence of microbial fossils or grains trapped well beyond the angle of
repose, are not currently found in the Walker Lake stromatolites (in fact, less than 2% non-carbonate
material is present in the structure,). While it cannot be ruled out that biology played a role in the
building of these stromatolites, there is also no direct evidence to support it.
Significance of the Walker Lake Stromatolites
Here, we have demonstrated with a certain degree of confidence that the columnar habit of
Walker Lake stromatolites is not a phototropic response. We do not assume that our conclusions hold
for ALL stromatolites. There may indeed be stromatolites that do record a phototropic response (e.g.,
Awramik and Vanyo, 1986). However, in the absence of additional data, demonstrating an unequivocal
44
phototropic response in ancient stromatolites remains quite difficult. Our study suggests that caution
must be applied when interpreting the morphology of ancient stromatolites as evidence for phototro-
pism.
While the view that columnar stromatolite growth reflects phototropism is still widely held in
the paleontologic community at large, it is clear that the mathematical models presented in the intro-
duction have pushed our understanding of stromatolite morphogenesis beyond the simple phototropic
model, where both abiotic and biotic processes can result in stromatolitic morphologies. That being
said, the Walker Lake stromatolites can be used to test the parameterization of the models. The Walker
Lake stromatolites at the top of the boulder are morphologically similar to the stromatolites on the
sides of the boulder. The similarity in morphology between the vertical and horizontal columns pres-
ents an interesting challenge to the current state of the art in stromatolite morphogenetic modeling. In
the models outlined in the introduction, downward motion of sediment towards a substrate under the
influence of gravity is a key component in the morphogenesis of branching columns (e.g., Grotzinger
and Knoll, 1999; Dupraz et al., 2006). This model would apply for the stromatolites formed on the
top of the boulder. However, the parameterization of the models would not predict the construction of
columnar branching morphologies laterally from a vertically inclined substrate, where sediment falling
under the influence of gravity would simply bypass the stromatolites on the sides of the boulder . Also,
the simple rules governing sediment smoothing behavior in the models would likely result in the sedi-
ment sloughing off in all cases, given the angle of growth. Therefore, the Walker Lake stromatolites
reveal that not all features responsible for stromatolite growth are considered with the current math-
ematical models that describe stromatolite morphogenesis.
CHAPTER 3 CONCLUSIONS
The common assumption that columnar stromatolitic structures are formed as a response of
photosynthetic bacteria to the availability of light was tested with Holocene stromatolites from Walker
Lake, Nevada. The resulting data reveal that a phototropic response was not the dominant control on
growth form or direction in these stromatolites. The data, whether categorized according to cardinal
direction or growth surface inclination, show only one discernable trend: a tendency towards surface
45
normal growth of the stromatolites. Although somewhat less reliable because slabbed stromatolites
cannot capture three-dimensional growth, the thicknesses of the stromatolites do not change in a sys-
tematic way that is consistent with phototropism. This does not rule out a biological origin for Walker
Lake stromatolites, but it does indicate that the activity of a photosynthetic microbial community
displaying phototropic behaviors is not necessary to generate a columnar morphology. It is clear from
these observations that the influence of phototrophic microorganisms on stromatolites cannot be as -
sumed, but must be taken on a case-by-case basis. Furthermore, the Walker Lake stromatolites can be
used to test the parameterization of mathematical growth models, which do not predict lateral growth
of branching columns from a vertical substrate.
46
CHAPTER 4: MAGNETIC SUSCEPTIBILITY AS A BIOSIGNATURE
CHAPTER 4 ABSTRACT
Microbialites—macroscopic sedimentary structures built or influenced by microor ganisms—
constitute a geobiologist’s dream and nightmare at the same time. As a visible structure built by mi-
croscopic organisms that are themselves too small to be imaged remotely, microbialites provide an
excellent target in the search for life in the ancient rock record on Earth as well as on other planets.
However, it is well known that abiotic processes can mimic microbialite morphology. Such signatures
as microfabric and isotope ratios (e.g., d
13
C) can be difficult to interpret, and organic matter is rarely
preserved in such structures. Here, I report a new biosignature based on the detrital magnetic mineral
component present in nearly all sedimentary rocks.
It is hypothesized that the distribution of detrital magnetic grains within a putative microbialite
will depend on the presence or absence of “sticky” microbial mats/biofilms. Magnetic grains in an abi -
otic structure should obey the laws of gravity/angle of repose (swept off peaks, concentrated in lows),
while magnetic grains adhered to a biofilm will seem to “defy” the laws of gravity . Recent advances in
our ability to measure miniscule magnetic fields open up the possibility to map the magnetic suscepti -
bility (the response of a sample to an induced magnetic field) of a putative microbialite sample.
This hypothesis was tested in two ways: 1) Parallel laboratory experiments in which magnetic
particles were introduced into a tank that contained either cyanobacterial mats or glass slides (on which
carbonate had abiotically precipitated) inclined at a variety of angles. 2) Using the results of the labora-
tory experiments, a variety of stromatolites of known and unknown biogenicities, ranging in age from
near-modern to ancient, were analyzed. The results of these experiments verify the hypothesis that
magnetic susceptibility can be used as a biosignature.
47
INTRODUCTION
One of the goals of geobiology is to understand the early evolution of life by examining the
nature of the most primitive organisms, the environment in which they evolved, and the way in which
they influenced that environment. Such endeavors assume that it is easy to dif ferentiate the biotic from
the abiotic, whether in ancient rocks on Earth or elsewhere in our solar system. But what constitutes a
robust biosignature? What may seem straightforward in theory is difficult in practice. Therefore, de -
termining what is, or is not, a biosignature represents one of the most significant challenges in geobiol -
ogy today. The focus of this research is on developing a new biosignature, based on magnetic suscep-
tibility of microbialites, taking advantage of recent technological advances applied in a novel way to
geobiological and astrobiological questions.
Microbialites are macroscopic sedimentary structures built by or influenced by microor gan-
isms. Stromatolites, as noted in Chapter 1, are probably the “celebrity” of the microbialite world, and
true biotic stromatolites would constitute a clear target for geobiology and astrobiology: a macroscopic
structure built by microscopic organisms, which are themselves too small to be imaged remotely.
However, as noted in previous Chapters, morphology can be deceiving. Abiotic structures that
mimic “real” stromatolites are known. Other abiotic processes, as common as mineral growth or as
esoteric as paint spatter or electroplating of metals, are now known to create structures indistinguish-
able from “real” stromatolites at some scales (McLoughlin et al., 2008). Furthermore, numerical
stromatolite growth models imply that microbial involvement may not be a prerequisite to form such
morphologies at all (Grotzinger and Rothman, 1996; Grotzinger and Knoll, 1999). Chapter 2 showed
that lamination in stromatolites, commonly assumed to be a result of the daily, yearly, or seasonal
activity of microbial communities, can instead be tied to longer-scale climate factors and local geo-
chemistry. Chapter 3 outlined the ways in which the domed growth habit of stromatolites (commonly
taken as a priori evidence of phototrophic microbial involvement) may not be tied to microbial activity
at all. Additionally, this domed growth pattern has been identified in structures that grew in the absence
of light (e.g., Rossi et al., 2010), and many phototrophic mats do not form domes.
The question can then be asked, how does one tell the abiotic posers from the bona fide micro-
48
bialites? Microscopic investigation can provide some relief, but many, if not most, putative microbi-
alites in the rock record have been subjected to post-depositional alteration that obscures the original
microfabric, rendering most putative microbialites ultimately ambiguous with respect to their bio-
genicity (as noted in Chapter 1). Most microbialites are composed of carbonate minerals, so carbon
isotopes are commonly cited as a potential biosignature. However, isotope ratios can also be ambigu-
ous; microbial phototrophic CO
2
fixation drives the surrounding carbon δ
13
C positive, and microbial
sulfate reduction drives it negative, making this a problematic biomarker. Organic matter is rarely
preserved in such structures, and actual microbial fossils are even more rare and usually require special
circumstances for preservation (e.g., early silicification). Here, a new biosignature is reported, based
on the inherent “stickiness” of microbial biofilms (as compared to abiotic systems), coupled with new
advances in the ability to measure minute concentrations of magnetic minerals: magnetic susceptibility
as a biosignature.
MAGNETIC SUSCEPTIBILITY
Magnetic susceptibility ( χ) is the response of a material to an applied magnetic field. A sample
with a large amount of magnetic material will produce a large response when an external magnetic
field is applied. Essentially, χ can be used to estimate the relative amount of magnetic material in a
sample. Given that magnetic mineral grains are present in nearly all depositional environments in
some fraction (discussed below), we hypothesized that the distribution (location and concentration)
of magnetic grains within a putative microbialite will depend on the presence or absence of “sticky”
microbial mats/biofilms versus an abiotic, non-sticky structure, as follows (Figure 4.1)
1) Magnetic grains in an abiotic structure should obey the laws of gravity and angle of repose,
which is typically about 33 degrees in air and slightly more in water (Carrigy, 1970). These
grains should be swept off of peaks and concentrated in troughs or along low-angled surfaces.
2) Magnetic grains adhered to biofilms will appear to “defy” the laws of gravity and appear in
positions inconsistent with simple physical sorting, such as steeply angled sides The microbial
communities that build microbialites are inherently sticky compared to the surrounding sedi-
49
ment.
The communities that build microbialites are inherently sticky compared to the surrounding sediment.
Typically, these microbes produce significant amounts of extracellular polymeric substances (EPS)
that aid in the trapping and binding of grains, and may provide a template for the nucleation of calcium
carbonate (Reid and Brown, 1991; Visscher et al., 1998 Laval et al., 2000; Reid et al., 2000; Konishi
et al., 2001). Modern marine stromatolites, such as those forming in the Bahamas, are known to form
by the trapping and binding activity of microbes in close association with diatoms and algae (Reid and
Brown, 1991; Reid et al., 2000). Additionally, trapping and binding ability is not limited to photosyn-
thetic organisms. Bailey et al. (2009) showed that mats in various environments have the ability to
trap and bind sediment. Thus, in standard surficial environments, detrital mineral grains (a fraction of
which will be magnetic) will adhere to the microbial mats that build microbialites.
Advantages of Magnetic Susceptibility as a Biosignature
There are several reasons why χ is potentially advantageous as a biosignature. First, magnetic
minerals reside in the insoluble fraction of carbonate rocks and would be less affected by certain post-
depositional alteration processes (such as heating) than the carbonate itself, organic matter, or micro-
bial fossils, which is often the case in stromatolites, especially for ancient samples. Secondly, due
to recent advances in technology allow the detection of minute concentrations of magnetic minerals,
experiments and analyses will not be sample–limited. χ can be discerned for samples as small as ~100
μg, assuring that the χ of a putative microbialite sample will be able to be mapped, even when the
grains are too small to be seen with a standard petrographic microscopic. Furthermore, the hypothesis
relies on the magnetic susceptibility of the microbialite, not its remanent magnetization (the direction
of the ancient magnetic field recorded at the time of deposition), which is potentially more susceptible
to later resetting. Magnetic susceptibility is simply dependent on the amount and type of magnetic
minerals in the sample. While the orientation of magnetic grains in a carbonate may be reset several
times over the course of its lifetime as the sample undergoes diagenetic alteration, the magnetic sus-
ceptibility will remain constant.
50
This is the first time magnetic susceptibility has been proposed as a biosignature in this way .
As with any new biosignature, strengths and weaknesses must be assessed. One potential pitfall is the
fact some types of bacteria can produce single domain magnetite crystals (Thomas-Keprta et al., 2000;
2001), (this has been especially well studied with respect to the famous Allen Hills meteorite (e.g.,
Weiss et al., 2000). It is possible that such bacteria may produce a magnetic signature and skew the
results of this analysis. However, Single domain magnetite requires the presence of certain kinds of
bacteria, and is not typically found as a detrital grain. Additionally, biogenic magnetite forms small
crystals that commonly dissolve in anoxic conditions, leaving little fossil record. As this study relies
on the detrital component of the putative microbialites, it ultimately represents a broadly useful tech-
nique.
PROOF OF CONCEPT – TAHITIAN MICROBIALITES
The concept for magnetic susceptibility as a biosignature grew out of observations made on
geologically young microbialites from Tahiti (Lund et al., 2010). Integrated Ocean Drilling Program
(IODP) Expedition 310 drilled 44 boreholes into post-glacial-max framework rocks of the Tahiti coral
reef to determine the timing and pattern of coral reef development associated with sea level rise at the
end of the last global glaciation. Over 600 meters of core were recovered, more than 60% of which
consisted of microbialites that grew within vugs (holes) in the coral reef. The microbialites grew
inward from all directions, and were well laminated (stromatolitic) at the outer surfaces of the vugs,
becoming more clotted (thrombolitic) as the vugs were occluded (Figure 4.2; Lund et al., 2010). The
paragenetic sequence revealed that the corals formed first, providing the framework, followed in suc -
cession by microbialite and finally a thin layer of isopachous aragonite cement.
Paleomagnetic samples collected from the microbialites all recorded an accurate and reproduc-
ible sense of the geomagnetic field at time of deposition, regardless of their position in the vug (at the
bottom, on the sides, or even at the top, hanging down). It was also determined that detrital magnetic
minerals from the Tahiti volcanic edifice (titanomagnetite) carried the magnetization. Even microbi -
alite textures growing straight down have notable concentration of detrital magnetic minerals (with
reasonable paleomagnetic orientation), which caused the authors to question: Why would samples that
51
grew straight down, from the top of a vug contain a detrital magnetic signature? Conventional wisdom
would lead one to believe that, due to gravity, the detrital grains would fall downward and be col-
lected in the bottoms of the vugs. The magnetic grains must have been held in place as the grains were
coalesced into the reef framework as the microbialite grew. The authors hypothesize that biofilms must
have carried out this task, by their inherent stickiness or by trapping and binding grains (Lund et al.,
2010). Thin section photomicrographs revealed that small (1-25 μm), opaque titanomagnetite grains
were bound within the microbialite, and not in the surrounding abiotic carbonate cements (Figure
4.2b,c). Magnetic Susceptibility tests revealed an average χ of ~ 4.0x10
-4
SI. The ‘high angle’ mi-
crobialites, as well those growing straight down retained at least 80% of the ‘bottom’ surface intensi-
ties (Lund et al., 2010), suggesting that microbialites growing at steep angles are able to trap and bind
detrital magnetic grains nearly as well as those that grow horizontally. It was these observations, made
by Lund et al. (2010) that suggested the research approach outlined below.
In order to assess the merits of χ as a biosignature, the concept was tested in several different
ways, both in the laboratory (abiotic carbonate precipitation and living biofilm experiments) and on
field samples of known and unknown biogenicity.
METHODS – LABORATORY EXPERIMENTS
Abiotic Carbonate Precipitation
Glass slides inclined at various angles (from 0 to 90 degrees, at 15 degree increments) were
placed in a 19-liter plastic tub and submerged in a slightly agitated, supersaturated calcium carbonate
solution (Figure 4.3) (only about half the tub was filled). The solution was made by adding 200 mg of
pure calcium carbonate per liter of water used. The mixture was chilled to 5°C and allowed to sit over-
night, dissolving the carbonate. The chilled, supersaturated carbonate solution was then passed through
a 15 μm filter to insure that no large undissolved grains were present. Grains smaller than 15 μm were
allowed to pass through to serve as carbonate nucleation sites. At 5°C, the saturation index of the solu-
tion with respect to carbonate (Ω
ca
) is 0.97. In order to precipitate carbonate, the solution was warmed
to room temperature (increasing Ω
ca

to 2.91). The solution was allowed to precipitate for 24 hours,
52
after which six grams of crushed magnetite (2 μm in diameter; suspended in deionized water to inhibit
clumping of the grains) were introduced into the fluid.The agitator was turned of f, and the magnetite
was allowed to settle for three hours. Samples and replicates were taken from each slide, scraped into
a sample vessel, and allowed to dry before being weighed in preparation for the χ measurement. Care
was taken to ensure that slides were handled in a manner that would avoid disturbing any magnetite
that may have collected on the surface.
Biofilm Experiments
Green, filamentous, cyanobacterial mats were collected at low tide from a lagoon in Catalina
Harbor, Catalina Island, California. The lagoon is a shallow, low energy area that consists of muddy
sand. In places, the muddy sand has a thin green layer microbes growing on top. Samples were con-
firmed to be filamentous, green cyanobacteria by microscopy (Figure 4.4c). These microbes were also
identified by the elevated presence of the nifH (nitrogenase reductase) gene in the surface mats (Ber -
tics et al., 2010), which is a hallmark of photosynthetic activity. After collection, these samples of mud
with thin biofilms were placed in circulating seawater at the USC Wrigley Marine Science Laboratory
on Catalina Island and allowed to grow for several days (Figure 4.4b). Mats were harvested, placed on
glass slides, and inclined at various angles (from 0 to 90 degrees, at 15 degree increments) at several
different points in a 30 gallon (~113 liter) tank of seawater (Figure 4.4a). Six grams of crushed mag-
netite (2 μm in diameter) was introduced into the fluid and allowed to settle out of suspension. After
allowing the magnetite to settle for 3 hours, samples and replicates were taken from all mats, as well as
‘control’ mats that were not exposed to magnetite. Samples were dried and weighed in preparation for
the χ measurement. A sample of the seawater was evaporated and tested to ensure that there was no
additional magnetic material being added.
Magnetic Susceptibility Measurement Procedure
Samples were placed in 1cm
3
plastic cubes, and had their χ intensities measured on a KLY-4s
Kappabridge belonging to Dr. Steve Lund at the University of Southern California. This apparatus
is appropriate for this study, as it has the ability to accurate measure χ intensities down to the order
53
of 10
-9
SI. Several blank sample cubes were also run along with each set of experiments. Both the
intensities of the sample cubes and the machine noise were subtracted from bulk χ readings (discussed
below). Finally, bulk intensities were adjusted for the measured weight of each sample. As magnetic
susceptibility is inherently a dimensionless number, all intensities are reported in the units “SI/g”.
Avoiding Potential Biases
Abiotic Experiments It is possible that the agitation could carry carbonate or magnetite to a certain
areas, leaving to a differential magnetic signature between slides inclined at opposite ends of the tank.
In order to avoid this, agitation of the solution was continued for 2 hours after the magnetite was added
to allow for an even distribution of grains in the solution. A visual inspection confirmed that carbon -
ate had precipitated directly on to the slides, and that a fairly even layer of magnetite had fallen over
the all portions of the tank (Figure 4.3c). Carbonate was allowed to precipitate for 24 hours before
the addition of magnetite in order to prevent the suspended magnetitic particles acting as nucleation
sites/seed crystals for carbonate in the water column. The experiment is designed to test whether or
not magnetite would adhere to recently precipitated carbonate, similar to the observations noted with
the abiotic aragonite cements in the Tahitian reef samples. It should also be noted that the amount of
magnetite added in this experiment is much larger than what would be found in a normal sedimentary
environment. Therefore, the measured intensities will be greater than what would be found in nature.
Biotic Experiments While equal amounts of magnetite (6 grams) were added to the biotic and abiotic
experiments, the tank used in the biotic experiment had a larger surface area and volume. This could
potentially cause χ intensities to be lower than those recorded from the smaller tank used in the abiotic
experiment, thus the discrepancy in surface areas of the tanks must be taken into account when consid-
ering the results of the experiments.
Sample Vessels and Machine Error Empty sample cubes were measured in order to assess their mag-
netic properties. In all cases, sample cubes are found to be slightly diamagnetic, meaning they create
a magnetic field in opposition to one that is externally applied. Thus, sample cubes have a χ that is
slightly negative, but usually at least two orders of magnitude lower in intensity than the samples being
54
measured (Table 4.1; 4.2; 4.3). While the χ of the vessels is subtracted from the intensity measure-
ment of the samples to ensure the most accurate analysis possible, it does not interfere with the χ of
the samples themselves. Additionally, machine noise was measured periodically throughout the testing
phases to assess whether or not the accuracy of the readings was drifting over the course of the mea-
surements. In all cases, the machine drift was measured to be on the order of 10
-9
SI, which was lower
than all samples (except those that were found to not have any magnetic signature – discussed below).
EXPERIMENTAL RESULTS
Abiotic Carbonate Precipitation
Results show a decrease in χ with an increase in the angle of the growing surface (Figure 4.5,
Table 4.1). Magnetic susceptibility dropped by an order of magnitude in samples that had been in-
clined greater than 40 degrees, and fell to zero when the growth surface was inclined near to or over 60
degrees, even though there was far more magnetite introduced into the tank than would be in a normal
sedimentary environment. A visual inspection of the slides inclined at high angles showed circular
areas of precipitated carbonate on the slides, and yet no magnetite held (Figure 4.5; 4.3c), in fact, piles
of magnetite could be seen collected at the bottom of the slides, indicating that carbonate alone is not
capable of binding magnetite at angles higher than 45 degrees. corroborating the results from the abi-
otic cements in the Tahiti samples which did not incorporate any opaque grains.
Biofilm Experiments
In the biotic experiments, there is a slight decline with increasing angle, but even the 90 degree
inclined mats held more than an order of magnitude more magnetite than any of the abiotic slides (oth-
er than the ones that were oriented horizontally) (Fig 4.6; Table 4.2), despite the fact that the tank was
twice the size versus the magnetite introduced. The measured intensities of the control mats, as well as
the evaporated seawater, were effectively zero. The results of the laboratory experiments indicate that
biofilms are much better at trapping and binding small grains than abiotic carbonate precipitates. .
55
DISCUSSION OF EXPERIMENTAL RESULTS
The results of the laboratory experiments are intriguing. On the one hand, they are exception-
ally straightforward: In abiotic systems, χ intensities will decrease with increasing surface steepness,
until the angle of repose is reached, and then no magnetic grains will adhere. In biotic systems, it is
more likely that there will not only be a larger overall amount of magnetic material bound into struc-
ture, and that it will stay fairly constant with increasing surface angle. It is interesting to note that, de-
spite the small area and the large amount of magnetite in the abiotic experiment, no magnetic material
held on to the carbonate at angles higher than 45 degrees. If carbonate alone could hold a significant
fraction of detrial magnetic material, it certainly would have been measurable under these conditions.
This signifies further that the hypothesis has merit, and may be useful in natural samples. However,
the real world seldom is as straightforward as the laboratory, and several issues must be considered.
One assumption made in these experiments is that every depositional system encountered will
have detrital material, a fraction of which is magnetic. It is true that there may be some areas without
a measurable detrital magnetic signature, but these areas are relatively rare. For instance, the Bahama
Bank is an isolated carbonate platform located relatively far from any source of magnetic particles (in
contrast to the Tahitian system’s proximity to volcanic edifaces). Regardless, studies have shown that
dust storms from the Sahara Desert in Africa are responsible for depositing a significant amount (50-
75 wt%) of heavy metals within the paleosols of the Bahamas (Moulin et al., 1997; Perry et al., 1997;
Prospero and Lamb, 2003), meaning it is unlikely that any depositional environment will be complete-
ly void of magnetic particles.
Though nearly all depositional systems will have some magnetic material in them, the amount
of magnetite added to each experiment is vastly more would typically be found in a surface environ-
ment. Additionally, each individual surface environment will receive a different amount of detrital
magnetic material depending on its regional geologic setting. Therefore, the absolute magnitude of χ
intensities cannot be directly compared across natural samples from different localities. Instead, rela-
tive intensities must be assessed at varying angles of inclination within natural samples. If χ intensi-
ties vary little with lamination inclination, then the sample likely had a biofilm in place to catch and
56
hold detrial material. However, if there are significant changes that vary as a function of degree of
inclination of the laminae, especially if there is a drop off in χ intensity at angles above 45 degrees (the
angle of repose for the abiotic experiement), it is likely that the structures are abiotic in origin.
In addition to measuring the changes in intensities between steep and shallow lamina in natu-
ral samples, the inter-columnar areas of stromatolites can be measured. These are areas between the
laminated structures that are filled with sediment from the depositional system. Potentially , these areas
would record the total amount of detrital magnetic material that had been delivered to the system over
the course of stromatolite accretion. When possible, the intensities of intercolumnar areas should be
measured and compared to the intensities of the laminae. In a biotic sample, it is expected that the
laminae, at all angles, would have χ intensities that are similar to the intercolumnar-areas (though per-
haps a little less). In biotic structures, horizontal laminae may record the same intensities as the inter-
columnar areas, but steep laminae would not.
Given the results thus far, it is clear that when examining natural samples, it will be important
to compare χ intensities within a single sample, between low and high angles (especially angles over
the angle of repose), and between column and intercolumnar areas.
NATURAL SAMPLES
Several different natural samples were collected and examined. One sample is of a known
biologic origin (silicious hot spring stromatolite); one is of known abiologic origin (hydrothermal vein
‘stromatolite’); and several are classified to be of ‘unknown’ biogenicity. The ‘unknown’ samples are
divided into two categories based on their microscopy: likely biotic, and likely abiotic (classification is
discussed in detail for each sample below).
Natural Sample Localities and Descriptions
Known Biotic Sample – Yellowstone Hot Spring Stromatolite Modern silicious stromatolites growing
in a hydrothermal spring in upper Hayden Valley in Yellowstone National Park (Figure 4.7) were col-
lected in association with Dr. John Spear, Colorado School of Mines. The stromatolites have two main
57
lithofacies: a fine, distinctly laminated main body composed of silicified non-heterocystous cyanobac -
terial sheaths (identified by 16S rRNA sequences, Pepe-Ranney et al., submitted) (Figure 4.7c), and
a ‘drape’ facies that is often sub-vertical, predominantly composed of the heterocystous phylotypes
Chlorogloeopsis and Fischerella (Pepe-Ranney et al., submitted) (Figure 4.7b). The very clear associa-
tion of cyanobacteria with the microstructure of the stromatolites makes these samples excellent biotic
end members for testing the utility of χ as a biosignature. The ‘drape’ laminations dome, and even fold
over at points (Figure 4.7b), ensuring that χ can be measured at a number of different angles, from 0
(horizontal laminae) to 90 (vertical laminae) degrees. It is expected from the laboratory results (and
the Tahiti field test) that χ intensities will vary somewhat between horizontally and vertically oriented
laminae, but that those variations will not be significant.
Known Abiotic Sample – Hydrothermal Vein Stromatolite Found in a hydrothermal vein that cross
cuts local bedding in the Devonian age Tempaiute Range in central Nevada (the samples themselves
are likely Tertiary in age) (Figure 4.9a,b), these samples appear to be domed, columnar stromatolites,
roughly 15 cm tall and finely (~100 micron) laminated. The laminations themselves are in a “light-
dark” pattern, appear to thin along sides, and thicken over the tops of columns, which is commonly
interpreted as an indication of biologic origin (Figure 4.9b). The stromatolites grew normal to the sur-
face of the vein, and likely precipitated deep in the Earth. While the macrostructure resembles many
Precambrian stromatolites, in thin section, the laminae are composed of interlocking calcite crystals
with bladed terminations and sweeping extinction (Figure 4.9c,d), suggesting a purely abiotic origin.
Raman spectroscopy reveals that the layers are purely calcite with some trace amounts of hematite, but
no traces of organic matter were found (Corsetti et al 2010).
Unknown Sample #1 – Likely Biotic Stromatolite: Johnnie Formation, Death Valley The Neoprotero-
zoic (Ediacaran) Johnnie Formation outcrops in the Death Valley region of eastern California/western
Nevada, USA, and contains mixed siliciclastic-carbonate lithofacies interpreted as a shallow marine
deposition (Summa, 1993; Corsetti and Kaufman, 2003) (Figure 4.10a,b) .The formation contains
abundant stromatolites within its carbonate units (e.g., Benmore, 1978). A form called Boxonia was
taken from the upper part of the Johnnie Formation. Boxonia represents an interesting case for χ as
58
a biosignature. While technically of ‘unknown’ biologic affinity (no microfossils or other ‘smoking
gun’ evidence for a biologic origin are present), Boxonia has ‘walls’, or an envelope around individual
columns that are highly suggestive of a microbial-driven morphogenesis (Fig. 4.10c) (Walter, 1992).
Interestingly, this sample preserves stromatolite columns, and inter-columnar areas that are filled with
sediment. In a stromatolite that formed as a result of biofilms, the intensity of χ measured on the
laminae should be the same as that of the surrounding sediment. In a non-biogenic stromatolite, detrial
grains would fall into the inter-columnar areas, leading to a higher χ measurement in the inter-colum-
nar spaces than in the laminae themselves.
In thin section, it is clear that the stromatolites have undergone a significant amount of di -
agenesis. Laminae, while having the wavy, crinkled appearance of microbial mats (Figure 4.11) are
composed of micrite, and re-crystalized carbonate cements (Figure 4.11). The stromatolite columns
are bordered by styolites, which are rimmed with a greenish mineral, most likely malachite (Figure
4.11e). There is abundant quartz, malachite, azurite, and dolomite in the inter-columnar areas (Figure
4.11d,f); indicating that hydrothermal fluids have interacted with the sample. Opaque grains that may
carry a magnetic signature can be seen embedded within the structure (Figure 4.11a-d) These grains
are rounded, and in no case interlocking, indicating that they have not been re-crystallized secondarily
in place.
The petrography is highly suggestive of a biologic origin for the laminae of these stromato-
lites, and it would be expected that χ intensities should be uniform around the structure. However, the
sample has gone through significant diagenesis, and may not retain its original magnetic signature, es -
pecially around the areas where the styolites are found. It is clear from the mineral assemblages found
that the sample has been affected by hydrothermal fluids, which are often reducing, and may have dis -
solved away magnetic minerals.
Unknown Sample #2 – Likely Biotic Stromatolite: Green River Formation, Wyoming The Green River
Formation is an Eocene lacustrine carbonate deposit that outcrops over much of Wyoming and Colora-
do (Figure 4.12; 4.14). Stromatolites can be found in several Members of the Green River Formation,
though the likely biotic ones studied here are from the lower Laney Member of the Tipton Road local-
59
ity (Figure 4.12), and are interpreted as having been formed in a near-shore transgressing lake system
(Bohacs et al., 2000; Lamond and Tapanila, 2003). The stromatolites consist of calcite columns, and
inter-columnar areas (Figure 4.12c). The laminations are composed of wavy micrite (often indicatice
of a microbial origin), and detrital grains can be seen bound within very steep (~70 degrees) laminae
(Figure 4.13). Intercolumnar areas are composed of detrital material (e.g. quartz grains, ooids) that is
encrusted in several generations of carbonate cement (Figure 4.13d,e) Accicular cements fringe clasts,
and euhydral cements infill voids (Figure 4.13d,e). While technically ‘unknown’, the petrography
indicates a biologic origin for these stromatolites, and it is expected that χ intensities for this sample
will be uniform throughout the structure.
Unknown Sample #3- Biotic/Abiotic Mix: Green River Formation, Wyoming Found near the contact
between the Tipton and Wilkins Peak Members of the Green River Formation at the Boar’s Tusk local-
ity in Southern Wyoming, these stromatolites are roughly 8 cm thick, and contain layers composed of
two alternating mineralogies (Figure 4.14). The first is a precipitated carbonate fan fabric that is likely
abiotic (Figure 4.15a-c). These horizons are finely laminated, however the laminations are crystal
termination boundaries (Figure 4.15c). In crossed nichols, the domed buildups show the sweeping
extinction characteristic of abiotic crystal fans. The second horizon consists of a micritic fabric that
includes trapped grains, and is likely biotic (Figure 4.15d).
These samples present an interesting test case, in that they contain layers that, while formed
in the same general depositional setting, alternate from a morphology that appears to be biologically
driven to one that appears to be abiologically driven and back again. If there are differences in χ
intensities based on biology, they should be easily seen in this sample. The scale of the likely abiotic
intervals prohibits the sampling of laminae based on degree (1 mm drill holes would record a mixture
of both flat lying and steeply angled laminae, Figure 4.15). Because of this, the relative intensities
between micrite and fan layers will be measured. It is expected that the micrite layers, which are likely
biotic, will have trapped and bound more magnetic grains than the abiotically precipitated fan layers.
Unknown Sample #4 – Likely Abiotic Stromatolite: Furnace Creek Formation, Death Valley The
Furnace Creek Formation is a Pliocene lacustrine deposit found in Death Valley, California (Figure
60
4.16a,b). The stromatolites found within the deposit are columnar, finely laminated, and composed of
carbonate (Figure 4.16c). The laminae are isopachous, meaning they do not change thickness along
the top or sides of the stromtolite column, which is commonly taken as evidence for an abiotic origin.
The deposits have a reddish stain, which is most likely due to iron oxides (such as hematite) (Figure
4.16c). The stromatolites are found growing off of a layer of reddish sediment (Figure 4.16c), allowing
for a comparison between the χ of the sediment, and the columns. In thin section, it can be seen that
stromatolite laminae are actually growth bands in larger crystal fans, that display sweeping extinction
and classic interference signals (Figure 4.17). It is expected that such a sample will show differences
in χ intensities between the collected sediment, and the crystal fans.
Unknown Sample # 5- Likely Abiotic Stromatolite: Walker Lake, Nevada Much information has al-
ready been given on the stromatolites of Walker Lake, Nevada. As the lamination of these samples is
not on the timescale of a microbial community (Chapter 2), and the growth directions and thicknesses
do not coincide with what one would expect if cyanobacterial were directly involved with their accre-
tion (Chapter 3), it is likely that abiotic factors were responsible for the formation of the finely laminat -
ed portion of the stromatolite. Two different samples were tested: One that originated from the top of
a boulder; and one that accreted off a near-vertical side. Each sample itself can be used to test whether
or not biofilms were present during accretion. Together, the samples compose a larger test case. In an
abiotic system, the sample from the near-horizontal top of the boulder would carry a detrial magnetic
signature that should be much higher than any carried in the structures from the steep side. In a biotic
system, the signature would be roughly the same at both scales.
METHODS – NATURAL SAMPLES
Samples were cut, polished, and holes were microdrilled along a single laminae. Samples were
collected from all angles, from steeply dipping to horizontal along the laminae. The resulting powder
(~10 mg per sample) was measured for χ. Intensities were normalized for weight. Thin sections of
each sample were analyzed to assess the timing and phases of magnetic minerals. Reported intensi-
ties are multiplied by 10
-4
ease of comparison. Multiplication factors will be explicitly stated for each
sample.
61
Avoiding Potential Biases
The amount of magnetite added to both experimental setups is much larger than what would be
found in a standard sedimentary environment. Additionally, the field samples have been obtained from
a number of different environments (marine, hot spring, lacustrine, etc.). It is therefore important to
remember that the measurements in the field samples are relative. χ intensities must be compared only
to other intensities within the same sample (high angle vs. low angle laminae of a single stromatolite),
and not to other experiments.
Because the field samples examined here range in age, from Modern to Proterozoic, many may
have undergone significant amount of diagenesis (like the Johnnie Formation stromatolites), which
could have potentially precipitated second generation magnetic minerals within the structure. For this
reason, the timing and phases of the magnetic minerals need to be assessed.
RESULTS – NATURAL SAMPLES
Known Biotic Sample – Yellowstone Hot Spring Stromatolite
Samples were taken at along the stromatolite laminae to create a map of magnetic susceptibil-
ity (Fig 4.18, measurements are multiplied by a factor of 10
-4
, results are shown in Table 4.3). As was
expected from the laboratory results, χ values were similar at every point in the stromatolite, ranging
from 3.45 to 4.09 SI/g, with an average of 3.59 ± 0.14 SI/g, even though some laminae were flat-lying
(completely horizontal), and some were completely vertical (Figure 4.18). Given only the χ data, and
the results from the laboratory experiments, one would comfortably call these stromatolites ‘biotic’.
Of course, when coupled with the petrographic data, all doubt as to the biogencity of the structures is
removed.
Known Abiotic Sample – Hydrothermal Vein Stromatolite
Results show that there little to no magnetic material bound within the structure of this stroma-
tolite (Figure 4.19; Table 4.3). The measured intensities were on the order of magnitude of machine
noise (10
-8
or 10
-9
SI). At these low intensities, differences in the data cannot be analyzed with any
62
real certainty. This is not altogether surprising, as a hydrothermal vein is not a typical surface sedi-
mentary environment, and would therefore have little to no detrital magnetic material. Any magnetic
grains present in the hydrothermal fluid would likely lock into place when the fluid cooled, and be less
susceptible to gravitational forces anyway. Petrographic analysis did reveal some opaque grains that
potentially could be carrying the small magnetic signature that was recovered , however, due to the
atypical growth direction and environment in which the specimen formed, it is difficult to extract any
useful information from the χ of this particular natural sample.
Unknown Sample #1 – Likely Biotic Stromatolite: Johnnie Formation, Death Valley

Results (Figure 4.20, multiplied by a factor of 10
-4
; Table 4.3) are consistent with a biotic origin
for the stromatolite. All areas of laminae showed roughly the same χ signature (ranging from 3.08 to
3.89) whether they were high angle or flat lying. Additionally, the χ intensities of the lamine matched
the intensities of the sediment in the inter-columnar areas, which ranged from 3.42 to 3.56 SI/g, with
an average of 3.51 SI/g (Figure 4.20). An area of the outer envelope of a column was also measured,
and was found to have a higher χ (roughly twice as much – 5.99 SI/g) than the laminae or the inter-
columnar areas, indicating that the envelope may have had a greater ability to trap and bind sediment,
which would be expected from the interpretation of the outer envelope as an EPS-rich encasement of
a biotic column. Given the χ data alone, one would be comfortable labeling this stromatolite ‘biotic’.
The other observable confirm this categorization, and the Johnnie Formation stromatolites can now be
more confidently classified ‘known biotic’ samples.
Unknown Sample #2 – Likely Biotic Stromatolite: Green River Formation, Wyoming
Results (Figure 4.21, multiplied by a factor of 10
-4
; Table 4.3) show similar concentrations on
all sides of laminae. All are found to be within the range of 3.8-4.1, except one sample from a near-
horizontal laminae that measures 5.60. Though the laminations range from near-vertical to near-hori-
zontal, there is very little variation in the measured intensities (the total average is 4.05 ± 0.40, without
the high outlying value it is 3.95 ±0.10, showing very little variance). If one only had the χ data, it
would be easy to interpret this a biotic stromatolite with a good degree of certainty. Given the χ and
63
the petrographic data, this sample can now be more confidently classified as a ‘known biotic’ stromato-
lite.
Unknown Sample #3- Biotic/Abiotic Mix: Green River Formation, Wyoming
Results (Figure 4.22, multiplied by a factor of 10
-4
, Table 4.3) show differences in χ intensities
between the micritic layer and the layer composed of crystal fans. The average χ for the micritic layer
is 0.966 ±0.17, while the average for the fans is 0.608 ±0.08, meaning that the micritic layer was better
at trapping and binding detrital material. It is interesting that the micritic layers reveal more variance,
whereas the precipitated layers have almost no variance. This may be due to the relative robustness
of the biofilm at different points in the accreting structure. A thinner biofilm may trap fewer detrital
grains. However, without the biofilm preserved, this explanation is conjectural. With this data, this
sample can now be more confidently assessed as a being made up of alternating layers of biogenic and
abiogenic stromatolites.
Unknown Sample #4 – Likely Abiotic Stromatolite: Furnace Creek Formation, Death Valley
Results (Figure 4.23, multiplied by a factor of 10
-4
; Table 4.3) are intriguing for the Furnace
Creek stromatolite. In all cases, the sediment off of which the stromatolites are growing has a higher χ
than the stromatolites themselves (and average of 3.82 for the sediment, when compared to an average
of 2.98 for the columns). While the columns were too small themselves to have χ measured as a func-
tion of the steepness of the laminae with much accuracy (steep sides without fail would include some
of the shallow tops within the 1 mm diameter of the drill hole), it is interesting to note that the lowest
intensity (1.78 SI/g) came from the steepest angle (Figure 4.23). The more horizontal laminations re-
corded intensities that were closer to those found in the underlying sediment (3.2 -3.3 SI/g) (Table 4.3).
If one was given only this information about the sample, I would conclude that it was likely abiotic,
though still a bit uncertain. When the χ data is coupled with the petrographic data, it seems sure that
the stromatolites of Furnace Creek were not made under the direct influence of microbes, and can now
be classified as ‘known abiotic’ samples.
64
Unknown Sample # 5- Likely Abiotic Stromatolite: Walker Lake, Nevada
The first Walker Lake stromatolite measured (from the steep side of the boulder), like the
Hydrothermal Vein sample, contained very little magnetic material (on the order of 10
-7
SI/g, Figure
4.24; Table 4.3). Studies of Walker Lake sediment from the time period of stromatolite growth reveal
significant χ intensities (ranging from 5 to 20 SI) (Yuan et al., 2006b), indicating that there is a flux of
magnetic detrital material to the lake, and that it is preserved in the sediments. The fact that none of
the magnetic material was bound into the stromatolite structure indicates that no biofilm could have
been present.
A stromatolite from the top of the boulder was also tested (Figure 4.25; Table 4.3) for compari-
son. It was thought that if any stromatolites from Walker Lake contained magnetic material, it would
be there. In this case, χ was able to be measured. Flat-lying areas averaged 1.240 (10
-4
SI/g), while
steep sides averaged 0.7185. These are observations expected from an abiotic structure.
This constitutes two different scales of data for the Walker Lake stromatolites: At the boulder
scale, the stromatolties on the top record a measurable χ intensity, while the ones on the near-vertical
sides do not. Within the sample from the top, the flat laminae record a higher intensity than do steep
laminae. This combination of data allows for a more confident classification of Walker Lake stromato-
lites as abiotic.
DISCUSSION – NATURAL SAMPLES
Overall, the results of the tests on natural samples conformed to the expectations derived from
the laboratory experiments. The only sample that could not be definitively asses with magnetic sus -
ceptibility was the known abiotic stromatolite, which formed in a hydrothermal vein, not in a typical,
surfacial depositional system. All other samples could be more confidently classified as ‘known biotic’
or ‘known abiotic’ in the light of the χ intensity measurements.
It is clear, however, that there are, at this point, still limits to the utility of this biosignature.
Absolute intensity cannot compared across samples from different depositional environments. Mag-
65
netic susceptibility will vary in each depositional system, and results must be taken in context. In any
natural sample, the relative intensities should be measured as a function of angle. If a sample has areas
that may be biotic, and others that may be abiotic (as was the case in the second unknown sample from
the Green River Formation), the relative intensities between those areas should be measured. If at all
possible, relative intensities should be compared between widely varying angles. If a near-horizontal
surface is compared with a surface inclined at only 15 degrees, there may not be a measurable differ-
ence in their relative intensities. However, a near-horizontal surface and a near-veritcal surface may
have significant differences.
When at all possible, some idea of the overall magnetic flux into the system should also be
quantified. Whether this be with published sediment records (as in the case of Walker Lake), or with
data measured from intercolumnar areas, it is always useful to know how much magnetic material the
stromatolite is holding on to as compared to how much is entering the system overall.
Finally, it is important to investigate the diagenesis of the samples. The older natural samples
are, the more likely it is that they have been subject to heat, pressure, and hydrothermal fluids, that
can either dissolve away magnetic material (if the are reducing in nature), or precipitate new mineral
phases secondarily. A sample that has been exposed to these conditions may have had the amount
of magnetic material in the structure increased, lessened, or removed entirely. For instance, if it was
found that the steep sides of the Johnnie Formation stromatolite (which were close to styolites and hy-
drothermally precipitated minerals) contained no magnetic signature, one could not say, in light of the
diagenesis, that there were never any magnetic materials present. Magnetic susceptibility measure-
ments must be looked at in the context of diagenesis to avoid a false interpretation of results.
Given the data presented here, I am confident that, even with the limitations presently associ -
ated with this method, magnetic susceptibility has great utility as a biosignature.
66
CHAPTER 4 CONCLUSIONS
As discussed earlier, the development of unambiguous biosignatures that give us the ability to
differentiate biotic from abiotic structures is crucial to the study of geobiology, in respect to looking
for life elsewhere in the solar system as well as to the study of ancient life on Earth. Traditional evi-
dence of the trapping and binding ability of biofilms is not always preserved in the fossil record. Di -
agenesis obscures original fabric, and recrystalizes carbonate and siliciclastic grains. Fortunately, the
minute magnetic fraction of trapped and bound detrital grains remains, and can be directly measured.
Abiotic experiments showed that mineral precipitation alone is not enough to hold onto signifi -
cant amounts of detrital material at high angles. Biofilms, on the other hand, can hold lar ge fractions
of magnetite even when inclined at near-vertical angles. In most natural samples studied here, the mag-
netic susceptibility of a sample confirmed the biogencity that was suspected from studying petrology .
It is clear from the field test in Tahiti, the laboratory experiments, and the testing of natural samples,
that magnetic susceptibility has great potential as a biosignature.
67
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79
FIGURE CAPTIONS
Chapter 1
Figure 1.1: Stromatolite form diversity through time, modified after Awramik and Sprinkle,
1999. Stromatolites first appear in the Archean, rise in form diversity through the Protererozoic,
reaching a peak in the Mesoproterozoic, and decline in the Neoproterozoic.
Figure 1.2: Stromatoites from Shark Bay, Western Australia (from Grotzinger and Knoll, 1999).
A: Modern stromatolite mounds, scale bar is 40 cm. B: Cross section of Shark Bay stromatolite,
showing coarse lamination. Knife is 7.5 cm long.
Figure 1.3: Hand sample and thin section of a modern Bahamian stromatolite, from Reid et al.,
2000. A: Stromatolite hand sample. B: Reconstruction of laminae. Scale bar is 10 mm C; D:
Thin section photomicrograph of laminae, scale is 100 μm.
Figure 1.4: Pavillion lake Microbialites, from Laval et al., 2000. Top Panel: A: microbialite at
10-15 m depth, decimeters in diameter. B: 20 m deep microbialite, meters in diameter. C: 20 m
deep microbialites, 20-30 m in diameter. D: Deep water microbialite mound, > 30 m in diameter.
Bottom Panel: Microbialite microscture. A, B: SEM images of calcified filamentous microfossils
and trapped sediment. C: optical thin section of an early Cambrian dolomitic stromatolite, simi-
lar to forms found in Pavillion lake.
Figure 1.5: Microbialites of Green Lake, from Thompson et al., 1990. A: Surface view of mas-
sive, non-laminated microbialites. B: Underwater mosses haging from underside of non-laminat-
ed microbialites. Scale is 0.5 m.
Fgire 1.6: Microbialites from Lake Clifton, central Australia. Top: Photograph of eastern mar-
ing of Lake Clifton, showing growing microbialites. Bottom: A, B; C, D: Cross sections and cor-
responding thin sections of two Lake Clifton Microbialites. Scale bars are 20 mm and 10 mm,
respectively. Microbialites are clotted and non-laminated, unlike Precambrian forms.
Figure 1.7: Microbialites from Lake Tanganyika, East African Rift Valley, from Cohen et al., 1997.
A: Large stromatolite forming around rocky ledges at 20 m depth. Field of view is 2 m. B: Dom-
al stromatolites at 10 m water depth, large dome in center is 0.7 m. C: Small encrustations grow-
ing at 15 m depth. Fish in center is 10 cm. D: Close up of individual stromatolite column. Field
of view is 20 cm. E: Poorly laminated columns and columnar braching stromatolite. Scale divi-
80
sion is mm. F: Meso and microstructure of stromatolite. Scale is 1 mm. G: Coarse lamination
of stromatolites, scale is 1mm. H: Coated grain and peloidal allochems infilling spaces between
stromatolite columns. Scale bar is 5 mm.
Figure 1.8: Hand sample of stromatolites from the ~850 Ma Bitter Springs Formation in central
Australia, from Williams et al., 2007.
Figure 1.9: Thin sections of the Bitter Springs stromatolite. A, B: Stromatolite column in plane
polarized light and crossed polars, respectively. Reddish grains are dolomite rhombs, indicating
the sample has gone through significan diagenesis. Scale bar is 500 μm. C, D: Edges of columns
and inter-columnar area in polarized light and crossed polars, respectively. Lamination is seen
to be on the order of <50 μm, even though diagenesis has significantly altered the sample. Scale
bars are 500 μm. E, F: Close up on column, and inter-columnar area. Scale bars are 100μm.
Figure 1.10: Field photo of a stromatolite from the Deep Spring Formation in Nevada. Bob
Douglas’ hand for scale.
Figure 1.11: Thin section photomicrographs of a Deep Spring Formation stromatolite. A: Di-
agenesis has all but obscured original fabric. Laminations are indistinct at best, though on the
order of ~100 μm. Scale is 500 μm. B: Stromatolite is made up of interlocking crystals, with
little detrital material, and no fossil cells. Scale is 100 μm.
Figure 1.12: Hand sample of the Gunflint Formation stromatolites. Scale bar is 1 cm.
Figure 1.13: Thin section photomicrographs of a stromatolite from the Gunflint Formation
(~1.8 Ga). A: Columns and intercolumnar areas. Scale is 1000 μm. B: Laminations are defined
by fine grained material, and thicken over the tops. Scale is 500 μm. C: Laminations can be seen
to be made up of small, filamentous remains of bacteria. Scale is 50 μm.
81
Chapter 2
Figure 2.1: Geologic map of Walker Lake and surrounding in western Nevada at 1:50,000 scale.
Area inside of box is focused on in Figure 2.2. USGS, 1957.
Figure 2.2: Close up of area inside box in Figure 2.1. Star on the southwest corner marks the
location where the stromatolites are found. USGS, 1957.
Figure 2.3: Google Earth Satellite image of Walker Lake, outside the town of Hawthorne, Nevada.
Box indicates the location of the stromatolites. Scale bar is 4.5 km.
Figure 2.4: A: Close up of area inside box on Figure 2.2. Red box indicates the location of stro-
matolites (note the position at the base of an alluvial fan coming off the Wassuk Range). B: Close
up of area in red box from A. Scale bar is 2.0 km. Beach on which the stromatolites are located is
marked by the green box. Scale bar is 0.5 km.
Figure 2.5: Hydrocast of major dissolved chemical species in Walker Lake collected by Lisa Col-
lins and Will Berelson from August 2005. Oxygen is depleated at depths below 20 m, and Cal-
cium, though variable, never reaches above 0.2 mM.
Figure 2.6: A: Field photo of Walker Lake. Stromatolites are located just above current lake level.
B: Boulder with encrusted stromatolites on all sides (arrow points to stromatolites growing off a
boulder).
Figure 2.7: Internal structure of Walker Lake stromatolites. The lowermost section of the stro-
matolite is finely laminated like many Precambrian stromatolites. The fine lamination grades
into a weakly laminated middle section, and a non-laminated upper section. The base of this
sample was the former boulder-stromatolite contact. Scale bar is 5 mm.
Figure 2.8: Photomicrogrpahs of the lower, finely laminated portion of Walker Lake stromato-
lite WL_6. Laminations are defined by the density of the carbonate/the amount of pore space.
A: 2.5X Magnification, in plane polarized light. B: 2.5X Magnification in crossed polars. C: 10X
maginification, in plane polarized light. D: 10X magnification, in crossed polars. Blue color is
epoxy used to hole the sample together during processing.
82
Figure 2.9: Photomicrographs of the lower, finely laminated portion of Walker Lake stromatolite
WL_6. A: 20X magnification in plane polarized light. B: 20X magnification in crossed polars. At
this scale, the carbonate crystals are seen to be interlocking.
Figure 2.10: Photomicrographs of the lower transition zone (from finely laminated to weakly
laminated) of the stromatolite WL_6. A: 2.5X Magnification, in plane polarized light. B: 2.5X
Magnification in crossed polars. C: 10X maginification, in plane polarized light. D: 10X magni-
fication, in crossed polars. The laminations begin to dome, become less well defined, and cannot
be counted as easily.
Figure 2.11: Photomicrographs of the upper transition zone (from weakly laminated to non-
laminated) of the stromatolite WL_6. A: 2.5X Magnification, in plane polarized light. B: 2.5X
Magnification in crossed polars. C: 10X maginification, in plane polarized light. D: 10X magnifi-
cation, in crossed polars. Lamination becomes clotted, and increasingly hard to discern. Vugs are
visible in the structure, though there is still very little detrital materia13l.
Figure 2.12: Photomicrographs of the upper transition zone (from weakly laminated to non-lam-
inated) of the stromatolite WL_6. A: 20X magnification in plane polarized light. B: 20X magnifi-
cation in crossed polars.
Figure 2.13: Photomicrograps of the non-laminated portion of stromatolite WL_6. A: 2.5X
Magnification, in plane polarized light. B: 2.5X Magnification in crossed polars. C: 10X magini-
fication, in plane polarized light. D: 10X magnification, in crossed polars. While the carbonate
crystallography is the same as in the other portions of the stromatolite, the differences in pore
space (and density) have disappeared, giving the stromatolite a non-laminated appearance.
Figure 2.14: Photomicrograps of the non-laminated portion of stromatolite WL_6. A: 20X magni-
fication in plane polarized light. B: 20X magnification in crossed polars. The interlocking car-
bonate crystals are the same as in other zones, but without differences in pore space.
Figure 2.15: Compilation of all
14
C dates, given in Calibrated Years Before Present (1950), from
four samples, and plotted as a function of their distance from the top of the structure. The major
morphological transitions are marked in grey. Transition zone 1 denotes the change from finely
laminated to weakly laminated. Transition zone 2 denotes the change from weakly laminated
to non-laminated crust. Average growth rates are calculated from the equation of a best fit line
through the pooled data. Growth rate is not consistent throughout the structure, and in fact
drops over an order of magnitude in the finely laminated portion with no obvious changes in
morphology.
83
Figure 2.16: Assembled photomosaic of sample WL_6, with drill holes preserved from
14
C analy-
sis, and resulting
14
C ages (calibrated and corrected).
Figure 2.17: Close up on lowest section of WL_6 (Zone I), spanning 3307-3000 ybp.
Figure 2.18: Close up on the second section of WL_6 (Zone II), spanning 3000-2823 ybp.
Figure 2.19: Close up on the third section of WL_6 (Zone III), spanning 2823-2672 ybp.
Figure 2.20: Close up on the highest section of WL_6 (Zone IV), spanning 2672-2475 ybp.
Figure 2.21: Lamination periods of a Walker Lake stromatolite compared with the structure of
the stromatolite and changes in lake level during the time of formation. A: Lake level changes
over the course of stromatolite accretion in accordance with Yuan et al. (2006b) and Adams
(2007); Stromatolite accretion began when lake level was high. The structure grew as lake level
fluctuated. B: Variations in lamination period during the time of stromatolite accretion for a lake
with a constant reservoir effect of 300 years. C: Variations in the lamination period given a reser-
voir effect that varies as a function of lake level. Solid error lines represent the error associated
with the calibrations of the
14
C ages. Dashed lines show the possible (though unlikely) added
error from the number of laminations included in the span of one drill hole. All ages are given
in Calibrated Years Before Present. The stromatolite started forming in about 40 m of water and
grew as lake level fluctuated. Lamination period is shown to be non-linear through the growth
of the stromatolite. When lake level is high, the period of lamination is around 4-6 years. When
lake level is low, lamination increases to a more annual rate. Varying the amount of the reser-
voir effect has no bearing on the overall results of this study. These lamination rates are more
consistent with an ENSO-like response than that of a microbial community, which would be daily,
seasonal, or at most, yearly. The apparent break in the upper portion of the stromatolite in zone
IV is an artifact of sample preparation. The sample was originally continuous, and no sample was
lost during the processes.

84
Chapter 3
Figure 3.1: Comparison of Walker Lake stromatolites (c) to a Proterozoic form from the John-
nie Formation in Death Valley (a) and a modern marine stromatolite from the Bahamas (b). The
sample from Walker Lake is a much better textural analogue to the Proterozoic stromatolite than
modern marine forms.

Figure 3.2: A: Hypothetical distribution of growth angles from a phototrophic community. B:
Hypothetical distribution of growth angles from a non-phototrophic community or abiotic pre-
cipitation. If the morphology of the stromatolites was influenced by a growing photic communi-
ty, the distribution of growth angles would range up towards 90 degrees, with very few samples
growing above 90, or in the ‘down’ direction. If the morphology does not depend on photosyn-
thesis, it is expected that the distribution will cluster around 90 degrees, or surface-normal.
C: Diagram of sample methodology. The growth direction is measured from the surface, so 90
degrees always represents ‘surface normal’. Angles lower than 90 degrees represent growth in
the ‘up’ direction, while angles higher than 90 degrees represent growth in the ‘down’ direction.
Figure 3.3: A: Hypothetical distribution of thickness of stromatolites in a phototrophic com-
munity. The data would show that samples on the top of the boulder are thicker, due to receiv-
ing more sunlight. Samples taken from the sides of boulders (steeply inclined) would be skewed
towards smaller thicknesses. B: Hypothetical distribution of thicknesses of stromatolites for a
non-phototrophic community. Data would not be clearly skewed toward thicker or thinner on
either the top or sides of boulders.
Figure 3.4: GPS locations of boulders and samples used in data analyses in Chapter 3. A: Boul-
der 1. B: Boulder 2. C: Boulder 3. D: Boulder 4.
Figure 3.5: Distribution of the growth angles from all samples, 305 in total. Mean and median
are both 83 degrees
Figure 3.6: Distributions of growth angles based on the cardinal direction samples were facing.
All show the same trend.
Figure 3.7: Distributions of growth angles based on the inclination of the surface the samples
were accreting off of.
85
Figure 3.8: Distributions of thicknesses from all samples, 199 in total. Mean is 1.41 ± 0.54 cm.
Figure 3.9: Distributions of thicknesses by cardinal direction.
Figure 3.10: Distributions of thicknesses by inclination of the surface the samples were accret-
ing off of.
Figure 3.11: Comparison of average thicknesses as a function of facing direction and boulder
surface inclination for the north-south plane. Shaded box is the error of all measurements. It is
predicted that for a phototrophic community, an asymmetrical trend would emerge, where all
samples where thicker on the south side than the north, and that shallowly inclined, south facing
samples would be the thickest of all. This is not supported by the data. Some variation in thick-
ness is visible, however all measurements fall within the overall error, meaning the variation
seen is not statistically significant.
Figure 3.12: Comparison of average thicknesses as a function of facing direction and boulder
surface inclination for the east-west plane. Shaded box is the error of all measurements. Some
variation in thickness is visible, however all measurements fall within the overall error, meaning
the variation seen is not statistically significant. Additionally, there is no data for angles be-
tween 0 and 90 degrees, which does not allow for the extrapolation of a valid trend.
86
Chapter 4
Figure 4.1: Conceptual theory behind the use of Magnetic Susceptibility (CHI) as a biosignature.
A: Hypothetical distribution of magnetic grains in a biotic system. The presence of a ‘sticky’
biofilm leads to an even distribution of magnetic grains around the structure. B: Hypothetical
distribution of magnetic grains in an abiotic system. Detrital grains are concentrated in lows in
accordance with physical sorting laws.
Figure 4.2: A: Tahitian microbialite recovered from IODP leg 310. The microbialte grows off
of all sides of the coral reef into vugs (holes), sometimes growing straight down (arrow shows
growth direction). B: Photomicrograph of the microbialite in plane polarized light. C: Micro-
bialite in crossed polars. Arrow points to opaque grains of titanomagnetite bound within the
microbialite. Grains are only found in association with the microbialite, and are not included in
the surrounding cements.
Figure 4.3: A: Abiotic carbonate precipitation experiment setup. Magnetite was introduced into
a slightly agitated solution after carbonate was allowed to precipitate on slides inclined at vary-
ing angles. B: Empty tank and set up. C: Magnetite piles at the bottom of a slide inclined at 60
degrees, even though carbonate has precipitated and held.
Figure 4.4: A: Biologic experimental setup. B: Mats collected from Catalina Harbor. C: Filamen-
tous cyanobacteria that make up the bulk of the mats.
Figure 4.5: Results of abiotic experiments. Magnetic Susceptibility decreases with increasing
angle of inclination, dropping to zero at angles higher than 45 degrees. Square represents the
average signature of the sample vessles (negative due to being diamagnetic).
Figure 4.6: Results of biofilm experiments. Magnetic Susceptibility decreases slightly with
increasing angle of inclination, but never drops to zero. High angles of inclination (≥ 60) still
recorded notable signatures. Red square represents the average signature of the sample vessles
(negative due to being diamagnetic). Green square represents the average signature of control
mats (those that were not introduced to any magnetite).
87
Figure 4.7: Location, hand sample, and thin section photomicrographs of Yellowstone Hot
spring Stromatolite (known biotic sample). A: Stromatolites growing around the edge of a hot
spring in Yellowstone National Park. B: Cross section of the stromatolite, showing both flat lying
lamina and drap facies lamina. C: Photomicrograph of stromatolite laminae. Light laminae are
defined by vertically oriented, silicified, filamentous sheaths. Dark laminae are defined by hori-
zontally oriented sheaths. Scale bar is 1000 μm.
Figure 4.8: Location of Hydrothermal Vein stromatolites in the Tempaiute Range, Central Ne-
vada. A: Tempaiute Range. B: Close up of area in A. Box is the location where the hydrothermal
vein is found. Scale bars are 0.5km.
Figure 4.9: Field photo, hand sample, and thin section photomicrographs of Hydrothermal Vein
Stromatolite. A: Vein in Devonian limestone (Will Berelson for scale). B: Field photo of hydro-
thermal vein, with “up” direction noted. C: Photomicrograph of hydrothermal stromatolite in
plane polarized light. Scale is 500 μm. D: Photomicrograph of hydrothermal stromatolite in
crossed polars. Sweeping extinction is evident, and lamination is seen to be defined by the termi-
nation boundaries of crystals. Scale is 500 μm.
Figure 4.10: Location and hand sample of Johnnie Formation stromatolite (unknown, likely bi-
otic). A: Area of Death Valley, east of the California/Nevada border where Boxonia was collected.
B: Close up of box in A. Boxonia was collected roughly 1km east of the Gunsight mine, courtesy
of Preston Cloud (Cloud and Semikhatov, 1969). Scale is 0.5 km. C: Hand sample of Boxonia,
showing fine lamination. Arrow point to ‘envelope’ that suggests a biologic origin.
Figure 4.11: Thin section photomicrographs of Johnnie Formation stromatolite. A: Wavy lami-
nae. Scale bar is 500 μm. B: Close up of laminae. Light laminae are recrystalized, but rounded
opaque grains can be seen bound into the structure. Scale bar is 500 μm. C: Laminae and side of
column. Opaque grains can be seen bound at high angles. Scale bar is 500 μm. D: Inter-colum-
nar area. Azurite can be seen replacing malachite, and dolomite crystals are distributed through-
out. Scale bar is 200 μm. E: Close up on styolite running up the side of the stromatolite column.
Malachite has precipitated around the edges of the styolite. Scale bar is 200 μm. F: Close up on
inter-columnar area. Azurite, malachite, and dolomite are visible. Scale bar is 100 μm.
Figure 4.12: A: Location of the Tipton Road Locality of the Green River Formation in Wyoming.
B: Close up of area highlighted in A. Box indicates area where the tube-forming stromatolite
samples are found. Scale bar is 0.5 km. C: Hand sample of the tube-forming stromatolites.
88
Figure 4.13: Photomicrographs of tube-forming stromatolites from the Green River Formation
in Wyoming. A: Thin section of laminae and inter-columnar area (large grains). Scale bar is 1000
μm. B: Laminae in plane polarized light. Scale bar is 1000 μm. C: Laminae in crossed polars.
Detrial quartz grains can be seen bound into the structure. Scale bar is 1000 μm. D: Make up
of inter-columnar area in plane polarized light. Ooids and other detrial grains are encased in
several generations of secondary carbonate. Scale bar is 100 μm. E: Inter-columnar material in
crossed polars.
Figure 4.14: A: Location Boar’s Tusk Locality of the Green River Formation in Wyoming. B:
Close up of area highlighted in A. Box indicates area where the Boar’s Tusk stromatolite is found.
Scale bar is 0.5 km. C: Hand sample of the Boar’s Tusk stromatolite.
Figure 4.15: Photomicrographs of the Boar’s Tusk stromatolite. A: Carbonate crystal fans grow-
ing off of a micritic layer in plane polarized light. Scale bar is 500 μm. B: Fan layer and micritic
layer in crossed polars. Fans show sweeping extinction typical of abiotic precipitation. Micritic
layers have detrial quartz grains bound into the structure. Scale bar is 500 μm. C: Close up of
crystal fans in crossed polars. Laminae are defined by crystal termination boundaries. Scale bar
is 100 μm. D: Close up of micritic lamina. Carbonate grains are interlocking, and detrital mate-
rial is bound into the structure. Scale bar is 100 μm.
Figure 4.16: A: Location of the Pliocene Furnace Creek stromatolite from Death Valley, Califor-
nia. B: Close up of area highlighted in A. Box indicates location of Furnace Creek stromatotlites.
Scale bar is 0.5 km. C: Hand sample of Furnace Creek stromatolite.
Figure 4.17: Photomicrographs of the Furnace Creek stromatolite. A: The structure of the
stromatolite is defined by crystal fans. Scale bar is 1000 μm. B: Fans in crossed polars. The fans
exhibit sweeping extinction. Scale bar is 1000 μm. C: Close up of crystal fans in plane polarized
light. Scale bar is 100 μm. D: Close up of crystal fans in crossed polars, with sweeping extinction.
Scale bar is 100 μm.
Figure 4.18: Results from the Yellowstone hot spring stromatolite. Magnetic susceptibility is
similar regardless of laminae inclination.
Figure 4.19: Results from the hydrothermal vein stromatolite. Very little magnetic material is
found within the structure.
Figure 4.20: Results from the Johnnie Formation stromatolite. Magnetic susceptibility is similar
regardless of laminae inclination, indicating a biologic origin for the structure.
89
Figure 4.21: Results from the Green River Formation tube-forming stromatolties. Magnetic sus-
ceptibility is similar regardless of laminae inclination, indicating a biologic origin for the struc-
ture.
Figure 4.22: Results from the Boar’s Tusk stromatolite. Crystal fan layers have less magnetic
material than micritic layers, idicating that crystal fans are abiotic, while micrite layers formed
under biologic influence.
Figure 4.23: Results from the Furnace Creek stromatolite. Crystal fan layers have less magnetic
material than the sediment they are accreting off of, indicating that the fans are abiotic.
Figure 4.24: Results from the Walker Lake stromatolite that was found growing off the steep
side of a boulder. Very little magnetic material is contained within the structure.
Figure 2.25: Results from the Walker Lake stromatolite that was found growing off the top of a
boulder. There is a higher magnetic susceptibility intensity on the top of the strucure than there
is in the steeply inclined laminae, indicating an abiotic origin for the structure.

90
Figure 1.1: Stromatolite form diversity through time,
modified after Awramik and Sprinkle, 1999
0
100
200
300
400
500
500 0 1000 1500 2000 2500
Total number of stromatolite forms
Ma
MR
Archean
Paleo-
proterozoic
Si Rh Or St ER
Neo-
proterozoic
Meso-
proterozoic
Proterozoic
P M C
Phanerozoic
V LR
Diversity of stromatolite forms through time
FIGURES - CHAPTER 1
Figure 1.1: Stromatolite form diversity through time (after Awramik and
Sprinkle, 1999)
91
Figure 1.2: Modern stromatolites from Shark Bay,
Western Australia, from Grotzinger and Knoll, 1999.
Figure 1.2: Modern stromatolites from Shark Bay, Western Australia
(Grotzinger and Knoll, 1999)
92
Figure 1.3: Hand sample and thin section of a modern
Bahamian stromatolite, from Reid et al., 2000.
Figure 1.3: Hand sample and thin section of a modern Bahamian
stromatolite (Reid et al., 2000)
93
Figure 1.4: Pavillion Lake Microbialites, from Laval et
al., 2000
Figure 1.4: Pavilion Lake micrbialites (Laval et al., 2000)
94
Figure 1.5: Green Lake Microbialites, from Thompson
et al., 1990
Figure 1.5: Green Lake microbialites (Thompson et al., 1990)
95
Figure 1.6: Lake Clifton Microbialites, from Wacey et
al., 2010
Figure 1.6: Lake Clifton microbialites (Wace et al., 2010)
96
Figure 1.7: Lake Tanganyika Microbialites, from
Cohen et al., 1997
Figure 1.7: Lake Tanganyika microbialites (Cohen et al., 1997)
97
Figure 1.8: Hand sample of Bitter Springs ormation
stromatolite
Figure 1.8: Bitter Springs Formation stromatolite
98
Figure 1.9: Thin sections of the Bitter Springs
stromatolite (~850 Ma)
A B
C
E
A
F
D
Figure 1.9: Thin sections of Bitter Springs stromatolites (~850 Ma)
99
Figure 1.10: Field photo of the Deep Spring Forma-
tion stromatolite
Figure 1.10: Deep Spring Formation stromatolite
100
Figure 1.11: Thin section photomicrographs of the
Deep Spring Formation stromatolites
A
B
Figure 1.11: Thin section photomicrographs of the Deep
Spring Formation stromatolites
101
Figure 1.12: Hand sample of the ~1.8 Ga Gunflint Formation stromatolites
Figure 1.12: Gunflint Formation stromatolites (~1.8 Ga)
102
A B
C
Figure 1.13: Thin section photomicrographs of the ~1.8 Ga Gunflint Formation stromatolites
Figure 1.13: Thin seciton photomicrographs of the Gunflint Formation
stromatolites
103
FIGURES - CHAPTER 2
Figure 2.1: Geologic map of Walker Lake and surrounding area in
western Nevada at 1:50,000 scale. Area inside of box is focused on in
Figure 2.2
104
Figure 2.2: Close up of area inside box in Figure 2.1. Star on the
southwest corner marks the location of stromatolites. USGS, 1957
105
Figure 2.3: Google Earth satelite image of Walker Lake, outside the
town of Hawthorn, Nevada. Box indicates location of stromatolites
106
Figure 2.4: A: Close up of area inside box on Figure 2.3. Box indi-
cates stromatolite location. B: Close up of area in box from A. Beach
on which the stromatolites are located is marked by the green box.
107
Figure 2.5: Hydrocast of major dissolved chemical species in Walker Lake, col-
lected by Lisa Collins and Will Berelson in August 2005. Oxygen is depleated at
depths below 20m. Calcium, though variable, never reaches above 0.2 mM
108
Figure 2.6: A: Field photos of Walker Lake. B: stromatolites encrusting
boulders at the current shoreline
109
Figure 2.7: Internal structure of Walker Lake stromatolites
110
Figure 2.8: Photomicrographs of the lower, finely laminated portion of
Walker Lake stromatolite WL_6
111
Figure 2.9: Photomicrograph of the lower, finely laminated poriton of
WL_6 at 20X magnification
112
Figure 2.10: Photomicrographs of the lower transition zone of WL_6
113
Figure 2.11: Photomicrographs of the upper transition zone of WL_6 at
20X magnification
114
Figure 2.12: Photomicrographs of the upper transition zone of WL_6
at 20X magnification
115
Figure 2.13: Photomicrographs of the non-laminated portion of WL_6
116
Figure 2.14: Photomicrographs of the non-laminated portion of WL_6
at 20X magnification
117
Figure 2.15: Compilation of all 14C dates (Calibrated YBP) from four samples,
plotted as a function of their distance from the top of the structure. Major
morphological transitions are marked in grey
118
Figure 2.16: Assembled photomosaic of WL_6 (drill holes are preserved
from 14C analysis), and measured 14C ages (Calibrated YBP)
119
Figure 2.17: Zone I of WL-6, spanning 3307-3000 YPB
120
Figure 2.18: Zone II of WL_6, spanning 3000-2832 YBP
121
Figure 2.19: Zone III of WL_6, spanning 2832-2672 YBP
122
Figure 2.20: Zone IV of WL_6, spanning 2672-2275 YBP
123
Figure 2.21: Lamination periods of WL-6 compared with the structure of the stro-
matolite and changes in lake level during the time of foramtion. A: Lake level-
changes over the course of stromatolite accretion (Yuan et al., 2006b; Adams,
2007). B: Variations in lamination period given a constant carbon reservoir effect
of 300 years. C: Variations in lamination period given a carbon reservoir effect
that scales as a function of lake level
124
Modern Marine
Walker Lake
Proterozoic
1cm
1cm
a b
c
Figure 3.1: Comparison of Walker Lake stromatolites (c) to a Proterozoic form from the
Johnnie Formation in Death Valley (a) and a modern marine stromatolite from the Bahamas
(b). The sample from Walker Lake is a much better textural analogue to the Proterozoic
stromatolite than modern marine forms.
FIGURES - CHAPTER 3
Figure 3.1: Comparision of WL stromatolites (c) to a Proterozoic form from the Johnnie
Formation in Death Valley (a) and a modern Bahamian stromatolite (b)
125
0 45 90 135 180
Sample Count
Growth Angle
0 45 90 135 180
Sample Count
Growth Angle
Phototrophic Community
Non-Phototrophic Community
Surface Inclination
Angle of growth
from surface
surface normal =
90
o
0-89
o
91-180
o
a
b
c
Figure 3.2: A: Hypothetical distribution of growth angles from a phototrophic community. B:
Hypothetical distribution of growth angles from a non-phototrophic community or abiotic
precipitation. C: Diagram of sample methodology.
Figure 3.2: A: Hypothetical distribution of growth angles from a phototrophic
community. B: Hypothetical distribution of growth angles from a non-phototrophic
community or abiotic structure. C: Diagram of sample methodology
126
0 90 90 45 45 0 90 90 45 45
S N
Thickness
E W
Surface Inclination
0 90 90 45 45 0 90 90 45 45
S N
Thickness
E W
Surface Inclination
A
B
Figure 3.3: : A: Hypothetical distribution of thickness of stromatolites in a phototrophic
community. B: Hypothetical distribution of thicknesses of stromatolites for a non-
phototrophic community.
Figure 3.3: A: Hypothetical distribution of thickness of stromatolites in a
phototrophic community. B: Hypothetical distribution of thickness of stromatolites
for a non-phototrophic community or abiotic structure
127
Sample Count
Growth Angles - All Measurements
Growth Angles (90 = surface normal)
0
15
30
45
60
0 45 90 135 180
N = 304
Average = 82.13
StDv = 11.67
Figure 3.4: :Distribution of the growth angles from all
samples, 305 in total.
Figure 3.4: Distribution of the growth angles from all (305)
samples
128
Sample Count
All samples, Top excluded
Growth Angle
0
10
20
30
40
50
0 45 90 135 180
Top Samples
Sample Count
0
2
4
6
8
10
12
14
0 45 90 135 180
Growth Angle
West Facing Samples
0
5
10
15
20
0 45 90 135 180
North Facing Samples
0
5
10
15
0 45 90 135 180
South Facing Samples
0
5
10
15
20
25
30
0 45 90 135 180
0
2
4
6
8
10
12
14
0 45 90 135 180
Growth Angle
Sample Count
Sample Count
Growth Angle Growth Angle Growth Angle
East Facing Samples
Sample Count Sample Count
N = 48
Average = 80.67
StDv = 9.94
N = 153
Average = 81.78
StDv = 12.19
N = 52
Average = 88.69
StDv = 10.06
N = 72
Average = 82.85
StDv = 9.93
N = 252
Average = 80.81
StDv = 11.55
N = 30
Average =80.07
StDv = 8.53
Figure 3.5: Distributions of growth angles based on the car-
dinal direction samples were facing.
Figure 3.5: Distributions of growth angles based on the cardinal facing
direction of samples
129
0
5
10
15
20
0 90 180
Initial inclination - 90 degrees
60 90 120 150 180
Sample Count
Growth Angle
0
2
4
6
8
10
12
14
Initial inclination - 0 degrees
Sample Count
Growth Angle
0
5
10
15
20
Initial Inclination - 50 degrees
Growth Angle
Sample Count
0
5
10
15
20
Initial Inclination - 80 degrees
Sample Count
Growth Angle
0 180 90 0 180
90
0 180 90
N = 90
Average = 81.78
StDv = 13.19
N = 50
Average = 85.94
StDv = 6.29
N = 108
Average = 77.18
StDv = 110.82
N = 52
Average = 88.69
StDv = 10.06
Figure 3.6: Distributions of growth angles based on the inclination of the sur-
face the samples were accreting off of.
Figure 3.6: Distributions of growth angles based on the inclination of
the growth surface
130
0
10
20
30
40
50
0 0.5 1 1.5 2 2.5 3 3.5
Total Thickness Data
Count
Thickness (cm)
N = 199
Average = 1.41 cm
StDv = 0.55 cm
Figure 3.7: Distributions of thicknesses from all samples,
Figure 3.7: Distribution of thickness, all samples
131
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
Top Samples - Thickness
Count
Thickness (cm)
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
East Facing Samples - Thickness
Count
Thickness (cm)
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
South Facing samples - Thickness
Count
Thickness (cm)
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
North Facing Samples - Thickness
Count
Thickness (cm)
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
West Facing Samples - Thickness
Count
Thickness (cm)
N = 18
Average = 1.39 cm
StDv = 0.36 cm
N = 82
Average = 1.28 cm
StDv = 0.47 cm
N = 21
Average = 1.12 cm
StDv = 0.31 cm
N = 51
Average = 1.44 cm
StDv = 0.60 cm
N = 27
Average = 1.98 cm
StDv = 0.53 cm
Figure 3.8: Distributions of thicknesses by cardinal direction.,
Figure 3.8: Distributions of thickness by cardinal direction
132
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
90 Degree Inclination - Thickness
Count
Thickness (cm)
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
80 Degree Inclination - Thickness
Count
Thickness (cm)
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
0 Degree Inclination - Thickness
Count
Thickness (cm)
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5
50 Degree Inclination -Thickness
Count
Thickness (cm)
N = 27
Average = 1.98 cm
StDv = 0.53 cm
N = 36
Average = 1.33 cm
StDv = 0.33 cm
N = 72
Average = 1.30 cm
StDv = 0.55 cm
N = 63
Average = 1.35 cm
StDv = 0.49 cm
Figure 3.9: Distributions of thicknesses by inclination of the surface the
samples were accreting off of.
Figure 3.9: Distributions of thickness by inclination of sample growth
surface
133
90 45 0 45 90
South North
Surface Inclination (Degree)
Thickness (cm)
0 1.0 2.0 2.5
N = 160
Figure 3.10: Comparison of average thicknesses as a function of facing direction and boulder
surface inclination for the north-south plane. Shaded box is the error of all measurements.
Figure 3.10: Comparison of average thickness as a function of cardinal facing direction
and growth surface inclination for the north-south plane. Shaded box represents the
error of all measurements
134
90 45 0 45 90
East West
Surface Inclination (Degree)
Thickness (cm)
0 1.0 2.0 2.5
N = 66
Figure 3.11: Comparison of average thicknesses as a function of facing direction and boulder
surface inclination for the east-west plane. Shaded box is the error of all measurements.
Figure 3.11: Comparison of average thickness as a function of cardinal facing
direction and growth surface inclination for the east-west plane. Shaded box
represents the error of allmeasurements
135
Figure 4.1 - Theory of Magnetic Susceptibility as a Biosignature
Biotic Systems
Abiotic Systems
Detrital Magnetic Grains
(size greatly exaggerated)
“Sticky” biofilm entraps grains
microbialite
abiotic
precipitate
Magnetic Grains concentrated
in topographic lows
Magnetic Grains washed
off tops
A
B
FIGURES - CHAPTER 4
Figure 4.1: Theory of Magnetic Susceptibility as a Biosignature
136
A
B
C
Figure 4.2 - Tahitian Microbialites
Figure 4.2: Tahitian microbialites
137
magnetite grains introduced Water
Agitator
Figure 4.3 - Abiotic Carbonate Experimental Setup
A
B C
Figure 4.3: Abiotic carbonate experimental setup
138
A C
B
Figure 4.4 - Biotic Experiment
Figure 4.4: Biotic experiment
139
Magnetic Susceptibility per gram
Angle of Inclination
0.0005
0.0000
0.0004
0.0003
0.0002
0.0001
-0.0001
90 45 15 30 60 75
Samples
Average of
Sample Vessles
Figure 4.5 - Abiotic Results
Figure 4.5: Abiotic results
140
Magnetic Susceptibility per gram
Angle of Inclination
0.00145
0.0000
0.00115
0.00085
0.00055
0.00025
-0.0001
90 45 15 30 60 75
Samples
Average of
Sample Vessles
Average of
Control Samples
Figure 4.6 - Biotic Results
Figure 4.6: Biotic results
141
Figure 4.7 - Yellowstone Hotspring Stromatolite
A
B C
Figure 4.7: Yellowstone hotspring stromatolite
142
Figure 4.8 - Google Location of Hydrothermal Vein Stromatolites,
Tempaiute Range, Central NV
A
B
Figure 4.8: Google Earth locaiton of hydrothermal vein stromatolites,
Temaiute Range, Central NV
143
Up Direction
Figure 4.9 - Hydrothermal Vein Field photo, Hand Sample, and Thin Sections
A
B
C D
Figure 4.9: Hydrothermal vein field photo (A), hand sample (B), and
thin sections (C,D)
144
California
Nevada
A
B
C
Figure 4.10 - Location and Hand Sample of Johnnie Formation Stromatolite
Figure 4.10: Location (A,B) and hand sample (C) of Johnnie Formation
stromatolite
145
C
A
F E
D
B
Figure 4.11 - Johnnie Formation thin sections
Figure 4.11: Johnnie Formation thin section
146
Figure 4.12 - Google Location of Green River Formation at the Tipton
Road Locality, and Hand Sample of the Tube-Forming Stromatolite
A
C
B
Wyoming
Colorado
Figure 4.12: Location (A, B) and hand sample of the Green River Formation
tube-forming stromatolite
147
A
D
B C
E
Figure 4.13 - Green River Thin section (Tube Forming Stromatolites)
Figure 4.13: Thin section photomicrogrpahs of tube-forming stromatolites
148
Figure 4.14 - Boar’s Tusk Location and Hand Sample
Wyoming
Montana Idaho
A B
C
Figure 4.14: Boar’s Tusk location (A, B), and hand sample (C)
149
A
D
B
C
Figure 4.15 - Boar’s Tusk thin sections
Figure 4.15: Boar’s Tusk thin section photomicrographs
150
Figure 4.16 - Location and Hand Sample of Furnace Creek Stromatolite
B
C
California
Nevada
A
Figure 4.16: Location (A, B) and hand sample (C) of Furnace Creek stromatolite
151
A
D
B
C
Figure 4.16- Furnace Creek Thin Section
Figure 4.17: Thin section photomicrographs of Furnace Creek stromatolites
152
1.816
3.690
3.585
3.492
3.554
3.468
3.514
3.518
3.453
3.611
3.558
Figure 4.18: Results from the Yellowstone Hotspring stromatolite
Figure 4.18: Yellowstone hotspring stromatolite results
153
0.029
0.069
0.034
0.026
0.041
0.061
0.060
0.025
0.038
Figure 4.19: Hydrothermal vein stromatolite results
Up Direction
154
Up Direction
3.556
3.418
3.548
3.649
3.550
3.403
3.575
3.478
3.548
3.477
3.434
5.991
Figure 4.20 - Johnnie Formation Results
Figure 4.20: Johnnie Formation stromatolite results
155
4.136
3.930
3.999
3.807
3.898
3.814
3.896
3.887
3.989
5.603
3.862
3.989
Figure 4.21 - Green River Results
(Tube Forming Stromatolite)
Figure 4.21: Tube-forming stromatolite results
156
1.156
0.902
0.840
0.680
0.528
0.616
Figure 4.22 - Boar’s Tusk Results
Figure 4.22: Boar’s Tusk stromatolite results
157
Up Direction
3.576
3.442
4.455
3.259
3.315
3.199
1.780
3.341
Figure 4.23 - Furnace Creek Results
Figure 4.23: Furnace Creek stromatolite results
158
Up Direction
0.927
1.033
-0.019
-0.026
-0.052
-0.022
0.231
0.034
-0.066
0.849
0.734
0.031
0.053
0.159
0.999
0.008
0.038
0.051
Figure 4.24 -Walker Lake Side Results
Figure 4.24: Walker Lake stromatolite - side results
159
Up Direction
Figure 4.25 -Walker Lake Top Results
Figure 4.25: Walker Lake stromatolite - top results
160
TABLES
Chapter 2
Table 2.1: Reported raw
14
C ages given in Years Before Present (BP) and associated error.
Table 2.2: Corrections of raw
14
C dates using the method of Stuiver et al. (1998).
Table 2.3: Lamination Periods and Standard Deviations for a lake with a constant reservoir effect of
300 years, Sample WL_6.
Table 2.4: Lamination Periods and Standard Deviations for a lake with a varying reservoir effect,
Sample WL_6.
Chapter 3
Table 3.1: Hypothetical results and the type of response they would indicate.
Table 3.2: Measurements of Growth Direction and Thickness of Walker Lake stromatolites.
Chapter 4
Table 4.1: Results of abiotic carbonate precipitation experiment.
Table 4.2: Results of biofilm experiment.
Table 4.3: Results of tests on Natural Samples.
161
TABLES - CHAPTER 2
2.1: Raw
14
C data and error (uncalibrated; for calibrated dates, see Table 2.2). Data from
WL_6 (in red) is used in lamination counts.
UCIAMS #
Fraction Mod-
ern ±
D14C
(‰) ±
14C age
(YBP) ±

WL_6
48934 0.6626 0.0014 -337.4 1.4 3305 20
48953 0.6739 0.0014 -326.1 1.4 3170 20
48954 0.6853 0.0016 -314.7 1.6 3035 20
48935 0.7071 0.0015 -292.9 1.5 2785 20
48955 0.7370 0.0015 -263.0 1.5 2450 20
WL_5
48947 0.6703 0.0015 -329.7 1.5 3215 20
48948 0.6724 0.0016 -327.6 1.6 3190 20
48949 0.6875 0.0015 -312.5 1.5 3010 20
48950 0.6886 0.0015 -311.4 1.5 3000 20
48951 0.7412 0.0016 -258.8 1.6 2405 20
48952 0.7651 0.0016 -234.9 1.6 2150 20
WL_F
48945 0.6874 0.0014 -312.6 1.4 3010 20
48946 0.6947 0.0014 -305.3 1.4 2925 20
WL_8_1a
18325 0.9134 0.0016 -92.7 1.6 730 15
18326 0.7525 0.0013 -252.4 1.3 2285 15
18328 0.7154 0.0014 -289.4 1.4 2690 20
18329 0.6898 0.0012 -314.8 1.2 2985 15
18330 0.6769 0.0012 -327.5 1.2 3135 15
18331 0.6756 0.0014 -328.9 1.4 3150 20
162
Table 2.2: Calibrated
14
C data (2σ, using the method of Stuiver et al., 1998). A con-
stant 300 year reservoir effect of carbon is assumed. Dates are then 2σ calibrated
(maximum and minimum dates from the 2σ calibration are shown). Dates are re-
ported in Calibrated Years Before Present (YBP) (Calibrated years before 1950). The
average values were used to construct Figure 2.5.
UCIAMS
#
2σ
Max
YBP
2σ
Min

YBP
Average
YBP
mm from top
of stromatolite
WL_6
48934 3323 3292 3307.5 32
48953 3073 2927 3000 25
48954 2868 2778 2823 19
48935 2714 2629 2671.5 14
48955 2301 2248 2274.5 11
WL_5
48945 3158 3149 3153.5 44
48946 3138 3132 3135 35
48947 2851 2765 2808 27
48948 2847 2761 2804 21
48949 2141 2035 2088 8
48950 1863 1846 1854.5 4
WL_F
48951 2851 2765 2808 28
48952 2770 2738 2754 18
WL_8_1a
18325 512 502 507 0.5
18326 1987 1979 1983 6.8
18328 2455 2411 2433 13
18329 2839 2830 2834.5 23
18330 2993 2946 2969.5 33
18331 3079 2949 3056 42
163
Ave.
Age
(CYBP)
Max.
Age
(CYBP)
Min.
Age
(CYBP)
Reservoir
Effect
(yrs)
Ave.# of
Lams.
Between
Dates
Max
#
Lams
Min.
#
Lams.
Ave.
Period
(yrs/lam)
Max.
Period
(yrs/lam)
Min.
Period
(yrs/lam)
Likely
StDv
(Fig.
3)
Max.
StDv
(Fig.
3)
3307 3323 3292 300
55 68 44 5.58 7.2 4.0 1.6 2.9
3000 3073 2927 300
63 80 51 2.81 4.7 0.9 1.9 2.5
2823 2868 2778 300
96 110 81 1.58 2.5 0.7 0.9 1.2
2671.5 2714 2629 300
89 97 81 4.46 5.2 3.7 0.8 1.2
2274.5 2301 2248 300

Table 2.3: Lamination counts between sets of drill holes for sample WL_6, as well as average lamination period (years/
lamination) and the standard deviation of the measurement calculations assuming a constant reservoir effect for carbon
of 300 years. Maximum rate was calculated to be the average number of laminations over the least amount of time, with
minimum rate being the average number of laminations over the most amount of time accounting for the 2σ calibration er-
ror (the solid error bars on Figure 2.6b). The possible—though unlikely—error associated with the number of laminations
that are covered by one set of drill holes is also taken into account, creating a ‘worst-case scenario’ error, shown as the
dashed error bars on Figure 2.6b. This maximum rate is calculated by taking the maximum number of laminations over the
least amount of time, while the minimum is calculated by taking the minimum number of laminations over the maximum
time period.
164
Table 2.4: Lamination Periods and Standard Deviations for a lake with a varying reservoir effect,
Sample WL_6.
Ave.
Age
(CYBP)
Max.
Age
(CYBP)
Min.
Age
(CYBP)
Reservoir
Effect
(yrs)
Ave.# of
Lams.
Between
Dates
Max
#
Lams
Min.
#
Lams.
Ave.
Period
(yrs/lam)
Max.
Period
(yrs/lam)
Min.
Period
(yrs/lam
Likely
StDv
(Fig.
3)
Max.
StDv
(Fig.
3)
3041 3057 3026 567
55 68 44 3.78 5.4 2.2 1.6 2.5
2833 2906 2760 467
63 80 51 2.27 4.1 0.4 1.9 2.4
2690 2735 2645 433
96 110 81 0.89 1.8 0.0 0.9 1.1
2604.5 2647 2562 367
89 97 81 4.46 5.2 3.7 0.8 1.2
2207.5 2234 2181 367

165
Table 3.1: Hypothetical results and the type of response they would indicate
Result Phototrophic Response Non-Phototrophic Response
Growth Inclined toward top x
Growth Inclined toward
south x
Surface normal growth x
Thickest at top x
Thickest on South x
Thicker at shallower angles x
Thicker at steeper angles x
Even Thickness x

TABLES - CHAPTER 3
166
Table 3.2: Data used for Chapter 3 analyses. Samples are identified by the boulder the originated
from (1,2,3, or 4); the large sample taken from that boulder; the subsample of that block, and the in-
dividual stromatolite column from each subsample. Due to sample frailty, thicknesses of the finely
laminated portion could not be accurately measured on every stromatolite. Thicknesses are listed
only for samples where no amount of carbonate was missing from the bottom portion.
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
1 A a 1 0 Top 80
1 A a 2 0 Top 86 1.6
1 A a 3 0 Top 89 1.4
1 A a 4 0 Top 115
1 A b 1 0 Top 81
1 A b 2 0 Top 75 2.1
1 A b 3 0 Top 91
1 A b 4 0 Top 66
1 A c 1 0 Top 96
1 A c 2 0 Top 100
1 A c 3 0 Top 103
1 A c 4 0 Top 120
1 B a 1 90 East 81 1.2
1 B a 2 90 East 78 0.80
1 B a 3 90 East 92 0.88
1 B a 4 90 East 73 1.0
1 B a 5 90 East 71 1.5
1 B a 6 90 East 83
1 B a 7 90 East 74
1 B b 1 90 East 80
1 B b 2 90 East 81
1 B b 3 90 East 91 0.92
1 B b 4 90 East 68
1 B b 5 90 East 87 1.4
1 B b 6 90 East 81 0.84
1 B b 7 90 East 83 0.88
1 B b 8 90 East 84
1 B c 1 90 East 84
1 B c 2 90 East 80 0.82
1 B c 3 90 East 97 1.4
1 B c 4 90 East 68 1.3
1 B c 5 90 East 89
167
1 B c 6 90 East 81 1.2
1 B c 7 90 East 63 2.0
1 B c 8 90 East 84 1.1
1 B d 1 90 East 70 1.2
1 B d 2 90 East 81 0.77
1 B d 3 90 East 90 0.88
1 B d 4 90 East 79 1.0
1 B d 5 90 East 89 1.3
1 B d 6 90 East 62
1 B d 7 90 East 78 0.88
1 C a 1 50 SW 81 1.3
1 C a 2 50 SW 84 1.0
1 C a 3 50 SW 79
1 C a 4 50 SW 85 1.0
1 C a 5 50 SW 97 1.2
1 C a 6 50 SW 73 1.3
1 C a 7 50 SW 82
1 C a 8 50 SW 85
1 C a 9 50 SW 86
1 C b 1 50 SW 90 1.8
1 C b 2 50 SW 91 1.0
1 C b 3 50 SW 84 1.4
1 C b 4 50 SW 82 1.1
1 C b 5 50 SW 90 1.6
1 C b 6 50 SW 83 0.94
1 C c 1 50 SW 91
1 C c 2 50 SW 89 1.3
1 C c 3 50 SW 81 1.3
1 C c 4 50 SW 78 1.2
1 C c 5 50 SW 91
1 C c 6 50 SW 87
1 C c 7 50 SW 92
1 C c 8 50 SW 75
1 C c 9 50 SW 70
1 C d 1 50 SW 90 1.5
1 C d 2 50 SW 92 1.3
1 C d 3 50 SW 84 2.1
1 C d 4 50 SW 91 1.5
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
168
1 C d 5 50 SW 88 1.6
1 C e 1 50 SW 98 1.1
1 C e 2 50 SW 85 1.0
1 C e 3 50 SW 87 0.85
1 C e 4 50 SW 93 1.2
1 C e 5 50 SW 88 1.5
1 C e 6 50 SW 77 1.7
1 C f 1 50 SW 81
1 C f 2 50 SW 92
1 C f 3 50 SW 95
1 C f 4 50 SW 93
1 C g 1 50 SW 84 1.1
1 C g 2 50 SW 86 1.1
1 C g 3 50 SW 83 1.2
1 C g 4 50 SW 93 0.94
1 C g 5 50 SW 80 1.1
1 C g 6 50 SW 87 1.4
1 C g 7 50 SW 81 1.1
1 C g 8 50 SW 94 2.1
1 C g 9 50 SW 92 1.7
1 C g 10 50 SW 81 1.7
1 C g 11 50 SW 76 2.1
1 D a 1 76 South 86
1 D a 2 76 South 94
1 D a 3 76 South 84
1 D a 4 76 South 75
1 D b 1 76 South 92 1.1
1 D b 2 76 South 84 1.1
1 D b 3 76 South 80
1 D b 4 76 South 71
1 D b 5 76 South 78
2 B a 1 90 SE 84
2 B a 2 90 SE 84 1.2
2 B a 3 90 SE 93
2 B a 4 90 SE 120
2 B a 5 90 SE 91 0.96
2 B a 6 90 SE 111
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
169
2 B a 7 90 SE 76
2 B a 8 90 SE 88 1.0
2 B b 1 90 SE 62
2 B b 2 90 SE 81
2 B b 3 90 SE 110
2 B c 1 90 SE 110 1.2
2 B c 2 90 SE 101 1.6
2 B c 3 90 SE 85 1.2
2 B c 4 90 SE 96 1.2
2 B c 5 90 SE 100 1.5
2 B c 6 90 SE 78 1.6
2 B c 7 90 SE 85 1.8
2 B c 8 90 SE 95 1.7
2 B c 9 90 SE 66 1.6
2 B c 10 90 SE 64 3.2
2 B d 1 90 SE 65
2 B d 2 90 SE 68
2 B d 3 90 SE 74 0.94
2 B d 4 90 SE 88 1.2
2 B d 5 90 SE 87 0.89
2 B d 6 90 SE 61 1.8
2 B e 1 90 SE 102 1.3
2 B e 2 90 SE 87 1.3
2 B e 3 90 SE 88
2 B e 4 90 SE 103 0.89
2 B e 5 90 SE 88 1.5
2 B e 6 90 SE 89
2 B e 7 90 SE 98
2 B e 8 90 SE 84 1.5
2 B e 9 90 SE 90
2 B e 10 90 SE 73 2.1
2 B e 11 90 SE 65 3.3
3 A a 1 0 Top 91
3 A a 2 0 Top 97
3 A b 1 0 Top 103
3 A b 2 0 Top 85
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
170
3 B a 1 >80 North 67 2.2
3 B a 2 >80 North 70 2.1
3 B a 3 >80 North 90 0.57
3 B a 4 >80 North 92 0.75
3 B a 5 >80 North 87 1.7
3 B b 1 >80 North 99 0.56
3 B b 2 >80 North 89 0.88
3 B b 3 >80 North 80 1.3
3 B b 4 >80 North 79 2.1
3 B b 5 >80 North 80 2.2
3 B b 6 >80 North 77 2.0
3 B c 1 >80 North 80 1.8
3 B c 2 >80 North 90 0.71
3 B c 3 >80 North 78 1.7
3 B c 4 >80 North 90 0.0
3 B c 5 >80 North 92 1.0
3 B c 6 >80 North 90 1.4
3 B c 7 >80 North 99
3 B c 8 >80 North 80 1.9
3 B d 1 >80 North 75 1.8
3 B d 2 >80 North 77 2.2
3 B d 3 >80 North 65 2.5
3 B d 4 >80 North 87 0.75
3 B d 5 >80 North 73 1.2
3 B d 6 >80 North 75 1.5
3 B d 7 >80 North 73 1.0
3 B d 8 >80 North 68 1.1
3 B e 1 >80 North 65 2.2
3 B e 2 >80 North 70 2.1
3 B e 3 >80 North 81 1.9
3 B e 4 >80 North 90 1.6
3 B e 5 >80 North 78 1.7
3 B e 6 >80 North 80 2.1
3 B e 7 >80 North 89 1.9
3 B e 8 >80 North 55 2.4
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
171
3 B f 1 >80 North 80 0.89
3 B f 2 >80 North 79 0.74
3 B f 3 >80 North 83 1.4
3 B f 4 >80 North 78 1.6
3 B f 5 >80 North 79 1.9
3 B f 6 >80 North 80 1.7
3 B f 7 >80 North 75 1.5
3 B f 8 >80 North 70 1.4
3 B f 9 >80 North 65 2.3
3 C a 1 0 Top 81 1.9
3 C a 2 0 Top 86 2.3
3 C a 3 0 Top 82 1.8
3 C b 1 0 Top 79 2.3
3 C b 2 0 Top 98 2.7
3 C b 3 0 Top 89
3 C b 4 0 Top 80 1.6
3 C b 5 0 Top 87
3 C b 6 0 Top 80 2.2
3 C c 1 0 Top 85 1.0
3 C c 2 0 Top 94 1.7
3 C c 3 0 Top 91 2.2
3 C c 4 0 Top 80 0.75
3 C c 5 0 Top 87 2.5
3 C c 6 0 Top 77 2.5
3 C d 1 0 Top 102
3 C d 2 0 Top 91 2.0
3 C d 3 0 Top 90 2.4
3 C d 4 0 Top 90 1.2
3 C d 5 0 Top 100 2.3
3 C d 6 0 Top 77 1.9
4 A a 1 90 West 72 1.3
4 A a 2 90 West 68 1.4
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
172
4 A a 3 90 West 78 1.2
4 A b 1 90 West 75 1.1
4 A b 2 90 West 65 1.0
4 A b 3 90 West 75
4 A b 4 90 West 66 1.3
4 A b 5 90 West 76 1.4
4 A d 1 90 West 70 1.8
4 A d 2 90 West 65 1.7
4 A d 3 90 West 72 1.9
4 A d 4 90 West 95 1.6
4 A d 5 90 West 105
4 A d 6 90 West 110
4 A e 1 90 West 83
4 A e 2 90 West 74 0.93
4 A e 3 90 West 75 1.0
4 A e 4 90 West 64 1.8
4 A e 5 90 West 56 2.0
4 A f 1 90 West 70 1.8
4 A f 2 90 West 74 0.94
4 A f 3 90 West 80 1.0
4 B a 1 >80 SE 90
4 B a 2 >80 SE 73
4 B a 3 >80 SE 71
4 B a 4 >80 SE 60 1.0
4 B a 5 >80 SE 63 0.82
4 B b 1 >80 SE 66 0.85
4 B b 2 >80 SE 65 1.0
4 B b 3 >80 SE 85 0.75
4 B b 4 >80 SE 94
4 B c 1 >80 SE 65
4 B c 2 >80 SE 88
4 B c 3 >80 SE 75
4 B c 4 >80 SE 74
4 B c 5 >80 SE 66
4 B c 6 >80 SE 59
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
173
4 B d 1 >80 SE 62
4 B d 2 >80 SE 69
4 B d 3 >80 SE 81
4 B d 4 >80 SE 100
4 B d 5 >80 SE 91
4 B e 1 >80 SE 70 0.73
4 B e 2 >80 SE 69 0.77
4 B e 3 >80 SE 85 0.62
4 B e 4 >80 SE 60 1.1
4 B e 5 >80 SE 74 0.77
4 B e 6 >80 SE 58 0.96
4 B f 1 >80 SE 62 0.89
4 B f 2 >80 SE 90
4 B f 3 >80 SE 94 0.82
4 B g 1 >80 SE 73
4 B g 2 >80 SE 54
4 B h 1 >80 SE 75 0.74
4 B h 2 >80 SE 68 0.78
4 B h 3 >80 SE 64 1.1
4 B h 4 >80 SE 61 1.1
4 B h 5 >80 SE 60 1.3
4 B i 1 >80 SE 59 0.83
4 B i 2 >80 SE 73 1.0
4 C a 1 underhang NE 100
4 C a 2 underhang NE 93
4 C a 3 underhang NE 90
4 C g 1 underhang NE 90
4 C h 1 >80 NE 91
4 C h 2 >80 NE 81
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
174
4 C i 1 >80 NE 91
4 C i 2 >80 NE 73 0.96
4 C i 3 >80 NE 75 0.83
4 C i 4 >80 NE 65
4 C i 5 >80 NE 71
4 C i 6 >80 NE 74
4 C j 1 >80 NE 95
4 C j 2 >80 NE 84 0.88
4 C j 3 >80 NE 75 1.0
4 C j 4 >80 NE 71 1.5
4 C k 1 >80 NE 80 0.72
4 C k 2 >80 NE 74 0.68
4 C k 3 >80 NE 74 0.88
4 C k 4 >80 NE 78
4 D a 1 0 Top 86
4 D a 2 0 Top 79
4 D a 3 0 Top 90
4 D a 4 0 Top 90
4 D a 5 0 Top 90 1.9
4 D a 6 0 Top 90
4 D b 1 0 Top 86 1.7
4 D b 2 0 Top 99
4 D b 3 0 Top 85
4 D b 4 0 Top 91
4 D b 5 0 Top 95 2.8
4 D c 1 0 Top 83
4 D c 2 0 Top 84
4 D c 3 0 Top 76
4 D c 4 0 Top 94 2.7
4 D c 5 0 Top 70 1.6
Boulder Sample Subsample Column Inclination
(degrees)
Facing
Direction
Growth
Angle
Thickness
(cm)
 
Table 3.2 continued
175
Table 4.1: Results of Abiotic Precipitation experiments. CHI decreases with increasing
angle. Beyond 45 degrees, no magnetic material bound to the carbonate on any slides.
Angle Sample
Cube
( g)
Cube +
sample
(g)
Sample
wt(g) Bulk CHI
Minus
Blank
Ave Minus Noise CHI/G
0 1 5.88 5.98 0.1 3.54E-05 3.95E-05 4.30242E-05 0.000430242
0 2 5.95 6.06 0.1 0.0000387 4.28E-05 4.63142E-05 0.000463142
15 1 5.87 5.87 0.1 2.20E-05 2.60E-05 2.95842E-05 0.000295842
15 2 5.83 5.83 0.1 0.00002487 2.89E-05 3.24842E-05 0.000324842
30 1 5.97 6.07 0.1 2.06E-05 2.46E-05 2.81742E-05 0.000281742
30 2 5.86 5.96 0.1 2.31E-05 2.71E-05 3.06642E-05 0.000306642
45 1 5.93 6.03 0.1 -5.04E-06 -9.79E-07 2.57816E-06 2.57816E-05
45 2 5.95 6.95 0.1 -5.08E-06 -1.02E-06 2.53416E-06 2.53416E-05
60 1 5.87 5.97 0.1 -6.97E-06 -2.91E-06 6.4516E-07 6.4516E-06
60 2 5.91 6.01 0.1 -6.77E-06 -2.71E-06 8.4916E-07 8.4916E-06
75 1 5.84 5.94 0.1 -7.38E-06 -3.32E-06 2.3716E-07 2.3716E-06
75 2 5.72 5.82 0.1 -7.50E-06 -3.44E-06 1.1916E-07 1.1916E-06
90 1 5.99 6.09 0.1 -0.000007098 -3.04E-06 5.1616E-07 5.1616E-06
90 2 5.84 5.94 0.1 -7.12226E-06 -3.07E-06 4.919E-07 0.000004919
Blanks
1 0 -0.000003969
2 15 -0.000004165
3 30 -0.000004178
4 45 -0.000004149
5 60 -0.000003988
6 75 -0.000003892
Average -4.05683E-06
Noise 1 -0.000003581
2 -0.000003554
3 -0.000003537
Average -3.55733E-06
TABLES - CHAPTER 4
176
Degree Sample
Cube
wt(g)
Sample
and
cube(g)
Sam-
ple (g) CHI (Bulk) Minus Blank Ave Minus Noise CHI/g
0 CH01a 5.762 6.23 0.468 0.0003767 0.00038075 0.000384536 0.000821657
0 CH0 1b 5.772 6.19 0.418 0.001261 0.00126505 0.001268836 0.003035492
0 CH0 1c 5.757 6.55 0.793 0.0004346 0.00043865 0.000442436 0.000557926
0 CH0 1d 5.789 6.08 0.291 0.0005083 0.00051235 0.000516136 0.001773662
0 CH0 2a 5.797 6.15 0.353 0.0001626 0.00016665 0.000170436 0.000482821
0 CH0 2b 5.782 6.12 0.338 0.000161 0.00016505 0.000168836 0.000499514
0 CH0 2c 5.776 6.6 0.824 0.0001256 0.00012965 0.000133436 0.000161937
0 CH0 2d 5.766 6.23 0.464 0.0001227 0.00012675 0.000130536 0.000281327
15 CH15a 5.771 6.11 0.339 0.0001227 0.00012675 0.000130536 0.000385061
15 CH15b 5.776 6.36 0.584 0.0001287 0.00013275 0.000136536 0.000233794
15 CH15c 5.795 6.37 0.575 0.0004764 0.00048045 0.000484236 0.000842149
15 CH15d 5.745 6.21 0.465 0.0001696 0.00017365 0.000177436 0.000381582
30 CH30a 5.748 6.15 0.402 0.0001191 0.00012315 0.000126936 0.00031576
30 CH30b 5.762 6.21 0.448 0.0001188 0.00012285 0.000126636 0.000282669
30 CH30c 5.801 6.2 0.399 0.0001051 0.00010915 0.000112936 0.000283047
30 CH30d 5.758 7.25 1.492 0.0004651 0.00046915 0.000472936 0.000316981
45 CH45 1a 5.763 6.14 0.377 0.0000745 7.85497E-05 8.23357E-05 0.000218397
45 CH45 1b 5.76 7.29 1.53 0.0003013 0.00030535 0.000309136 0.000202049
45 CH45 1c 5.774 6.14 0.366 0.0000918 9.58497E-05 9.96357E-05 0.000272229
45 CH45 1d 5.786 6.26 0.474 0.0001117 0.00011575 0.000119536 0.000252185
45 CH45 2a 5.773 6.12 0.347 0.00006987 7.39197E-05 7.77057E-05 0.000223936
45 CH45 2b 5.789 6.42 0.631 0.0007436 0.00074765 0.000751436 0.001190865
45 CH45 2c 5.781 6.13 0.349 0.0001877 0.00019175 0.000195536 0.000560274
45 CH45 2d 5.789 6.17 0.381 0.00006653 7.05797E-05 7.43657E-05 0.000195186
60 CH60a 5.789 5.94 0.151 0.00002616 3.02097E-05 3.39957E-05 0.000225137
60 CH60b 5.738 6.06 0.322 0.00004777 5.18197E-05 5.56057E-05 0.000172689
60 CH60c 5.749 6.01 0.261 0.00004491 4.89597E-05 5.27457E-05 0.000202091
60 CH60d 5.743 6.46 0.717 0.0001879 0.00019195 0.000195736 0.000272993
Table 4.2: Results of Biofilm Experiments
177
75 CH75 1a 5.76 5.89 0.13 0.0000231 2.71497E-05 3.09357E-05 0.000237967
75 CH75 1b 5.752 5.98 0.228 0.00002209 2.61397E-05 2.99257E-05 0.000131253
75 CH75 1c 5.733 6.07 0.337 0.0000368 4.08497E-05 4.46357E-05 0.00013245
75 CH75 1d 5.77 6.09 0.32 0.00003099 3.50397E-05 3.88257E-05 0.00012133
75 CH75 2a 5.751 5.89 0.139 0.00003792 4.19697E-05 4.57557E-05 0.000329178
75 CH75 2b 5.788 5.97 0.182 0.00003684 4.08897E-05 4.46757E-05 0.000245471
75 CH75 2c 5.771 5.96 0.189 0.00004262 4.66697E-05 5.04557E-05 0.000266961
75 CH75 2d 5.777 6.04 0.263 0.00004896 5.30097E-05 5.67957E-05 0.000215953
90 CH90a 5.749 5.92 0.171 0.0000138 1.78497E-05 2.16357E-05 0.000126525
90 CH90b 5.767 6.04 0.273 0.00001944 2.34897E-05 2.72757E-05 9.9911E-05
90 CH90c 5.779 5.96 0.181 0.00001341 1.74597E-05 2.12457E-05 0.00011738
90 CH90d 5.794 5.94 0.146 0.000007887 1.19367E-05 1.57227E-05 0.00010769
Seawa-
ter CHSW 5.85 6.16 0.31 -0.00000395 9.97E-08 3.8857E-06 1.25345E-05
CHCa 5.788 6.13 0.342 0.00001065 1.46997E-05 1.84857E-05 5.40518E-05
Con-
trol CHCb 5.736 5.85 0.114 0.000002444 6.4937E-06 1.02797E-05 9.01728E-05
CHCc 5.787 6.16 0.373 0.00001405 1.80997E-05 2.18857E-05 5.86748E-05
CHCd 5.772 5.99 0.218 0.000005772 9.8217E-06 1.36077E-05 6.24206E-05
Blanks 1 5.87 5.87 -0.000004576
2 5.87 5.87 -0.000003993
3 5.87 5.87 -0.00000395
4 5.87 5.87 -0.000003912
5 5.87 5.87 -0.000003916
6 5.87 5.87 -0.000003951
Aver-
age -4.04967E-06
Noise 1 -0.000003847
2 -0.000003746
3 -0.000003765
Aver-
age -0.000003786
Table 4.2 Continued
Degree Sample
Cube
wt(g)
Sample
and
cube(g)
Sam-
ple (g) CHI (Bulk) Minus Blank Ave Minus Noise CHI/g
178
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000

Walker
Lake Side WL_4_1_A 5.78 5.79 0.01 -0.000006987 -2.9373E-06 8.487E-07 0.00008487 0.8487
Western
Nevada WL_4_1_B 5.76 5.77 0.01 -0.000007102 -3.0523E-06 7.337E-07 0.00007337 0.7337
Holocene WL_4_1_C 5.78 5.79 0.01 -0.000007805 -3.7553E-06 3.07E-08 3.07E-06 0.0307
WL_4_1_D 5.76 5.77 0.01 -0.000007783 -3.7333E-06 5.27E-08 5.27E-06 0.0527
WL_4_1_E 5.77 5.78 0.01 -0.000007858 -3.8083E-06 -2.23E-08 -2.23E-06 -0.0223
WL_4_1_F 5.76 5.77 0.01 -0.000007677 -3.6273E-06 1.587E-07 1.587E-05 0.1587

WL_4_2_A 5.85 5.86 0.01 -0.000006909 -2.8593E-06 9.267E-07 0.00009267 0.9267
WL_4_2_B 5.96 5.97 0.01 -0.000006803 -2.7533E-06 1.0327E-06 0.00010327 1.0327
WL_4_2_C 5.99 6 0.01 -0.000007855 -3.8053E-06 -1.93E-08 -0.00000193 -0.0193
WL_4_2_D 5.92 5.93 0.01 -0.000007605 -3.5553E-06 2.307E-07 0.00002307 0.2307
WL_4_2_E 5.93 5.94 0.01 -0.000007902 -3.8523E-06 -6.63E-08 -6.63E-06 -0.0663
WL_4_2_F 5.97 5.98 0.01 -0.000007802 -3.7523E-06 3.37E-08 3.37E-06 0.0337

WL_4_3_A 5.93 5.94 0.01 -0.000006836 -2.7863E-06 9.997E-07 0.00009997 0.9997
WL_4_3_B 5.87 5.88 0.01 -0.000007862 -3.8123E-06 -2.63E-08 -2.63E-06 -0.0263
WL_4_3_C 5.75 5.76 0.01 -0.000007828 -3.7783E-06 7.7E-09 7.7E-07 0.0077
WL_4_3_D 5.99 6 0.01 -0.000007888 -3.8383E-06 -5.23E-08 -5.23E-06 -0.0523
WL_4_3_E 5.89 5.9 0.01 -0.000007798 -3.7483E-06 3.77E-08 3.77E-06 0.0377
WL_4_3_F 5.97 5.98 0.01 -0.000007785 -3.7353E-06 5.07E-08 5.07E-06 0.0507
Table 4.3: Results from Natural Samples
179

Blank 1 -0.000004576
Blank 2 -0.000003993
Blank 3 -0.00000395
Blank 4 -0.000003912
Blank 5 -0.000003916
Blank 6 -0.000003951
Blank Aver-
age -4.04967E-06
Noise Aver-
age -0.000003786
Johnnie
Formation JF_1_A 5.99 6 0.01 -0.000003824 9.133E-08 3.64933E-06 0.000364933 3.64933
Death Val-
ley JF_1_B 5.91 5.92 0.01 -0.000003962 -4.667E-08 3.51133E-06 0.000351133 3.51133
Neopro-
terozoic JF_1_C 5.86 5.87 0.01 -0.000003923 -7.67E-09 3.55033E-06 0.000355033 3.55033
JF_1_D 5.84 5.85 0.01 -0.000004397 -4.8167E-07 3.07633E-06 0.000307633 3.07633
JF_1_E 5.85 5.86 0.01 -0.000003974 -5.867E-08 3.49933E-06 0.000349933 3.49933
JF_1_F 5.97 5.98 0.01 -0.00000407 -1.5467E-07 3.40333E-06 0.000340333 3.40333

Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
180
JF_2_A 5.86 5.87 0.01 -0.000001482 2.43333E-06 5.99133E-06 0.000599133 5.99133
JF_2_B 5.96 5.97 0.01 -0.000003804 1.1133E-07 3.66933E-06 0.000366933 3.66933
JF_2_C 5.97 5.98 0.01 -0.000004039 -1.2367E-07 3.43433E-06 0.000343433 3.43433
JF_2_D 5.9 5.91 0.01 -0.000003958 -4.267E-08 3.51533E-06 0.000351533 3.51533
JF_2_E 5.9 5.91 0.01 -0.000003584 3.3133E-07 3.88933E-06 0.000388933 3.88933
JF_2_F 5.84 5.85 0.01 -0.000003898 1.733E-08 3.57533E-06 0.000357533 3.57533

JF_3_A 5.96 5.96 0 -0.000003996 -8.067E-08 3.47733E-06 0.000347733 3.47733
JF_3_B 5.85 5.85 0 -0.000003912 3.33E-09 3.56133E-06 0.000356133 3.56133
JF_3_C 5.98 5.99 0.01 -0.000004001 -8.567E-08 3.47233E-06 0.000347233 3.47233
JF_3_D 5.86 5.86 0 -0.000003935 -1.967E-08 3.53833E-06 0.000353833 3.53833
JF_3_E 5.72 5.73 0.01 -0.000003837 7.833E-08 3.63633E-06 0.000363633 3.63633
JF_3_F 5.97 5.97 0 -0.000003995 -7.967E-08 3.47833E-06 0.000347833 3.47833

JFI1 5.86 5.87 0.01 -0.000003917 -1.67E-09 3.55633E-06 0.000355633 3.55633
JFI2 5.99 6 0.01 -0.000004055 -1.3967E-07 3.41833E-06 0.000341833 3.41833
JFI3 5.86 5.87 0.01 -0.000003925 -9.67E-09 3.54833E-06 0.000354833 3.54833

Noise Aver-
age -0.000003558
Blank1 -0.000003928
Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
181
Blank2 -0.000003899
Blank 3 -0.000003919
Blank Aver-
age -3.91533E-06
Tempiute
Vein TV_1_A 5.93 5.95 0.02 -0.000004779 -0.000003277 5.8E-08 2.9E-06 0.029
Central
Nevada TV_1_B 5.91 5.93 0.02 -0.000004699 -0.000003197 0.000000138 6.9E-06 0.069
Devonian
host rock TV_1_C 5.97 5.99 0.02 -0.000004769 -0.000003267 6.8E-08 3.4E-06 0.034

TV_2_A 5.96 5.98 0.02 -0.000004784 -0.000003282 5.3E-08 2.65E-06 0.0265
TV_2_B 5.85 5.86 0.01 -0.000004796 -0.000003294 4.1E-08 4.1E-06 0.041
TV_2_C 5.83 5.84 0.01 -0.000004776 -0.000003274 6.1E-08 6.1E-06 0.061

TV_3_A 5.97 5.99 0.02 -0.000004788 -0.000003286 4.9E-08 2.45E-06 0.0245
TV_3_B 5.97 5.98 0.01 -0.000004777 -0.000003275 6E-08 6E-06 0.06
TV_3_C 5.92 5.93 0.01 -0.000004799 -0.000003297 3.8E-08 3.8E-06 0.038

Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
182
Noise Aver-
age -0.0000015
Blank 1 -0.000001504
Blank 2 -0.000001502
Blank 3 -0.000001502
Blank Aver-
age 0.000003335
Hot spring YA_1_A 5.84 5.85 0.01 -0.000003929 -0.000000361 3.63167E-06 0.000363167 3.63167
Yellowstone
NP YA_1_B 5.85 5.86 0.01 -0.000003871 -0.000000303 3.68967E-06 0.000368967 3.68967
Modern YA_1_C 5.95 5.96 0.01 -0.000004042 -0.000000474 3.51867E-06 0.000351867 3.51867
YA_1_D 5.9 6 0.1 -0.000003986 -0.000000418 3.57467E-06 0.000357467 3.57467
YA_1_E 5.98 5.99 0.01 -0.000003472 0.000000096 4.08867E-06 0.000408867 4.08867
YA_1_F 5.97 5.98 0.01 -0.000004046 -0.000000478 3.51467E-06 0.000351467 3.51467

YA_2_A 5.95 5.96 0.01 -0.000004107 -0.000000539 3.45367E-06 0.000345367 3.45367
YA_2_B 5.87 5.88 0.01 -0.000003952 -0.000000384 3.60867E-06 0.000360867 3.60867
YA_2_C 5.88 5.89 0.01 -0.000004006 -0.000000438 3.55467E-06 0.000355467 3.55467
YA_2_D 5.9 5.91 0.01 -0.000004004 -0.000000436 3.55667E-06 0.000355667 3.55667
YA_2_E 5.88 5.89 0.01 -0.000004002 -0.000000434 3.55867E-06 0.000355867 3.55867
YA_2_F 5.86 5.87 0.01 -0.000003976 -0.000000408 3.58467E-06 0.000358467 3.58467
Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
183

YA_3_A 6 6.01 0.01 -0.000004068 -0.0000005 3.49267E-06 0.000349267 3.49267
YA_3_B 5.83 5.84 0.01 -0.000003991 -0.000000423 3.56967E-06 0.000356967 3.56967
YA_3_C 5.93 5.94 0.01 -0.000003994 -0.000000426 3.56667E-06 0.000356667 3.56667
YA_3_D 5.95 5.96 0.01 -0.000004052 -0.000000484 3.50867E-06 0.000350867 3.50867
YA_3_E 5.87 5.88 0.01 -0.00000395 -0.000000382 3.61067E-06 0.000361067 3.61067
YA_3_F 5.97 5.98 0.01 -0.000004092 -0.000000524 3.46867E-06 0.000346867 3.46867

Noise Aver-
age -0.000003568
Blank 1 -0.000003992
Blank 2 -0.000003982
Blank 3 -0.000004004
Blank Aver-
age -3.99267E-06
Green
River GR_1_A 5.74 5.75 0.01 -0.0000037 3.497E-07 4.1357E-06 0.00041357 4.1357
Wyoming GR_1_B 5.76 5.76 0 -0.000003822 2.277E-07 4.0137E-06 0.00040137 4.0137
Eocene GR_1_C 5.8 5.81 0.01 -0.000003906 1.437E-07 3.9297E-06 0.00039297 3.9297
GR_1_D 5.8 5.81 0.01 -0.00000395 9.97E-08 3.8857E-06 0.00038857 3.8857
GR_1_E 5.78 5.79 0.01 -0.000003811 2.387E-07 4.0247E-06 0.00040247 4.0247
GR_1_F 5.79 5.8 0.01 -0.000003836 2.137E-07 3.9997E-06 0.00039997 3.9997
Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
184
GR_2_A 5.78 5.79 0.01 -0.000003949 1.007E-07 3.8867E-06 0.00038867 3.8867
GR_2_B 5.74 5.74 0 -0.000003712 3.377E-07 4.1237E-06 0.00041237 4.1237
GR_2_C 5.79 5.8 0.01 -0.000003847 2.027E-07 3.9887E-06 0.00039887 3.9887
GR_2_D 5.74 5.74 0 -0.000003777 2.727E-07 4.0587E-06 0.00040587 4.0587
GR_2_E 5.74 5.74 0 -0.000003808 2.417E-07 4.0277E-06 0.00040277 4.0277
GR_2_F 6 6.01 0.01 -0.000004029 2.07E-08 3.8067E-06 0.00038067 3.8067

GR_3_A 5.89 5.9 0.01 -0.00000394 1.097E-07 3.8957E-06 0.00038957 3.8957
GR_3_B 5.98 5.99 0.01 -0.000002233 1.8167E-06 5.6027E-06 0.00056027 5.6027
GR_3_C 5.91 5.92 0.01 -0.000003974 7.57E-08 3.8617E-06 0.00038617 3.8617
GR_3_D 5.89 5.9 0.01 -0.00000397 7.97E-08 3.8657E-06 0.00038657 3.8657
GR_3_E 5.98 6 0.02 -0.000004022 2.77E-08 3.8137E-06 0.00038137 3.8137
GR_3_F 6.03 6.04 0.01 -0.000003938 1.117E-07 3.8977E-06 0.00038977 3.8977

Blank 1 -0.000004576
Blank 2 -0.000003993
Blank 3 -0.00000395
Blank 4 -0.000003912
Blank 5 -0.000003916
Blank 6 -0.000003951
Blank Aver-
age -4.04967E-06
Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
185
Noise Aver-
age -0.000003786
Furnace
Creek FC 1A 5.85 5.86 0.01 -0.000001261 0.000000241 0.000003576 0.0003576 3.576
Death Val-
ley FC 1B 5.89 5.9 0.01 -0.000001395 0.000000107 0.000003442 0.0003442 3.442
Pliocene FC1C 5.91 5.92 0.01 -3.821E-07 1.1199E-06 4.4549E-06 0.00044549 4.4549

FC2A 5.92 5.93 0.01 -0.000001496 6E-09 0.000003341 0.0003341 3.341
FC2B 5.84 5.86 0.02 -0.000001278 0.000000224 0.000003559 0.00017795 1.7795
FC2C 5.99 6 0.01 -0.000001578 -0.000000076 0.000003259 0.0003259 3.259

FC3A 5.95 5.96 0.01 -0.000001638 -0.000000136 0.000003199 0.0003199 3.199
FC3B 5.94 5.95 0.01 -0.000001522 -0.00000002 0.000003315 0.0003315 3.315

Blank 1 -0.0000015
Blank 2 -0.000001504
Blank 3 -0.000001502
Average -0.000001502
Noise Aver-
age -0.000003335
Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
186
Table 4.3 Continued
Location Sample
Cube
wt
(g)
Sam-
ple +
Cube
(g)
Sam-
ple wt
(g) Mag Sus (bulk)
Minus Blank
Ave
Minus Noise
Ave Mag sus/g
Mag sus/g
*10000
Boar’s Tusk
Green
River BT1 3.96 3.99 0.03 -0.00000289 7.7E-08 0.000003466 0.000115533 1.155333333
Wyoming BT2 3.96 4 0.04 -0.00000275 0.000000217 0.000003606 9.015E-05 0.9015
Eocene BT3 3.94 3.98 0.04 -0.000002998 -3.1E-08 0.000003358 8.395E-05 0.8395
BT4 3.98 4.03 0.05 -0.000002955 1.2E-08 0.000003401 6.802E-05 0.6802
BT5 3.99 4.06 0.07 -0.000002663 0.000000304 0.000003693 5.27571E-05 0.527571429
BT6 3.96 4.02 0.06 -0.000002663 0.000000304 0.000003693 6.155E-05 0.6155
Walker
Lake Top
Nevada WLT1 3.97 4.01 0.04 -0.000002072 0.000000895 0.000004284 0.0001071 1.071
Holocene WLT2 4.01 4.06 0.05 -0.00000284 0.000000127 0.000003516 7.032E-05 0.7032
WLT3 3.99 4.04 0.05 -0.000002686 0.000000281 0.00000367 7.34E-05 0.734
WLT4 3.94 3.97 0.03 -0.000002125 0.000000842 0.000004231 0.000141033 1.410333333
Blanks -
WLT and
Boar’s Tusk
Blank 1 -0.000002981
Blank 2 -0.000002975
Blankd 3 -0.000002945
Averaie -0.000002967
Noise Aver-
age - WLT;
BT -0.000003389

Abstract (if available)

Abstract Stromatolites are commonly defined as laminated organo-sedimentary structures built by the trapping and binding and/or precipitation of minerals via microbial processes. They are thought to represent evidence of some of the oldest life on Earth, and are thus popular targets for geobiologic studies. Despite their high profile in the geobiologic community, the processes that control the different aspects of stromatolite morphology (i.e. form, growth rate, texture, and lamina formation) are poorly understood. If stromatolites are going to be held up as proof of the earliest life on Earth, some of the confusion involved in their study must be resolved. For my thesis, I use Holocene lacustrine stromatolites to address common assumptions about lamination, growth rate, and growth form. Additionally, I report a new stromatolite biosignature based on the detrital magnetic mineral component present in nearly all sedimentary rocks. ❧ Lamination in stromatolites is commonly interpreted to record the periodic response of a microbial community to daily, seasonal, or perhaps yearly environmental forcing. Here, high resolution 14C dating of Holocene stromatolites from Walker Lake, Nevada is used to construct a record of lamination rate over the course of accretion. Laminations formed at a rate of 5.6 ± 1.6 yrs/lamination at the base of the structure, 2.8 ± 1.9 yrs/lamination and 1.6 ± 0.9 yrs/lamination in the middle, and 4.5± 0.8 yrs/lamination at the top. Thus, much of the stromatolite grew in response to a forcing with a 4-6 year periodicity. This indicates that lamination formation is likely more closely related to local climate factors (e.g., ENSO) for these stromatolites than microbial metabolism. These results show that generalizations regarding the influence of microbial mats on stromatolite formation should be used with extreme caution. ❧ Typically, stromatolites have a domed morphology that is commonly taken as an indication of microbial phototropism (growth towards incident light). The stromatolites of Walker Lake can be found on all sides of boulders, making them an ideal test case for this commonly held assumption. Angle of growth measurements on over 300 stromatolites demonstrate that the structures grew nearly perpendicular to their growth surface, regardless of the initial angle of inclination. Furthermore, there are no significant differences in the distribution of growth angles between north, south, east or west facing samples. This evidence suggests that phototactic growth towards incident light was not a dominant factor in controlling stromatolite morphogenesis, and that the presence of a domed macrostructure cannot be taken as evidence of photosynthetic life. ❧ Given that the results of the lamination and growth direction studies contradict the prevailing theories of stromatolite morphogenesis, a definitive way to test the biogenicity of stromatolites both ancient and modern is still needed. Here, I report a new biosignature that can be used to test for the presence or absence of a biofilm at the time of formation of a stromatolite. It is hypothesized that the distribution of detrital magnetic grains within a putative microbialite will depend on the presence or absence of “sticky” microbial mats/biofilms. Magnetic grains in an abiotic structure should obey the laws of gravity/angle of repose (swept off peaks, concentrated in lows), while magnetic grains adhered to a biofilm will seem to “defy” the laws of gravity. To test this hypothesis, parallel laboratory experiments were undertaken in which magnetic particles were introduced into a tank that contained either cyanobacterial mats or glass slides (on which carbonate had abiotically precipitated) inclined at a variety of angles. Using the results of the laboratory experiments, a variety of stromatolites of known and unknown biogenicities, ranging in age from near-modern to ancient, were analyzed. The results of these experiments verify the hypothesis that magnetic susceptibility can be used as a biosignature. The sum total of this study sheds light on the processes that form stromatolites, and leads us a step further towards the unambiguous understanding of the biogenicity of stromatolties.

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Asset Metadata

Creator Petryshyn, Victoria A. (author)

Core Title Stromatolites in the ancient and modern: new methods for solving old problems

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publication Date 05/23/2013

Defense Date 05/23/2013

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

Tag magnetic susceptibility,microbialite,OAI-PMH Harvest,stromatolite,Walker Lake

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Corsetti, Frank A. (committee chair), Berelson, William M. (committee member), Bottjer, David J. (committee member), Capone, Douglas G. (committee member), Lund, Steven P. (committee member)

Creator Email v.petryshyn@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-257740

Unique identifier UC11293233

Identifier etd-PetryshynV-1698.pdf (filename),usctheses-c3-257740 (legacy record id)

Legacy Identifier etd-PetryshynV-1698.pdf

Dmrecord 257740

Document Type Dissertation

Format application/pdf (imt)

Rights Petryshyn, Victoria A.

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 a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA