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The Triassic organic-rich flat clam biotope: A synthesis of paleoecological and climate modeling analyses

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The Triassic organic-rich flat clam biotope: A synthesis of paleoecological and climate modeling analyses

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Content THE TRIASSIC ORGANIC-RICH FLAT CLAM BIOTOPE: A SYNTHESIS OF
PALEOECOLOGICAL AND CLIMATE MODELING ANALYSES
by
Tran Thoai Huynh
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(EARTH SCIENCES)
August 2003
Copyright 2003 Tran Thoai Huynh
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UMI Number: 1417925
Copyright 2003 by
Huynh, Tran Thoai
All rights reserved.
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UNIVERSITY OF SOUTHERN CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELES, CALIFORNIA 90089-1695
This thesis, written by
TRAN THOAI HUYNH
under the direction of he r thesis committee, and
approved by all its members, has been presented to and
accepted by the Director of Graduate and Professional
Programs, in partial fulfillment of the requirements for the
degree of
MASTER OF SCIENCE
Director
Date M A Y 1. 2003
Thesis Committee
Chair
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ACKNOWLEDGEMENTS
This project would not have been completed without the encouragement,
support, advice, and patience of some very key people.
First and foremost are my thesis committee members. I am truly indebted to
Dr. Dave Bottjer, whose unflagging encouragement and guidance - despite my many
detours - have helped me find my way through academic life and to develop into a
better scientist. It was he who first took me into the PaleoLab and fostered my love
for the “critters.” I will always be grateful for the intellectual freedom he has
provided me. To Dr. Chris Poulsen I owe great thanks for introducing me to the
world of climate modeling. He has not only offered me another tool with which to
view the past, but has also given me space in his lab where I can further expand my
horizons as a geologist. And last, but not least, thank you to Dr. Frank Corsetti for
arming me in class, in lab, and in the field with the techniques that no geologist
should be without. This thesis grew out of the many insights, comments, and
discussions with my committee members.
During the course of this project, I also benefited greatly from time spent
with various Earth Science faculty at USC, including Bob Douglas, Donn Gorsline,
A 1 Fischer, Doug Hammond, Will Berelson, and Lowell Stott, who created an
environment that nurtured intellectual growth and whose expertise helped me to
formulate some of the ideas for this project. Without the administrative staff - Cindy
Waite, Vardui Ter-Simonian, John Yu, and John McRaney - to assist me against the
intrusions of the bureaucratic details, I would not have been able to focus on the
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science. Thank you to my field assistants, Nicole Fraser, Pat McDonald, Jennifer
Pien, Seth Finnegan, and Bob Staines, for putting up with my tyrannical ways while
in the field. Without their eyes, strength, and patience, I would not have rocks to
study.
An old Chinese proverb says, "No river can return to its source, yet all rivers
must have a beginning.” And so finally, to my family and close friends I owe much
gratitude for they are my source. It is their enthusiasm, support, and constancy in my
life that has provided me with the self-confidence to pursue and overcome the
obstacles of academic life. To my parents, who have been my role models from the
very beginning, I owe the greatest acknowledgement for their sacrifices and infinite
support. I am likewise extremely grateful to Oliver Dully for his limitless love, deep
understanding of me, and countless hours spent listening to my ideas. To Nicole
Fraser, who guided me in all aspects of this project (including inspiration, field work,
data analyses, and writing), I will always be beholden. But even greater is my debt
to her for the immense support and unfailing faith she has given me through the
years.
This research was supported by grants from the American Association of
Petroleum Geologists, the Geological Society of America, the Sigma-Xi Foundation,
the University of Southern California Friends of the Wrigley Institute, and the
University of Southern California Department of Earth Sciences.
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iv
TABLE OF CONTENTS
Acknowledgements .................................................................................................ii
List of Figures........................................................................ vi
Abstract ................................................................................................................. ix
CHAPTER 1: INTRODUCTION. ..................................................................... 1
1.1 The Mesozoic Marine Radiation................................................................1
1.2 Mesozoic Flat Clams................................................................................... 3
1.3 Mesozoic Setting....................................................................................... 6
1.4 Overview of this Study......................................................................... 8
CHAPTER 2: PALEOECOLOGY OF DAONELLA BIVALVES FROM THE
FOSSIL HILL MEMBER................................................................................ 10
2.1 Introduction...................................................................................................10
2.2 Phylogeny..................................................................................................... 14
2.3 Geologic Setting........................................................................................... 16
2.4 Paleoecological Analysis.............................................................................23
2.5 Results.......................................................................................................... 24
2.5.1 Morphology......................................................... 24
2.5.2 Preservation.................................................................................. 26
2.5.3 Fossil Association........................................................................ 31
2.6 Discussion............................. 33
2.7 Conclusion....................................................................................................42
CHAPTER 3: SENSITIVITY OF LATE TRIASSIC CLIMATE TO CHANGES
IN ATMOSPHERIC C 02..................................................... 44
3.1 Introduction..................................................................................................44
3.2 The Model and Simulations........................................................................48
3.3 Results.......................................................................................................... 52
3.3.1 Global Warming............................................ 52
3.3.2 Sensitivity to C 0 2 Changes.........................................................61
3.4 Discussion.......................................... 67
3.4.1 Model Comparisons .................................................... 67
3.4.2 C 02 Sensitivity.............................................................................72
3.4.3 Implications for Biota...................................................... 73
3.4.4 Model Limitations and Future Studies ...........................75
3.5 Conclusion ........................................................................................ 76
CHAPTER 4: DISTRIBUTION OF LATE TRIASSIC HALOBDD
ACCUMULATIONS..................................................................................................... 79
4.1 Introduction............................................................. 79
4.2 Methodology............... 84
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4.3 Results...........................................................................................................86
4.3.1 Environmental Tolerance.............................................................86
4.3.2 Bivalve Accumulations and Upwelling Zones............................87
4.4 Discussion..................................................................................... 91
4.5 Conclusions.................................................................................................. 95
CHAPTER 5: CONCLUSIONS................................... 96
5.1 Summary of Results..................................... 96
5.2 Future Research............... 100
BIBLIOGRAPHY.................................................................. 102
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vi
LIST OF FIGURES
Figure 1.1. Marine diversity curve, modified from Erwin (1993)...............................1
Figure 1.2. Global marine familial diversity, modified from Sepkoski (1992)..........2
Figure 1.3. Summary of ranges and modes of life of the more common lower
dysaerobic taxa of the Phanerozoic......................................................... 3
Figure 1.4. Triassic paleogeographic reconstruction, modified from
Scotese (2000).................................................... ....7
Figure 2.1. Proposed life mode for flat clams..................... 11
Figure 2.2. Daonella concentrations in the Middle Triassic Fossil Hill Member,
exposed in north central Nevada....................................................................................14
Figure 2.3. Tectonic reconstruction of part of Western Pangea during Triassic
times, modified from Speed (1978).............................................................................. 17
Figure 2.4. Strati graphic nomenclature of relevant lithologic units outcropped in
the Southern Humboldt Range and the Augusta Mountains........................................18
Figure 2.5. Measured section on the south side of Fossil Hill, southern Humboldt
Range, central Nevada.................................................................................................... 20
Figure 2.6. Facies of the Fossil Hill section.................................................................21
Figure 2.7. Photomicrographs of A) pyrite grains and B) micro-grading in black
micritic limestone facies of the Fossil Hill Member........................................... 22
Figure 2.8. Morphology of halobiid bivalves.............................................................. 25
Figure 2.9. Daonella in the Fossil Hill Member......................................................... 27
Figure 2.10. A) Articulated Daonella preserved in layer above rusty micritic
limestone layer. B) Compacted shell layer in black micritic limestone facies 28
Figure 2.11. Size frequency of shells in laminated black micritic limestone facies,
collected at 1, 4, 7, and 10m......................................... 29
Figure 2.12. Size frequency of shells in siltstones (2 and 9m) and silty limestone
facies (6 and 11m)...........................................................................................................30
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vii
Figure 2.13. Associated fossils in the Fossil Hill Member.........................................32
Figure 2.14. Model for oxygen-restricted faunas based on the dynamically unstable
dysoxic environments of some modem shelves, modified from Wignall (1994).... 38
Figure 2.15. Shell concentrations grouped into categories according to accumulation
histories and time scales................................................................................................. 40
Figure 3.1. Schematic diagram of a coupled general circulation model (GCM) 45
Figure 3.2. Paleogeographic reconstruction of the Late Triassic (216 Ma), modified
from Scotese (1994)........................................................................................................49
Figure 3.3. Zonally averaged mean annual surface air temperature for Late Triassic
simulations (black) and present day (PD)..................................................................... 53
Figure 3.4. Mean annual surface air temperature (A-C) and temperature range
(D-F) of Late Triassic simulations................................................................................. 54
Figure 3.5. Annual average sea surface temperature (A-C) and salinity (D-F) for
the top 50m of the ocean of the Late Triassic simulations.......................................... 55
Figure 3.6. Zonally averaged mean annual sea surface temperature (A), salinity (B),
east-west wind stress (C), and north-south wind stress of Late Triassic simulations
(4X, 6X, 8X) compared to present day ran (PD)..........................................................56
Figure 3.7. Mean annual surface air temperature in July (A-C) and January (D-F)
of Late Triassic simulations........................................................................................... 57
Figure 3.8. Zonally average mean annual surface air temperature (A), greenhouse
forcing (B), precipitation minus evaporation (C), and specific humidity (D) of Late
Triassic simulations (4X, 6X, 8X) compared to present day run (PD)..................... 58
Figure 3.9. Mean annual precipitation minus evaporation rate (A-C) and total
precipitation rate (D-F) of Late Triassic simulations................................................... 59
Figure 3.10. Meridional heat transport by the oceans of Late Triassic
simulations.......................................................................................................................61
Figure 3.11. Difference in air surface temperature of Late Triassic runs:
(A) 6X minus 4X, (B) 8X minus 6X, and (C) 8X minus 4X................................ 63
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Figure 3.12. Differences in surface air temperature (solid black line) and greenhouse
forcing (dashed line) between Late Triassic simulations: (A) 6X minus 4X, (B) 8X
minus 6X, and (C) 8X minus 4X. Units for greenhouse forcing are W/m“.............. 64
Figure 3.13. Meridional overturning in the ocean of Late Triassic simulations......65
Figure 3.14. Difference in sea surface temperature (left) and sea surface salinity
(right) of Late Triassic runs: (A,D) 6X minus 4X, (B,E) 8X minus 6X, and (C,F) 8X
minus 4X......................................................... 67
Figure 4.1. Two bivalve genera from the family Halobiidae, Daonella and
Halobia, and their strati graphic range............... 80
Figure 4.2. Model for halobiid bivalve accumulation, modified from Parrish
etal. (2001)......................................................................................................................81
Figure 4.3. Cretaceous organic carbon burial history, sea level history, and oceanic
anoxic events in the Western Interior Basin, U.S.........................................................82
Figure 4.4. Distribution of halobiid bivalves during the Norian stage, based on
Tozer (1984) and McRoberts (1997)............................................................................ 85
Figure 4.5. Average sea temperature and salinity for the top 175m of the ocean... 86
Figure 4.6. Average sea temperature (A-D) and salinity (E-H) of four depth
levels of the ocean from top to bottom: 0-25m, 25-75m, 75-125m, and 125-175m
depth.......................................................................................................... ... .................88
Figure 4.7. Average vertical velocity of the top 175m of the ocean for 4xCC> 2
simulation.........................................................................................................................89
Figure 4.8. Average vertical velocity of the top 175m of the ocean for 6xCC> 2
simulation .......................................................................................................... 90
Figure 4.9. Average vertical velocity of the top 175m of the ocean for 8XCO2
simulation.........................................................................................................................90
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ix
ABSTRACT
Bivalves of the ‘flat clam’ facies are unique to Mesozoic strata. These thin-
shelled, cosmopolitan bivalves are adapted to low-oxygenated shelf settings that
were once widespread and persistent, but no longer prevalent today. This study uses
Daonella accumulations preserved in Nevada to define characteristics of a Triassic
oxygen-deficient biotope and conditions under which it can be preserved. The
paleoceanographic setting that enabled emergence, radiation, and extinction of low
oxygen-tolerant taxa is explored using climate model simulations. Results show that
the Triassic flat clam biotope consists of epibenthic, opportunistic bivalves capable
of rapidly colonizing dysoxic environments. Dense shell concentrations represent
multiple colonization events and resulted from preservation under suboxic bottom
water conditions and binding by bacterial mats. Triassic flat clams existed when the
oceans were significantly warmer and less saline than today. Constriction of their
preferred habitats due to atmospheric CO2 increases may have led to their demise
during the Late Triassic.
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CHAPTER 1. INTRODUCTION
1.1. THE MESOZOIC MARINE RADIATION
The end-Permian mass extinction caused one of the most complete
reorganizations of marine communities in the history of metazoan life. Many groups
that filled niches left vacant after this mass extinction persisted throughout the
Mesozoic and into the present day. Biotic recovery from the end-Permian mass
extinction, as measured by paleoecological and global diversity data, lasted
throughout the Early Triassic, with true expansion into the Mesozoic marine
radiation only beginning in the Middle Triassic (Erwin, 1993; Sepkoski, 1992;
Hallam and Wignall, 1997). (Figure 1.1) Thus, the first stage of the Middle Triassic
marks the initiation of the Mesozoic Marine Radiation (MMR). It has been
suggested that at this time normal benthic conditions returned and various groups
were re-established following the end-Permian mass extinction, so aptly called the
‘Paleozoic Nemesis’ (Hallam and Wignall, 1997). For example, diverse groups, in-
EXPANSION ' 4 -*— LA G -*T*REBOUND*-
V
V .
Early ffliskte //
PERMIAN TRIASSIC /JURASSIC
Figure 1.1. Marine diversity curve, modified from Erwin (1993).
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2
eluding marine reptiles, foraminifera, and scleractinians, radiated rapidly while algal-
sponge reefs became re-established (Hallam and Wignall, 1997). Some groups
radiated later in the Middle Triassic, including some bivalves, brachiopods, and
bryozoa that were derived from previously insignificant Paleozoic lineages (Hallam
and Wignall, 1997). Furthermore, it was during this time that the Modem Marine
Fauna, as defined by Sepkoski (1992) really begins to diversify (Figure 1.2). While
global diversity trends are useful in defining the initiation of the MMR, they do not
elucidate how ecosystems change through time. Environments respond differently to
Mgsrizriie:7|:riz PALEOZOiC
400 200
Geologic Time (MYBP)
Figure 1.2. Global marine familial diversity, modified from Sepkoski
(1992). The ‘Big Five’ mass extinctions in the Phanerozoic are
numbered from oldest to youngest.
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environmental perturbations. For example, the pelagic groups fared extremely well,
in comparison to the bulk of the benthos, after the end-Permian crisis (Hallam and
Wignall, 1997). A fundamental legacy of the end-Permian crisis is that the re
organizations during the Middle Triassic set the foundation for the diversification
patterns for the rest of the Mesozoic.
1.2 MESOZOIC FLAT CLAMS
Dysaerobic facies have been found throughout the fossil record, particularly
in Mesozoic strata (Figure 1.3). The taxa comprising facies have changed through
time from trilobite to brachiopod to bivalve-dominated communities. Dysaerobic
taxa were especially prevalent during the initial rise of the Modem Fauna and most
of the Mesozoic. During the Triassic, bivalves displaced brachiopod communities in
deep-water black shale environments (Jablonski et al., 1983). A similar
displacement occurs in other marine settings, but much later (Allison et al., 1995).
Among the black shale dysaerobic fauna, the flat clam facies is a unique to the
Mesozoic.
In Mesozoic rocks worldwide, thick accumulations of flat clams are found in
organic-rich, laminated sediments (e.g. Oschmann, 1988; Rohl et al., 2001; Parrish,
1987, Kauffman, 1991; Campbell, 1994; Kobayashi, 1963; McRoberts, 1993). The
bivalves of this facies are typically very thin-shelled, cosmopolitan taxa that are
preserved in very low diversity assemblages but in high abundances (McRoberts,
2000; Kauffman, 1978; Parish et al., 2001; Sageman and Bina, 1997; Morris, 1990;
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/ Plemchtwnia
pragcardiaccans
TRSI OBI ITS
BIVALVES
Plerinopeeien DunbareUu Cktiriu
Daonella. Anlacomyelh
. Halohta
Bositra/Posidonia
protocardids
Svncoidea chonetids
ARTICULATE
BRACHIOPODS
corbuJids
IN A R IfC LILA lT
BRACHIOPODS
Figure 1.3. Summary of ranges and modes of life of the more common lower dysaerobic taxa of the
Phanerozoic. The faunas show a secular change from trilobite to brachiopod to bivalve-dominated
forms in the Paleozoic. Inarticulate brachiopods form a constant component of lower dysaerobic
communities. Note the representatives of the ‘flat clam facies,’ including Clairia, Daonella,
Halobia, Aulacomyella, and inoceramid bivalves are found throughout the Mesozoic. Figure
modified from Allison et al. (1995).
Hallam, 1987). Examples of such deposits include bivalves from the family
Halobiidae (Triassic), Posidoniidae (Jurassic), and Inoceramidae (Cretaceous).
(Figure 1.3) The dense deposits containing such high abundances of flat clams have
been interpreted as representing oxygen-depleted environments (Rohl et al., 2001;
Parrish et al., 2001; McRoberts, 2000; Oschmann, 1993; Sageman et al., 1991).
These flat clam assemblages found in commonly laminated strata are an oxygen-
deficient marine biotope, that was once widespread and persistent, but have no direct
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analog in the present day. The global nature of such deposits suggests that Mesozoic
oceans experienced times of suboxic to dysoxic waters, and that only in the course of
Cenozoic times did suboxic waters become restricted to isolated bodies of water such
as the Black Sea (Etter, 2002; Allison et al., 1995; Wignall, 1994; Fischer and
Bottjer, 1995). Consequently, oxygen-deficient habitats are not as widespread today
as they were in the past.
Poorly aerated water masses are attractive to potential opportunistic
colonizers for various reasons (Wignall, 1994; Fischer and Bottjer, 1995; Allison et
al., 1995). Typically a ready food supply exists in low oxygenated environments
because these areas have high amounts of available organic matter (Wignall, 1994;
Allison et al., 1995; Fischer and Bottjer, 1995). The redox gradients that exist in
oxygen-deficient habitats can be an additional source of energy that can be extracted
with the help of chemosynthetic symbionts (Wignall, 1994; Vetter et al., 1991;
Southward, 1986; Reid and Brand, 1986). Dysoxia and suboxia can also provide a
chemical barrier to potential predators. However, organisms needed to develop
adaptations that would enable survival in such low oxygenated habitats. Such
adaptions would include 1) reducing expenditure of energy, 2) increasing the
efficiency of oxygen extraction and transport, and 3) use of other sources of
chemical energy (e.g. chemotrophic symbionts) (Wignall, 1994; Allison et al., 1995;
Fischer and Bottjer, 1995). It has been suggested that potential low-oxygen habitats
not only attracted colonizers, but also invited their evolutionary elaboration (Fischer
and Bottjer, 1995). Bivalves meeting these criteria include monospecific populations
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Aviculopecten, Monotis, Pseudomonotis, Buchia, Bositra, Posidonomya, and
Inoceramus, all of which are found in Mesozoic laminated strata (Fischer and
Bottjer, 1995). All these groups produced short-lived re-radiations that progressively
disappeared with the reduction of the oxygen-deficient habitats, leaving only relicts
of faunas adapted to oxygen-deficient settings today (Fischer and Bottjer, 1995).
The question remains as to how the low-oxygen biotope emerged, what the
paleoenvironmental conditions were that enabled them to radiate, and why such
facies disappeared from the geologic record.
1.3 MESOZOIC SETTING
Geologic evidence points to Mesozoic climate being much warmer than
today (e.g. Gordon, 1975; Habicht, 1979; Busson, 1982; Crowley, 1983; Ronov et
al., 1989; Robinson, 1973; Hallam, 1985; Shubin et al., 1991; Retallack, 1999).
Warm temperate climates extended to the poles during most of the Triassic (e.g.
Retallack, 1999; Ronov et al., 1989; Robinson, 1973; Hallam, 1985, Habicht, 1979).
The warmth during the early Mesozoic has been attributed increased atmospheric
carbon dioxide level, estimated to be 4-6 times the present day level (Berner, 1991;
1994; Berner and Kothavala, 2001; McElwain et al., 1999; Retallack, 2001). Thus,
the Mesozoic was a hot-house time. Two major landmasses, Laurasia and
Gondwanaland formed the supercontinent of Pangea, which was assembled during
the Paleozoic and rifted apart during the Mesozoic (Scotese and Golonka, 1992;
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Olsen et al, 1996; Marzoli et al., 1999; Hames et al., 2000; Hesselbo et al., 2001;
Cohen and Coe, 2002). For most of the Triassic and Jurassic, Pangea was
Figure 1.4. Triassic paleogeographic reconstruction, modified from Scotese (2000).
nearly symmetrical about the equator. The combination of the gigantic landmass and
lowered sea level during the early Mesozoic produced extremely continental climates
with associated aridity (Gordon, 1975; Habicht, 1979; Chandler et al., 1992; Parrish,
1993; Crowley, 1994). Pangea was surrounded on all sides by the world ocean,
Panthalassa, and the Tethys Sea (Figure 1.4). During the early Mesozoic Pangea
began to rupture producing profound effects on the climate and biotic responses at the
time (e.g. Yapp and Poths, 1996; McElwain et al., 1999; Marzoli et al., 1999; Hallam,
2000; Palfey et al., 2000; Hames et al., 2000; Hesselbo et al., 2001; Cohen and Coe,
2002). Associated with the more equable Mesozoic times is the widespread existence
of cosmopolitan taxa, including fauna from the flat clam facies (Oschmann, 1988;
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Rohl et al., 2001; Parrish, 1987, Kauffman, 1991; Campbell, 1994; Kobayashi, 1963;
McRoberts, 1993).
1.4 OVERVIEW OF THIS STUDY
In order to address the question of what paleoenvironmental conditions were
prevalent to invite the evolutionary elaboration of taxa adapted to low-oxygenated
environments, it is important to 1) define the characteristics of an oxygen-deficient
biotope, and 2) determine the prevailing paleoceanographic conditions. This study
examines a flat clam facies preserved in Middle Triassic strata in central Nevada,
and explores their significance during Triassic times. Furthermore, a first order
attempt is made to elucidate the paleoceanographic conditions through numerical
simulations of the Triassic climate system.
A climate model is employed here because it can provide a useful framework
for interpretation of existing data; while at the same time it can generate testable
hypotheses for regions where data are poorly known. The mechanisms driving
climate change through time can be explored using climate models to 1) simulate a
time slice, 2) assess the sensitivity of the climate system to changes of specific
variables, or 3) examine how the ocean and atmosphere react to extreme forcing.
This study is divided into three parts. Chapter 2 examines the paleoecology
of the bivalve Daonella, a representative of the flat clam facies from Triassic rocks,
as preserved in the Fossil Hill Member in central Nevada. First, the controversy
about their life habit is addressed and a resolution is proposed to their life mode in
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the context of the depositional setting represented by the Fossil Hill Member. The
results are then used to define an oxygen-depleted biotope from the Triassic. The
widespread nature of deposits similar to the Fossil Hill Member throughout the
world suggests that the conditions that enabled the appearance of Daonella to evolve
and radiate were not limited to western North America. Therefore, a range of
possible paleoceanographic conditions are simulated for the Triassic using a coupled
ocean-atmosphere general circulation model, and results are presented in Chapter 3.
The climate simulations are those of the Late Triassic, a time that coincides to the
height of the radiation of the oxygen-depleted biotope presented and prior to their
decline and disappearance at the end of the Triassic. Because CCL-induced global
warming due to volcanic outgassing has been invoked as a cause for the end-Triassic
mass extinction, a sensitivity experiment is performed using a climate model to
explore how the Triassic climate system responds to changes in atmospheric CO2.
The final part of this research focuses on the distribution of the bivalves from the
family Halobiidae, specifically Halobia deposits during the Late Triassic. Chapter 4
combines the results of the climate simulations with the paleoecological study to
suggest where the ‘lost biotope’ of the Triassic could have been preserved and why.
In addition, the climate simulations enable a hypothesis for the extent of
environmental tolerance shown by these cosmopolitan bivalves. The results are all
summarized in Chapter 5 with concluding remarks, and a discussion for further
avenues of research.
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CHAPTER 2. PALEOECOLOGY OF DAONELLA BIVALVES FROM THE
FOSSIL HILL MEMBER, NORTH CENTRAL NEVADA
2.1 INTRODUCTION
High-density accumulations of flat clams are found throughout the geologic
record, particularly from Mesozoic strata. These very thin, hard-shelled bivalves are
typically preserved in organic-rich units that contain very low diversity fauna
(usually monotypic). The genera, however, are often global in distribution.
Examples of such flat-clam facies include “ Lucina ” miniscula in the Jurassic
Kimmeridge Clay from northwestern Europe (Oschmann, 1988), Halobia in the
Triassic Shublik Formation from Artie Alaska (Parrish, 1987), and Bositra in the
Jurassic Posidonienschiefer of Germany (Kauffman, 1978; 1981). While flat clam
accumulations are well known from the fossil record, intense debate still exists on
the origin of these bivalves. How the flat clams accumulated and concentrated is
problematic for several reasons. Morphological features that could aid in assessing
the paleoecology and phylogeny of the bivalves are often not preserved. Very few, if
any, modem analogs to the flat clam accumulations are known. As a consequence,
the origin of these flat-clam facies in the fossil record centers on various life habit
interpretations.
Figure 2.1 illustrates the various interpretations for the mode of life of flat
clams found in the fossil record. Some researchers have interpreted the clams as
benthic organisms whose thin shells would maximize oxygen exchange, and allow
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NEKTONIC . - 'L \
PSEUDOPLANKTONIC / 1
PLANKTONIC
LARVAE
EPIBENTHIC
CHEMOSYMBIONTS
EPIBENTHIC
MUDSTICKERS,
EPIBENTHIC
Figure 2.1. Proposed life mode for flat clams. Bivalves of the ‘flat-clams facies’ have been
interpreted as either pelagic (pseudoplanktonic, nektonic) or benthic dwellers (epibyssate,
chemosymbionts, mudstickers). It has also been suggested that these bivalves have a planktonic
larvae stage. Figure modified from Wignall (1994).
them to be highly tolerant of low oxygen conditions (e.g. Oschmann, 1988;
Kauffman, 1978; 1981). Oschmann (1988) proposed that some groups of “ Lucina”
were infaunal suspension feeders adapted for consuming H2S-oxidizing bacteria.
Thus the high-density accumulations are then a result of the O2 -H2S boundary
shallowing above the sediment water interface driving the organisms to the surface
and resulting in their deaths. Kauffman (1978, 1981), on the other hand, believed
that the flat clams from the Jurassic Posidonienschiefer were simply epifaunal
suspension feeders that were highly tolerant of dysoxic to anoxic conditions.
Seilacher (1990) suggested that the byssal tube of Halobia was a pump for hydrogen
sulfide, and thus suggested that some flat clams were chemosymbiotic with sulfur
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12
reducing algae or bacteria. Other flat clam morphological features also allowed them
to adapt to life on the seafloor. The broad flat valves prevented the bivalves from
sinking into the soft substrate, like modem ‘snow-shoe strategists’ (Hollingworth and
Wignall, 1992; Whitham, 1991). However, this interpretation is also debatable.
Antia and Wood (1977) suggested that Bositra lived as mudstickers in the benthic
realm, like some modem Pinna.
Opponents to the benthic life habit interpretations argue that such dense
concentrations of benthic organisms are unlikely on the seafloor because oxygen is at
a premium (Parrish, 1987; 1998). Parrish has argued that the high accumulations of
Halobia in the Triassic Shublik Formation represent mass kills that occur when
anoxic waters are upwelled to the surface and suffocate the animals living there
(Parrish et al., 2001). Based on distributional and morphological observations, some
researchers have suggested that the flat clams are nektoplanktonic or
pseudoplanktonic. Jeffries and Minton (1965) proposed an active swimming
existence for Bositra because of the widespread geographic occurrences. Moreover,
experiments have shown that active swimming is possible among the flat clams
(Hayami, 1969). Wignall and Simms (1990), among others, have suggested a
pseudoplanktonic life habit for such flat clams. The bivalve concentrations are the
remains of dead organisms that have become detached from their floating substrates
(e.g. algae, driftwood, or other invertebrates). Rather than spending the entire life
cycle as pseudoplankton, Oschmann (1993) and McRoberts (2000) concluded that
Bositra buchi and Halobia, respectively, have a teleplanic larval phase and an adult
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13
benthic phase. As a consequence of such diverse opinions, it is still unclear as to
how flat clams lived and how they become so densely packed in the fossil record.
Halobiid bivalve accumulations are excellent examples of the flat-clam
facies. Bivalves from the family Halobiidae, which includes Daonella, Aparimella,
and Halobia, first appeared in the Middle Triassic and were present all the seas by
the Late Triassic. Interestingly, halobiids are among the most short-lived and
cosmopolitan bivalves of the Mesozoic (Tozer, 1984; McRoberts, 2000). For this
reason, they are often utilized as index fossils, with time resolutions that may exceed
those of ammonoids (McRoberts, 1997). These very thinned-shelled bivalves
typically occur in black shale facies of anoxic or dysoxic environments, and are,
therefore, useful indicators of particular environmental conditions. As in other
Mesozoic thin-shelled bivalves, however, the preservation is often quite low, and
little is known about their paleoecology.
Daonella species are preserved in strata of the Star Peak Group, exposed
throughout north central Nevada. They occur most abundantly in the Middle
Triassic Fossil Hill Member of the Prida Formation (Figure 2.2). The aim of this
project is to investigate the occurrence and preservation of these halobiid bivalves in
the Fossil Hill Member, at its type locality, in order to obtain clues on the
paleoecology of Daonella in particular, and other flat clam accumulations at large.
Regardless of the life habit interpretation, these flat clam facies are significant
because they represent a “lost biotope” (Fischer and Bottjer, 1995) from the
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14
Figure 2.2. Daonella concentrations in the Middle Triassic Fossil Hill Member, exposed
in north central Nevada.
Mesozoic that is no longer prevalent in the modem day. Thus, to understand the
development of such flat clam accumulations in the fossil record is to understand an
important time in bivalve evolution.
2.2 PHYLOGENY
The members of the family Halobiidae are defined as thin-shelled pteriaceans
with low valve convexity, primitive radial plications, and commarginal rugae that are
retained from a posidoniid ancestor. Three genera are recognized in the family
Halobiidae (Daonella, Aparimella, and Halobia) on the basis of the hinge region and
byssal system. A fourth genus, Enteropleura, may be placed in the Halobiidae
pending further study (Campbell, 1994). Daonella lacks an anterior auricle,
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15
Aparimella possesses an upper anterior auricle, and Halobia has a two-fold anterior
auricle.
It was first proposed by Campbell (1994) that Daonella, Aparimella, and
Halobia represent a simple evolutionary series, whereby Halobia evolved from
Aparimella, which descended from Daonella. However, debate still exists on
whether or not Halobia is a natural taxon. Suggestions have been made that Halobia
is polyphyletic (Gruber, 1976; Polubotko, 1988) because several upper Camian to
lower Norian Halobia exhibit posidoniid morphology. Also, there is evidence that
the ligament types (a conservative feature in bivalve evolution) vary among Halobia
species (Newell and Boyd, 1987). McRoberts (2000) recently described a new
halobiid species, Halobia daonellaformis, with external ornamentation similar to
Daonella lommeli, but with a poorly developed anterior auricle. From this
observation he argues for an alternative phylogeny for Halobiidae. While
McRoberts agrees with Campbell that Aparimella and Halobia are descendants of
Daonella, he believes that they are sister taxa descended from the common ancestor
Daonella. Thus, the true phylogenetic relationship among the members of
Halobiidae is still problematic. What is known is that Daonella first appeared in the
fossil record during the earliest Middle Triassic and disappeared in the earliest Late
Triassic. Halobia, on the other hand, emerged in the latest Middle Triassic and
prevailed until the end of the Triassic, when the family Halobiidae disappeared from
the fossil record. Both these members of Halobiidae quickly emerged, radiated, and
invaded all Triassic seaways at the time of their existence. Here, the occurrence of
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16
Daonella in western equatorial Pangea, as preserved in outcrops of the Fossil Hill
Member in central Nevada, is investigated.
2.3 GEOLOGIC SETTING
Figure 2.3 is a tectonic reconstruction of part of western Pangea during the
Triassic, modified after Speed (1978). The Early Triassic deformation of western
Nevada resulted from the collision of the oceanic plate, Sonomia, with the
continental margin. The resultant subsiding marine trough became the site for later
Triassic and Jurassic sedimentation. The Triassic shelf terrane, which is represented
by the strata of the Star Peak Group (Figure 2.4), is comprised of rocks deposited in
the eastern margin of the gently subsiding basin.
The Star Peak Group represents a carbonate complex, initially deposited
during the Late Spathian transgression, and ranges in age from the latest Early
Triassic to the middle Late Triassic. In some areas the Star Peak Group is as much
as 1200m thick (Nichols and Silberling, 1977). It overlies the predominantly
volcanic rocks of Koipato Group deposited during the Spathian age. Above the Star
Peak Group sits the terrigenous clastic rocks of the Auld Lang Syne Group.
The Prida Formation makes up the lower part of the Star Peak Group in the
northern East Range, Humboldt Range, and northern Stillwater Range in central
Nevada. The Prida Formation, originally described by Muller and others (1951),
consists of three lithologic units: the Lower Member, Fossil Hill Member, and Upper
Member.
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17
OREGON
NEVADA
' / S J I N f l T F P r * ,c
VOLC ArtlC % f
\ Ak - TEPRAM-
J -
K M
Figure 2.3. Tectonic reconstruction of part of Western Pangea during Triassic times,
modified from Speed (1978). The Early Triassic deformation of western Nevada resulted
from the collision of the oceanic plate, Sonomia, with the continental margin. The
subsiding marine trough became the site for later Triassic and Jurassic sedimentation. The
shelf terrane is comprised of rocks deposited in the eastern margin of the gently subsiding
basin. The Fossil Hill Member, part of the shelf terrance, is exposed in the southern
Humboldt Range in central Nevada.
The Lower Member of the Prida Formation has been interpreted to represent
open, relatively deep-marine conditions during the Late Spathian (Nichols and
Silberling, 1977). Localized uplift early in the Anisian was succeeded by general
subsidence and deposition of relatively deep-water sediments, including the Fossil
Hill Member (Nichols and Silberling, 1977; Carey, 1984). During Ladinian times, a
carbonate platform prograded across the shelf and resulted in deposition of the
overlying Upper Member (Nichols and Silberling, 1977; Carey, 1984).
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18
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Figure 2.4. Stratigraphic nomenclature of relevant lithologic units outcropped in the
Southern Humboldt Range and the Augusta Mountains. Time of deposition is constrained
by conodont and ammonoid biostratigraphy. Dashed lines represent unconformities
between rock units.
Silberling and Wallace (1969) defined the Fossil Hill Member of the Prida
Formation. Its type locality is located in the southern Humboldt Range. The Fossil
Hill Member was deposited during the Anisian, the first stage of the Middle Triassic,
and is unique in that it hosts some of most well preserved and most diverse Middle
Triassic fauna in the world (Smith, 1914; Nichols and Silberling, 1977; Tozer, 1982;
Bucher, 1992). Furthermore, outcrops of the Fossil Hill Member are fairly
homogeneous and have an areal extent of almost 5000 km3 (Nichols and Silberling,
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19
1977). At its type locality on the south side of Fossil Hill in the Humboldt Range,
the Fossil Hill Member can be up to 60m in thickness. It consists largely of laterally
discontinuous thin- or medium-thick beds of dense gray-weathering limestone or
black micritic limestone mixed with a wide variety of platy-weathering calcareous
mudstone, calcareous siltstone, and silty limestone (Silberling and Wallace, 1969;
Nichols and Silberling, 1977). The Upper Member of the Prida Formation
gradationally overlies the Fossil Hill Member. It is a monotonous sequence of dark-
gray limestone, which is mostly recrystallized, sparsely quartz-silty, lime wackestone
(Silberling and Wallace, 1969; Nichols and Silberling, 1977).
For this study, a detailed section was measured at Fossil Hill, in the southern
Humboldt Range, that included the Fossil Hill Member and the Upper Member of
the Prida Formation. The section measured at Fossil Hill was over 45m thick. It can
be divided lithologically into three distinct parts (Figure 2.5). The bottom third of
the section consists of lenticular limestones interbedded with calcareous siltstones,
shales, and silty limestones. These facies are finely laminated, and show no
evidence of bioturbation (Figure 2.6.A and B). Upsection are thicker bedded,
grainier limestones. Concretions and evidence of hardgrounds are found within this
interval of the Fossil Hill Member. Thin silica stringers are also indicative of greater
silificification in this part of the section (Figure 2.6.C). The limestones are cross-
stratified (Figure 2.6.D), with some signs of hummocky cross stratification to
indicate storm wave base level. The top third of the measured section consists of
dark gray limestone beds with increasing chert content. Thick chert layers are found
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20
L ithology Sedimentary
Structures
m m
m m
I B M P
M i
« ln »
MMMMM
F o ssils
H P
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m assive black lim estone conglom erate, clasts
of underlying lithology
thick grainy dark gray lim estone beds and
cherty lim estone b ed s {10-50 cm], planar
b ed d ed
slightly fossiliferous (ammonite-rich), mainly
fragm ents
g reater silicification and evidence of
hardg ro u n d s,ch ert layers (cm's thick)
1
large concretions, hardgrounds,silicification
discontinuous lam inations
fossiliferous (am m onite-rich)
grainy light to dark gray lim estones (1 Q ’s cm)
with cross-stratification (including
hum m ocky cross stratification)
T
lenticular black micritic lim estones (5-10 cm)
with calcareous siltstones, shales, and silty
lim estone interbeds (cm's)
highly fossiliferous (bivalve-rich)
finely lam inated, lacking bioturnation
1
Sym bols
C onglom erate tstone
m
Daonella sp.
Lim estone Interbedded silstone and shale Bivalve (benthic)
m
Lim estone, grainy U p s '” Nautiloid
m
Ammonite
Figure 2.5. Measured section on the south side of Fossil Hill, southern Humboldt Range, central
Nevada. The Fossil Hill Member is a shallowing upward sequence, representing an offshore shelf
environment. Also depicted is the relative abundance of Daonella bivalves throughout the measured
section and associated fossils. Daonella is preferentially preserved in the facies representing a low
energy, dysoxic environment.
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21
Figure 2.6. Facies of the Fossil Hill section. A) At 7m, a finely laminated limestone with a
distinct horizon of fossil accumulation above a silty limestone. B) At 14m, finely laminated
black micritic limestone. C) At 23m, grainy limestone with silica stringers, denoted by white
arrow. D) At 20 meters, cross-stratified grainy limestone. E) At 37m, grainy limestone with
thick chert layer. F) Between 28m to 37m, planar bedded dark gray limestone beds.
in some of the limestones near the top of the section (Figure 2.6.E). The rock units in
this part of the section are planar-bedded grainy limestones (Figure 2.6.F). A massive
limestone conglomerate overlies the whole measured section, and is composed of
clasts of the underlying lithology. Thus, the section measured at Fossil Hill
represents an overall shallowing upward sequence.
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22
Figure 2.7. Photomicrographs of A) pyrite grains (indicated by white
arrows) and B) micro-grading in black micritic limestone facies of the
Fossil Hill Member.
The absence of high-energy allochems and bioturbation in the commonly
laminated or thinly bedded facies of the Fossil Hill Member suggests deposition in a
deep-water setting. The abundance of pyrite grains, particularly in the black micritic
limestone facies, throughout the section suggests that very low levels of oxygenation
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23
existed during the time of deposition (Figure 2.7.A). The fine-grained nature of the
Fossil Hill Member points to a predominantly low energy, probably offshore shelf
setting. In thin section, the micritic limestone facies shows episodic micro-grading
(Figure 2.7.B). Thus, this depositional environment likely experienced dysoxic to
anoxic conditions, which were periodically interrupted by storm events that
increased the flux of detrital input.
2.4 PALEOECOLOGICAL ANALYSIS
In order to assess the life mode of Daonella, their morphology, associated
occurrence, and mode of accumulation must be closely examined. These parameters
were investigated at the measured section on Fossil Hill, and compared to field
observations made at Favret Canyon in the Augusta Mountains, located 25 km to the
east. Bulk samples were collected at lm intervals from the measured section at Fossil
Hill for analysis in the laboratory. An additional 19 samples were collected from
various lithologies with abundant fossils for closer examination of the relationship
between fossil occurrence and lithology in the Fossil Hill Member. Each sample
weighed approximately 4 to 5 kg. Where possible, the bulk sample collected for
paleoecological analysis consisted of one large piece of material kept intact with duct
tape. Orientation was noted for all samples collected. Laboratory analysis of the
bulk samples collected at Fossil Hill included observations of Daonella occurrence
in the various lithologies along with associated sedimentary structures and fauna.
The size, orientation, articulation, and abundance of Daonella were recorded in
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2 4
addition to the preservation of the shells from bedding planes derived from splitting
the bulk samples. The relative abundance of organisms, mainly halobiid bivalves
and ceratite ammonoids, from each sample along with the average size of halobiid
bivalves were then plotted against the stratigraphic column to assess the degree of
size sorting and selection, as well as the occurrence of shell concentrations in the
Fossil Hill Member. The sedimentologic and paleoecological investigations were
then compared and combined to produce a holistic interpretation of both the mode of
life Daonella, as preserved in the Fossil Hill Member.
2.5 RESULTS
2.5.1 Morphology
Daonella have a very simple morphology. (Figure 2.8) The outer shell lacks
ornamentation except for some radial plications, presumably retained from their
posidoniid ancestor. The valves contain commarginal rugae, and there is a marked
transition from the juvenile shell to the adult shell. Daonella from the Fossil Hill
Member have very thin shells, almost always <1 mm thick. The valves range in
height from several millimeters to 3-4 cm in thin section. They have very low valve
convexity, a feature exacerbated by evidence of compaction in the sediment. Details
of the ligament area, where the two valves are joined, are not preserved in the
observed specimens of Daonella. Thus, no conclusion can be drawn from the
ligament design that may impose constraints on the shell geometry and morphology
to allow for a particular mode of life interpretation. Furthermore, because of the
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2 5
Daonella Halobia
beak
beak
furrow
furrows
umbonat
region
posterior auricle
D a o n ella anterior latara) view
Figure 2.8. Morphology of halobiid bivalves. Daonella have very thin, flat shells that range
in height from 3-4 cm in the Fossil Hill Member. Daonella lacks an anterior auricle, whereas
Halobia possess a two-fold antierior auricle.
poor preservation of the thin shell material in such dense limestones, which make
disaggregating samples without destroying the shell impossible, Daonella
musculature is undetermined from these samples. While data is lacking on the
ligament area and musculature of Daonella in the Fossil Hill Member at this time,
other clues to their functional morphology can be ascertained. Unlike Halobia, the
genus Daonella lacks an anterior auricle (Figure 2.8). Halobia possesses a two-fold
anterior auricle. This feature has been interpreted previously by Campbell (1994) to
represent a byssal gape, where the byssus emerged from the shell and attached to
some substrate in the water column. No such structure is present in Daonella,
however, and the flat nature of the shells preserved in the Fossil Hill Member would
not indicate the presence of a byssal gape between the valves.
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26
2.5.2 Preservation
The Fossil Hill Member is a shallowing upward sequence, representing an
offshore shelf environment. Figure 2.5 shows the occurrence of Daonella bivalves
throughout the section. The highest abundances occur in the lower third of the
measured section, which consists of lenticular limestones interbedded with
calcareous silstones, shales and silty limestones. These facies are finely laminated
and show no evidence of bioturbation. Higher up in the section, where a higher
energy regime is represented, Daonella occur sporadically as whole organisms or
they are found only as fragments. The highest concentrations of Daonella are found
in the laminated black micritic limestone facies (Figure 2.9.A and D). These
concentrations are very dense and also contain highly recrystallized ammonite and
nautiloid shells (Figure 2.9.A). In the sparry limestones, where the ammonite and
nautiloid shells are most abundant, only fragments of Daonella exist (Figure 2.9.B).
An abrupt transition between the silty limestone facies to the dense accumulations of
valves in the micritic limestone facies is not uncommon, and is indicative of rapid
accumulation (Figure 2.6.A). Few Daonella valves are preserved in the silty
limestones and calcareous limestone facies, which contain more detrital input. Little
to no Daonella specimens are found in the cherty limestone facies.
The most abundant accumulations of Daonella occur in the bottom portion of
the measured section consisting of interbedded black micritic limestones and
calcareous shale, silt, and silty limestones. Above these rock units, the limestones
are grainier and at times cross-stratified. These features indicate a higher energy re-
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27
Figure 2.9. Daonella in the Fossil Hill Member. A) Dense accumulations occurring with
nautiloids and ammonites on several bedding planes in black micritic limestone facies. B)
Fragments of Daonella occurring with recrystallized ammonites in black sparry limestone
facies. C) Daonella preserved convex up, without size sorting in black micritic limestone
facies. D) Black micritic limestone facies containing only Daonella bivalves.
gime, and greater potential for mass transport. This conclusion is verified by the
increasingly greater amount of bivalve fragments in the samples collected above 17
meters. To more accurately assess the life habit of Daonella it is important to
concentrate on units where the potential for transport is low and, as a consequence,
in-situ preservation has a greater likelihood. Thus, the mode of life interpretations
made herein relies on observations made from Daonella accumulations in the lower
third of the measured section where wave-action was presumably at a minimum.
Daonella can occur in very high abundances, thousands on a single bedding
plane of laminated black micritic limestone. The valves are almost always convex
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2 8
Figure 2.10. A) Articulated Daonella preserved in layer above rusty micritic limestone
layer. B) Compacted shell layer in black micritic limestone facies.
up (Figure 2.9), and in vertical cross-section some appear articulated (Figure 2.10).
In the black micritic limestones the whole shell is preserved and there is little to no
fragmentation in hand sample. In thin section, however, there is evidence for
compaction of the shells (Figure 2.10).
Various Daonella shell sizes, from juvenile forms up to adult forms that
attain 4 cm in height, can be found within the same bedding plane. In fact, the shells
are so thin that they are often packed on top of one another to form the laminations
found in the limestones. Figure 2.11 shows the shell size frequency distribution of
four black, laminated, micritic limestone facies in the bottom of the measured
section, collected at different intervals. The shells include all macrofossils found on
that particular bedding plane. Typically, however, more than 95% of the organisms
were Daonella specimens, making the assemblage almost monotypic. The mean size
varies from 6.9 to 10.4 cm, and as many as 42 shells are found in 200 cm2. The
number of total organisms depends largely on the number of small organisms
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29
30
1m
o 15
N=119
M ean=6.9748
Median=6
10 15 20 25 30
Shell Height (mm)
N=97
Mean=7.2990
Median=7
10 15 20 25
Shell Height (mm)
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Median=6
10 15 20 25
Shell Height (mm)
30
10m
N=65
Mean=10.4154
Median=10
10 15 20 25
Shell Height (mm)
Figure 2.11. Size frequency of shells in laminated black micritic limestone facies, collected at 1, 4,
7, and 10m. The shells included all macrofossils found on an area of approximately 200 cm2. The
shell height is defined in the same manner as Figure 2.8. N is the total number of specimens
counted.
preserved. What is especially interesting is that within each bedding plane, there is
little to no size sorting of the shells that would indicate sediment transport. Rather,
the size distributions are more suggestive of natural populations, dominated
primarily by smaller organisms, Daonella juvenile specimens in this case. The shell
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30
to
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2m
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N=21 N=26
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Median=13
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She!! Height (mm)
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30 10 15 20
Shell Height (mm)
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Median=8
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Shell Height (mm)
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11m
N=33
Mean=10.3333
Median=10
10 15 20 25
Shell Height (mm)
30
Figure 2.12. Size frequency of shells in siltstones (2 and 9m) and silty limestone facies (6 and
11m). The shells included all macrofossils found on an area of approximately 200 cm2. N is the
total number of specimens counted.
size frequency distributions from 4 siltstone and silty limestone beds show a similar
pattern (Figure 2.12). While the average size appears larger in these facies (from
8.1-14.7 cm), as expected in the higher energy regime represented by these
lithologies, they do not exhibit much sorting either. Furthermore, fewer individuals
are found on the same area in these facies (between 21-36 specimens), as compared
to the black micritic limestone facies. The siltstone sample collected at 9m is
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31
exceptional in that it did not contain any Daonella. All the shells preserved in this
sample were clams from the family Corbulidae, a benthic bivalve tolerant of low
oxygen condition. Nevertheless, the shell sizes also exhibit a normal distribution. In
sum, the majority of Daonella accumulated in the lower part of the Fossil Hill
Member. The Daonella preserved together represent a wide range of sizes,
indicative of natural populations where juvenile forms predominated. In the
laminated facies, Daonella are the most abundant, often representing the only taxon
within a bed.
2.5.3 Fossil Association
While Daonella dominate the Fossil Hill section in numbers, other fossils are
also found within these rock units (Figure 2.13). It has long been noted that
nautiloids and ceratiid ammonites occur in association with Daonella (Smith, 1914).
The cephalopods are found throughout the entire measured section at Fossil Hill,
even in units where there is only evidence of bivalve fragments. While the
cephalopods occur in association with the daonellid bivalves, Daonella do not appear
to have been attached to them. In other words, the ammonites and nautiloids were
not substrates of attachment for the flat clams. Nor is there evidence of wood, algae,
or any other type of fossils that may have served as attachment substrates.
Vertebrate bone fragments and fish scales are also preserved in the Fossil Hill.
While Daonella is the dominant taxon within the black micritic limestone facies,
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32
other bivalves are also found throughout the section, albeit more sporadically.
Bivalves are from the families Buchiidae, Modiomorphidae, Corbulidae, Pectinidae,
Figure 2.13. Associated fossils in the Fossil Hill Member. A) Corbulid bivalves. B) Entolium sp.
found in close proximity to Daonella bivalves. C) Abundant buchiid bivalves. D) Preserved fossil
tetrapod skin tissue.
and Oxytomidae (Cox and Newell, 1969; Wignall, 1994). These bivalves are more
inflated, and can be both larger and smaller than the Daonella found in the section.
Thus far, however, there is no evidence of trace fossils. Although not investigated
here, Carey (1984) and others have noted the abundance of conodonts within the
Fossil Hill Member, which they used to biostratigraphically confine the time of
deposition to the Anisian.
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33
2.6 DISCUSSION
Daonella have very thin flat valves, and lack a byssal gape. They occur most
abundantly in the well-laminated, black micritic limestone facies. Often they are
preserved in a convex up orientation and are articulated. There is a large range of
shell sizes accumulating on the same bedding plane. Noteworthy is their association
with other bivalves that have been shown by other workers to be benthic dysoxic
fauna. The ways in which Daonella is preserved within the Fossil Hill Member, and
its associated fauna are significant clues to their paleoecology.
Daonella were not likely pelagic organisms. With no apparent means of
attachment, because they lack a byssal gape, or obvious attachment substrates, a
pseudoplanktonic interpretation seems highly unlikely. Futhermore, the clams do
not have a wide facies distribution, as one would expect of pseudoplanktonic or
nektonic forms. Rather, Daonella are mainly concentrated in the black micritic
limestone facies.
Swimming by self-propulsion is a conspicuous behavior in free-living
scallops. According to Hayami (1991), actively swimming species commonly show
four features: 1) larger umbonal angle and a larger aspect ratio than bysally attached
species, which tend to increase with ontogenetic growth; 2) the presence of gapes
along antero-and postero-dorsal margins, which are exits for propulsive jet currents;
3) growth in the striated part of the adductor muscle is positive allometrically with
shell size and is markedly oblique to the commissure plane; and 4) very thin, but
tough, shells that become progressively thinner with ontogenetic growth. Daonella
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34
do not meet these requirements, save the last one. It is important to note that the
scallops studied by Hayami (1991) employed active swimming as a strategy against
predation, and thus searches for potential predators of Daonella may also be fruitful.
However, since there is no evidence for predation in the preserved Daonella shells
from the Fossil Hill Member, the general morphology of swimming scallops may not
apply for Daonella. This evidence, coupled with the facies distribution, suggests that
the nektonic life mode for Daonella seems improbable.
The size distributions and the presence of articulated specimens do not
support the proposition that Daonella accumulations are death assemblages of
pelagic organisms, since their valves are not size sorted. As yet, however, a
planktonic larval stage cannot be ruled out. A planktonic or pseudoplanktonic
strategy at some stage in ontogenetic development is one possible explanation for the
rapid dispersal of halobiid bivalves in the Triassic oceans, considering their
extremely short range in the rock record. McRoberts (2000) suggested that the larval
shell morphology of Halobia daonellaformis indicates a planktotrophic larval stage
based on the criterion outlined by Jablonski and Lutz (1983) using the ratio between
the prodissoconch-I and prodissoconch-II sizes for planktotrophic bivalves.
Although this criterion was not tested for in this study of Daonella from Fossil Hill,
it is a worthy avenue of investigation into the possible planktonic strategy of this
bivalve at the larval stage.
More likely, Daonella were benthic organisms based on their preservation
within the Fossil Hill Member. They were probably not benthic mudstickers since
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35
upon death the valves would have to undergo greater deformation or fragmentation
in order to be preserved along the laminae. This is not the case for the flat clams
within the Fossil Hill Member. Thus, an epibenthic snow-shoe strategy seems
appropriate for Daonella based on the size frequency distributions and articulated
specimens. Moreover, the Daonella are found in association with other bivalves that
have been interpreted as dysoxic fauna. However, a chemosymbiotic life mode is
difficult to test since many of the shells are highly recrystallized.
These conclusions are similar to ones drawn by Etter (2002a), who examined
Daonella deposited in the Middle Triassic Lagerstatte of Monte San Giorgio, located
in the southernmost part of Switzerland, traditionally interpreted to have formed in a
stagnant basin that was severely oxygen depleted in general (Bemasconi, 1991;
1994). He noted that Daonella in these deposits are very abundant, never associated
with floating objects, never sorted by size, and the majority of the shells are
preserved convex side up. As a result, he interpreted these Daonella as epibenthic
bivalves extremely tolerant of low oxygen in the bottom waters, which also
experienced very gentle currents along the bottom. These bivalves occur between
laminae, which may be products of microbial mats that grew on the seafloor with
strongly dysoxic bottom-waters. The low oxygen values at Monte San Giorgio
prevented establishment of a more diverse and burrowing benthic fauna. Thus, the
accumulations of Daonella in this deposit represent colonization events when
improved water circulation or turbidites temporarily rejuvenated bottom water
oxygen levels. Furthermore, the excellent preservation of the Daonella shells is
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36
attributed to the severely dysoxic bottom waters and sealing of the carcasses of dead
organisms by microbial mats, and accumulation of these bivalves under quiet bottom
waters was enhanced by the lack of scavenging organisms.
Very few modem analogs exist for low oxygen tolerant epibenthic bivalves.
A possible example inhabits the California Continental Borderland basins, where
very low subsill circulation and high surface productivity in these basins result in
very depleted oxygen bottom waters (Etter, 2002b). Almost 16 basins (ranging from
600-2500 meters deep) make up the California Continental Borderland off the coast
of Southern California. Sills, ridges, and islands separate one basin from another in
this area. The rare Delectopecten randolphi is a pectinid bivalve found from only 22
box cores from the more than 800 boxcores sampled by the BLM survey undertaken
in the Borderland basins between 1975 and 1977 (Etter, 2002b). According to Etter,
D. randolphi have characters which are well correlated with excellent swimming
abilities, including “the airfoil shape in cross section, the large umbonal angle and
near symmetrical auricles, the hooked dorsal shell margin, the strongely oblique
arrangement of the adductor muscle components, and the huge muscularized velum
of the inner mantle fold, combined with a pallial line far away from the shell margin”
(Etter, 2002b). These characters are also found in reclining species on soft bottom
substrates in low-energy environments, where swimming can be used as a quick
escape mechanism from predators (Etter, 2002b).
However, D. randolphi can also attach to sandy and pebbly substrates with its
byssus, and exhibit an opportunistic behavior (Etter, 2002b). Larval shell analysis
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37
suggests a small egg size, large number of offspring, and a planktotrophic larval
stage lasting at least several weeks. This strategy allows for wide dispersal and
enhances potential for opportunistic colonization. Live specimens have been
captured from deep basin stations with oxygen values as low as 0.3 ml O2 /I (Etter,
2002b), and D. randolphi shells have been found preserved in life position in
laminated sediments where oxygen levels have to be well below 0.1 ml O2 /I in order
to exclude the burrowing infaunal species that could disrupt the laminae. Thus, D.
randolphi appears to be an opportunistic organism with an extreme tolerance of low
oxygen levels and a very broad ecological niche. In one particular box core,
approximately 100 individuals of the same size class were found on an area of 1 m ,
and indicate that this bivalve can colonize substrates in high numbers and survive in
high densities for several years given favorable conditions (Etter, 2002b).
While D. randolphi can be used as a modem analog for interpreting the life
mode of bivalves such as Daonella from “flat clam facies”, the rarity of D. randolphi
clams in the sediments studied from the California Continental Borderland in these
restricted basins still leaves the question of high density clam accumulations in the
fossil record unresolved. Most of the occurrences of bivalves in shelf and basin
sediments today are rare, and occur in very low concentrations (Etter, 2002b),
whereas fossil flat clam accumulations can be up to meters in thickness and span a
vast area. In order to address the question of how such dense accumulations can
occur, it is important to, first, understand the distribution of oxygen-restricted faunas
in shelf settings and, secondly, how shell concentrations are accumulated.
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38
Today, oxygen deficient environments are frequently encountered in many
modem shelf areas, particularly in temperate latitudes where a summer thermocline
develops. Various models of oxygen-restricted fauna have been proposed (Rhoads
and Morse, 1971; Byers, 1977; Savrda et al., 1984; Tyson and Pearson, 1991).
Figure 2.14 shows a newer model for oxygen-restricted faunas based on the
dynamically unstable dysoxic environments of some modem shelves. Under declin-
epifau n a
sulfur b acteria
- ii r S shallow infauna
O '
RPD zo n e
lam in ated s e d im e n t I :
if)
~ o
% 5 yrs
< u
1 1y r
c
CD
O)
> .
g 1 mo
2.0 0.5
A verage B enthic D issolved O xygen V alu es (mi/t)
ANAEROBIC
LOW ER
D YSAEROBIC
U P P E R
DYSAEROBIC
LOW ER
POIKILOAEROBIC
U P P E R
POIKILOAEROBIC
A EROBIC
Figure 2.14. Model for oxygen-restricted faunas based on the dynamically
unstable dysoxic environments of some modern shelves, modified from
Wignall (1994). The depth at which infauna can bioturbate and irrigate their
burrows depends on the depth of the redox potential discontinuity (RPD)
zone, the sharp boundary between reduced and overlying oxygenated
sediment.
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39
mg bottom water oxygen levels, there is a progressive loss of benthic species until
practically all disappear at 0.3-0.5 ml O2 /I, the redox boundary rises and reduces the
depth at which infaunal organisms can bioturbate and irrigate, and the size of the
burrows declines. A dysoxic faunal gradient in shelf settings is recording not so
much the oxygen levels as the duration of habitable conditions. Lowest dysoxic
conditions allow only very brief benthic colonization events, of a few months only,
while normal benthic communities require between five and eight years of normal
conditions in order to become established (Rosenberg, 1976; Leppakoski, 1975).
According to the model, anaerobic laminated sediments are produced under
conditions where bottom-water oxygen levels remain permanently below 0.5 ml O2 /I.
Lowest dysaerobic sediments are also laminated, but can contain low diversity shell
pavements (typically comprised of suspension feeding bivalves) and
microbioturbation horizons representing transient colonization events. These
colonization events last for less than a year typically since prolonged benthic faunal
activity results in destruction of fine laminae (Soutar et al., 1981).
The preservation of shell material in the fossil record depends on the
mineralogy of the shell material, dynamics of sediment accumulation, the
paleoenvironmental gradients, and the extent of time averaging. Kidwell (1991) has
classified shell concentrations into four main categories based on their stratigraphies
and inferred histories of accumulation (Figure 2.15). Event concentrations record
ecologically brief episodes, which have the scale of laminae and beds. Composite or
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4 0
ecologically brief episode
of shell concentration
EVENT
CONCENTRATION
preservation
as discreet events
amalgamation or
- accretion of multiple
^ events
average or
expanded section
j L
COMPOSITE
CONCENTRATION
condensed
section
HIATAL
CONCENTRATION
truncation of significant section by erosion/corrosion,
strong taphonomic cidling of biaclasts
±
LAG CONCENTRATION
Figure 2.15. Shell concentrations grouped into categories according to
accumulation histories and time scales. Concentrations range from simple event-
concentrations to accretionary, multiple-event accumulations (both composite-
concentrations of “normal” or expanded thickness, and hiatal-concentrations that
are stratigraphically condensed), and highly derived, strati graphically disjunct lag-
concentrations. Each category includes biogenic, sedimentological, and mixed-
origin coquinas; these may be composed of any proportion of local and exotic
shells. Figure modified from Kidwell (1991).
multiple-event concentrations, ranging from beds and bedsets to reservoir-scale
bioclastic facies, record the accretion and amalgamation of multiple generations
and/or event-concentrations. Hiatal or condensed concentrations are complex
accumulations that are thin relative to coeval strata owing to slow net rates of
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41
accumulation. Lag concentrations are very thin, highly residual concentrations
associated with surfaces of significant strati graphic truncation.
The model of oxygen-restricted faunas, based on modem shelf settings, and
shell concentration classification scheme presented by Kidwell (1991) can be used to
interpret the accumulations of Daonella in the Fossil Hill Member that outcrop in the
Southern Humboldt Range. In the Fossil Hill Member, Daonella is preserved most
abundantly in the laminated black limestone facies found interbedded with
calcareous silt, calcareous shale, and silty limestones. The lack of bioturbation in
these facies suggests that accumulations of Daonella are similar to fauna of the lower
dysaerobic found in modem shelf settings, where bottom oxygenation levels are
dynamically unstable. The enhanced preservation of shell material and laminated
sediments may have resulted from low bottom oxygen conditions and the binding
from bacterial mat formation. The high abundance and dominance of smaller
individuals suggests that Daonella may have been an opportunistic species that
colonized the substrate when conditions became favorable. The occurrence of
Daonella in other fine-grained facies indicates that these clams could inhabit
different types of substrate. The horizons of black limestones, where dense
accumulations occur, represent many transient colonization events. The thick
fossiliferous limestone beds resulted from the slow winnowing of the shell
concentrations, from multiple colonization events, by gentle wave action at the
seafloor. Daonella preservation and accumulation is further enhanced by bottom
water dysoxia, which excluded both predators and bioturbating organisms.
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4 2
2.1 CONCLUSION
Functional morphological and paleoecological analysis, preservational
features, and associated faunal occurrences of Daonella in the Fossil Hill Member,
exposed in central Nevada, suggest that Daonella are opportunistic epibenthic
organisms, capable of rapidly colonizing shelf environments when bottom water
dysoxia prevailed. Their excellent preservation and dense accumulation are due to
the low oxygenation levels and hydrographic conditions that existed on the seafloor.
The great areal extent of the Fossil Hill Member, as mapped by Nichols and
Silberling (1977), and relative homogeneity of this lithologic unit throughout central
Nevada suggests that dysoxic conditions were prevalent in this area.
Evidence from other Daonella localities is suggestive of widespread dysoxia
during the Middle Triassic. Daonella have been found in other parts of the world,
including Italy (Mojsisovics, 1874; De Lorenzo, 1896, Cafiero and De Capoa
Bonardi, 1982), Switzerland (Kittl, 1912; Reiber, 1968; Etter, 2001, 2002), Bulgaria
(Encheva, 1978); Montenegro (Cafiero and De Capoa Bonardi, 1980); Russia
(Polubotko, 1984); Indonesia (Rothpletz, 1892), China (Chen, 1982), Japan (Bando,
1964), Vietnam (Khuc, 1990), Malaysia (Kobayashi, 1963), New Zealand
(Trenchmen, 1918; Marwick, 1953; Campbell, 1994), Afghanistan (Farsan, 1972),
Turkey (Freneix, 1972), and North and South America (Smith, 1914, Tozer, 1982;
Gruber, 1983; McRoberts, 1993). The deposits are typically fine-grained and
laminated and contained very dense accumulations of bivalves, very similar to the
character of the Fossil Hill Member exposed in central Nevada. Moreover, Daonella
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4 3
bivalves later gave rise to other halobiid bivalves that eventually dispersed into the
Tethys, Panthalassic, and Arctic seas by the Late Triassic. Likewise, the halobiid
bivalve deposits show dense accumulations of flat clams in fine-grained laminated
deposits also interpreted to represent low oxygenated environments. The
paleoceanographic conditions that allowed for both the emergence and proliferation
of this family of Halobiidae in the Triassic have yet to be explored. Future research
should examine paleoceanographic conditions of the Middle and Late Triassic in
order to assess the necessary environments for the success of such opportunistic
species. It is interesting to note that other flat-clam facies (e.g. Bositra, Inoceramus,
and “ Lucina ”) are associated with episodes of oceanic anoxia. The question
remains as to whether the widespread distribution of bivalves from the family
Halobiidae is indicative of similar paleoceanographic conditions. Thus, the
emergence of the family Halobiidae during the Middle Triassic may have been a
prelude to other Mesozoic flat clam facies.
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4 4
CHAPTER 3. LATE TRIASSIC CLIMATE AND ITS SENSITIVITY TO
CHANGES IN ATMOSPHERIC C 02:
Preliminary Results from a Fully Coupled Ocean-Atmosphere Model
3.1 INTRODUCTION
Climate models can be used to provide a framework for interpretation of
existing data, while also generating testable hypotheses for regions where data are
poorly known. Modeling climate is also vital to understanding the sensitivity of the
ocean-atmosphere system and the mechanisms driving climate change through time.
Results derived from climate models are obtained from running either energy
balance models (EBMs) or general circulation models (GCMs). EBMs are
thermodynamic models that resolve the land-sea distribution and provide information
on the seasonal cycle of surface temperatures, depending on the sophistication of the
model. GCMs, on the other hand, are dynamic three-dimensional global models
based on standard equations of motion and thermodynamics that govern the
circulation of atmosphere and ocean.
General circulation models use numerical methods to solve the equations of
motion and thermodynamics and compute physical fields such as wind velocity,
temperature, and moisture. GCMs can be further classified as atmospheric, oceanic,
or coupled ocean-atmosphere general circulation models. The differences between
various GCMs are due to their complexity, resolution, and parameterization of the
physical processes that occur on a subgrid scale (e.g. cloud formation, radiation, pre
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45
evaporation
*
solar luminosity
••h solar
J radiation a
longw ave
radiation
' <
* modol gria
h e a lin g a n d c o o lin g c f a t m o s p h e i e
change ti> composition of atmosphere
B B B W W M
— , — . — ™. ■ .
' -
**------
sum acs current
sn o w
cover
tnermonaline
ocean
model grid mode
Figure 3.1. Schematic diagram of a coupled general circulation model (GCM). The components of
the model (adapted and modified from Suenaka (1992)) include an atmosphere, an ocean, and a land
surface model of varying grid resolutions. A coupler can be used to link the different components of
the GCM. Physical fields (temperature, moisture, salinity, wind velocity, etc.) are computed by
solving equations of motion and thermodynamics. Interactions between components of the GCM are
either calculated or parameterized. Inputs into the GCM include solar luminosity, orbital
parameters, atmospheric gas composition, continental distribution and topography, bathymetry of the
seafloor, terrestrial vegetation type and distribution, and land and sea ice coverage. Prescribed ocean
temperature and salinity profiles are used to initialize the GCM.
cipitation, convection, hydrology, and other surface processes). Figure 3.1 is a
schematic showing the complexity of a coupled GCM and how various components
conditions and initial conditions. Boundary conditions are not part of the equations
used to calculate the physical fields. Depending on the complexity of the GCM, they
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46
may include solar radiation incident at the top of the atmosphere, orbital parameters,
atmospheric gas composition, continental distribution and topography, bathymetry of
the seafloor, terrestrial vegetation type and distribution, and land and sea ice
coverage. Typically, prescribed ocean temperature and salinity distributions are used
to initiate climate models since ocean residence times are much longer than the
atmosphere. The climate model is then run preferably until the principle components
of the model reach an equilibrium state, which is representative of the average
climate dynamics of the system simulated. These models are used 1) to simulate a
time slice, 2) assess the sensitivity of the climate system to changes of specific
variables, or 3) examine how the ocean and atmosphere react to extreme forcing.
In this chapter, I present preliminary results of numerical simulations of Late
Triassic climate, a time coincident with the third largest biotic crisis of the
Phanerozoic. During the Late Triassic, the continued rifting of the supercontinent
Pangea likely produced profound effects on the climate, such as global warming,
increased aridity, and strong monsoon circulation. However, the scarcity of the
global paleoclimate dataset for this time interval requires the use of climate models
to understand the paleoceanographic conditions for the time. Specifically, I examine
the sensitivity of the Norian climate system to atmospheric CO2 forcing, and explore
the link between increases of atmospheric CO2 and climate change during this time.
Also, the implications increasing pCCL has on biotic turnover at the T-J boundary are
briefly discussed.
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4 7
At the end of the Triassic, about 80% of all species went extinct (Sepkoski,
1992). Massive turnover occurred in both the marine and terrestrial realm: 30% of
marine genera (Raup and Sepkoski, 1986; Palfy et al., 2000) and 50% of tetrapod
species (Colbert, 1986) went extinct. Many microfloral species also exhibited
turnover (Lundblad, 1959) and more than 95% of megafloral species in Europe
(Visscher and Bragman, 1981) and North America (Fowell, 1994) disappeared at the
end of the Triassic.
This mass extinction event coincides with a major perturbation of the global
carbon cycle as indicated by a large negative carbon isotopic excursion, recorded in
marine carbonates and organic matter (Ward et al., 2001), and paleosols (e.g. Tanner
et al., 2001; Retallack, 2001; Ekart et al., 1999; Yapp and Poths, 1996).
Furthermore, studies of stomatal characters of fossil leaves (McElwain et al., 1999),
changes in sea level (Hallam and Wignall, 1997), and flood basalt volcanism
(Marzoli et al, 1999; Hesselbo et al., 2002) suggest that atmospheric CO2 increased
significantly at the end of the Triassic. These environmental perturbations also
occurred at broadly the same time as the emplacement of the Central Atlantic
Magmatic Province flood basalts (Marzoli et al., 1999). Thus, CCE-induced global
warming due to volcanic outgassing has been invoked as a cause for the mass
extinction at the end of the Triassic (Stothers, 1993; Courtillot, 1994; Marzoli et al.,
1999; Yapp and Poths, 1996; McElwain et al., 1999; Hallam, 2000). However, it is
unclear how changes in atmospheric CO2 may be linked to the biotic crisis at the end
of the Triassic. It has been postulated that atmospheric CO2 increases can have
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48
significant influence on the climate system by inducing an increased greenhouse
effect, and thus driving biotic turnover. But the data used to infer paleoclimatic
conditions of the Late Triassic are scarce and the link between CO2 and climatic
changes is still tenuous for the Triassic-Jurassic boundary.
3.2 THE MODEL AND SIMULATIONS
The experiments were run using a fully coupled mixed-resolution ocean and
atmosphere GCM, called the Fast Ocean Atmosphere Model (FOAM) version 1.5.
The atmospheric component is the parallelized version of the Community Climate
Model 2 (CCM2) developed by NCAR and Oak Ridge and Argonne National
Laboratories. The radiative and hydrological physics have been upgraded by Rob
Jacob to be equivalent to the CCM3 version 3.2 (described in detail by Kiehl et al.,
1996). The atmospheric component divides the globe into 40 latitude and 48
longitude grids (4.5° x 7.5°), with 18 vertical levels. The ocean component, OM3,
has been optimized for performance and scalability on massively parallel processing
computers. The horizontal resolution of OM3 is a 128 x 128 grid. While the ocean
component of FOAM 1.5 can be run at 16 or 24 vertical levels, for these simulations
the ocean was divided in 16 levels. A coupler links the atmospheric component of
FOAM to the ocean model OM3, and is described in detail by Jacob (1997).
To test the effect of increased CO2 on the Late Triassic climate system,
atmospheric CO2 levels were varied while all other boundary and initial conditions
were kept identical between simulations of climate conditions for the time. The Late
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4 9
Triassic lasted for -22 million years, from 227.4 to 200 Ma (International
Strati graphic Chart). The continental distribution used for the simulations were
based on the Scotese (1994) reconstruction for the Norian (216 Ma), which include
coarse estimates of topography and bathymetry (Figure 3.2).
i Mountains 1 Landmass ( f Continental Margin □ Deep Water
Figure 3.2. Paleogeographic reconstruction of the Late Triassic (216 Ma), modified from Scotese
(1994). Solid black lines demarcate present day continental margins.
Two major landmasses, Laurasia and Gondwanaland, joined near the equator
to form the supercontinent of Pangea. North America, Greenland, and major parts of
Eurasia formed Laurasia in the northern hemisphere, while South America, Africa,
India, Australia, and Antarctica formed Gondwanaland in the southern hemisphere.
For most of the Triassic and Jurassic, Pangea was nearly symmetrical about the
equator. Fifty-five percent of the total exposed land area was in the southern
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5 0
hemisphere by Norian times (Parrish, 1985). The Panthalassa Ocean and Tethys Sea
surrounded Pangea on all sides.
Because of the limited data on Late Triassic vegetation distribution, uniform
land surface characteristics were prescribed for all grid cells containing land so as
not to impose any subjective judgment about the distribution of moist and arid
regions. Solar luminosity was reduced to 98% of the present-day level, as suggested
by Endel and Sofia (1981). The CH4 and N2O concentrations were set to modem
values of 1714 ppbv and 311 ppbv, respectively. The model eccentricity, obliquity,
precession, rotation rate, and ozone concentrations were similarly set to present day
values because of the lack of knowledge of these parameters for the Late Triassic.
However, considering the simulated time spans millions of years, at some point
during the Late Triassic, these parameters were comparable to present day values.
The ocean model was initialized using modem day temperature and salinity profiles.
The Late Triassic simulations only differed in the prescribed atmospheric
CO2 concentrations. Many values for atmospheric CO2 have been postulated for the
Late Triassic and Early Jurassic period and controversy still remains over the
magnitude of increasing CO2 levels across the T-J boundary. Atmospheric CO2
concentrations can be reconstructed from fossil stomatal characteristics (McElwain
et al., 1999; Retallack, 2001), geochemical proxies (Pearson and Palmer, 2000; Ekart
et al., 1999), and geochemical modeling (Bemer and Kothavala, 2001). Throughout
the Mesozoic, atmospheric CO2 levels were high (Ekart et al., 1999; Retallack, 2001;
Bemer and Kothavala, 2001). For example, Retallack (2001), who used fossil plant
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51
cuticles to determine atmospheric CO2 levels, suggested that for most of the
Mesozoic era, CO2 levels were quite high (1000-2000 ppmv), with transient
excursions to even higher concentrations (>2000 ppmv). Estimates of carbon
dioxide concentrations, using a standard diffusion model C 0 2 paleobarometer on
pedogenic carbonates, show that CO2 increased through the Triassic to
approximately 3000 ppmv (Ekart et al., 1999). Significant controversy over the
pCC> 2 estimates stems from the magnitude of change across the T-J boundary. Using
changes in stomatal density of fossil leaf cuticles of Gingkgoales and Cycadales
plants, McElwain et al. (1999) determined that CO2 levels increased fourfold across
the T-J boundary, from 600 to 2100-2400 ppmv. Bemer and Kothavala’s
geochemical modeling of the long-term carbon cycle suggests that at the T-J
boundary, the atmospheric CO2 concentrations were 3-4 times the pre-industrial level
of 300 ppmv. Tanner et al. (2001), contradict these findings. They argue that
atmospheric CO2 levels across the boundary were relatively stable, rising only by
about 250 ppmv, based on estimates from carbon isotopic compositions of pedogenic
calcite from paleosol. In response, Beerling (2001) suggested that by taking into
account the natural variance of stable isotopic values in terrestrial plant leaves -
which Tanner et al. (2001) did not - pCCh actually rose up to 1000 ppmv across the
T-J boundary.
The CO2 sensitivity of the Late Triassic climate considered here tested
atmospheric carbon dioxide levels ranging from 1200-2400 ppmv, or 4-8 times the
pre-industrial level, which bracket most of the estimates for CO2 concentrations
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52
found in the literature. Three simulations were run at 4, 6, and 8 times the pre
industrial value of atmospheric CO2 (referred to as 4X, 6X, and 8X simulations
herein) for the duration of 100 model years. After approximately 50 years, the
atmosphere component of FOAM had equilibrated, but the deep ocean temperatures
have yet to reach steady state. Thus, only surface ocean processes are considered
here. The last 5 years of the model runs were averaged and analyzed for the effects
of varying pC02 on Late Triassic simulations. The results were compared to FOAM
simulations of modem day (MD) climate, the COADS1 , and paleoclimate indicators
from the geologic record. It was noted that when modem temperatures were
compared to COADS, the FOAM simulated modem day climate is much cooler in
the mid-high latitudes. The sea-ice component of FOAM version 1.5 has a tendency
to simulate colder temperatures. However, because the Late Triassic simulations
showed that conditions could not maintain permanent ice, the cold temperatures
simulated were limited to a few months annually and had no significant impact on
the overall climate patterns.
3.3 RESULTS
3.3.1 Global Warming
Preliminary model results suggest that increased global warming, extreme
seasonality, and high aridity are characteristic of the Late Triassic climate. The
1 The Comprehensive Ocean Atmosphere Data Set (COADS) is a compilation of surface marine data
for the period of 1854-1979, gathered in collaboration by the National Climate Data Center, the
Environmental Research Laboratories, and the Cooperative Institute for Research in Environmental
Sciences, and the National Center for Atmospheric Research.
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53
simulated Triassic climate is up to 7°C warmer than the modem day temperatures
derived from COADS (Figure 3.3). The mean annual surface temperatures range
from 21.50-22.05°C for the Triassic simulations. The most intense warming occurs
27 -
O
O
P O
S 3
8. -13 -
S
£
-23
4X PD (FOAM)
6X PD (COADS)
8X
-33
-43
-80 -40 0 40 80
Latitude
Figure 3.3. Zonally averaged mean annual surface air temperature for Late Triassic
simulations (black) and present day (PD). Present day temperatures are derived from
FOAM results and compilations from the Comprehensive Ocean Atmosphere Data
Set (COADS).
in the mid- to high-latitudes where temperatures are up to 14°C hotter during the
Triassic than the modem day simulation, but even the tropics are characterized by
temperatures that are 4-6°C warmer. The highest mean annual temperatures (>32°C)
are found on the tropical lowlands in the interior of Pangea (Figure 3.4A-C).
Significantly warmer ocean temperatures, as compared to the present day, indicate
that global warming during the Late Triassic is not restricted to the continents. In the
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-13 -3 7 17 27 37°C
Surface Air Temperature
0 10 20 30 40 50°C
Annual Temperature Range
Figure 3.4. Mean annual surface air temperature (A-C) and temperature range (D-F) of Late Triassic
simulations. Results are derived from 4X simulation (top), 6X simulation (middle), and 8X
simulation (bottom). Solid black lines demarcate Late Triassic continental margins.
tropics, the mean temperature for the top 200m of the ocean is as high as 32°C
(Figure 3.5), which is almost 4-6°C greater than the present day ocean temperatures
(Figure 3.6A). At the high latitudes of the Late Triassic, mean ocean temperatures
are never below freezing, and no sea ice is able to form. The Tethys Ocean in all
simulations is slightly warmer than other areas in the ocean at the same latitude
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Sea Surface Temperature Sea Surface Salinity
Figure 3.5. Annual average sea surface temperature (A-C) and salinity (D-F) for the top 50m of the
ocean of the Late Triassic simulations. Results are derived from 4X simulation (top), 6X simulation
(middle), and 8X simulation (bottom). White shaded areas demarcate Late Triassic continental
margins.
(Figure 3.5). The warm Tethys becomes a source of warmth for the continental
interiors, where strong western boundary currents, due to the increasing E-W wind
stress component, continually maintain the warm temperatures (Figure 3.6C).
The large size of Pangea contributes to the extreme seasonality characteristic
of Late Triassic climate, especially in the southern hemisphere. The high latitudes
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56
A. C.
-80 0 40 80 -40
B.
Latitude
34-
30-
26-
2 2 -
80 ■ 8 0 -40 0 40
Latitude
-4-
-40 -80
Latitude
D.
Latitude
< < r 6
e
Z 4
2 2
80 -40 0 40 -80
Figure 3.6. Zonally averaged mean annual sea surface temperature (A), salinity (B), east-west
wind stress (C), and north-south wind stress of Late Triassic simulations (4X, 6X, 8X) compared
to present day run (PD).
experience the largest annual temperature range (Figure 3.4D-F). In the interior of
Pangea the annual average range is as high as 48°C. Even in the tropics, the annual
temperature range is up to 10°C, as compared to ~5°C for the present day. These
temperature ranges simulated for the Late Triassic are comparable, if not higher,
than the modem Eurasian continent. Summer temperatures in excess of 30°C are
typical for the tropics (Figure 3.7A-C). Maximum summer temperatures occur in
the western tropical interior of Pangea, where they can reach as high as 50°C.
Above freezing temperatures extend to the poles in the summer hemisphere. Thus,
no permanent ice sheet is maintained during the Triassic. Winter freezing temper-
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57
July Surface Air Temperature January Surface Air Temperature
Figure 3.7. Mean annual surface air temperature in July (A-C) and January (D-F) of Late Triassic
simulations. Results are derived from 4X simulation (top), 6X simulation (middle), and 8X
simulation (bottom). Solid black lines demarcate Late Triassic continental margins.
atures do not extend as far equatorward in the Late Triassic as they do in the modem
day (Figure 3.8A). In the southern hemisphere, freezing temperatures reach
equatorward to about 50-80° latitude along the west coast of Pangea, 40-50° latitude
in the interior, and 40-45° latitude along the eastern coast. In the northern
hemisphere, the freezing temperatures only extend equatorward to 40-50° latitude
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58
(Figure 3.7D-F). Thus, Late Triassic climate is characterized by cold winter
temperatures and warm summer temperatures.
Vast areas of Pangea receive very little moisture and experience arid to
semiarid conditions throughout most of the year. The difference in zonally averaged
mean annual precipitation minus evaporation of the Triassic simulations does not
vary much from the present day run (Figure 3.8C). However, most of Pangea
experiences negative P-E values, indicating that evaporation tended to exceed
precipitation on land from equator to the poles (Figure 3.9). The exceptions are the
land bordering the Tethys Sea, a narrow belt in the northern tropics, and the polar
A. C.
300-
290-
280
270
1 260-
250
240-
230
-80
B.
— 4X P iJ '/O A M ;
- M l T O IC OA D S)
-40 0 40
Latitude
80
o 300
250-
200 -
100-
50-
-50
Latitude
a.
-2 -
0 40 80 -40 -80
D.
Latitude
8
2 o
o
&
0
Latitude
Figure 3.8. Zonally average mean annual surface air temperature (A), greenhouse forcing
(B), precipitation minus evaporation (C), and specific humidity (D) of Late Triassic
simulations (4X, 6X, 8X) compared to present day run (PD).
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59
D
-0.5 0 0.5 !.(>x!07 m /s 0 4 8 12 1 6 x l0 7 m /s
Precipitation Minus Evaporation Precipitation
Figure 3.9. Mean annual precipitation minus evaporation rate (A-C) and total precipitation rate
(D-F) of Late Triassic simulations. Results are derived from 4X simulation (top), 6X simulation
(middle), and 8X simulation (bottom). Solid black lines demarcate Late Triassic continental
margins.
coasts. The warmer temperatures will raise the saturation vapor pressure (see “The
Clausius-Clapeyron Equation”, Wallace and Hobbs, 1977). But an increase in
atmospheric water vapor content does not translate into increased moisture on the
supercontinent of Pangea in this case due to the increased evaporation potential that
also results from higher global temperatures. The moisture received by precipitation
is located mostly along the coast, particularly in western tropical Pangea, and over
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6 0
the oceans (Figure 3.9). In the Triassic simulations, the concentration of convective
cloud cover is primarily over the oceans, and is reduced compared to the present day
simulation (not shown). Since convective precipitation is responsible for most of the
precipitation globally, this indicates that the net moisture transfer from ocean to land
was less than present day.
The warm global temperatures are partially attributed to an increased
greenhouse effect (Figure 3.8A-B) arising from higher levels of atmospheric CO2.
The enhanced greenhouse effect largely results from the large increase in water
vapor content in the atmosphere, as shown by the higher values of specific humidity,
because water vapor is also an effective greenhouse gas (Figure 3.8D). Furthermore,
the high global temperatures are too warm to sustain permanent land or sea ice cover,
which reduces the planetary albedo and acts as a positive feedback to the global
warming during the Late Triassic. Additionally, warming at higher latitudes during
the Late Triassic is maintained by increased ocean heat transport. Houghton et al.
(1996) determined that the present oceans transport a maximum of 2-3 PetaWatts
poleward for the modem day. The FOAM simulation of present day ocean heat
transport is much larger at the low latitudes (Figure 3.10). Regardless, the Late
Triassic simulations show that the oceans transported even greater amounts of heat
(Figure 3.10). The Triassic simulations suggest that the ocean transported almost 8-
10 times more heat that what is observed in the present day. In sum, the warm
temperatures during the Late Triassic are a result of 1) an increased greenhouse
effect resulting from increasing atmospheric C02 and water vapor content, 2) greater
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61
meridional heat transport by the ocean, and 3) a positive feedback from reduced
planetary albedo because of the lack of permanent ice cover.
16-
o.
4X
6X
8X
PD
-16-
40 s o -80 -40 0
Latitude
Figure 3.10. Meridional heat transport by the oceans of Late
Triassic simulations. Positive numbers indicate northward ocean
heat transport, while negative values indicate southward transport
of heat. Units are in PetaWatts.
3.3.2 Sensitivity to CO? Changes
The Late Triassic modeling results indicate that, although raising levels of
atmospheric CO2 only slightly increases the greenhouse effect and surface air
temperatures, higher levels of carbon dioxide in the air are sufficient to change ocean
circulation patterns. The Triassic climate system simulated here indicates that
increasing CO2 in the atmosphere does not necessarily result in ubiquitously warmer
temperatures. While higher atmospheric C 02 concentrations (relative to the present
day) do result in global warming during the Late Triassic, the global average
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62
temperature does not vary significantly between the three Triassic simulations, even
when the CO2 concentration was doubled. Average planetary temperatures are
21.50°C for CO2 atmospheric concentration of 1200 ppmv, 21.54°C for 1800 ppmv,
and 22.05°C for 2400 ppmv. However, regional patterns differ significantly between
Triassic simulations due to the role of ocean dynamics in controlling surface
temperatures. Figure 3.3 shows that surface air temperatures are higher in the tropics
and subtropics when atmospheric C 0 2 is increased, but lower in the high latitudes
when C 02 concentrations are raised from four times the pre-industrial level to eight
times. Since raising the C 0 2 content between the 4X simulation and the 6X
simulation results in warming almost the entire planet, most of the high latitude
cooling occurs when C 02 is raised from six times to eight times the pre-industrial
level (Figure 3.11). The lower average temperatures at high latitudes are due to high
summer and low winter temperatures, resulting in larger annual temperature ranges
when C 02 levels are increased (Figure 3.4). In addition, the increasing C 02 levels
do not have significant impact on the aridity on land, since most of Pangea was
already quite dry. Although the increased temperatures are partially attributed to
higher atmospheric C 02 content, doubling C 02 in the atmosphere from 1200 to 2400
ppmv results in raising the zonally averaged greenhouse effect by only 5 W/m2 in
any region (Figure 3.8A). Raising the C 02 concentration between the 4X and 6X
simulation results in both increasing the average surface temperature and the
greenhouse effect (Figure 3.12A). In general, increases in greenhouse forcing sim
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63
H i
-12 -8 -4 0 4 8°C
Surface Air Temperature
Figure 3.11. Difference in air surface temperature of Late Triassic runs: (A) 6X
minus 4X, (B) 8X minus 6X, and (C) 8X minus 4X. Dashed contour line
represents 0°C difference in surface air temperature.
ulations tend to correspond to increases in temperature at most latitudes, with the
exception of the polar regions (Figure 3.12). Thus, the Triassic simulations show
that the link between the increased CO2 levels in the atmosphere and the climate is
complicated.
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6 4
Temperature (°C)
Q -2
Greenhouse Forcing ( W/in2)
S O -80 -40 0 40
Latitude
6
4
2
0
■ 2
-4
■ 6
80 -80 -40 0 40
Latitude
Q -2
80 -40 40 -80 0
Latitude
Figure 3.12. Differences in surface air temperature (solid black line) and
greenhouse forcing (dashed line) between Late Triassic simulations: (A) 6X minus
4X, (B) 8X minus 6X, and (C) 8X minus 4X. Units for greenhouse forcing are
W/m2.
The surface temperature distribution demonstrates that larger differences
exist between the 4X and 6X simulation than between the 6X and 8X simulation.
These differences can be attributed to changes in ocean dynamics between Late
Triassic simulations. When atmospheric CO2 is increased, meridional ocean heat
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-80 -40 0
Latitude
B.
a
G .
a j
Q
- 0.0
- 1.0
- 2.0
-3.0
-4.0
-5.0
" T V ■ ' ?
■■MM
40 80
■HHHF
m a m
M ' •■ •■ ': c
' l ^
v
* 1 1
l i a l i H H H H M W I
-80 -40 0
Latitude
40
c.
a
a *
&
< U
P
.0.0 - ^ ~ - \
- 1.0
- 2.0
J ' ° f
-4.0 I
-5.0 - ■
■ ■ • ' f7T
v ; : I f v ' ,
-80
-Z'
-40
iS S S
80
il* fc
0 40 80
Latitude
-100 -50 0 50
Meridional Overturning
100 Sv
Figure 3.13. Meridional overturning in the ocean of Late
Triassic simulations. Results are derived from 4X
simulation (top), 6X simulation (middle), and 8X simulation
(bottom). Solid contour lines are positive values indicating
flow in the counterclockwise direction and dashed contour
lines are negative values showing flow in the clockwise
direction. The flow direction is always parallel to the
contour lines. Units are in Sverdrups (106 cubic meters/s),
and contour interval is 5 Sv.
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66
transport decreases, implying that the ocean in the 6X and 8X simulation have a
reduced capacity for carrying heat to the higher latitudes. At the southern high
latitudes, ocean heat transport is diminished by almost eight times when CO2 levels
are raised from 1200 ppmv to 1800-2400 ppmv (Figure 3.9). The trend is not as
severe in the northern Hemisphere, where the existence of land surrounding the
Tethys Sea restricts ocean circulation. Figure 3.13 shows the stream functions for
meriodional overturning for the Late Triassic simulations. Meridional overturning is
the measure of mass transport in the ocean. The 4X simulation exhibits more
rigorous overturning (50-60 Sv, or almost 3 times greater) than the 6X and 8X
simulations, especially in the Southern Panthalassic Ocean. With increasing CO2,
the oceans become more stagnant. Warm deep-water temperatures in the high
latitudes characterize the Late Triassic. The warm high latitude deep waters are
formed primarily from the sinking of warm surface waters and the export of warm
saline waters from the Tethys Sea in the northern hemisphere. In the simulations
with higher atmospheric CO2 concentrations, the near surface temperatures become
progressively warmer and less saline (Figure 3.14A-C) due to the increased
precipitation at these latitudes (Figure 3.14D-F). The result is a decrease in deep-
water formation at high latitudes and less vigorous overturning with increasing CO2.
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67
Sea Surface Temperature Sea Surface Salinity
Figure 3.14. Difference in sea surface temperature (left) and sea surface salinity (right) of Late
Triassic runs: (A,D) 6X minus 4X, (B,E) 8X minus 6X, and (C,F) 8X minus 4X.
3.4 DISCUSSION
3.4.1 Model Comparisons
The supercontinent configuration of Pangea and its impact on the climate
system have been explored both through observational and modeling studies since it
was first recognized that the land/ocean distribution are significant in shaping ancient
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68
climates (Lyell, 1837; Koppen and Wegener, 1924). While the geological record of
Pangean climate has been explored for decades, modeling studies have only more
recently become available, and only very few studies exist that simulate the Late
Triassic climate system. All modeling studies of Pangean climate simulate global
warming, intense aridity and high seasonality on land (e.g. Kutzbach and Gallimore,
1989; Chandler et al., 1992; and Crowley, 1994).
These modeling results are consistent with the geologic record showing that
during the Triassic and Jurassic aridity was extensive over much of Pangea and the
planet experienced widespread warmth (e.g. Robinson, 1973; Ziegler et al., 1979;
Frakes, 1979; Bambach et al., 1980, Hay et al., 1981; Parrish et al., 1982; Crowley,
1983, and Lloyd, 1983). According to Gordon (1975), this time had “an apparently
unique level of evaporite deposition,” where global evaporite formation was at its
peak in the Phanerozoic (Gordon 1975; Habicht, 1979; Busson, 1982). The
extensive aridity is further verified by the relative ubiquity of similarly distributed
red bed deposits and the common occurrence of eolian sand deposits (Habicht, 1979;
Crowley, 1983; Ronov et al., 1989), as well as changes in clay mineralogy at this
time to suggest increased dryness (Tucker and Benton, 1982; Hallam et al., 1991).
However, humid zones did exist during the Triassic in the high latitudes, indicated
by the presence of coals and bauxite deposits at these areas (Robinson, 1973;
Habicht, 1979; Hallam, 1985; Ronov et al., 1989). This evidence for warmth, and
the existence of fairly cosmopolitan biota during the Late Triassic and Early Jurassic
times have led some researchers to suggest that climates were more ‘equable’
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69
worldwide (Hallam, 1985; Shubin et al., 1991). For instance, Hallam (1985) asserted
that the existence of diverse floral assemblages at high latitudes and the lack of
glacial evidence indicate that polar regions were warmer and wetter during the Late
Triassic than they are today. While this observation may be true, modeling results in
this study and elsewhere (e.g. Crowley et al., 1989; Kutzbach and Gallimore, 1989;
Chandler et al., 1992) suggest that large seasonal temperature fluctuations exist over
mid- and high-latitude on the continental interiors, where summer temperatures are
quite warm while winter temperatures are rather cool. This discrepancy between the
fossil record and model results requires further investigation.
One of the first modeling studies of Late Triassic climate was performed by
Wilson et al. (1994) using the GENESIS AGCM, coupled with a slab ocean, with
prescribed ocean heat transport. They modeled the Late Triassic during the Camian,
(225 Ma), and obtained results that suggest no apparent polar ice sheets existed
during the Late Triassic, and most of the seasonal precipitation fell on major
highland areas of Pangea, primarily over the tropical and subtropical highlands.
Also, the continental interiors exhibited very large seasonal temperature ranges
(>45C C), and strong monsoonal circulation. The simulations done for this study also
reproduced this result, although the atmospheric model used herein was coupled with
a dynamic ocean GCM, and are consistent with the geological data. The results from
this preliminary study are also consistent with Chandler et al. (1992), who modeled
the Early Jurassic using the GCM developed at the Goddard Institute for Space
studies. They showed that the Early Jurassic was 5-10°C warmer than the present
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70
day, whereas the results from this study only show a warming of 6-7°C relative to
the modem day. Chandler et al. (1992) also concluded that the global warming
simulated for the Early Jurassic resulted primarily from increased ocean heat
transport and high levels of atmospheric CO2. The global warming was further
sustained by 1) decreases in planetary albedo due to a reduction of sea ice, snow
cover, and low clouds, and 2) increases in atmospheric water vapor.
While the results of this study largely agree with simulations of Pangean
climate done by Chandler et al. (1990), they contradict the suggestion of Kutzbach et
al. (1990). Kutzbach et al. (1990) were among the first to model the circulation
patterns of the world ocean Panthalassa using a low-resolution dynamic ocean
model. They used a highly idealized ocean basin configuration to represent Pangean
times, and obtained results to suggest that ocean heat transport during the existence
of the Pangean supercontinent is similar to present day values. The maximum ocean
heat transport is about 1.7 PetaWatts and occurs at about 30° latitude in their
simulation. Furthermore, Kutzbach et al. (1990) showed that the ocean during
Pangean time exhibited relatively strong thermohaline circulation, with annual
average meridional circulation reaching as high as 20 Sverdrups in both
hemispheres. The Late Triassic experiments in this study show that meridional
overturning is stronger in the southern hemisphere than the northern hemisphere at
relatively low levels of atmospheric CO2, and the converse is true when CO2 is
increased (Figure 3.13). Only in the northern hemisphere is the meridional
overturning comparable to those simulated by Kutzbach et al. (1990). In the
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71
southern hemisphere, the 4X simulation shows mass transport is much greater than
the Kutzbach et al. (1990) results, whereas the 6X and 8X simulations show that
overturning is much weaker. However, Kutzbach et al. (1990) also simulated
warmer deeper water than present, with temperatures below about 1000 m to be as
warm as 8-10°C. In the Late Triassic simulations, deep ocean temperatures are also
warmer than present day. However, they are not as warm as those simulated by
Kutzbach et al. (1990). Reasons for the observed differences in ocean circulation in
this study and those of Kutzbach et al. (1990) may lie in the fact that the models used
were very different. Kutzbach et al. (1990) utilized a low-resolution model (5° x 5°)
with 8 vertical levels, and forced it using results of surface fields derived from an
atmospheric GCM that was coupled to a 50 m mixed-layer ocean (Kutzbach and
Gallimore, 1989). Furthermore, Kutzbach et al. (1990) ran their model for a longer
period of time, of 400 model years, whereas the results presented here are derived
from only 100 years of model integration time. It is important to note that although
there are differences between the results from this study and those done by Kutzbach
et al. (1990), the qualitative trends of Pangean ocean circulation are similar: very
warm and deep waters, deep water formation occurs at high latitudes where
relatively fresh polar waters are cooled and sink, and the Tethys is a source of both
warm deep water and heating for the continents. It would be interesting to see if
these qualitative trends continue when FOAM 1.5 is allowed to run long enough for
the ocean component to equilibrate.
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72
3.4.2 CO? Sensitivity
Much research exists about the importance of past carbon dioxide
fluctuations as a cause for climate change in the Phanerozoic. Early work by Eric
Barron, Robert Berner, and colleagues suggested that increases in atmospheric CO2
are required to explain warm time periods in Earth’s history (Berner et al., 1983;
Berner, 1991, 1994; Barron and Washington, 1985; Barron et al., 1995; Sloan and
Rea, 1996). A recent study comparing the model estimates with proxy climate
indicators of atmospheric carbon dioxide levels and global temperatures shows that
there is good first order-agreement between changes in CO2 and climate for the
global-scale temperature change (Crowley, 2000). Despite the good agreement
between global temperatures and atmospheric CO2 levels, additional factors (such as
changes in orography, ocean circulation, and vegetation) are often required to
explain climate change. Manabe and Bryan (1985) showed that temperature of the
surface polar waters largely determines the temperature of the deep water, and that
the intensity of the N-S THC and the magnitude of the oceanic ocean heat transport
were relatively insensitive to increases in the temperature caused by increasing CO2.
This is in direct disagreement with the results from this study, which suggests that
changes in C 02 levels do have significant impact on ocean heat transport and ocean
dynamics during the Late Triassic. In addition to having little impact on ocean heat
transport, other studies have shown that changes in atmospheric CO2 have little
effect on continental warmth which could offset the large winter cooling simulated
(Manabe and Bryan, 1985; Schneider et al., 1985, Chandler et al., 1992). Rather,
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73
warming caused by increasing CO2 results, primarily from climate feedbacks such as
surface temperature warming, sea-ice decrease, and cloud changes. In this study,
however, simulated surface temperatures on land do respond to increased
atmospheric CO2, particularly in the high latitudes, where decreased ocean heat
transport results in significantly lowered temperatures. The Late Triassic scenario
simulated here suggests that changes in CO2 do have an impact on climate by
changing the dynamics in the ocean.
3.4.3 Implications for Biota
The mass extinction at end of the Triassic ultimately gave rise to the
dinosaurs (Tucker and Benton, 1982; Olsen, 1987), wiped out the last of the
Paleozoic holdovers from the end-Permian mass extinction in both the marine and
terrestrial realm (Raup and Sepkoski, 1987; Palfy et al., 2000), and set the stage for
the emergence of the Modem Fauna (Sepkoski, 1992), including such organisms as
the scleractinian corals, found commonly today. It has been suggested that the
drastic increases in carbon dioxide input to the atmosphere due to volcanic
outgassing from the emplacement of the Central Magmatic Province flood basalts
caused the mass extinction (Stothers, 1993; Courtillot, 1994; Marzoli et al., 1999;
Yapp and Poths, 1996; McElwain et al., 1999; Olsen, 1999; Hallam, 2000). For
example, McElwain et al. (1999) suggested that a fourfold increase in atmospheric
carbon dioxide (from 600 to 2100-2400 ppmv) would increase the mean global
temperature by 3-4°C. This type of global warming would be sufficient to cause
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74
high -temperature injury to leaves by decreasing their ability to transpirationally cool,
resulting in the extinction of may large-leaved species at the T-J boundary. The
results from this modeling study of Late Triassic climate show that a two-fold
increase in atmospheric CCL (from 1200 to 2400 ppmv) only warms the mean global
temperature by 0.5°C. This result brings into question the C02-induced global
warming as a plausible mechanism for the end-Triassic mass extinction.
It has long been noted that Late Triassic climate was more ‘equable’ (Hallam,
1985; Shubin et al., 1991) because of the existence of fairly cosmopolitan biota at
this time. Diverse Late Triassic fossil floral assemblages have been found at much
higher latitudes than where they occur presently (Hallam, 1985; Retallack, 2001).
The simulated climate in this study demonstrates that mid- to high-latitude areas
showed extreme seasonal fluctuations, of high summer temperatures and cold winter
temperatures. The existence of such a cosmopolitan biota may imply that organisms
that existed during the Late Triassic were able to tolerate a much wider range of
environmental conditions. Similarly, the results shown here indicate that high levels
of atmospheric carbon dioxide could reduce ocean circulation, as well as raise the
temperature of the deep ocean. Both these observations could result in lowered
oxygenation levels in the ocean, and may have immense repercussions for the marine
fauna.
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3.4.4 Model Limitations and Future Studies
The Late Triassic climate simulated contradicts some previous modeling
results and observational studies, and further modeling attempts are required to make
for better data-model comparisons. Better modeling of Late Triassic climate should
include more realistic land surface characteristics since vegetation has significant
impact on albedo and transpiration. In particular, expansion of vegetation to high
latitudes during the Late Triassic could have resulted in greater absorption of
incoming solar radiation, which would increase high latitude temperatures.
Additionally, the effect Milankovitch variations have on temperature should be
investigated. Some geologic evidence exists for higher-frequency Milankovitch-
scale fluctuations on Pangea: lake deposits from Triassic rift system in eastern North
America (Olsen, 1986) and other cyclic deposits (Van Houten, 1964; Fischer, 1964).
Crowley et al. (1992) showed that Milankovitch variations can modulate the
magnitude of summer warming by as much as 14-16°C on Pangea, with large
changes occurring in both the northern and southern hemispheres at mid- and high-
latitudes. The differences between the ‘hot’ and ‘cold’ summer orbit configurations
that they used resulted in seasonal insolation values differing by as much as 160
W/m2 for mid-latitudes. Since seasonal temperatures differ so greatly on
supercontinents for different orbital configurations, comparisons between GCM
simulations and observations when only one configuration is used may lead to
erroneous conclusions.
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76
Changes in geography (e.g. Barron and Peterson, 1990) also impact ocean
circulation patterns, as well as the opening and closing of ocean gateways such as the
Central American isthmus, Drake Passage, and Indonesian straits (Maier-Reimer et
al., 1990; Hirst and Godfrey, 1993; Mikolajewicz et al., 1993). The Late Triassic
was a time of development of the rift system that ultimately ruptured Pangea to form
the present day continental configurations. The rift system developed in the modem
day Central Atlantic Ocean. Thus, the changing ocean circulation patterns that
resulted from the rifting of Pangea should be further investigated.
Changes in ocean circulation also have great impact on the chemistry of the
ocean. In order to understand the mechanisms that cause biotic changes in the
oceans during the Late Triassic it is necessary to understand how simulated ocean
circulation patterns affect the fauna. As suggested earlier, the warmer ocean
temperatures and reduced ocean circulation arising from increased CO2 may have
impacted the oxygenation levels in the ocean and possibly contributed to the mass
extinction at the end of the Triassic. This hypothesis can be explored by
incorporating a biogeochemical component to the general circulation model used
here to assess relative and absolute oxygenation levels in the oceans during the Late
Triassic.
3.5 CONCLUSION
In this chapter, numerical simulations of Late Triassic climate were
performed using a coupled ocean-atmosphere GCM to examine the sensitivity of the
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77
Norian world to atmospheric CO2 forcing. This approach enables us to explore the
possible linkages between changes in CO2 and the biotic response since the time
slice simulated is coincident with continued rifting of the supercontinent Pangea,
which likely produced profound effects on the climate, and one of the ‘Big Five’
mass extinctions of the Phanerozoic. Late Triassic experiments were performed in
which all other boundary conditions were held constant while atmospheric CO2 was
varied from 4,6, and 8 times pre-industrial values.
All experiments show that intense warming, extreme seasonality, and high
aridity characterize Late Triassic Pangean climate. These features of the Late
Triassic world are maintained by a significantly increased greenhouse effect and
ocean heat transport, as compared to present day values. An increase in the
greenhouse effect results from both greater atmospheric CO2 levels and increased
atmospheric water vapor content due to higher temperatures. But an increase in
atmospheric water vapor content does not translate into increased moisture on the
supercontinent of Pangea because of higher evaporation potential that also results
from higher global temperatures. Thus, the continental interiors remain very arid in
all Triassic simulations.
While increasing the atmospheric CO2 concentrations only raises global
average temperatures slightly during the Late Triassic, significant changes can occur
regionally due to the changes in ocean dynamics. Compared to the present day
simulation, the Late Triassic oceans are significantly warmer and less saline due to
intensified precipitation patterns from increased atmospheric CO2. Consequently,
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78
Triassic oceans are generally more sluggish than in the modem simulations, as
shown by the reduced meriodional overturning in Triassic simulations. Furthermore,
increased atmospheric C 02 levels are also very effective in changing global ocean
circulation patterns by reducing the ocean heat transport. Thus it appears that higher
levels of C 0 2 were effectively making the world ocean Panthalassa more stagnant
during the Late Triassic.
Global warming due to an increased greenhouse effect because of elevated
levels of CO2 may not be a sufficient mechanism for the mass extinction at the end
of the Triassic since global average temperature do not increase significantly even
with doubled atmospheric C 02 levels. These results demonstrate, however, that
changing ocean dynamics may play an important role in the evolution of some
marine fauna at the time since increased ocean temperatures and a more stagnant
ocean could have resulted in more oxygen depletion in the water column. Additional
simulations using a climate model that incorporates a biogeochemical component
should be used to test this hypothesis.
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CHAPTER 4. DISTRIBUTION OF LATE TRIASSIC
HALOBIID BIVALVE ACCUMULATIONS
4.1 INTRODUCTION
The association of the “flat clam facies” in organic-rich deposits is a
signature of the Mesozoic rock record. The flat clam accumulations typically
contain very cosmopolitan faunas with very rapid evolutionary rates (Tozer, 1984;
McRoberts, 1997). These bivalves occur in organic-rich, laminated, fine-grained
deposits (Oschmann, 1988; Rohl et al., 2001; Parrish, 1987, Kauffman, 1991;
Campbell, 1994; Kobayashi, 1963; McRoberts, 1993) representing very oxygen
deficient shelf environments, where bottom waters may have ranged from anoxic to
dysoxic conditions for extended periods of time (Rohl et al., 2001; Parrish et al.,
2001; McRoberts, 2000; Oschmann, 1993; Sageman et al., 1991).
Halobiid bivalve accumulations, particularly deposits consisting of the genera
Daonella and Halobia, are an example of such an organic-rich flat clam facies from
the Triassic. The earliest of such accumulations, containing the bivalve Daonella,
are found in deposits of Middle Triassic age throughout the world (Figure 4.1).
Halobia accumulations, which are mostly Late Triassic in age, have a similar global
distribution. When Triassic paleogeographic reconstructions are utilized, it is shown
that Halobiidae are found in all the Triassic seas (Tozer, 1984; McRoberts, 1997).
Halobiid bivalves are unique in that they are among the most short-lived and
cosmopolitan bivalves found in the fossil record, with evolutionary rates some times
exceeding those of ammonoids (Tozer, 1984; McRoberts, 1997). Furthermore,
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Figure 4.1. Two bivalve genera from the family Halobiidae, Daonella and
Halobia, and their stratigraphic range.
evidence has shown that these bivalves were benthic organisms capable of living in
very low oxygenated environments (for detailed discussion, see Chapter 2). The
widespread distribution of halobiid bivalves during the Middle and Late Triassic
stages may suggest that oxygen-deficient bottom waters were prevalent during the
Triassic. However, the origin of organic-rich deposits is still contested, and various
models have been proposed for the accumulation of organic-rich rocks.
It has long been recognized that oceanographic processes control the
distribution of organic-rich sediments. Different models have been proposed for the
distribution of organic-rich deposits, particularly in Mesozoic strata. Researchers
have explained high accumulations of organic-rich sediments using ocean stagnation
models with several subvariants (Fischer and Arthur, 1977; Ryan and Cita, 1977;
Thierstein and Berger, 1978; Arthur and Schlanger, 1979; de Graciansky et al., 1984;
TIME
Daonella sp.
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81
and others), increased carbon supply models (Dean et al., 1978; Jenkyns, 1980;
Pedersen and Calvert, 1990; Parrish, 1993; and others), and the warm saline bottom
waters hypothesis (Brass et al., 1982). Many researchers have suggested that
organic-rich sediments can accumulate in areas of high productivity where there is
an increase in the carbon supply, such as upwelling zones (e,g. Trask, 1932;
Brongersma-Sanders, 1948; McKelvey, 1959; Dott and Reynolds, 1969; Dow, 1978;
Parrish et al., 1979; Demaison and Moore, 1980; Parrish, 1982; and numerous
others). In upwelling zones, persistent vertical currents continuously supply of
nutrients to the photic zone to sustain high bioproductivity at the surface and allow
for the accumulation of rocks rich in organic matter below these zones (Figure 4.2).
Alternative explanations for the distribution of organic-rich deposits in the fossil
record include the oceanic anoxic event (OAE) model and the transgression model.
ORGANIC-RICH LIMESTONE / MUDSTONE PHOSPHATtC LIMESTONE / SANDSTONE
(finely lam inated) PHOSPHORITE (bioturbatton, sh elly horizons)
Figure 4.2. Model for halobiid bivalve accumulation, modified from Parrish et al. (2001).
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82
Record of
Global Sea Level
(H aq e t al., 1987)
Record of
Transgression-Regression
(K auffm an, 1977)
124.5
131.8
135.0
1.40.7
Figure 4.3. Cretaceous organic carbon burial history, sea level history, and
oceanic anoxic events in the Western Interior Basin, U.S.
Schlanger and Jenkyns (1976) first proposed the OAE model to explain the
widespread distribution of black shales during certain times of the Cretaceous.
During three episodes in the Cretaceous, particularly the Aptian/Albian (OAE I), the
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Cenomanian/Turoni an (OAE II), and possibly the Santonian/Campanian (OAE M)
the oceans experienced widespread oxygen depletion likely due to higher seawater
temperatures. The bottom waters contained insufficient oxygen to oxidize the
carbon at the sediment-water interface, and thus allowed for greater preservation of
organic matter and increased accumulation of organic-rich sediments.
Furthermore, the accumulation of organic-rich sediments has been associated
with changes in sea level. However, the nature of this relationship is complex. A
rapid increase in organic-rich deposits during the Middle Jurassic to Middle
Cretaceous (Parrish and Curtis, 1982) corresponds to more or less a steady rise in sea
level (Vail et al., 1977). Episodes of high organic carbon burial in the Cretaceous
rock record are associated with global sea level highstands (Figure 4.3). However,
the fluctuation in the number of organic-rich rocks during the Triassic and Early
Jurassic bears no apparent relation to sea level changes (Parrish and Curtis, 1982).
The global distribution of organic-rich rocks containing halobiid bivalves
during the Late Triassic raises the question as to the origin of these accumulations.
Since oceanographic processes control the distribution of organic-rich sediments,
climate simulations can be used to test the hypotheses for the accumulation of
organic-rich sediments in the geologic record. This chapter focuses on the
distribution of halobiid bivalve accumulations during the Late Triassic in organic-
rich deposits and their possible origins. Climate model simulations are used to
assess the environmental tolerance of halobiid bivalves during the Late Triassic, and
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84
the feasibility of the hypothesis that organic-rich sediments containing dense
accumulations of halobiid bivalves were deposited in upwelling zones is examined.
4.2 METHODOLOGY
The results from the simulation of Late Triassic climate, as described in the
previous chapter, are used herein to assess halobiid bivalve distributions. For a
detailed description of the model and experiments, please refer to “The Model and
Simulations” in Chapter 3. The majority of the results presented here are obtained
from the 4xCC>2 experiment, when atmospheric CO2 levels were set to 1200 ppmv.
The results from this experiment were chosen because the atmospheric CO2
concentration was thought to have been about this level, prior to the emplacement of
the Central Magmatic Province, when Halobia was prevalent in the fossil record.
The results from the climate simulation using FOAM 1.5 are compared with the
halobiid bivalve distribution for Norian times compiled by Tozer (1984) and
McRoberts (1997) (Figure 4.4). The comparison between bivalve occurrences and
paleoceanographic parameters allowed for the estimation of bivalve tolerances of
temperature and salinity.
In addition, climate models can be used to predict the location of upwelling
regions, and hence the likely areas where organic-rich deposits may accumulate.
Major upwelling currents are driven by winds. There are two types of persistent,
wind-driven upwelling currents: coastal upwelling and oceanic divergences. Coastal
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85
Figure 4.4. Distribution of halobiid bivalves during the Norian stage, based on
Tozer (1984) and McRoberts (1997). Localities are demarcated as solid black
circles. Paleogeographic reconstruction is from Scotese (1994).
upwelling occurs under a steady longshore wind direction oriented with the land on
the left in the Northern Hemisphere and on the right in the Southern Hemisphere.
Friction and the Coriolis effect combine to create a net offshore flow of water in the
upper several tens of meters. The water diverging from the coast is replaced from
nutrient-laden waters from below. High biologic productivity results when the water
reaches the surface. Oceanic divergences occur in areas of low atmospheric
pressure, and similarly results in upwelling and high biological productivity. Thus,
the halobiid distributions were compared to simulated areas of upwelling to test the
hypothesis that these Late Triassic organic-rich deposits are found at or near
upwelling zones.
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86
4.3 RESULTS
4.3.1 Environmental Tolerance
Halobiid bivalve accumulations are found from low to high latitudes during
the Norian stage of the Triassic. The majority of the deposits are along the edges of
Eastern Pangea, primarily around the Tethys Sea. Figure 4.5 shows the sea
temperature and salinity averaged for the top 175m of the ocean, with the halobiid
occurrences. (Only the top 175m of the ocean was examined since modem shelf
settings are typically less than 175m in depth, and ample evidence exists for flat clam
facies representing shelf environments.) The Halobia accumulations are present in
regions where ocean temperatures are colder than other localities at the similar
latitude for all depth levels plotted. The bivalves occur in a wide range of latitude
(45°S to 75°N), where waters temperatures varied from 6° to 25° and salinities from
28 to 36 psu. This suggests that halobiids had a very large range of tolerance with
\
M
-2 6 14 22 30 38°C
Sea Temperature
24 26 28 30 32 34 36 38 40 psu
Sea Salinity
Figure 4.5. Average sea temperature and salinity for the top 175m of the ocean. Solid black circles
demarcate halobiid bivalve occurrences.
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87
respect to temperatures and salinity, and is consistent with evidence for Halobia
being a cosmopolitan genus. However, depending on the interpretation of life mode,
Halobia may or may not have been able to tolerate such varied temperature and
salinity conditions in the ocean since these parameters vary greatly with depth. The
ocean component of FOAM is divided into 16 depth levels, and the upper surface
levels can be examined to further assess the degree of temperature and salinity
tolerance possible for Halobia during the Late Triassic which have direct bearing on
the life mode controversy surrounding bivalves of the “flat clam facies” (see Chapter
2 for a detailed discussion). Figure 4.6 shows the average sea temperature and
salinity calculated for four depth ranges (0-25m, 25-75m, 75-125m, and 125-175m)
simulated by FOAM. The difference between surface ocean temperatures and that at
depths of 175m is as large as 6°C at most halobiid localities. The difference in
salinity is about 2 psu. Whether Halobia was a pseudoplanktonic organism living in
the water column or a benthic bivalve is important since a large range in
environmental conditions is simulated between these two habitats.
4.3.2 Bivalve Accumulations and Upwelling Zones
In the simulations of the Late Triassic, upwelling zones occur in equatorial
regions, along the coasts of Pangea in the low to mid latitudes, and at divergence
areas in the mid to high latitudes (Figure 4.7). Equatorial regions have the highest
upwelling velocity in the Late Triassic simulations (greater than 0.3xl0'5 m/s), while
most of the other regions of upwelling have vertical velocities less than 0.3xl0"5 m/s.
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88
A -
- , ^ 4* * *4 k , ‘ V
BESi
, 7 - -
ift ...
< y >
MtfPH
Sea Temperature Sea Salinity
Figure 4.6. Average sea temperature (A-D) and salinity (E-H) of four depth levels of the ocean from
top to bottom: 0-25m, 25-75m, 75-125m, and 125-175m depth. Solid black circles demarcate halobiid
bivalve occurrences.
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89
Plotting the Halobia bivalve accumulations along with the vertical velocities
simulated for the Late Triassic shows that the majority of the deposits (40 out of 48)
occur in upwelling regions, with great a number of the halobiid localities occurring
in the Tethys Sea. At these localities, cold bottom waters are upwelled to the
surface, as shown in the temperature profiles in Figures 4.5 and 4.6. Interestingly,
when the Halobiid localities are compared to the simulated Late Triassic climate
where atmospheric CO2 was increased to 1800 and 2400 ppmv, the number of
localities occurring in areas of upwelling decreases to 36 and 31, respectively
(Figures 4.8 and 4.9). Raising the atmospheric pCCL in the simulations resulted in
the contraction of upwelling regions during the Late Triassic.
Vertical Velocity
Figure 4.7. Average vertical velocity of the top 175m of the ocean for 4xC02 simulation.
Dashed lines indicate velocity of 0 m/s, or no net vertical current movement. Positive values
indicate vertical current movement upwards towards the surface. Solid black circles demarcate
halobiid bivalve occurrences. Dashed contour lines represent 0 m/s.
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-1.5 -0.9 -0.3 0.3 0.9 1.5xl0-5 m/s
Vertical Velocity
Figure 4.8. Average vertical velocity of the top 175m of the ocean for 6xC02 simulation.
Dashed lines indicate velocity of 0 m/s, or no net vertical current movement. Positive values
indicate vertical current movement upwards towards the surface. Solid black circles demarcate
halobiid bivalve occurrences. Dashed contour lines represent 0 m/s.
Vertical Velocity
Figure 4.9. Average vertical velocity of the top 175m of the ocean for 8xCQ2 simulation. Dashed
lines indicate velocity of 0 m/s, or no net vertical current movement. Positive values indicate
vertical current movement upwards towards the surface. Solid black circles demarcate halobiid
bivalve occurrences. Dashed contour lines represent 0 m/s.
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91
4.4 DISCUSSION
The simulated results of the Late Triassic paleoceanographic conditions
presented here were obtained after running FOAM for 100 model years. While after
approximately 50 years, the atmospheric component of the model had equilibrated,
the deep ocean temperatures had yet to reach steady state. Thus, only surface ocean
processes were considered here. Although the ocean component of FOAM had not
equilibrated, qualitative statements can still be made for the possible environmental
tolerances of such organisms during the Late Triassic and the origin of halobiid
bivalve deposits with respect to their occurrences in regions of upwelling. The
climate simulations have shown this time is characterized by increased warming
throughout the ocean due in part to increased poleward ocean heat transport.
Halobiid bivalve accumulations had a wide latitudinal extent during the Late
Triassic. The results shown here suggest that Halobia may have had great tolerance
for a wide range of temperatures and salinity in the oceans even though the equator
to pole gradient is diminished. While a large environmental tolerance is customary
for cosmopolitan organisms such as Halobia, the range of conditions these bivalves
were able to inhabit must be carefully examined.
The widespread occurrence of halobiid accumulations may have resulted
from increased preservation of such deposits during the Late Triassic. Halobia
concentrations are typically found in organic-rich, high carbonate content facies from
Late Triassic rocks. Other carbonate deposits, such as reefs, were also known to
have great latitudinal extent during the Late Triassic, ranging from 38°S to more than
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92
40°N (Kiessling, 2001). Like the occurrences of Halobia accumulations, the
majority of reefs were limited to western boundary settings, with the notable
exceptions of prolific reef growth at the western margin of Pangea and northern
margin of Australia, where reefs are associated with eastern boundary currents and
are close to predicted upwelling zones (Kiessling et al., 1999). Reefs, however, are
composed of organisms with very limited environmental tolerances. Typically,
carbonate production of reefs is low, with notable exceptions during the mid-
Silurian, Givetian-Frasnian, Late Triassic, Late Jurassic, mid-Cretaceous, and
Neogene (Kiessling et al., 2000). The Middle to Late Triassic has also been shown
to be a period of intense carbonate precipitation, as reflected by the increased
abundance of marine cements, ooids, and calcified cyanobacteria (Riding, 1996).
Geological evidence and climate simulations have shown significant global warming
during the Late Triassic. Raising temperature will: 1) by equilibrium effects, directly
increase CaCCL precipitation; and 2) by kinetic effects, speed up equilibration.
Furthermore, the simulations using FOAM have shown that the Late Triassic is
characterized by increased rainfall, which could result in enhanced terrigenous
weathering and greater delivery of calcium and bicarbonate into the oceans. The net
result is increased rates of carbonate precipitation. Thus, greater occurrences of
halobiid bivalves, which are preserved predominately in facies of high calcium
carbonate content, during the Late Triassic may be a result of increased preservation
of such facies in the rock record.
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93
Comparison of halobiid bivalve localities with simulated vertical current
velocity indicates that the majority of the deposits are found in upwelling regions.
Upwelled waters bring nutrients to the surface to stimulate biological productivity
and increasing the supply of organic matter to the sea floor. If Halobia are indeed
benthic bivalves, the increased supply of organic matter to the sea floor can also
promote the growth of such opportunistic species. Increased oxygen utilization by
organisms at the surface would diminish oxygen levels in the water column and
enhance the preservation of organic matter in the sediments. The FOAM simulations
show that ocean temperatures are several degrees warmer than the present day ocean.
Warmer ocean temperatures decrease oxygen solubility in the seawater, thus,
enhancing the oxygen depletion in the water column. The net result would be
increased accumulation and preservation of dysaerobic taxa like Halobia in organic-
rich deposits.
Fischer and Arthur (1977) and, subsequent workers, have stressed that
productivity patterns in space and time are the key to understanding the distribution
of organic-rich deposits. It is still unclear as to the origin of halobiid bivalves in the
fossil record. The coincidence of halobiid occurrences with upwelling zones
suggests that the accumulation of halobiid bivalves in the fossil record may have
resulted from increased carbon supply during the Late Triassic. Decreased bottom
water oxygenation and increased supply of organic matter to the sea floor are
conditions that select for the proliferation of opportunistic organisms capable of
inhabiting dysoxic environments. However, the Late Triassic was also a time of
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9 4
increased carbonate precipitation and the conditions in the bottom waters also
favored enhanced preservation of organic-rich carbonate deposits. It is interesting to
note that upwelling areas are diminished in the simulations with higher atmospheric
pC 02. Evidence exists for a carbon dioxide spike across the Triassic-Jurassic
boundary, a time coincident to the disappearance of Halobiidae from the fossil
record.
The results presented here are consistent with the upwelling model for the
accumulation of organic-rich deposits containing halobiid bivalves. The ocean
stagnation model cannot be ruled out since evidence also exists for a more sluggish
ocean with increasing levels of atmospheric pC02 (see Results Figure 3.13 from
Chapter 3). In order to better determine the origin of such organic-rich
accumulations in the geologic record, future research should focus on the interplay
between carbon supply at the surface, carbon degradation in the water column, and
carbon preservation at the sea floor. While understanding the oceanographic
processes during the Late Triassic can give us a starting point in determining the
origin of organic-rich deposits, biological and chemical processes are major
influences in the accumulation of such sediments. Thus, future simulations of Late
Triassic paleoceanographic conditions require a biogeochemical component in order
to understand the complex interactions that result in accumulation of organic-rich
sediments during such times. Furthermore, truly understanding the oceanographic
and biogeochemical conditions controlling the accumulation of organic-rich rocks
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95
requires detailed studies of basin evolution histories and basin-by-basin comparisons
of organic facies and paleogeographic settings.
4.5 CONCLUSION
Comparison of the global distribution of organic-rich rocks containing
halobiid bivalves to results derived from climate model simulations of the Late
Triassic indicate that Halobia are cosmopolitan organisms with wide environmental
tolerances. The majority of the deposits are found within simulated upwelling
regions. The climate simulations suggest that the Late Triassic oceans were warm
and sluggish. These conditions are enhanced with increased atmospheric carbon
dioxide levels. Warm water temperatures and diminished thermohaline circulation
may have exacerbated the oxygen depletion in the oceans during the Late Triassic
and resulted in increased accumulation of dysoxic and anoxic facies containing
halobiid bivalves. However, caution must be taken before attributing the
accumulation of flat clam facies during the Late Triassic solely to widespread
oxygen depletion since this time oceanographic conditions also favored enhanced
carbonate precipation and preservation of such deposits. Thus, further research
should implement a biogeochemical model to the simulation of Late Triassic climate
to improve the understanding of the origin of organic-rich deposits during this time.
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CHAPTER 5. CONCLUSIONS
5.1 SUMMARY OF RESULTS
Massive reorganization of marine communities that occurred after the end-
Permian mass extinction set the foundation for the diversification patterns of the rest
of the Mesozoic and gave rise to the Modem Fauna (sensu Sepkoski). Among the
Modem Fauna, the flat clam facies is a unique signature of Mesozoic strata, and a
representative dysaerobic fauna from the fossil record. The flat clam assemblages
found in Mesozoic deposits represent an oxygen-deficient biotope that was once
widespread and persistent, but no longer prevalent. The goal of this research was: 1)
to examine the paleoecology of a representative taxon of the Mesozoic flat clam
facies, namely the bivalve Daonella, in order to define an oxygen-depleted biotope
from Triassic times; 2) to simulate the paleoceanographic conditions that may have
existed during the Late Triassic, the time of the height of the oxygen-depleted
biotope examined here, just prior to their decline at the end of the Triassic, which
could give rise to such a biotope; and 3) to combine the climate simulation results
with the paleoecological study to obtain a hypothesis as to why the dysaerobic flat
clam facies was prevalent during the Late Triassic.
The representative flat clam assemblage examined in this research is the
high-density accumulations consisting of Daonella bivalves from Middle Triassic
Fossil Hill Member, which outcrops in central Nevada. The Fossil Hill Member was
deposited in a quiet shelf environment that periodically experienced dysoxic to
anoxic conditions. In other words, the shelf environment has been interpreted to be
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97
dynamically unstable. The Daonella bivalves preserved in the Fossil Hill Member
were epibenthic organisms adapted to living in oxygen-deficient environments.
Daonella accumulations represent a dysaerobic biofacies composed of opportunistic
bivalves capable of rapidly colonizing substrates because of favorable bottom water
environments. Favorable conditions included a ready food supply due to the high
supply of organic matter and exclusion of predators due to the dysoxia and suboxia
that existed in such waters. Horizons of Daonella within the Fossil Hill Member
represent colonization events, and the dense accumulations are multiple events
concentrated through time by gentle wave action and winnowing at the seafloor.
Excellent preservation results from suboxic conditions and the binding of Daonella
horizons by bacterial mat formation. The similarity between the Daonella deposits
found throughout the world and accumulations containing bivalves from the family
Halobiidae suggest that dysaerobic facies were prevalent during the Middle and Late
Triassic.
The prevalence of such dysaerobic facies during the Triassic, and their
paucity in modem settings, suggests that specific paleoceanographic conditions must
have existed that selected for the proliferation, accumulation, and preservation of
high-density flat clam accumulations during the Middle and Late Triassic. The use
of a coupled atmosphere-ocean general circulation model to simulate paleoclimate
indicates that the Late Triassic climate was characterized by intense global warming,
extreme seasonality, and high aridity on land. The Late Triassic oceans are also
significantly warmer, less saline due to the resultant increases in precipitation
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98
patterns, and generally more sluggish than the modem simulations. The warm
climate of the Late Triassic is maintained by an increased greenhouse effect (due to
elevated levels of atmospheric carbon dioxide and water vapor), greater meriodional
heat transport by the ocean, and reduced planetary albedo resulting from the lack of
permanent ice cover.
The end of the Triassic was coincident with a major perturbation of the global
carbon cycle, and ample evidence exists to suggest that atmospheric CO2 increased
significantly across the Triassic-Jurassic boundary. These changes may have greatly
influenced the emergence and later disappearance of Triassic flat clams at the end of
the Triassic. The Late Triassic experiments demonstrate that the climate is sensitive
to changes in atmospheric CO2, in that the ocean dynamics differ depending on the
carbon dioxide concentration in the atmosphere. Raising the pC 02 levels results in
1) decreasing the heat transported to the poles by the oceans, 2) reduction of deep-
water formation in the high latitudes, and 3) diminished overturning of the ocean.
The paleoenvironmental conditions simulated using the coupled atmosphere-ocean
model are consistent with conditions favorable to the accumulation of organic-rich
flat clam facies in the geologic record.
Bivalves from the family Halobiidae make up the primary components of flat
clam facies of Middle and Late Triassic age. They are globally distributed, found in
all the Triassic seas, and typically occur in organic-rich, laminated sediments
containing high amounts of calcium carbonate. The Late Triassic climate
simulations were compared the halobiid accumulations collected worldwide, and
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9 9
results are consistent with halobiid bivalves being cosmopolitan organisms able to
withstand a wide range of environmental conditions. The halobiid accumulations
have a large latitudinal extent, which encompass a wide range to temperatures and
salinity. Halobiid bivalves have been shown to be tolerant of very low-oxygenated
environments, and the prevalence of such organic-rich deposits during the Late
Triassic suggests that the oceans may have experienced general oxygen depletion.
Alternatively, the greater abundance of the flat clam facies may be a result of greater
carbon supply to the seafloor or the increased preservation potential for such flat
clam assemblages at this time. When the halobiid localities are compared to the
vertical current velocities in the ocean that are simulated by the climate model, it has
been shown that the majority of the organic-rich deposits are found in simulated
upwelling zones. In the simulations where atmospheric CO2 was raised, the total
number of halobiid accumulations coinciding with upwelling regions is progressively
diminished. The climate simulations with varying levels of atmospheric pCCL have
also shown the oceans to be warm and sluggish, conditions that may contribute to
depleting the oxygen in the water column. Combined with the general global
warming and likelihood of increased continental weathering due to increased rainfall,
the results derived from Late Triassic climate simulations are consistent with other
observations suggesting that this time experience enhanced carbonate precipitation.
The general warming of the ocean, possible oxygen depletion, and increased
carbonate precipitation potential may have all contributed to the increased potential
for preserving the flat clam facies in the fossil record. The disappearance of halobiid
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100
deposits at the end of the Triassic may then be linked to the major perturbation in the
global carbon cycle. The re-emergence of the flat clam facies, comprised of other
bivalve lineages, later in the Mesozoic in similar settings under comparable
oceanographic conditions suggests that the appearance of the family Halobiidae
during the Middle Triassic can be viewed as a prelude to other Mesozoic flat clam
facies. Thus, the bivalves from the family of Halobiidae are significant players in
shelf settings during the initiation of the Marine Mesozoic Radiation.
5.2 FUTURE RESEARCH
Many questions still remain unanswered with respect to the origin of the flat
clam facies of the Mesozoic and the accumulation of organic rich deposits in the
geologic record. What adaptations, including functional morphology and life
strategies, enabled halobiid bivalves and other flat clams to not only survive, but also
thrive in oxygen-deficient environments? To what extent do the thick accumulations
of halobiid bivalves preserved in the fossil record represent the population structure
of the living assemblages? Is the prevalence of such assemblages in the Mesozoic
fossil record a product of enhanced preservation due to ideal oceanographic
conditions, or a result of intense proliferation of significant players in the history of
life in the marine realm? It is still yet unclear if the paleoceanographic conditions
that allowed for both the emergence and proliferation of flat clam assemblages of the
Mesozoic were basinally restricted or globally prevalent. How well has the Triassic
paleoceanography been simulated using this particular climate model? Futhermore,
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101
the necessary environments that selected for the success of such an opportunistic
species is little understood. To what extent were the paleoenvironments that the
bivalves inhabited depleted in oxygen or experienced increased carbon supply? Why
did oxygen-deficient habitats that housed flat clam assemblages during the Mesozoic
disappear in more modem settings?
In order to address these questions, efforts should be taken to better
understand strategies employed by Halobiidae in their paleoenvironments, and
careful attention should be paid to examine how these bivalve accumulations differ
between localities in order to assess the global extent of such paleoceanographic
conditions. Better modeling studies of the Triassic should be developed, which
include more realistic land surface characteristics and incorporate the effects of
varying Milankovitch parameters on the climate during the Triassic. The extent of
oxygen depletion and increased organic carbon supply due to changes in weathering
can be modeled by incorporating a biogeochemical model to the general circulation
model. Including such a component will also allow for better testing of the
hypotheses for the organic-rich carbon deposits in the geologic record. Finally,
modeling times in geologic history when these habitats were prevalent and more
modem settings, when they are not, would further explain the disappearance of
oxygen-deficient settings by comparing the two scenarios.
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102
BIBLIOGRAPHY
Allison, P. A., Wignall, P. B., & Brett, C. E. (1995). Palaeo-oxygenation: effects
and recognition. In D. W. J. Bosence & P. A. Allison (Eds.), Marine
Palaeoenvironmental Analysis from Fossils. Geological Society Special
Publication 83 (pp. 97-112). London, UK: Geological Society of London.
Arthur, M. A., & Schlanger, S. O. (1979). Cretaceous “oceanic anoxic events” as
causal factors in development of reef-reservoired giant oil fields. American
Association of Petroleum Geologists Bulletin, 63, 870-885.
Bambach, R. K., Scotese, C. R., & Ziegler, A. M. (1980). Before Pangea: The
geographies of the Paleozoic world. American Scientist, 68, 26-38.
Bando, Y. (1964). On some lower and middle Triassic ammonoids from Japan.
Transactions and Proceeding o f the Palaeontological Society of Japan, 56,
332-345.
Barron, E. I., & Peterson, W. H. (1990). Mid-Cretaceous ocean circulation: Results
from model sensitivity studies. Paleoceanography, 2, 729-739.
Barron, E. J., Fawcett, P. J., Peterson, W. H., Pollard, D., & Thompson, S. L. (1995).
A “simulation” of Mid-Cretaceous climate. Paleoceanography, 10, 953-962.
Barron, E. J., & Washington, W. M. (1985). Warm Cretaceous climates: High
atmospheric CO2 has plausible mechanism. In E. T. Sundquist & W. S.
Broecker (Eds.), The Carbon Cycle and Atmospheric CO2 ' Natural
Variations Archean to Present. Geophysical Monograph No. 32 (pp. 546-
553). Washington, DC: American Geophysical Union.
Beerling, D. (2002). CO2 and the end-Triassic mass extinction. Nature, 415, 386-
387.
Bemasconi, S. M. (1991). Geochemical and microbial controls on dolomite
formation and organic matter production/preservation in anoxic
environments: A case study from the Middle Triassic Grenzbitumenzone,
Southern Alps (Ticino, Switzerland). Ph.D. thesis, Eidgenossische
Technische Hochschule Zurich.
Bemasconi, S. M. (1994). Geochemical and microbial controls on dolomite
formation in anoxic environments: A case study from the Middle Triassic
(Ticino, Switzerland). Contributions to Sedimentology, 19, 1-109.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
103
Bemer, R. A. (1991). A model for atmospheric CO2 over Phanerozoic time.
American Journal o f Science, 291, 339-376.
Bemer, R. A. (1994). GEOCARB II: A revised model for atmospheric CO2 over
Phanerozoic time. American Journal of Science, 294, 56-91.
Bemer, R. A., & Kothavala, Z. (2001). GEOCARB HI: A revised model of
atmospheric CO2 over Phanerozoic time. American Journal of Science, 301,
182-204.
Bemer, R. A., Lasaga, A. C., & Garrels, R. M. (1983). The carbonate-silicate
geochemical cycle and its effect on atmospheric carbon dioxide over the last
100 million years. American Journal of Science, 283, 641-683.
Bucher, H. (1992). Ammonoids of the Hyatti Zone and the Anisian transgression in
the Triassic Star Peak Group, northwestern Nevada, USA.
Palaeontographica, 223, 137-166.
Brongersma-Sanders, M. (1948). The importance of upwelling water to vertebrate
paleontology and oil geology. K. Nederl. Akad. Wetens., Afd. Natuurkd., 45,
1- 112.
Brass, G. W., Southam, J. R., & Peterson, W. H. (1982). Warm saline bottom water
in the ancient ocean. Nature, 296, 620-623.
Busson, G. (1982). Le Trias comme periode salifere. Geologische Rundschau, 71,
857-880.
Byers, C. W. (1979). Biogenic structures of black shale paleoenvironments.
Postilla, 174, 1-43.
Cafiero, B., & De Capoa Bonardi, P. (1980). Stratigraphy of the pelagic Triassic in
the Budva-Kotor area (Cma-Gora, Montenegro, Yugoslavia). Bolletino della
Societa Paleontologica Italiana, 19, 179-204.
Cafiero, B., & De Capoa Bonardi, P. (1982). Biostratigrafia del Trias pelagico della
Sicilia. Bolletino della Societa Paleontologica Italiana, 21, 1-71.
Campbell, H. J. (1994). The Triassic bivalves Halobia and Daonella in New
Zealand, New Caledonia, and Svalbard. Institute o f Geological & Nuclear
Sciences Monograph, 4, 1-66.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10 4
Carey, S. P. (1984). Conodont biofacies of the Triassic of northwestern Nevada. In
D. L. Clark (Ed.), Conodont Biofacies and Provincialism. Geological
Society of America Special Paper 196 (pp. 295-305). Boulder, CO:
Geological Society of America.
Chandler, M. A., Rind, D., & Ruedy, R. (1992). Pangean climate during the Early
Jurassic: GCM simulations and the sedimentary record of paleoclimate.
Geological Society of America Bulletin, 104, 543-559.
Chen, J. (1982). Assemblages of Daonella (Bivalvia) in South Guizhou Province.
Scientia geologica sinica, 4, 235-238.
Cohen, A. S., & Coe, A. L. (2002). New geochemical evidence for the onset of
volcanism in the Central Atlantic Magmatic Province and environmental
change at the Triassic-Jurassic boundary. Geology, 30, 267-270.
Colbert, E. H. (1986). Mesozoic tetrapod extinctions: a review. In D. K. Elliot
(Ed.), Dynamics of Extinction (pp. 49-62). New York: John Wiley and Sons.
Courtillot, V. (1994). Mass extinctions in the last 300 millions years: one impact
and seven flood basalts? Israel Journal of Earth Sciences, 43, 255-266.
Cox, L. R. & Newell, N. D. (1969). Family Posidoniidae. In R. C. Moore (Ed.),
Treatise on Invertebrate Paleontology, Part N, Volume 1, Mollusca 6,
Bivalvia, N342-N344. Geological Society of America Incorporated and
University of Kansas.
Crowley, T. J. (1983). The geologic record of climatic change. Reviews of
Geophysics and Space Physics, 21, 828-877.
Crowley, T. J. (1994). Pangean climates. In G. D. Klein (Ed.), Pangea:
Paleoclimate, Tectonics, and Sedimentation During Accretion, Zenith, and
Breakup o f a Supercontinent. Geological Society of America Special Paper
288 (pp. 25-39). Boulder, CO: Geological Society of America.
Crowley, T. J. (2000). Carbon dioxide and Phanerozoic climate. In B. T. Huber, K
G. McLeod, & S. L. Wing (Eds.), Warm Climates in Earth History (p. 425-
444). Cambridge, UK: Cambridge University Press.
Crowley, T. J., Hyde, W. T., & Short, D. A. (1989). Seasonal cycle variations on the
supercontinent of Pangaea. Geology, 17, 457-460.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
105
Dean, W. E., Gardner, J. V., Jansa, L. F., Cepek, P., & Siebold, E. (1978). Cyclic
sedimentation along the continental margin of northwest Africa. Initial
Report o f the Deep Sea Drilling Project, 41, 965-989.
de Graciansky, P. C., Herbin, L., Montadert, C., Muller, A., Schaaf, A., & Sigal, J.
(1984). Ocean-wide stagnation episode in the Late Cretaceous. Nature, 308,
346-349.
De Lorenzo, G. (1896). Fossili del Trias medio di Lagonegro. Palaeontographica
italica, II, 113-148.
Demaison, G. J., & Moore, G. T. (1980). Anoxic environments and oil source bed
genesis. American Association o f Petroleum Geologists Bulletin, 64, 965-
986.
Dott Sr., R. H., & Reynolds, G. T. (1969). Sourcebook for petroleum geology.
American Association o f Petroleum Geologists Memoir, 5, 471 pp.
Dow, W. G. (1978). Petroleum source beds on continental slopes and rises.
American Association of Petroleum Geologists Bulletin, 62, 1584-1606.
Doyle, P., & Whitham, A. G. (1991). Palaeoenvironments of the Nordenskjold
Formation: an Antarctic Late Jurassic-Early Cretaceous black shale-tuff
sequence. In R.V. Tyson and T. H. Pearson (Eds.), Modem and ancient
continental shelf anoxia. Geological Society Special Publication 58 (pp.
397-414). Boulder, CO: Geological Society of America.
Ekart, D. D., Cerling, T. E., Montanez, I. P., & Tabor, N. J. (1999). A 400 million
carbon isotope record of pedogenic carbonates: Implications for
paleoatmospheric carbon dioxide. American Journal of Science, 299, 805-
827.
Encheva, M. G. (1978). Phylogenetic development of the Family Posidoniidae and
the genera Daonella and Halobia (Bivalvia; Triassic). Geologica blacanica,
8, 55-67.
Endel, A. S., & Sofia, S. (1981). Rotation in solar-type starts, I, Evolution models
fro the spin-down of the sun. Astrophysical Journal, 243, 625-640.
Erwin, D. H. (1993). The Great Paleozoic Crisis: Life and Death in the Permian.
New York: Columbia University Press, 327pp.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 0 6
Etter, W. (2002a). Monte San Giorgio: Remarkable Triassic marine vertebrates. In
D. J. Bottjer, W. Etter, J. W. Hagadom, & C. M. Tang (Eds.), Exceptional
Fossil Preservation: A Unique View in the Evolution o f Marine Life (pp. 221-
250). New York: Columbia University Press.
Etter, W. (2002b). Black shale bivalves - a modern analogue at last. In prep.
Farsan, M. (1972). Stratigraphische und Palaogeographische stellung der Khenjan-
seri und deren Pelecypoden (Trias, Afghanistan). Palaeontographica,
Abteilungen A, 140, 131-191.
Fischer, A. G. (1964). The lofer cyclothems of the alpine Triassic. Kansas
Geological Survey, 169, 107-149.
Fischer, A. G., & Arthur, M. A. (1977). Secular variations in oceanic circulation.
American Association of Petroleum Geologists Annual Meeting Abstracts, 2,
23.
Fischer, A. G., & Bottjer, D. J. (1995). Oxygen-depleted waters: a lost biotope and
its role in ammonite and bivalve evolution. Neues Jahrbuch fur Geologie
und Palaontologie Abhandlung, 195, 133-146.
Fliigel, E., & Senowbary-Daryan, B. (2001). Triassic reefs of the Tethys. In G. D.
Stanley, Jr. (Ed.), The History and Sedimentology o f Ancient Reef Systems
(pp. 217-249). New York: Kluwer Academic/Plenum Publishers.
Fowell, S. J. (1994). Palynology of Triassic/Jurassic boundary sections from the
Newark Supergroup of eastern North America: implications for catastrophic
extinction scenarios. PhD thesis, Columbia University, NY.
Frakes, L. A. (1979). Climates Throughout Geologic Time, 310 pp. New York:
Elsevier Scientific.
Freneix, S. (1972). Daonella indica (Bivalvia) de la region d’Antalya (Bordure sud
du Taurus, Turquie). Microstructure du test. Notes et Memoires sur le
Moyen-Orient, Museum National D’Ehstoire Naturelle, Paris, XIII, 1-11.
Gordon, W. A. (1975). Distribution by latitude of Phanerozoic evaporite deposits.
Journal of Geology, 83, 671-684.
Gruber, B. (1976). Neue Ergebnisse auf dem Gebiete der Okologie, Stratigraphie
und Phylogenie der Halobien (Bivalvia). Mitteilungen der Gesellschaft der
Geologie und Bergbaustudenten in Osterreich, 23, 181-198.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
107
Gruber, B. (1983). Revision der im Union Canyon/Nevada und Eagle Creek/Oregon
gefundenen Halobiiden. In E. Kristan-Tollman, A. Tollman, & B. Gruber
(Eds.), Tethys-Faunenelemente in der Trias der U. S. A. Mitteilungen der
Osterrreichischen Geologischen Gesellschaft, 76, 241-248.
Habicht, J. K. A. (1979). Paleoclimate, paleomagnetism, and continental drift.
American Association o f Petroleum Geologists Studies in Geology, 9, 1-32.
Habicht, J. K. A. (1979). Paleoclimate, paleomagnetism, and continental drift.
American Association of Petroleum Geologists, 9, 1-32.
Hallam, A. (1985). A review of Mesozoic climates. Geological Society of London
Journal, 142, 433-445.
Hallam, A. (1987). Mesozoic marine organic-rich shales. In J. R. V. Brooks & A. J.
Fleet (Eds.), Marine Petroleum Source Rocks. Geological Society Special
Publication 26 (pp.251-261). London, UK: Geological Society of London.
Hallam, A. (2000). Mass extinctions and sea-level changes. In C. Koeberl (Ed.),
Catastrophic Events and Mass Extinctions: Impacts and Beyond (pp. 65).
LPI Contribution. Houston, TX: Lunar and Planetary Institute.
Hallam, A., & Wignall, P. B. (1997). Mass extinctions and Their Aftermath.
Oxford: Oxford University Press, 320 pp.
Hallam, A., Grose, J. A., & Ruffell, A. H. (1991). Palaeoclimatic significance of
changes in clay mineralogy across the Jurassic-Cretaceous boundary in
England and France. Palaeogeography, Palaeoclimatology, Palaeoecology,
81, 173-187.
Hames, W. E., Renne, P. R., & Ruppel, C. (2000). New evidence for geologically
instantaneous emplacement of earliest Jurassic Central Atlantic Magmatic
Province basalts on the North American margin. Geology, 28, 859-862.
Haq, B.U., Hardenbol, J., & Vail, P.R. (1987). Chronology of fluctuating sea levels
since Triassic. Science, 235, 1156-1166.
Hay, W. W., Barron, E. J., Sloan II, J. L., & Southam, J. R. (1981). Continental
drift and the global pattern of sedimentation. Geologische Rundschau, 70,
302-315.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
108
Hay, W. W., Behensky Jr., J. F., Barron, E. J., & Sloan n, J. L. (1982). Late
Triassic-Liassic paleoclimatology of the proto-Central North Atlantic rift
system. Palaeogeography, Palaeoclimatology, Palaeoecology, 40, 13-30.
Hay ami, I. (1969). Notes on Mesozoic “planktonic” bivalves (in Japanese with
English abstract). Journal of the Geological Society o f Japan, 75, 375-385.
Hay ami, I. (1991). Living and fossil scallop shell as airfoils: an experimental study.
Paleobiology, 11, 1-18.
Hesselbo, S. P., Grocke, D. R., Jenkyns, H. C., Bjerrum, C. J., Farrimond, P.,
Morgans Bell, H. S., & Green, O. R. (2000). Massive dissociation of gas
hydrate during a Jurassic oceanic anoxic event. Nature, 406, 392-395.
Hesselbo, S. P., Robinson, S. A., Surlyk, F., & Piasecki, S. (2002). Terrestrial and
marine extinction at the Triassic-Jurassic boundary synchronized with major
carbon-cycle perturbation: A link to initiation of massive volcanism?
Geology, 30, 251-254.
Hirst, A. C., & Godfrey, J. S. (1993). The role of Indonesian throughflow in the
global ocean GCM. Journal of Physical Oceanography, 23, 1057-1086.
Hollingworth, N. T. J., & Wignall, P. B. (1992). The Callovian-Oxfordian boundary
in Oxfordshire and Wiltshire based on two new temporary sections.
Proceedings of the Geologists ’ Association, 103, 15-30.
Houghton, J. T., Filho, J. T. M., Callander, B. A., Harris, N., Kattenberg, A., &
Masked, K. (1996). Climate Change 1995: The Science of Climate Change.
Cambridge: Cambridge University Press.
Hubbard, R., & Boulter, M. C. (2000). Phytogeography and paleoecology in
Western Europe and Easter Greenland near the Triassic-Jurassic boundary.
Palaios, 15, 120-131.
Jablonski, D., & Lutz, R. A. (1983). Larval ecology of marine benthic invertebrates:
paleobiological implications. Biological Reviews, 58, 21-89.
Jablonski, D., Sepkoski, J. J. Jr, Bottjer, D. J., & Sheehan, P. M. (1983). Onshore-
offshore patterns in the evolution of Phanerozoic shelf communities.
Science, 222, 1123-1125.
Jacob, R. (1997). Low frequency variability in a simulated atmosphere ocean
system. PhD thesis, University of Wisconsin-Madison.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 0 9
Jeffries, R. P. S., & Minton, P. (1965). The mode of life of two Jurassic
“ Posidonia ” (Bivalvia). Palaeontology, 8, 156-85.
Jenkyns, H. C. (1980). Cretaceous anoxic events - from continents to oceans.
Journal of Geological Society of London, 137, 171-188.
Karhu, J., & Epstein, S. (1986). The implications of the oxygen isotopic records in
coexisting cherts and phosphates. Geochimica et Cosmochimica Acta, 50,
1745-1756.
Kauffman, E. G. (1978). Benthic environments and paleoecology of the
Posidonienschiefer (Toarcian). Neues Jahrbuch fur Geologie und
Palaontologie Abhandlung, 157, 18-36.
Kauffman, E. G. (1981). Ecological reappraisal of the German Posidonienschiefer
(Toarcian) and the stagnant basin model. In J. Gray, A. J. Boucot, and W. B.
N. Berry (Eds.), Communities o f the Past (pp. 311-381). Stroudsburg, PA:
Hutchinson Ross Publishing.
Kauffman, E. G. (1977). Geological and biological overview: Western Interior
Cretaceous Basin. In E. G. Kauffman (Ed.), Cretaceous Facies, Faunas, and
Paleoenvironments across the Western Interior Basin (pp. 75-99). The
Mountain Geologist 14. Denver, CO: Rocky Mountain Association of
Geologists.
Khuc, V. (1990). Characteristic assemblages of the Triassic fauna of Viet Nam.
Triassic biostratigraphy and paleogeography of Asia, Mineral Resources
Development Series, 59. 56-72.
Kidwell, S.M. (1991). The stratigraphy of shell concentrations. In P. A. Allison
and D. E. G. Briggs (Eds.), Taphononomy: Releasing the Data Locked in the
Fossil Record (pp. 211-261). New York: Plenum Press.
Kiehl, J. T., Hack, J. J., Bonan, G. B., Boville, B. A., Briegleb, B. P., Williamson, D.
L., & Rasch, P. J. (1996). Description of the NCAR Community Climate
Model (CCM3), NCAR Technical Note NCAR/NT-420+STR, Boulder, CO.
Kiessling, W. (2001). Phanerozoic reef trends based on the paleoreef database. In
G. D. Stanley, Jr. (Ed.), The History and Sedimentology of Ancient Reef
Systems (pp. 41-88). New York: Kluwer Academic/Plenum Publishers.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
110
Kiessling, W., Fliigel, E., & Golonka, J. (1999). Paleoreef maps: Evaluation of a
comprehensive database on Phanerozoic reefs. AAPG Bulletin, 83, 1552-
1587.
Kiessling, W., Fliigel, E. & Golonka, J. (2000). Fluctuations in the carbonate
production of Phanerozoic reefs. In E. Insalaco, P. Skelton, & T. J. Palmer
(Eds.), Carbonate Platform Systems: Components and Interactions.
Geological Society Special Publications 178 (pp. 191-215). London, UK:
Geological Society of London.
Kittl, E. (1912). Materialen zu einer Monographic der Halobiidae und Monotidae
der Trias, Resultate der Wissenschaftelichen Erforschung des Balatonsees, I
Band, I Teil. Paleontologie Anhang., Wien, 229 p.
Kobayashi, T. (1963). On the Triassic Daonella beds in central Pahang, Malaya:
Contributions to the geology and palaeontology of southeast Asia HI. Recent
Progress of Natural Sciences in Japan, 34, 101-112.
Koppen, W., & Wegener, A. (1924). Die Klimate der geologischen Vorzeit, 255 pp.
Berlin: Verlag von Gebruder Bomtraeger.
Kutzbach, J. E., & Gallimore, R. G. (1989). Pangaean climates: Megasoons of the
Megacontinent. Journal of Geophysical Research, 94, 3341-3357.
Kutzbach, J. E., Guetter, P. J., & Washington, W. M. (1990). Simulated circulation
of an idealized ocean for Pangean time. Paleoceanography, 5, 299-317.
Leppakoski, E. (1975). Assessment of pollution on the basis of macrozoobenthos in
marine and brackish environments. Acta Academiae Aboensis, Ser. B., 35, 1-
90.
Lloyd, C. R. (1983). Pre-Pleistocene paleoclimates: A summary of the geological
and paleontological evidence. Climate Research Institure Report 44.
Corvalis: Oregon State University.
Lundblad, B. (1959). The correlation and vertical distribution of the Rhaeto-Liassic
floras of N.W. Europe and E. Greenland. Stockholm Contributions in
Geology, 3, 87-90.
Lyell, C. (1837). Principles o f Geology, Volume 1. Philadelphia: James Kay, Jr. &
Brother.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
I l l
Maier-Reimer, E., Mikolajewicz, U., & Crowley, T. J. (1990). Ocean general
circulation model sensitivity experiment with an open Central American
isthmus. Paleoceanography, 5, 349-366.
Manabe, S., & Bryan, K., Jr. (1985). C02-induced change in a coupled ocean-
atmosphere model and its paleoclimate implications. Journal of Geophysical
Research, 90, 11689-11707.
Marwick, J. (1953). Divisions and faunas of the Hokonui system (Triassic and
Jurassic). New Zealand Geological Survey Paleontological Bulletin, 21, 1-
142.
Marzoli, A., Renner, P. R., Piccirillo, E. M., Emesto, M., Bellieni, G., & De Min, A.
(1999). Extensive 200-million-year-old continental flood basalts of the
Central Atlantic Magmatic Province. Science, 284, 616-618.
McElwain, J. C., Beerling, D. J., & Woodward, F. I. (1999). Fossil plant and global
warming at the Triassic-Jurassic boundary. Science, 285, 1386-1390.
McKelvey, Y. E. (1959). Relation of marine upwelling waters to phosphorite and
oil. Geological Society of America Bulletin, 70, 1783-1784.
McRoberts, C. A. (1993). Systematics and biostratigraphy of halobiid bivalves from
the Martin Bridge Formation (Upper Triassic), northeast Oregon. Journal of
Paleontology, 67, 198-210.
McRoberts, C. A. (1997). Late Triassic North American Halobiid bivalves:
stratigraphic distribution, diversity trends, and their circum-Pacific
correlation, p. 198-208. In J. M. Dickins (Ed.), Late Paleozoic and Early
Mesozoic circum-Pacific Events (pp. 198-208). Cambridge: Cambridge
University Press.
McRoberts, C. A. (2000). A primitive Halobia (Bivalvia: Halobiodea) from the
Triassic Northeastern British Columbia. Journal of Paleontology, 74, 599-
603.
McRoberts, C. A., & Stanley, G. D., Jr. (1989). A unique bivalve-algae life
assemblage from the Bear Gulch Limestone (Upper Mississippian) of central
Montana. Journal o f Paleontology, 63, 578-581.
Mikolajewicz, U., Maier-Reimer, E., Crowley, T. J., & Kim, K.-Y. (1993). Effect
of Drake and Panamanian gateways on the circulation of an ocean model.
Paleoceanography, 8, 409-426.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
112
Morante, R., & Hallam, A. (1996). Organic carbon isotopic record across the T-J
boundary in Austria and its bearing on the cause of the mass extinction.
Geology, 24, 391-394.
Morris, K. A. (1980). Comparison of major sequences or organic-rich mud
deposition in the British Jurassic. Journal of the Geological Society, 137,
157-170.
Mojsisovics, E. von. (1874). Uber die triadischen Pelecypoden Gattungen Daonella
und Halobia. Abhandlungen der Kaiserlich Koniglich geologischen
Reichsansta.lt, 7, 1-38.
Muller, S. W., Ferguson, H. G., & Roberts, R. J. (1951). Geology of the Mount
Tobin quadrangle, Nevada. U.S. Geological Survey Quadrangle Map GQ-7.
Newell, N. D., & Boyd, D. W. (1987). Interation of ligament structures in
pteriomorphian bivalves. American Museum Novitates, 2875, 1-11.
Nichols, K. M., & Silberling, N. J. (1977). Stratigraphy and depositional history of
the Star Peak Group (Triassic), northwestern Nevada. Geological Society of
America Special Paper 178, 73 pp.
Olsen, P. E. (1986). A 40-million-year lake record of early Mesozoic orbital climate
forcing. Science, 234, 842-848.
Oschmann, W. (1988). Kimmeridge clay sedimentation - a new cyclic model.
Palaeogeography, Palaeoclimatology, Palaeoecology, 65, 217-251.
Oschmann, W. (1993). Environmental oxygen fluctuations and the adaptive
response of marine benthic organisms. Journal o f Geological Society, 150,
187-191.
Palfy, J., Mortensen, J. K., Carter, E. S., Smith, P. L., Friedman, R. M., & Tipper, H.
W. (2000). Timing the end-Triassic mass extinction: first on land, then in
the sea? Geology, 28, 39-42.
Parrish, J. T. (1982). Upwelling and petroleum source beds, with reference to the
Paleozoic. American Association of Petroleum Geologists Bulletin, 66, 750-
774.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
113
Parrish, J. T. (1985). Latitudinal distribution of land and shelf and absorbed solar
radiation during the Phanerozoic. U. S. Geological Survey Open-File Report,
85-31, 21pp.
Parrish, J. T. (1987). Lithology, geochemisty, and depositional environment of the
Shublik Formation (Triassic), northern Alaska. In I. L. Tailleur and P.
Weimer (Eds.), Alaskan North Slope Geology (p. 391-396). Anchorage:
Pacific Section, Society of Economic Paleontologists and Mineralogists and
the Alaska Geological Society.
Parrish, J. T. (1993). Climate of the supercontinent Pangea. The Journal of
Geology, 101, 215-233.
Parrish, J. T. (1998). Interpreting Pre-Quaternary Paleoclimate from the Geologic
Record. Columbia University Press, New York, 338 p.
Parrish, J. T., & Curtis, R. C. (1982). Atmospheric circulation, upwelling, and
organic-rich rocks in the Mesozoic and Cenozoic eras. Palaeogeography,
Palaeoclimatology, Palaeoecology, 40, 31-66.
Parrish, J. T., & Gautier, D. L. (1993). Sharon Springs Member of Pierre Shale:
Upwelling in the Western Interior Seaway? In Caldwell, W. G. E., &
Kauffman, E. G. (Eds.), Evolution of the Western Interior Basin. Geological
Association of Canada Special Paper 39 (pp. 319-332). Toronto, Canada:
Geological Society of Canada.
Parrish, J. T., Droser, M. L., & Bottjer, D. J. (2001). A Triassic upwelling zone:
The Shublik Formation, Arctic Alaska. Journal o f Sedimentary Research,
71,272-285.
Parrish, J. T., Hansen, K. S., & Ziegler, A. M. (1979). Atmospheric circulation and
upwelling in the Paleozoic, with reference to petroleum source beds.
American Association o f Petroleum Geologists Bulletin, 63, 507-508.
Pearson, P. N., & Palmer, M. R. (2000). Estimating atmospheric pC02- In B.
Schmitz, B. Sundquist, & F. P. Andreas son (Eds.), Early Paleogene Warm
Climates and Biosphere Dynamics: Short Papers and Extended Abstracts
(pp. 127-128). GFF 122. Stockholm, Sweden: Geological Society of
Sweden.
Pedersen, T. F., & Calvert, S. E. (1990). Anoxia vs. productivity: What controls the
formation of organic-carbon-rich sediments in sedimentary rocks? American
Association of Petroleum Geologists Bulletin, 74, 454-466.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
114
Polubotko, I. V. (1984). Zonal and correlation significance of Late Triassic
halobiids. Soviet geology, 6, 40-51.
Polubotko, I. V. (1989). On the morphology and systematics of the Late Triassic
Halobiidae (bivalve mollusks). Annual of the All-Union Paleontological
Society, 31, 90-103.
Raup, D. M., & Sepkoski, J. J., Jr. (1986). Periodic extinctions of family and
genera. Science, 231, 833-836.
Reiber, H. (1968). Die Artengruppe der Daonella elongate Mojs. Aus der
Grenzbitumenzone der mittleren Trias des Monte San Giorgio (Kt. Tessin,
Schweiz). Palaontologische Zeitschrift, 42, 33-61.
Reichow, M. K., Saunders, A. D., White, R. V., Pringle, M. S., Al’Mukhamedov, A.
I , Medvedev, A. I , & Kirda, N. P. (2002). 4 0 Ar/3 9 Ar dates from the West
Siberian Basin: Siberian flood basalt province doubled. Science, 296, 1846-
1849.
Reid, R. G. B., & Brand, D. G. (1986). Sulfide-oxidizing symbiosis in lucinaceans:
implications for bivalve evolution. Veliger, 29, 3-24.
Renne, P. (2002). Flood basalts - bigger and badder. Nature, 296, 1812-1813.
Retallack, G. J. (2001). A 300-million-year record of atmospheric carbon dioxide
from fossil plant cuticles. Nature, 411, 287-290.
Retallack, G. J. (2002). Triassic-Jurassic atmospheric CO2 spike. Nature, 415, 387-
388.
Retallack, G. J., & Krull, E. S. (1999). Landscape ecological shift at the Permian-
Triassic boundary in Antarctica. Australian Journal o f Earth Sciences, 46,
785-812.
Rhoads, D. C., & Morse, I. W. (1971) Evolutionary and ecologic significance of
oxygen-deficient marine basins. Lethaia, 4, 413-428.
Riding, R. (1996). Long-term changes in marine CaCOs precipitation. Mem. Soc.
geol. Fr., 169, 157-166.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
115
Robinson, P. L. (1973). Palaeoclimatology and continental drift. In D. H. Tarling
and S. K. Runcorn (Eds.), Implications of Continental Drift to the Earth
Sciences (p. 451-476). San Diego, CA: Academic Press.
Rohl, H. J., Schmid- Rohl, A., Oschmann, W., Frimmel, A., & Schwark, L. (2001).
The Posidonia Shale (Lower Toarcian) of SW-Germany: an oxygen-depleted
ecosystem controlled by sea level and paleoclimate. Palaeogeography,
Palaeoclimatology, Palaeoecology, 165, 27-52.
Ronov, A., Khain, V., & Balukhovsky, A. (1989). Atlas of lithological-
paleogeographical maps of the world: Mesozoic and Cenozoic of continents
and oceans (79 pp.). Leningrad, USSR: Ministry of Geology.
Rosenberg, R. (1976). Benthic faunal dynamics during succession following
pollution abatement in a Swedish estuary of West Sweden. Journal of
Experimental Marine Biology and Ecology, 26, 107-133.
Rothman, D. H. (2001). Global diversity and the ancient carbon cycle. Proceedings
of the National Academy of Sciences, 98, 4305-4310.
Rothpletz, A. (1892). Die perm-, Trias- und Jura- Formation auf Timor and Rotti im
indischen Archipel. Palaeontographica, 39, 57-106.
Ryan, W. B. F., & Cita, M. B. (1977). Ignorance concerning episodes of ocean-
wide stagnation. Marine Geology, 23, 197-215.
Sageman, B. B. & Bina, C. (1997). Diversity and species abundance patterns in
Late Cenomanian black shale biofacies: Western Interior, U.S. Palaios, 12,
449-466.
Sageman, B. B., Wignall, P. B., & Kauffman, E. G. (1991). Biofacies models for
organic-rich facies: tool for paleoenvironmental analysis, In G. Einsele, A.
Seilacher, & W. Ricken (eds.), Cycles and Events in Stratigraphy (p. 542-
564). Berlin: Springer Verlag.
Savrda, C. E., Bottjer, D. J., & Gorsline, D. S. (1984). Development of a
comprehensive oxygen-deficient marine biofacies model: evidence from
Santa Monica, San Pedro, and Santa Barbara Basins, California Continental
Borderland. American Association o f Petroleum Geologists Bulletin, 68,
1179-1192.
Schlanger, S. O., & Jenkyns, H. C. (1976). Cretaceous oceanic anoxic events: causes
and consequences. Geologie en Mijnbouw, 55, 179-184.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
116
Scotese, C. R. (2000). PALEOMAP Project, Earth History, Triassic Map down
loaded from http://www.scotese.com/newpage8.htm.
Scotese, C. R., & Golonka (1992). Paleogeographic Atlas: Arlington, PALEOMAP
Project. Department of Geology, University of Texas, Arlington.
Seilacher, A. (1990). Aberrations in bivalve evolution related to photo- and
chemosymbiosis. Historical Biology, 3, 289-311.
Sepkoski, J. J., Jr. (1992). A compendium of fossil marine animal families.
Contributions in Biology and Geology, 83, 1-156.
Shubin, N. J., Crompton, A. W., Sues, H. D., & Olsen, P. E. (1991). New fossil
evidence on the sister-group of mammals and early Mesozoic faunal
distributions. Science, 251, 1063-1065.
Silberling, N. J., & Wallace, R. E. (1967). Geology of the Imlay quadrangle,
Pershing County, Nevada. U.S. Geological Survey Quadrangle Map GQ-
666 .
Sloan, L. C., & Rea, D. K. (1996). Atmospheric carbon dioxide and early Eocene
climate: A general circulation modeling sensitivity study. Palaeogeography,
Palaeoclimatology, Palaeoecology, 119, 275-292.
Smith, J. P. (1914). The middle Triassic invertebrate faunas of North America.
U.S. Geological Survey Professional Paper, 83, 1-143.
Soutar, A., Johnson, S. R., & Baumgartner, T. R. (1981). In search of modem
depositional analogs to the Monterey Formation. In R. E. Garrison & R. G.
Douglas (Eds.), The Monterey Formation and Related Siliceous Rocks of
California. Special Publication of the Pacific Section SEPM (pp. 123-147).
Los Angeles: SEPM, Pacific Section.
Southward, E. C. (1986). Gill symbionts in Thyasirids and other bivalve mollusks.
Journal o f the Marine Biological Association of the UK, 66, 889-914.
Stothers, R. B. (1993). Flood basalts and extinction events. Geophysical Research
Letters, 20, 1399-1402.
Suenaka, T. (2002). JAL Foundation, The Environmental Observation and
Monitoring, Climate Model Figure 12, downloaded from http://www.jal-
foundation.or.jp/html/TaikiKansoku/html/ezul2.htm.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 1 7
Tanner, L. H. (2002). Tanner replies. Nature, 415, 388.
Tanner, L. H., Hubert, J. F., Coffey, B. P., & Mclnemey, D. P. (2001). Stability of
atmospheric CO2 levels across the Triassic/Jurassic boundary. Nature, 411,
675-677.
Thierstein, H. R., & Berger, W. H. (1978). Injection events in ocean history.
Nature, 276, 461-466.
Tozer, E. T. (1982). Marine Triassic faunas of North America: Their significance
for assessing plate and terrane movements. Geologische Rundschau, 71,
1077-1104.
Trenchmen, C. T. (1918). The Trias of New Zealand. Quarterly Journal of the
Geological Society of London, 73, 165-245.
Tozer, E. T. (1984). The Trias and Its Ammonoids: The Evolution of a Time Scale.
Miscellaneous Report 35. Geological Survey of Canada.
Trask, P. D. (1932). Origin and Environment of Source Sediments of Petroleum.
Houston: Gulf Publishing Company, 323pp.
Tucker, M. E., & Benton, M. J. (1982). Triassic environments, climates, and reptile
evolution. Palaeogeography, Palaeoclimatology, Palaeoecology, 40, 361-
379.
Tyson, R.V, & Pearson, T. H. (1991). Modem and ancient continental shelf anoxia:
an overview. In R.V. Tyson & T. H. Pearson (Eds.), Modem and ancient
continental shelf anoxia. Geological Society Special Publication 58 (pp.l-
26). London, UK: Geological Society of London.
Vail, P. R., Mitchum Jr., R. M., & Thompson III, S. (1977). Seismic stratigraphy
and global changes of sea level, Part 4: Global cycles of relative changes of
sea level. In C. E. Payton (Ed.), Seismic Stratigraphy - Applications to
Hydrocarbon Exploration. American Association of Petroleum Geologist
Memoirs, 26, 83-97.
Van Houten, F. B. (1964). Origin of red beds: Some unsolved problems. In A. E.
M. Naim (Ed.), Problems in paleoclimatology (pp. 647-672). London:
Academic Press.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
118
Vetter, R. D., Powell, M. A., & Somero, G. N. (1991). Metazoan adaptations to
hydrogen sulfide. In C. Bryant (Ed.), Metazoan Life without Oxygen (pp.
109-128). London: Chapman and Hall.
Visscher, H., & Brugman, W. A. (1981). Ranges of selected palynomorphs in the
Alpine Triassic of Europe. Review of Palaeobotany and Palynology, 34,
115-128.
Wallace, J. M., & Hobbs, P. V. (1977). Atmospheric Science: An Introductory
Survey. New York: Academic Press.
Ward, P. D., Haggart, J. W., Carter, E. S., Wilbur, D., Tipper, H. W., & Evans, T.
(2001). Sudden productivity collapse associated with the Triassic-Jurassic
boundary mass extinction. Science, 292, 1148-1151.
Wignall, P.B. (1994). Black shales. Geology and Geophysics Monograph Series,
30. Oxford University Press, 136 pp.
Wignall, P. B. (2001). Large igneous provinces and mass extinctions. Earth Science
Reviews, 53, 1-33.
Wilson, K. M, Pollard, D., Hay, W. W„ Thompson, S. L„ & Wold, C. N. (1994).
General circulation model simulations of Triassic climates: Preliminary
results. In G. D. Klein (Ed.), Pangea: Paleoclimate, Tectonics, and
Sedimentation During Accretion, Zenith, and Breakup of a Supercontinent.
Geological Society of America Special Paper 288 (pp. 91-116). Boulder,
CO: Geological Society of America.
Yapp, C. J., & Poths, H. (1996). Carbon isotopes in continental weathering
environments and variations in ancient atmospheric CO2 pressure. Earth and
Planetary Science Letters, 137, 71-82.
Ziegler, A. M., Scotese, C. R., McKerrow, W. S., Johnson, M. E., & Bambach, R. K.
(1979). Paleozoic paleogeography. Annual Reviews o f Earth and Planetary
Science, 7, 473-502.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Creator Huynh, Tran Thoai (author)

Core Title The Triassic organic-rich flat clam biotope: A synthesis of paleoecological and climate modeling analyses

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