The Triassic Cow Branch Formation of the Dan River Basin is host to a world-class lagerstätte deposit of exceptionally preserved insects, among other organisms. The lagerstätte occurs within a cyclic, lacustrine sedimentary succession, hypothesized to have been driven by Milankovitch climate forcing. Through an integrated sedimentological and geochemical investigation, I present evidence that the lagerstätte was deposited during a lake transgression, under intermittently anoxic and ferruginous conditions. Sedimentological evidence shows a deepening followed by shoaling through a broad fining and subsequent coarsening of the sedimentary units of the sequence. This transition in grain size occurs at the lagerstätte. Despite relatively quartz-rich sediments sourced to the basin, silica-content in the studied cycle is exceptionally low. The replacement of silica by the zeolite mineral analcime, coupled with primary dolomite precipitation suggests alkaline lake water. Geochemical evidence, including total organic carbon (TOC), pyrite sulfur and iron speciation data suggest anoxic, ferruginous waters. At the lagerstätte interval, TOC content increases significantly, coinciding with the presence of darker, more laminated sedimentary lithofacies. At the interval of the highest TOC content, a spike in pyrite sulfur content occurs; likely the result of slowed sedimentation. Organic carbon-to-pyrite sulfur ratios suggest however, that the lake water was sulfate-poor and the deep waters never became euxinic (anoxic, H2S-containing). Iron proxy data show that the studied portion of the Cow Branch Formation deposited under intermittent to persistent anoxic conditions. These data suggest a confluence of factors — lake transgression, combined with alkaline and anoxic, ferruginous water chemistry — created an ideal scenario that led to lagerstätte formation. / Master of Science

Several previous Neoproterozoic microfossil diversity studies yield evidence for arelatively sudden biotic change prior to the first well‐constrained Sturtian glaciations. In an event interpreted as a mass extinction of eukaryotic phytoplankton followed by bacterial dominance, diverse assemblages of complex acritarchs are replaced by more uniform assemblages consisting of simple leiosphaerid acritarchs and bacteria. Recent data from the Chuar Group of the Grand Canyon (770‐742 Ma) suggest this biotic change was caused by eutrophication rather than the direct effects of Sturtian glaciation; evidence includes total organic carbon increases indicative of increasing primary productivity followed by iron speciation values that suggest sustained water column anoxia. A new data set (this study) suggests that this same eutrophication event may be recorded in shale units of the formation of Hades Pass and the Red Pine Shale of Utah’s Neoproterozoic Uinta Mountain Group (770‐742 Ma). Results of this study include a significant shift from a higher‐diversity (H’= 0.60) fauna that includes some ornamented acritarchs to a lower‐diversity (H’ = 0.11) fauna dominated by smooth leiosphaerids and microfossils of a bacterial origin (Bavlinella/ Sphaerocongregus sp.). This biotic change co‐occurs with a significant increase in total iii organic carbon values that directly follows a positive carbon‐isotopic excursion, suggesting increased primary productivity that may have been the result of elevated sediment influx and nutrient availability. Both the biotic change and period of increased total organic carbon values correspond with the onset of an interval of anoxia (indicated by total iron to aluminum ratios above 0.60) and a spike in sulfur concentration. Like those reported from the Chuar Group, these biotic and geochemical changes in the upper Uinta Mountain Group are independent of changes in lithofacies , and they suggest that either a eutrophication event or direct inhibition of eukaryotes by sulfide (or perhaps both) may have been the cause of the biotic turnover. These findings support current correlations between the Uinta Mountain and Chuar Groups, the idea that the biotic turnover preserved in both strata was at least a regional phenomenon, and current models of punctuated global ocean anoxia during mid‐ to late‐Neoproterozoic time. Whether or not this hypothesized eutrophication event was more than regional in extent remains a very interesting question and will certainly be a focus of future research.

The research presented here is an effort to characterize changes in marine oxygen availability across a portion of the later Cambrian noted for unique evolutionary dynamics and which includes a significant global oceanographic event known as the SPICE event (Steptoean Positive Carbon Isotope Excursion). Previous studies have revealed the SPICE caused large changes to the global cycles of carbon, sulfur, uranium, molybdenum and the overall trace metal content of seawater. Furthermore, the initiation of these changes appears to have been temporally coupled with marine extinctions across several paleocontinents raising the possibility of a common causal linkage between all these features. In particular, expanding marine anoxia has been invoked as the most parsimonious explanation for these co-occurring features. The research presented here tests this hypothesis directly across a range of spatial scales using the iron speciation paleoredox proxy to characterize redox conditions within individual basins and to facilitate comparison of conditions between basins. In addition to these analyses, we apply a new proxy, thallium stable isotopes to this interval to assess potential global changes in deoxygenation across the event. These iron speciation analyses showed shallow environments deoxygenated coincident with the initiation of the SPICE and extinction horizons, and these conditions were dominantly ferruginous. Importantly, this work also shows deeper-water environments were deoxygenated prior to and remained so across the event and these environments were also largely. Last we looked at changes in thallium isotopes during this same interval to see if this deoxygenation would be recorded as a positive shift across the interval if expanded anoxia were to impact the areal extent of manganese-oxide sedimentation and burial. We found it did record these changes, but with a different expression than during other more recent events explored using the isotope system. We attribute these differences to the unique chemical structure of the oceans during the Cambrian, which as documented herein were widely oxygen-deficient in their deeper depths. Given this recognition we suggest that thallium isotope studies in deep time should account for this redox structure of ancient oceans likely common under the less-oxygenated atmospheres of the ancient Earth. / Doctor of Philosophy / The research presented here is a story about oxygen in the oceans during an ancient portion of Earth history within the Cambrian Period (around 500 million years ago), soon after animal life first appears in the geologic record. The emerging biosphere of this time seems to have been particularly prone to extinctions, leading to the idea that environmental conditions, such as oxygen availability at the seafloor created difficult circumstances for animals in these ancient seas. This work seeks to quantify the levels of marine oxygenation at this time, however this remains a fundamental challenge because they cannot be directly measured from the rocks we study. Therefore, we rely on how the presence or absence of oxygen changed the chemistry of these rocks at the time they were sediments deposited on the seafloor. Here we use the behavior of two different elements, iron (Fe) and thallium (Tl), to understand changes in oxygen in the oceans around a large, globally-recorded extinction event called the SPICE event. Studying how much iron is concentrated in certain minerals in the rocks formed during this event allowed us to track how changes in oxygen may relate to these notable extinctions. We found that shallow coastal areas changed from oxygenated to deoxygenated at the same time of the extinctions, suggesting a direct role for this environmental shift in the biological crisis. Furthermore, we compared other locations from around the world using more new iron measurements in conjunction with previously published ones compiled by a collaborative geochemistry database project. This work revealed the deeper oceans were deoxygenated prior to and across the SPICE event and that the decline in oxygen in shallower environments was where most environmental change occurred during this time. Last we looked at changes in thallium isotopes during this same interval to see if this deoxygenation changed its global cycle. We found it did record global changes, but they were expressed differently than during other more recent events that have been studied. We attribute these differences to the unique chemical structure of the oceans during the Cambrian, which were widely oxygen-deficient in their deeper depths.

The Toarcian Oceanic Anoxic Event (T-OAE; ~183 million years ago) represents an interval during the Mesozoic when the emplacement of the Karoo-Ferrar Large Igneous Province (LIP) is thought to have resulted in significant environmental change. Associated with this interval was the widespread deposition of organic-rich sediments, carbon cycle and seawater chemistry changes, global warming, the development of marine anoxia, and major extinction events. The majority of studies of this event that have documented these responses have come from the Boreal and Tethyan regions of Europe, thus casting some doubt to the regional versus global significance of the event. Thus my dissertation has sought to reconstruct biogeochemical and paleoenvironmental changes across the T-OAE from a sedimentary succession that was deposited on the margins of a different ocean basin away from the well-studied European successions. Specifically, I have studied the chemostratigraphy of the Fernie Formation of the Western Canada Sedimentary Basin (WCSB), which was deposited on the eastern margin of the Panthalassa Ocean. The Toarcian carbon isotope excursions (CIEs) in the WCSB confirm that these features are global phenomena. I have suggested a new driver for small-scale CIEs observed during the event: the release of wetland-derived methane during progressive global warming. The osmium isotope record and numerical modeling of the osmium cycle suggests that continental weathering rates increased during the T-OAE by 230 – 540%. Rhenium abundance data also suggests that the increased geographic extent of marine anoxia during the T-OAE caused a global drawdown in the seawater rhenium inventory. Iron speciation data are used to reconstruct redox conditions within the WCSB, which suggest ferruginous conditions developed in the more distal locations at the onset of the T-OAE before returning to euxinic (anoxic and sulfidic) conditions. This is likely related to enhanced pyrite burial on a global scale, which caused the drawdown of the seawater sulfate inventory, thus limiting pyrite formation in the distal locations. The proximal setting remained euxinic across the T-OAE, and in all locations the iron speciation data suggest anoxic conditions persistent well after the interval that has been traditionally called the end of the T-OAE. / Ph. D.

The co-evolution of life and the environment stands as a cornerstone in Earth's 4.5-billion-year history. Environmental fluctuations have wielded substantial influence over biological evolution, while life forms have, in turn, reshaped Earth's surface and climate. This dissertation centers on a critical period in Earth's history—the Neoproterozoic Era—when profound environmental shifts potentially catalyzed pivotal eukaryotic evolutionary events. By delving deeper into Neoproterozoic paleoenvironments, I aim at a clearer understanding of life-environment co-evolution in this crucial era. The first chapter focuses on an important juncture—the transition from prokaryote to eukaryote dominance in marine ecosystems during the Tonian Period (1000 Ma to 720 Ma). To assess whether the availability of nitrate, an important macro-nutrient, played a critical role in this evolutionary event, nitrogen isotope compositions (δ¹⁵N) of marine carbonates from the early Tonian (ca. 1000 Ma to ca. 800 Ma) Huaibei Group in North China were measured. The data indicate nitrate limitation in early Neoproterozoic oceans. Further, a compilation of Proterozoic sedimentary δ¹⁵N data, together with box model simulations, suggest a ~50% increase in marine nitrate availability at ~800 Ma. Limited nitrate availability in early Neoproterozoic oceans may have delayed the ecological rise of eukaryotes until ~800 Ma when increased nitrate supply, together with other environmental and ecological factors, may have contributed to the transition from prokaryote-dominant to eukaryote-dominant marine ecosystems. Recognizing the spatial and temporal variations in Neoproterozoic oceanic environments, the second chapter lays the groundwork for a robust stratigraphic framework for the early Tonian Period. Employing the dynamic time warping algorithm, I constructed a global stratigraphic framework for the early Tonian Period using δ¹³C_carb data from the North China, São Francisco, and Congo cratons. This exercise confirms the generally narrow range of δ¹³C_carb fluctuations in the early Tonian, but also confirms the presence of a negative δ¹³C_carb excursion of notable magnitude (~9 ‰) at ca. 920 Ma in multiple records, suggesting that it was global in scope. This negative excursion, known as the Majiatun excursion, is likely the oldest negative excursion in the Neoproterozoic Era and marks the onset of the dynamic Neoproterozoic carbon cycle. Shifting focus to the late Neoproterozoic, the third chapter delves into the origins of Neoproterozoic superheavy pyrite, whose bulk-sample δ³⁴S values are greater than those of contemporaneous seawater sulfate and whose origins remain controversial. Two supervised machine learning algorithms were trained on a large LA-ICP-MS pyrite trace element database to distinguish pyrite of different origins. The analysis validates that two models built on the co-behavior of 12 trace elements (Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Au, Tl, and Pb) can be used to accurately predict pyrite origins. This novel approach was then used to identify the origins of pyrite from two Neoproterozoic sedimentary successions in South China. The first set of samples contains isotopically superheavy pyrite from the Cryogenian Tiesi'ao and Datangpo formations. The second set of samples contains pyritic rims from the Ediacaran Doushantuo Formation; these pyrite rims are associated with fossiliferous chert nodules and do not have superheavy sulfur isotopes. For the superheavy pyrite, the models consistently show high confidence levels in identifying its genesis type, and three out of four samples were inferred to be of sedimentary origins. For the pyritic nodule rims, the models suggest that early diagenetic pyrite was subsequently altered by hydrothermal fluids and therefore shows mixed signals. The third chapter highlights the importance of pyrite trace elements in deciphering and distinguishing the origins of pyrite in sedimentary strata. / Doctor of Philosophy / Understanding how life and the environment have shaped our planet's story over 4.5 billion years is like piecing together an intricate puzzle. On the one hand, changes in the environment kickstarted big shifts in how life evolved. On the other hand, living creatures have also left their mark on Earth's landscapes and climate. This dissertation focuses on unraveling the mysterious Neoproterozoic Era (1 billion to 538 million years ago), a time when Earth saw some of its most dramatic changes. A significant aspect of my investigation delves into the evolutionary dynamics within ancient marine ecosystems. Specifically, I'm exploring a critical juncture when organisms with more complex cellular structures, known as eukaryotes, became ecologically more important than prokaryotic life forms in many aspects of Earth systems. By examining ancient rock formations from China, I have found evidence suggesting that nitrate, a vital nutrient, was scarce in the Neoproterozoic oceans. However, around 800 million years ago, there appears to have been a significant surge in nitrate availability. This surge potentially catalyzed a pivotal phase in evolution, possibly driving the shift from prokaryote to eukaryote dominance in these ancient waters. Second, there is a challenge to delineate a robust timeline for the early Neoproterozoic Era. Imagine trying to piece together a story from a time when there were no calendars or clear dates. Employing advanced statistical methods and comparing chemical signals preserved in carbonate rocks from disparate global locations, I endeavor to craft a coherent timeline for this crucial period. Within this timeline, a noteworthy anomaly in the carbon cycle emerged around 920 million years ago known as the Majiatun excursion. This anomaly represents a significant shift in the Neoproterozoic carbon cycle. Furthermore, my investigation plunges into the geochemistry of sulfur, an important element in shaping ancient marine environments. Certain sedimentary rocks harbor anomalous sulfur isotope signatures in the mineral pyrite (also known as fool's gold), hinting at dramatic environmental transformations during the late Neoproterozoic. Employing advanced analytical techniques and machine learning methodologies, I seek to discern the origins and implications of these anomalous sulfur isotope signals found in pyrite, unraveling their significance in reconstructing the environmental dynamics of ancient oceans.