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abstract: Variations of 238U/235U in sedimentary carbonate rocks are being explored as a tool for reconstructing oceanic anoxia through time. However, the fidelity of this novel paleoredox proxy relies on characterization of uranium isotope geochemistry via laboratory experimental studies and field work in modern analog environmental settings. This dissertation systematically examines the fidelity of 238U/235U in sedimentary carbonate rocks as a paleoredox proxy focusing on the following issues: (1) U isotope fractionation during U incorporation into primary abiotic and biogenic calcium carbonates; (2) diagenetic effects on U isotope fractionation in modern shallow-water carbonate sediments; (3) the effects of anoxic depositional environments on 238U/235U in carbonate sediments. Variable and positive shifts of 238U/235U were observed during U uptake by primary abiotic and biotic calcium carbonates, carbonate diagenesis, and anoxic deposition of carbonates. Previous CaCO3 coprecipitation experiments demonstrated a small but measurable U isotope fractionation of ~0.10 ‰ during U(VI) incorporation into abiotic calcium carbonates, with 238U preferentially incorporated into the precipitates (Chen et al., 2016). The magnitude of U isotope fractionation depended on aqueous U speciation, which is controlled by water chemistry, including pH, ionic strength, carbonate, and Ca2+ and Mg2+ concentrations. Based on this speciation-dependent isotope fractionation model, the estimated U isotope fractionation in abiotic calcium carbonates induced by secular changes in seawater chemistry through the Phanerozoic was predicted to be 0.11–0.23 ‰. A smaller and variable U isotope fractionation (0–0.09 ‰) was observed in primary biogenic calcium carbonates, which fractionated U isotopes in the same direction as abiotic calcium carbonates. Early diagenesis of modern shallow-water carbonate sediments from the Bahamas shifted δ238U values to be 0.270.14 ‰ (1 SD) higher than contemporaneous seawater. Also, carbonate sediments deposited under anoxic conditions in a redox-stratified lake—Fayetteville Green Lake, New York, USA— exhibited elevated δ238U values by 0.160.12 ‰ (1 SD) relative to surface water carbonates with significant enrichments in U. The significant U isotope fractionation observed in these studies suggests the need to correct for the U isotopic offset between carbonate sediments and coeval seawater when using δ238U variations in ancient carbonate rocks to reconstruct changes in ocean anoxia. The U isotope fractionation in abiotic and biogenic primary carbonate precipitates, during carbonate diagenesis, and under anoxic depositional environments provide a preliminary guideline to calibrate 238U/235U in sedimentary carbonate rocks as a paleoredox proxy. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2018

Selenium (Se) is an essential trace element, which, with multiple oxidation states and six stable isotopes, has the potential to be a powerful paleo-environmental proxy. In this study, Se concentrations and isotopic compositions were analyzed in a suite of about 120 samples of fine-grained marine sedimentary rocks and sediments spanning the entire Phanerozoic. While the selenium concentrations vary greatly (0.22 to 72 ppm), the δ82/76Se values fall in a fairly narrow range from -1 to +1 , with the exception of laminated black shales from the New Albany Shale formation (Devonian), which have δ82/76Se values of up to +2.20 . Black Sea sediments (Holocene) and sedimentary rocks from the Alum Shale formation (Late Cambrian) have Se/TOC ratios and δ82/76Se values close to those found in modern marine plankton (1.72x10-6±1.55x10-7 mol/mol and 0.42±0.22 ). (Note: TOC = total organic carbon.) For the other sedimentary sequences, the Se/TOC ratios indicate enrichment in selenium relative to marine plankton. Additional input of isotopically light terrigenous Se (δ82/76Se ≈ -0.42 ) may explain the Se data measured in recent Arabian Sea sediments (Pleistocene). The very high Se concentrations in sedimentary sequences that include the Cenomanian-Turonian Ocean Anoxic Event (OAE) 2 possibly reflect a significantly enhanced input of volcanogenic Se to the oceans. As the latter has an isotopic composition (δ82/76Se ≈ 0 ) not greatly different from marine plankton, the volcanogenic source does not impart a distinct signature to the sedimentary Se isotope record. The lowest δ82/76Se values are observed in the OAE2 samples from Demerara Rise and Cape Verde Basin cores (δ82/76Se = -0.95 to 1.16 ) and are likely due to fractionation associated with microbial or chemical reduction of Se oxyanions in the euxinic water column. In contrast, a limiting availability of seawater Se during periods of increased organic matter burial is thought to be responsible for the elevated δ82/76Se values and low Se/TOC ratios in the black shales of the New Albany Shale formation. Overall, our results suggest that Se data may provide useful information on paleodepositional conditions, when included in a multi-proxy approach.

Understanding oxygenation throughout Earth history, particularly intervals where marine deoxygenation occurred, are crucial to investigating the changes in habitability on Earth. Marine deoxygenation events, in particular, can result in changes in the carbon, sulfur, and iron cycles on our planet. Changes in these elemental cycles lead to distinctive variation in the chemical composition of seawater that is recorded in marine sediments that are preserved into the sedimentary record. Our modern ocean is experiencing rapid deoxygenation, thus understanding the duration and extent of ancient deoxygenation events is vital to predicting future climate scenarios. Here I investigated the record of environmental change during the Early Jurassic Toarcian Oceanic Anoxic Event or T-OAE (~183 Ma). The first chapter of this dissertation investigates the record of marine anoxia across the Pliensbachian to Toarcian transition. Specifically, I investigate the temporal and geographic development of anoxia across three basins from the European Epicontinental Seaway. Through utilization of iron speciation, a local redox proxy, I identify anoxia developing before and persisting well after the negative carbon isotope excursion (NCIE) conventionally used to define the T-OAE. These data indicate an increase in the occurrence of anoxia at the Pliensbachian – Toarcian boundary, coincident with the initial phase of volcanism associated with the Karoo-Ferrar Large Igneous Province and an interval of heightened marine invertebrate extinction. Ultimately, our data support a greater temporal extent of anoxic conditions around the T-OAE, which support the greater sensitivity of marine oxygen levels to climatic change outside of the NCIE interval. The second chapter of this dissertation assesses the occurrence and extent of organic matter sulfurization (OMS), a biogeochemical feedback known to enhance the preservation and burial of OM. Because this process is accelerated when euxinic conditions develop in the water column, I investigated it as a mechanism promoting OM burial across two oceanic anoxic events of the Mesozoic. Importantly, I find that sulfurization does not occur uniformly across both events and propose a conceptual model of the depositional settings most favorable for sulfurization to occur and when throughout geologic time OMS is most likely to influence the global cycles of carbon and sulfur. / Doctor of Philosophy / Understanding past time intervals where there was widespread loss of oxygen in the oceans is crucial to understanding habitability on Earth. Since our modern oceans are experiencing a rapid loss of oxygen, understanding the duration and extent of ancient marine oxygen loss events is vital to predicting future habitability of the oceans. These ancient events can result in distinctive changes in the carbon, sulfur, and iron cycles on our planet. Variation in these elemental cycles lead to distinctive shifts in the chemical composition of seawater that is recorded in marine sediments that get preserved as rocks in the geologic record. Here, I investigated the record of environmental change during the Early Jurassic Toarcian Oceanic Anoxic Event or T-OAE (~183 Ma). The first chapter of this dissertation investigates the record of marine oxygen loss across the T-OAE. Specifically, I investigate the temporal and geographic development of oxygen loss across three ancient marine basins. Through utilization of a local tracer of water column oxygen loss (e.g. iron speciation) I identify oxygen loss developing before and persisting well after the conventional timeframe associated with the event. These data indicate oxygen loss first occurred before the T-OAE, coincident with the initial phase of volcanic eruptions from the Karoo-Ferrar Large Igneous Province and an interval of heightened marine extinction. Ultimately, these data support a longer time interval of oxygen loss around the T-OAE and the greater sensitivity of marine oxygen levels to climatic change. The second chapter of this dissertation assesses the occurrence and extent of organic matter sulfurization (OMS), a feedback known to enhance the preservation and burial of organic matter (OM). Because this process is accelerated when oxygen is lost and free sulfur builds up in the water column, I investigated its occurrence across two oceanic oxygen loss events of the Mesozoic Era. Importantly, I find that sulfurization does not occur uniformly across both events and propose a conceptual model of the settings most favorable for sulfurization to occur and also when in geologic time it is most likely to influence the global cycling of carbon and sulfur.

In this thesis I document the biostratigraphy of two Cretaceous sections in Pakistan, the Chichali Nala Section and the Moghal Kot Section. Furthermore, I document the stratigraphy of the so-called Oceanic Anoxic Events (OAEs) in the Moghal Kot Section. In addition, I establish potential links between the planktonic foraminiferal evolution and these OAEs in the Moghal Kot Section. Sea Surface Temperatures (SSTs) are established for the Valanginian time by using the TEX86 and δ18O proxies in the Chichali Nala Section. The new biostratigraphy of the Chichali Nala Section shows that the ages of the sediments are mainly Valanginian. The biostratigraphy of the Moghal Kot Section show ranges in age from the Early Aptian to Early Maastrichtian. Seven OAEs were recorded in the Moghal Kot Section based on the combined study of biostratigraphy, microfacies, and δ13C analysis. These OAEs correlate well with previously documented OAEs elsewhere, therefore the new record of the OAEs in the Moghal Kot Section confirms the widespread occurrence of these events, possibly all global in nature. A quantitative review of the planktonic foraminiferal evolution in the Moghal Kot Section indicates that the environmental changes along the OAE2 have strongly forced the evolution of the planktonic foraminifera. Conversely, no clear relationship is observed between other OAEs and planktonic foraminiferal evolution in the same section. The SST results based on the TEX86 in the Chichali Nala Section show that the surface ocean was consistently much warmer (10-12 oC) than today at the paleolatitude of ~-35o during the Valanginian time. Such warm conditions are also supported by the spore and pollen assemblages of the Chichali Nala Section. Collectively the two datasets indicate strongly that the Valanginian world was overall extremely warm. Such warming during the Valanginian is incompatible with previously suggested cooler conditions during this time period.

The applications of calcium-isotope measurements in marine carbonates are explored in several different contexts within this thesis. As a record of global ion fluxes, seawater calcium-isotope ratios can be used as tracers for large weathering imbalances, which develop as a feedback system in response to intervals of climate change. This approach provides valuable constraints on the complex climatic and oceanographic phenomena known as the Oceanic Anoxic Events. Over much longer timescales, the calcium-isotope ratio of seawater is influenced by steady-state processes that reflect the evolution of seawater chemistry. To understand these influences, the modern calcium-isotope budget is assessed quantitatively using a compilation of marine carbonate samples, revealing several distinctive components of the carbonate burial sink that can affect the steady-state balance of the calcium cycle. Changes in the major ion composition of seawater and in the organisms that contribute to sedimentary carbonate burial are shown to contribute significantly to the geological record of seawater calcium-isotope ratios. The importance of skeletal carbonate in the calcium cycle leads to another application of calcium isotopes towards understanding biomineralization. This large and complex topic is approached with calcium-isotope data from two unique growth experiments that constrain some of the mechanisms by which biogenic aragonite acquires its geochemical signatures. This range of topics presents a diverse, but by no means exclusive, sample of the topics that are accessible for investigation through calcium-isotope analysis. The potential of this isotopic tool is demonstrated by the breadth of environments and timescales represented in this work.

Oceanic anoxic events (OAEs) are periods in Earth's history when oceans were depleted in dissolved oxygen and characterized by deposition of organic-rich sediments. The Oceanic Drilling Program (ODP) has drilled through OAEs in a number of areas worldwide, collecting core and borehole log data. This project attempts to identify a characteristic signature from known ODP OAE sections using these data and to apply the signature to identify OAE intervals in Gulf of Mexico wells where cores are lacking. Additionally, pseudo density curves were generated from ODP logs and compared to bulk density logs to determine if the deviation between the two would aid identification of OAE intervals. A general, though not fool proof, signature of high gamma ray, uranium, neutron porosity and low density was seen in nearly all of the ODP holes. Using this signature 20 potential OAE intervals were identified in the Gulf of Mexico.

Fire is a natural process that has existed on our planet for more than ~350 million years, and is a process that continues to influence our everyday lives. On Earth, a relationship exists between the process of combustion and the natural functioning of the Earth system. Here, the process of combustion has been implicated in playing an essential role for life on Earth, where natural Earth system processes have been shown to influence ignition probability, fire spread and fire behaviour, and where fire can provide a variety of feedbacks, to the Earth system over different timescales. Over medium timescales of decades to hundreds of thousands of years, the likelihood and behaviour of fires are controlled by regional climate changes and vegetation type, whilst the occurrence of fire can play a crucial role in influencing biome persistence and development. Over long timescales (hundreds of thousands to multi-million year), the components influencing the probability of fire and fire behaviour not only involve processes occurring over local and regional spatial scales, and over short and medium timescales, but also long term processes occurring globally, such as changes in atmospheric oxygen concentration and the evolution of vegetation. Across these timescales in Earth’s past, combustion has been shown to impact global ecosystems, climate and the carbon cycle by generating feedbacks that influence Earth’s biogeochemical cycles. However, it is clear that our understanding of the role that fire plays in the Earth system, although improving is still developing. This thesis provides an analysis of these Earth system - fire relationships and feedbacks across medium and long timescales in deep time, in order to understand the role that fire may have played and what the record of fire can tell us about the functioning and re-equilibrating of the Earth system during and after significant carbon-cycle perturbation events occurring in Earth’s deep past. The results presented in this thesis contribute what is believed to be the first fossil evidence that rising atmospheric oxygen and fire feedbacks may have aided in the termination of a significant carbon-cycle perturbation event, termed the ‘Toarcian oceanic anoxic event’ that occurred ~183 million years ago during the Jurassic period, and the return of the Earth system towards ‘background functioning’. This thesis also provides an analysis of the record of wildfire in the form of fossil charcoal across the initiation of an anoxic event that occurred ~93 million years ago, during the Cretaceous period. The results illustrate that CO2 - climate driven changes in wildfire activity can be observed across medium timescales even during times of significant carbon-cycle perturbations, and modelled high atmospheric oxygen concentrations. These results illustrate how hypothesized changes in the hydrological cycle, and likely moisture content of fuel, appear to be the dominant control on wildfire activity during this period. Finally, this thesis provides an analysis of charcoal abundance variations occurring across natural, orbitally forced cycles, termed the Milankovitch cycles. The results presented illustrate that natural variations in charcoal abundance are possible over intermediate timescales within the geological record. This thesis therefore illustrates a need to take into consideration and incorporate ‘natural background’ fluctuations in fire activity occurring over medium timescales, when analysing and predicting past and future climate change patterns.

The Mesozoic is punctuated by several rapid global warming events that are marked by the worldwide deposition of organic-rich marine sediments. These events, known as oceanic anoxic events (OAEs), are characterised by intervals where the worldwide carbon cycle suffers a disruption due to major palaeoceanographic and climatic shifts, leading to anoxic marine environments and the creation of black shales. For this study, the Oceanic Anoxic Event 1a (OAE 1a), which occurred during the Early Cretaceous Aptian age (~120 Ma) was analysed. It was likely triggered by the Greater Ontong Java underwater volcanic event and is associated with major changes in marine environments and ecosystems, including nekton and plankton turnover, and sea water composition changes. The impact of this event on terrestrial land-based ecosystems is, however, less well understood. Here I document well preserved and diverse spore-pollen assemblages spanning OAE 1a from southern Sweden by examining the Höllviken I core. Before the OAE, palynofloras are dominated by conifers, suggestive of a relatively mild and dry coastal environment. At the onset of the OAE a fern spike occurs, where there is a shift to early successional stage vegetation. Gymnosperm diversity and abundance sharply decrease, and the palynofloral assemblages become dominated by ferns, indicating a shift to warm and wet conditions. Gymnosperms gradually recover thereafter, but the formerly abundant conifer pollen Classopollis does not recover and remains rare. Dinoflagellate cysts and microforaminiferal test linings increase in abundance after OAE 1a, suggesting a higher degree of marine influence. These findings show that OAE 1a had a substantial impact on the composition and diversity of high latitude terrestrial vegetation and marine plankton communities. / <p>The work for this thesis was financially supported by the Erasmus Mundus Joint Master Degree PANGEA programme.</p>