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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.

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.

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>

The Upper Cretaceous Tropic Shale of southern Utah captures critical oceanographic changes that occurred during Oceanic Anoxic Event 2 (OAE 2) and the transgression of the Greenhorn Sea. We investigated the response of planktic and benthic foraminifera in a shallow (<100 >m) marine environment stressed by the onset of OAE 2 during the Cenomanian-Turonian boundary (CTB; 93.9 Ma) to determine the oceanographic mechanisms controlling the observed turnovers in the foraminiferal record. This study is based on high-resolution quantitative foraminifera counts and isotope paleoecology (d18O and d13C) from a 40-m outcrop. The OAE 2 interval is identified by a distinctive d13Corg signature and by correlation of bentonites and limestones across the seaway. Results of assemblage analyses indicate discrete intervals of environmental perturbations across the CTB interval. At the onset of OAE 2, surface waters were dominated by planktic Guembelitra cenomana with minor species of Heterohelix. Benthic abundances increase and were initially dominated by oxygen tolerant infaunal Neobulimina albertensis. Epifaunal Gavelinella dakotaensis briefly proliferated during the core of the OAE 2 and coinciding with the planktic “Heterohelix shift” and increasing accumulation of organic matter. The peak of OAE 2 at ~17 m is marked by a rapid shift to infaunal Neobulimina dominance. We suspect incursion of oxygen-poor Tethyan intermediate waters with peak transgression during the early Turonian, coupled with high productivity in surface waters resulted in the rapid depletion of benthic oxygen. The foraminiferal record reveals strong cyclicity in planktic/benthic ratio resembling parasequences that correlate to the GSSP in central Colorado.

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 hallmark of oceanic anoxic event 1a (OAE1a) (early Aptian ~125 Ma) corresponds to worldwide deposition of black shales with total organic carbon (TOC) content > 2% and a d13C positive excursion up to ~5‰. OAE1a has been related to large igneous province volcanism and dissociation of methane hydrates during the Lower Cretaceous. However, the occurrence of atypical, coeval and diachronous organic-rich deposits associated with OAE1a, which are also characterized by positive spikes of the d13C in epicontinental to restricted marine environments of the Tethys Ocean, indicates localized responses decoupled from complex global forcing factors. The present research is a high-resolution, multiproxy approach to assess the paleoenvironmental conditions that led to enhanced carbon sequestration from the late Barremian to the middle Aptian in a restricted, Tethyan marginal basin prior to and during OAE1a. I studied the lower 240 m of the El Pui section, Organyà Basin, Spanish Pyrenees. The basin developed as the result of extensional tectonism linked to the opening of the Atlantic Ocean. At the field scale the section consists of a sequence of alternating beds of cm – m-scale, medium-gray to grayish-black limestones and marlstones with TOC up to ~4%. The results indicate that the lowest 85 m of the section, from latest Barremian –earliest Aptian, characterize a deepening phase of the basin concomitant with sustained riverine flux and intensified primary productivity. These changes induced a shift in the sedimentation pattern and decreased the oxygen levels in the water column through organic matter respiration and limited ventilation of the basin. The upper 155 m comprising the earliest – late-early Aptian document the occurrence of OAE1a and its associated geochemical signatures (TOC up to 3% and a positive shift in d13C of ~5‰). However, a low enrichment of redox-sensitive trace elements indicates that the basin did not achieve anoxic conditions. The results also suggest that a shallower-phase of the basin, coeval with platform progradation, may have increased ventilation of the basin at the same time that heightened sedimentation rates and additional input of organic matter from terrestrial sources increased the burial and preservation rate of TOC in the sediment.

Le géothermomètre Δ47 est basé sur la relation entre l’abondance des liaisons 13C–18O des carbonates et la température de calcification. Ce proxy contourne potentiellement les limites des autres thermomètres (δ18O, Mg/Ca) pour reconstruire les paléo-températures des océans, expliquant son développement exponentiel depuis dix ans. Cette thèse teste pour la première fois le potentiel et les limites de la thermométrie Δ47 sur les coccolithes, des nannofossiles calcaires produits par des organismes calcifiants dans la zone photique. Ces biominéraux calcitiques et ubiquistes constituent une part importante de l’archive sédimentaire. Des cultures in vitro nous ont permis d’établir que trois espèces de coccolithes actuelles enregistrent la même relation Δ47 – T que la calcite inorganique, alors qu'elles présentent de très larges effets vitaux en δ18O (±5‰). Nous concluons que ces espèces de coccolithes d'importance géologique ne présentent pas d’effets vitaux en Δ47. Nous avons ensuite appliqué le Δ47 à l’étude des sédiments enregistrant l’événement d’anoxie océanique du Toarcien (–183 Ma) au cours duquel les reconstructions de températures restent encore ambigües, notamment du fait de la méconnaissance de la composition isotopique en oxygène de l’eau de mer. Sur la base des données Δ47 acquises, nous proposons des températures élevées (de l’ordre de 36°C), mais restant relativement stables sur l'intervalle d'étude. En couplant ces températures aux données de δ18O des carbonates, nous suggérons une variation importante du δ18O de l'eau de mer dans le Bassin de Paris lors de la mise en place des faciès black shales. Enfin, sur des sédiments pélagiques subactuels, l’une des espèces étudiées présente des déséquilibres isotopiques en Δ47 non observés en culture et explicables par d’autres paramètres environnementaux comme l’intensité lumineuse. Cette thèse illustre le potentiel du thermomètre Δ47 des coccolithes en différents contextes, ouvrant un vaste champ d’application de reconstruction des paléo-environnements sur le Méso-Cénozoïque / The Δ47 geothermometer relies on the relationship between the 13C–18O abundance in carbonateand temperature of calcification. This proxy has the potential to overcome limitations of other thermometers(δ18O, Mg/Ca) to reconstruct oceanic paleotemperatures. This thesis evaluates for the first time the potentialand the limitations of the Δ47 thermometry of the coccoliths, the calcareous nannofossils produced byorganisms calcifying in the photic zone. These calcitic and ubiquitous biominerals represent an importantpart of the sedimentary archive. In vitro cultures allow us to establish that three modern coccoliths speciesrecord the same Δ47–T relation than inorganic calcite, although exhibiting substantial δ18O vital effects(±5‰). We conclude that these coccoliths species do not present any Δ47 vital effect. We subsequentlyapplied the Δ47 proxy to sediments from the Toarcian oceanic anoxic events (–183 Ma) during which thetemperatures reconstructions are still elusive, mainly due to the unknown oxygen isotopic composition ofseawater. On the basis of our Δ47 data, we infer relatively high, yet steady temperatures (around 36°C) duringthe examined interval. By combining these Δ47-derived temperatures to carbonate δ18O data, we can suggestdrastic change in the seawater δ18O composition in the Paris Basin at the onset of black shale deposition. Acore top calibration of Δ47 of coccoliths revealed that one of the studied cultured species exhibits Δ47disequilibrium that is accountable by other environmental parameters, such as light irradiance in the naturalenvironment. Thus, this thesis illustrates the potential of the coccolith Δ47 thermometer in different settings,opening a wide range of application to reconstruct the palaeoenvironments over the Meso-Cenozoic Eras

Carbonate platforms contain a wealth of information regarding the changing biota, sea level, ocean-chemistry, and climate of the Cretaceous Period. The Comanche platform of the northern Gulf of Mexico represents a vast, long-lived carbonate system that extended from west Texas through the Florida panhandle. In central and south Texas, excellent outcrops and an extensive suite of subsurface data provide an opportunity to document the evolution of this system, from the shoreline to the shelf-margin and slope. This study examines the changing facies, platform morphologies, and shelf-margin architectures of the mixed carbonate-siliciclastic, middle-Hauterivian to lower-Campanian interval. Stratigraphic results are integrated with stable-isotope geochemistry to document the detrimental effects of oceanic anoxic events on the carbonate platform. Seven second-order, transgressive-regressive supersequences of 3-14 Myr duration are defined in south Texas using sequence stratigraphic analysis of shelf-interior facies successions. Second-order supersequences are subdivided into several third-order depositional sequences of 1-3 Myr duration. In these sequences, facies proportions and stratal geometries of the shelf-interior are found to be the result of changing platform morphology and temporal evolution from distally-steepened ramp to rimmed-shelf depositional profiles. Shelf-margin trajectories, stratigraphic architectures, and facies proportions are a function of long-term accommodation trends expressed in second-order supersequences. These characteristics are modified by lateral variability in the underlying structural/tectonic setting and localized syndepositional faulting. The stratigraphic equivalents of oceanic anoxic events 1a, 1b, 1d, 2, and 3 are documented in the Cretaceous section of south Texas. These oceanic anoxic events coincided with maximum flooding zones of supersequences and are linked to carbonate platform drowning events on four separate occasions. The occurrence of oceanic anoxic events is found to be a fundamental driver of carbonate platform morphology, faunal composition, and facies evolution in transgressive-regressive supersequences of the northern Gulf of Mexico. / text

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