Browsing by Author "Caruthers, Andrew H."

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    Evaluating redox cycling across the Toarcian Oceanic Anoxic Event with implications for paleo-environmental reconstructions and organic matter sulfurization
    Marroquin, Selva Mariana (Virginia Tech, 2020-12-09)
    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.
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    Investigating the Sulphur Cycle during the End-Triassic Mass Extinction from Panthalassa
    Marroquín, Selva; Gill, Benjamin C.; Them, Theodore R.; Owens, Jeremy D.; Gröcke, Darren R.; Caruthers, Andrew H. (2018-08-15)
    The end-Triassic mass extinction (~201 Ma), one of the “Big Five” mass extinctions of the Phanerozoic,is estimated to have resulted in the loss of ~80%of known marine species. This interval is also characterized by a major perturbation to the carbon cycle, ocean acidification, and widespread oxygen deficiency within the oceans. While the specific causes of extinction and environmental changes are still debated, it is hypothesized that the event was triggered by massive volcanism associated with the emplacement of the Central Atlantic Magmatic Province. Despite the central role of sulphur in many of these potential mechanisms,little is known about what changes occurred in the sulphur cycle during this extinction event.Here,we will present sulphur isotope data from pyrite (δ34Spyrite),organic sulphur (δ34Sorg), and carbonate-associated sulphate (δ34SCAS)from sedimentary successions in Nevada, Alaska,and the United Kingdom to reconstruct the changes in the sulphur cycle over the end-Triassic mass extinction. Preliminary δ34Spyritedata from Alaska and the UK record large changes in local sulphur cycling. This appears to be linked to local bottom water redox conditions and organic carbon availability as tracked by iron speciation analyses and total organic carbon contents. δ34SCASand δ34Sorgdata will also be presented to track changes in the seawater sulphate reservoir and sulphur diagenetic pathways across the event.
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    Thallium isotopes reveal protracted anoxia during the Toarcian (Early Jurassic) associated with volcanism, carbon burial, and mass extinction
    Them, Theodore R.; Gill, Benjamin C.; Caruthers, Andrew H.; Gerhardt, Angela M.; Grocke, Darren R.; Lyons, Timothy W.; Marroquin, Selva M.; Nielsen, Sune G.; Alexandre, Joao P. Trabucho; Owens, Jeremy D. (National Academy of Sciences, 2018-06-26)
    For this study, we generated thallium (Tl) isotope records from two anoxic basins to track the earliest changes in global bottom water oxygen contents over the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) of the Early Jurassic. The T-OAE, like other Mesozoic OAEs, has been interpreted as an expansion of marine oxygen depletion based on indirect methods such as organic-rich facies, carbon isotope excursions, and biological turnover. Our Tl isotope data, however, reveal explicit evidence for earlier global marine deoxygenation of ocean water, some 600 ka before the classically defined T-OAE. This antecedent deoxygenation occurs at the Pliensbachian/Toarcian boundary and is coeval with the onset of initial large igneous province (LIP) volcanism and the initiation of a marine mass extinction. Thallium isotopes are also perturbed during the T-OAE interval, as defined by carbon isotopes, reflecting a second deoxygenation event that coincides with the acme of elevated marine mass extinctions and the main phase of LIP volcanism. This suggests that the duration of widespread anoxic bottom waters was at least 1 million years in duration and spanned early to middle Toarcian time. Thus, the Tl data reveal a more nuanced record of marine oxygen depletion and its links to biological change during a period of climatic warming in Earth’s past and highlight the role of oxygen depletion on past biological evolution.
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    Toarcian Mercury Anomalies Record Terrestrial Disturbance Rather Than Volcanic Activity
    Them, Theodore R.; Jagoe, C. H.; Gill, Benjamin C.; Caruthers, Andrew H.; Grasby, S. E.; Gröcke, Darren R.; Yin, R.; Owens, Jeremy D. (2018-08-15)
    Mercury (Hg) enrichments in ancient sedimentary successions are currently used to directly link the emplacement of large igneous provinces (LIPs) to environmental change and biological turnover in the geological record. It is suggested that increases in [Hg] and [Hg] normalized to total organic carbon (TOC), or Hg/TOC, is direct evidence for a LIP. However, there are several other major sources of Hg to the oceans that could operate on these timescales: i) enhanced hydrological cycle and weathering of continental material, ii) biomass burning and subsequent soil loss, and iii) thawing of permafrost. Furthermore, any changes in local sedimentary redox, which is a hallmark of many extinction events, can also affect local Hg signatures. Mercury isotopes have also recently been utilized to identify different potential sources of Hg during some of these events, but isotopic overlap between these sources complicates their differentiation. Here, we generated and compiled Hg concentration and isotope data from 11 globally distributed sites across the Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma). The TOAE Hg enrichments have been interpreted to be the result of Hg sourced from the Karoo-Ferrar LIP. However the data show a relationship between the proximity of the sites to landmasses and Hg enrichment. This notion is in agreement with the Hg isotope signatures from northeastern Panthalassa that suggest a terrestrial source of Hg. These data suggest that sedimentary Hg anomalies may not be a direct proxy for LIP emplacements, but instead can be the product of other feedbacks from LIPs.