Browsing by Author "Owens, Jeremy D."

Now showing 1 - 7 of 7
  • Thumbnail Image
    Evidence for rapid weathering response to climatic warming during the Toarcian Oceanic Anoxic Event
    Them, Theodore R.; Gill, Benjamin C.; Selby, David; Gröcke, Darren R.; Friedman, Richard M.; Owens, Jeremy D. (Nature Publishing Group, 2017-07-10)
    Chemical weathering consumes atmospheric carbon dioxide through the breakdown of silicate minerals and is thought to stabilize Earth’s long-term climate. However, the potential influence of silicate weathering on atmospheric pCO2 levels on geologically short timescales (103–105 years) remains poorly constrained. Here we focus on the record of a transient interval of severe climatic warming across the Toarcian Oceanic Anoxic Event or T-OAE from an open ocean sedimentary succession from western North America. Paired osmium isotope data and numerical modelling results suggest that weathering rates may have increased by 215% and potentially up to 530% compared to the pre-event baseline, which would have resulted in the sequestration of significant amounts of atmospheric CO2. This process would have also led to increased delivery of nutrients to the oceans and lakes stimulating bioproductivity and leading to the subsequent development of shallow-water anoxia, the hallmark of the T-OAE. This enhanced bioproductivity and anoxia would have resulted in elevated rates of organic matter burial that would have acted as an additional negative feedback on atmospheric pCO2 levels. Therefore, the enhanced weathering modulated by initially increased pCO2 levels would have operated as both a direct and indirect negative feedback to end the T-OAE.
  • Thumbnail Image
    Geochemical records reveal protracted and differential marine redox change associated with Late Ordovician climate and mass extinctions
    Kozik, Nevin; Gill, Benjamin C.; Owens, Jeremy D.; Lyons, Timothy W.; Young, Seth A. (2022-01-10)
    The Ordovician (Hirnantian; 445 Ma) hosts the second most severe mass extinction in Earth history, coinciding with Gondwanan glaciation and increased geochemical evidence for marine anoxia. It remains unclear whether cooling, expanded oxygen deficiency, or a combination drove the Late Ordovician Mass Extinction (LOME). Here, we present combined iodine and sulfur isotope geochemical data from three globally distributed carbonate successions to constrain changes in local and global marine redox conditions. Iodine records suggest locally anoxic conditions were potentially pervasive on shallow carbonate shelves, while sulfur isotopes suggest a reduction in global euxinic (anoxic and sulfidic) conditions. Late Katian sulfate-sulfur isotope data show a large negative excursion that initiated during elevated sea level and continued through peak Hirnantian glaciation. Geochemical box modeling suggests a combination of decreasing pyrite burial and increasing weathering are required to drive the observed negative excursion suggesting a ∼3% decrease of global seafloor euxinia during the Late Ordovician. The sulfur datasets provide further evidence that this trend was followed by increases in euxinia which coincided with eustatic sea-level rise during subsequent deglaciation in the late Hirnantian. A persistence of shelf anoxia against a backdrop of waning then waxing global euxinia was linked to the two LOME pulses. These results place important constraints on local and global marine redox conditions throughout the Late Ordovician and suggest that non-sulfidic shelfal anoxia— along with glacioeustatic sea level and climatic cooling—were important environmental stressors that worsened conditions for marine fauna, resulting in the second-largest mass extinction in Earth history and the only example during an icehouse climate.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Sulfur isotopes track the global extent and dynamics of euxinia during Cretaceous Oceanic Anoxic Event 2
    Owens, Jeremy D.; Gill, Benjamin C.; Jenkyns, Hugh C.; Bates, Steven M.; Severmann, Silke; Kuypers, Marcel M. M.; Woodfine, Richard G.; Lyons, Timothy W. (National Academy of Sciences, 2013)
    The Mesozoic Era is characterized by numerous oceanic anoxic events (OAEs) that are diagnostically expressed by widespread marine organic-carbon burial and coeval carbon-isotope excursions. Here we present coupled high-resolution carbon- and sulfurisotope data from four European OAE 2 sections spanning the Cenomanian-Turonian boundary that show roughly parallel positive excursions. Significantly, however, the interval of peak magnitude for carbon isotopes precedes that of sulfur isotopes with an estimated offset of a few hundred thousand years. Based on geochemical box modeling of organic-carbon and pyrite burial, the sulfur-isotope excursion can be generated by transiently increasing the marine burial rate of pyrite precipitated under euxinic (i.e., anoxic and sulfidic) water-column conditions. To replicate the observed isotopic offset, the model requires that enhanced levels of organic-carbon and pyrite burial continued a few hundred thousand years after peak organic-carbon burial, but that their isotope records responded differently due to dramatically different residence times for dissolved inorganic carbon and sulfate in seawater. The significant inference is that euxinia persisted post-OAE, but with its global extent dwindling over this time period. The model further suggests that only ∼5% of the global seafloor area was overlain by euxinic bottom waters during OAE 2. Although this figure is ∼30x greater than the small euxinic fraction present today (∼0.15%), the result challenges previous suggestions that one of the best-documented OAEs was defined by globally pervasive euxinic deep waters. Our results place important controls instead on local conditions and point to the difficulty in sustaining whole-ocean euxinia.
  • Thumbnail Image
    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.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Tracking Marine Deoxygenation during the Cambrian SPICE Event Using Thallium Isotopes
    LeRoy, Mathew; Gill, Benjamin C.; Them, Theodore R.; Owens, Jeremy D. (2018-08-15)
    The stable isotopic composition of thallium (ε205Tl) in organic-rich, sulfidic sediments has recently emerged as a new proxy for tracking changes in global marine redox conditions in the geologic record [1, 2]. Here we employ this proxy to interrogate the record of the Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) — a putative Paleozoic oceanic anoxic episode that coincides with a marine extinction event. We examined two stratigraphic successions which span the SPICE: the Alum and Outwoods Shales (from Sweden and the U.K, respectively). Both records reveal a shift to less negative ε205Tl values that correspond with the onset of the SPICE and extinction event. This suggests a reduction of Mn-oxide burial and the expansion of anoxic bottom waters as previously hypothesized from other global geochemical records. Interestingly, the shift to less negative ε205Tl values during the SPICE continues after the carbon isotope peak, suggesting expanded anoxia may have continued after the organic carbon burial event — a pattern also observed during OAE 2 of the Cretaceous [1]. This suggests a comparable timeline for organic carbon burial relative to marine deoxygenation during each event. Our results demonstrate that the Tl isotope proxy captures a detailed view of marine redox changes not recognized using previously employed proxies and improves our understanding of the biological response to marine deoxygenation during the SPICE and other similar events.