Department of Geosciences
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- Atmosphere-ocean oxygen and productivity dynamics during the Cambrian explosionDahl, Tais W.; Connelly, James N.; Li, Da; Kouchinsky, Artem; Gill, Benjamin C.; Porter, Susannah M.; Maloof, Adam C.; Bizzarro, Martin (2019-08-21)Large, motile animals proliferated during the Cambrian explosion,540–515 million years ago,and this period has been associated with both rising and declining O2levels on Earth [1–3]. To explore this conundrum, we reconstructed the global extent of seafloor oxygenation at ~sub-million year resolution based on U isotope compositions in 187 marine limestone samples from China, Siberia and Morocco,and coupled this record to simulations of atmospheric O2 levels using a mass balance model constrained by carbon, sulfur and strontium isotopes in the same rocks.The studied samples were investigated petrographically and geochemically to disentangle isotope fractionation processes occurring locally during diagenesis from secular changes in open seawater. We find that Sr/Ca, Mg/Ca and G18O help discriminate carbonate samples with a larger G238U offset from contemporaneous seawater, and decipher the signal that can be ascribed to global changes in the oceanic U pool. Our data demonstrate two significant expansions of seafloor anoxia in the aftermath of an interval with declining atmospheric pO2levels.The combination of atmospheric and oceanic O2records offers a self-consistent and highly dynamic picture of atmosphere-ocean oxygenation in which the evolving animal ecosystems might have both responded and contributed to global environmental change.Animals diversified possibly by a predator-prey arms race peaking in two pulses interrupted by these dramatic fluctuations in seafloor anoxia. When O2levels again rose, it occurred in concert with predicted high rates of photosynthetic production, both of which may have fueled more energy to predators and their armored prey in the evolving marine ecosystem.
- Environmental Divers of Decoupled Marine Sulfate Oxygen and Sulfur Isotope Trends during the Toarcian Oceanic Anoxic EventGomes, Maya; Waldek, Anna; Gill, Benjamin C.; Johnston, David (2019-08-12)The Toarcian Oceanic Anoxic Event was a major carbon cycle perturbation associated with two pulses of extinction and widespread environmental change. Studies of the biogeochemical sulfur cycle provide insight into the evolution of bottom-water redox conditions, which influence elemental cycling and habitability in the ocean. In particular, a ~6‰ positive excursion in the sulfur isotope composition of marine sulfate (d34SSO4) has been interpreted to reflect enhanced pyrite burialdue to the expansion of anoxic and euxinic sedimentary environmentsand/or changes in sulfur isotope fractionation associated with sulfate reduction. Similarly, the T-OAE preservesa6‰ positive oxygen isotope (d18OSO4) excursion that precedesthe d34SSO4 excursionatMonte Sorgenza.Using these observations, wemodel the behaviour of marine sulfate reservoir using both classic box models and a new flux balance approach to address the question of expanding euxinia and the role ofbiology.This study highlights how coupled 34SSO4 andd18OSOapproaches can provide insight into environmental changes associated with major perturbations to the carbon cycle.
- Evaluating Records of Environmental Change and Mass Extinctions during the Early PaleozoicMcKenzie, N. R.; Gill, Benjamin C.; Wernette, S. J.; Chen, J.; Park, T. -Y.; Myrow, P. M.; Hughes, N. C. (2019-08-21)The interval between the Cambrian ‘explosion’ and the Great Ordovician Biodiversification Event (GOBE) is marred by biotic turmoil.Several mass extinction events have been recognized throughout the Cambrian,with multiple Furongian (late) Cambrian extinctions mostly seen within trilobite fauna of North America.These extinctions appear to correspond with episodic ocean anoxia and carbon cycle perturbations, all of which are set to the backdrop of an extensive greenhouse climate.Here we will present new chemostratigraphic and compiled biostratigraphic data from Cambrian successions of the NorthChina/Sino-Korean block(NCB). These data help identify newly recognized trilobite extinctions in upper Cambrian carbonate platform deposits of the NCB that may be correlative to North American events, demonstrating their global extent.These extinctions also correspond with marked G13Cexcursions. The cessation of these repeated extinctions coincides with global reductions in arc-volcanism and climatic cooling, implying a causal linkage between baseline climate conditions and this extensive interval of biospheric perturbations.Collectively, these data help further elucidate the harsh environmental conditions that hindered early animal evolution.
- Investigating the Sulphur Cycle during the End-Triassic Mass Extinction from PanthalassaMarroquín, 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.
- Sulfur cycling within subduction zones: Insights from exhumed mafic and ultramafic slabSchwarzenbach, Esther M.; Li, J.; John, T.; Caddick, Mark J.; Petroff, Matthew; Gill, Benjamin C. (2019-04)Subduction zones significantly control the geochemical cycling and elemental transfer between the surficial andinternal reservoirs of the Earth. Amongst the most hydrophile elements sulfur is the fifth most abundant element on Earth. Subduction of oceanic lithosphere transports sulfur together with numerous other elements into Earth’s mantle and releases sulfur-bearing fluids modifying the redox state and the chemical budget of the mantle. Here, we present bulk rock sulfur geochemical data of exhumed mafic and ultramafic slab material to provide new insights into the sulfur cycle within subduction zones. Our data shows that small bodies of detached slab material are subject to metasomatic processes during exhumation (e.g., within a subduction zone channel), where fluids thatare circulating along the plate interface cause sulfur mobilization. Sulfur mobilization is thereby more pronounced within serpentinites compared to mafic rocks. In contrast, large sequences of obducted ophiolitic sections can retain their seafloor alteration signatures, particularly at moderate peak P-T conditions. Furthermore, dehydration-related vein systems in blue schists provide evidence for sulfur transfer within the subducted oceanic crust. These vein systems carry distinct sulfur isotopic signatures that reflect dehydration processes of underlying sequences suggesting a negative sulfur isotope signature of the dehydrating subducting slab. Our data provides new insights into the sulfur transfer between the slab and the mantle wedge, which may eventually control the formation of arc-related melts and porphyry deposits.
- Toarcian Mercury Anomalies Record Terrestrial Disturbance Rather Than Volcanic ActivityThem, 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.
- Tracking Marine Deoxygenation during the Cambrian SPICE Event Using Thallium IsotopesLeRoy, 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.