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|a Trabucho-Alexandre, J.
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|a Joint Program in Chemical Oceanography
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|a Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
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|a Woods Hole Oceanographic Institution
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|a French, Katherine L.
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|a Sepulveda, Julio
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|a Summons, Roger Everett
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|a French, Katherine L.
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|a Sepulveda, Julio
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|a Grocke, D. R.
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|a Summons, Roger Everett
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|a Organic geochemistry of the early Toarcian oceanic anoxic event in Hawsker Bottoms, Yorkshire, England
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|b Elsevier,
|c 2015-07-30T12:28:55Z.
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|z Get fulltext
|u http://hdl.handle.net/1721.1/97924
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|a A comprehensive organic geochemical investigation of the Hawsker Bottoms outcrop section in Yorkshire, England has provided new insights about environmental conditions leading into and during the Toarcian oceanic anoxic event (T-OAE; ~183 Ma). Rock-Eval and molecular analyses demonstrate that the section is uniformly within the early oil window. Hydrogen index (HI), organic petrography, polycyclic aromatic hydrocarbon (PAH) distributions, and tricyclic terpane ratios mark a shift to a lower relative abundance of terrigenous organic matter supplied to the sampling locality during the onset of the T-OAE and across a lithological transition. Unlike other ancient intervals of anoxia and extinction, biomarker indices of planktonic community structure do not display major changes or anomalous values. Depositional environment and redox indicators support a shift towards more reducing conditions in the sediment porewaters and the development of a seasonally stratified water column during the T-OAE. In addition to carotenoid biomarkers for green sulfur bacteria (GSB), we report the first occurrence of okenane, a marker of purple sulfur bacteria (PSB), in marine samples younger than ~1.64 Ga. Based on modern observations, a planktonic source of okenane's precursor, okenone, would require extremely shallow photic zone euxinia (PZE) and a highly restricted depositional environment. However, due to coastal vertical mixing, the lack of planktonic okenone production in modern marine sulfidic environments, and building evidence of okenone production in mat-dwelling Chromatiaceae, we propose a sedimentary source of okenone as an alternative. Lastly, we report the first parallel compound-specific δ[superscript 13]C record in marine- and terrestrial-derived biomarkers across the T-OAE. The δ[superscript 13]C records of short-chain n-alkanes, acyclic isoprenoids, and long-chain n -alkanes all encode negative carbon isotope excursions (CIEs), and together, they support an injection of isotopically light carbon that impacted both the atmospheric and marine carbon reservoirs. To date, molecular δ[superscript 13]C records of the T-OAE display a negative CIE that is smaller in magnitude compared to the bulk organic δ[superscript 13]C excursion. Although multiple mechanisms could explain this observation, our molecular, petrographic, and Rock-Eval data suggest that variable mixing of terrigenous and marine organic matter is an important factor affecting the bulk organic δ[superscript 13]C records of the T-OAE.
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|a NASA Astrobiology Institute
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|a Exobiology Program (U.S.)
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|a National Science Foundation (U.S.). Graduate Research Fellowship
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|a en_US
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|a Article
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|t Earth and Planetary Science Letters
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