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Organic geochemistry of the early Toarcian oceanic anoxic event in Hawsker Bottoms, Yorkshire, England

French, K.L.; Sepúlveda, J.; Trabucho-Alexandre, J.; Gröcke, D.R.; Summons, R.E.

Organic geochemistry of the early Toarcian oceanic anoxic event in Hawsker Bottoms, Yorkshire, England Thumbnail


Authors

K.L. French

J. Sepúlveda

J. Trabucho-Alexandre

R.E. Summons



Abstract

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 View the MathML source record in marine- and terrestrial-derived biomarkers across the T-OAE. The View the MathML source 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 View the MathML source records of the T-OAE display a negative CIE that is smaller in magnitude compared to the bulk organic View the MathML source 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 View the MathML source records of the T-OAE.

Citation

French, K., Sepúlveda, J., Trabucho-Alexandre, J., Gröcke, D., & Summons, R. (2014). Organic geochemistry of the early Toarcian oceanic anoxic event in Hawsker Bottoms, Yorkshire, England. Earth and Planetary Science Letters, 390, 116-127. https://doi.org/10.1016/j.epsl.2013.12.033

Journal Article Type Article
Acceptance Date Dec 23, 2013
Online Publication Date Jan 23, 2014
Publication Date Mar 15, 2014
Deposit Date May 19, 2015
Publicly Available Date Mar 29, 2024
Journal Earth and Planetary Science Letters
Print ISSN 0012-821X
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 390
Pages 116-127
DOI https://doi.org/10.1016/j.epsl.2013.12.033
Related Public URLs https://darchive.mblwhoilibrary.org/handle/1912/6678

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