Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 390 (2014): 116-127, doi:10.1016/j.epsl.2013.12.033.
Description:
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 δ13C record in marine- and terrestrial-derived biomarkers across the
T-OAE. The δ13C 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 δ13C records of the T-OAE display a negative CIE
that is smaller in magnitude compared to the bulk organic δ13C 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 δ13C records of the T-OAE.
Description:
Funding support for work at MIT was provided by grants from the
NASA Astrobiology Institute and the NASA Exobiology Program to RES and an NSF
graduate fellowship to KLF. This project was partly funded by a NSERC Discovery Grant (# 288321) and NERC Standard Grant (NE/H021868/1) to DRG. JTA is funded by
the NERC Standard Grant (NE/H021868/1) to DRG.
Keywords:
Toarcian oceanic anoxic event
;
Lipid biomarkers
;
Okenane
;
Photic zone euxinia
;
Stable carbon isotopes
;
Hawsker Bottoms
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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