Organic geochemistry of the early Toarcian oceanic anoxic event in Hawsker Bottoms, Yorkshire, England
French, Katherine L.
Grocke, Darren R.
Summons, Roger E.
MetadataShow full item record
KeywordToarcian oceanic anoxic event; Lipid biomarkers; Okenane; Photic zone euxinia; Stable carbon isotopes; Hawsker Bottoms
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.
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.
Showing items related by title, author, creator and subject.
Multidecadal Indian Ocean variability linked to the Pacific and implications for preconditioning Indian Ocean dipole events Ummenhofer, Caroline C.; Biastoch, Arne; Böning, Claus W. (American Meteorological Society, 2017-02-15)The Indian Ocean has sustained robust surface warming in recent decades, but the role of multidecadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature ...
Detection of an unusually large hydrothermal event plume above the slow-spreading Carlsberg Ridge : NW Indian Ocean Murton, Bramley J.; Baker, Edward T.; Sands, Carla M.; German, Christopher R. (American Geophysical Union, 2006-05-31)About 90% of Earth's volcanism occurs along the global mid-ocean ridge system. Here, sporadic volcanic and tectonic activity is thought to cause cataclysmic release of hydrothermal fluids, forming event plumes. Each plume ...
Scientific sampling event log from ARSV Laurence M. Gould LMG1110 in the Southern Ocean from Nov. 2011 (Salp_Antarctic project) Bucklin, AnnThe event log to record all instrument deployments during the cruise. For a complete list of measurements, refer to the supplemental document 'Field_names.pdf', and a full dataset description is included in the supplemental ...