Towards an understanding of the carbon isotopic changes across the Toarcian oceanic anoxic event

Abstract

A combination of bulk carbon, biomarker and compound specific isotopic analyses were used in order to investigate the changes which accompanied the deposition of black shales during the upper tenuicostatum and lower falciferum zones of the Toarcian (early Jurassic, 183 Ma) ocean anoxic event (OAE). In this study, we reveal that apparent negative isotopic excursions in bulk organic and carbonate carbon were the result of compositional changes of organic matter and diagenesis, respectively. Organic petrology and Rock-Eval pyrolysis of organic matter from the Jet Rock, Hawsker Bottoms, Yorkshire, England, show that the upper tenuicostatum zone contains very large amounts of terrigenous debris. A careful review of the carbonate carbon record, as reported in the literature, indicates that a large negative isotopic excursion in bulk carbonate is likely the result of diagenesis, rather than reflective of seawater isotopic conditions. Biomarker distributions and isotopic composition of primary production biomarkers show little variation during the largest changes in the bulk records. Biomarker source indicators vary little throughout the section, indicating little change in biota or redox structure of the water column during this widespread deposition of black shales. Isotopic compositions of algal short chain n-alkanes, pristane and phytane also remain steady across the section.

(cont.) Long chain n-alkanes, biornarkers for higher plants, also do not change during the event. Isorenieratane, a biomarker for green sulphur bacteria and an indicator of photic zone euxinia, however, show a strong peak in concentration coincident with the maximum abundance of organic carbon. Because we have found no evidence for significant isotopic variation on land or in the ocean, we must infer that there were no major redistributions of carbon in the ocean-atmosphere system during the Toarcian OAE. Therefore, oceanic overturn or large input of methane are not plausible explanations for this event. This deposition of black shales was a result of periodic episodic euxinia, which resulted in the increased preservation of organic matter. We believe that this event was not a large, one-time occurrence, but a characteristic response to paleogeography and oceanic circulation patterns of the Mesozoic.