CAUSES, EXTENT AND CONSEQUENCES OF MARINE ANOXIA DURING THE TOARCIAN (EARLY JURASSIC) OCEANIC ANOXIC EVENT

The Toarcian oceanic anoxic event (183 Ma) is characterised by the widespread burial of organic matter, a C-isotope excursion of -7 per mil; and geochemical and isotopic evidence for a rise in ocean anoxia, global temperature and the rate of chemical weathering (Kemp et al. 2005, Cohen et al. 2007, Pearce et al. 2008). Milankovitch cycles controlled both the four abrupt C-isotope shifts (each of -2 to 3 per mil) that constitute the overall C-isotope excursion and the changes in the extent of marine anoxia.

Data sets from sections around the world indicate three possible marine extinction horizons associated with the Toarcian oceanic anoxic event. However, taking into account the Signor-Lipps effect it is likely that only the youngest extinction horizon is a true extinction event. This youngest horizon coincides with the initial C-isotope shift, and with the start of the rapid increase in global temperature and global expansion of seawater anoxia (Caswell et al. 2009). Invertebrate species range data from sections in England show distinct relationships with high-resolution geochemical datasets. There was an almost complete absence of marine fauna for over 10,000 years immediately after each of the four abrupt C-isotope shifts and only one bivalve species, Pseudomytiloides dubius, occurs in high abundance at other times throughout the event. Increased epifaunal bivalve diversity and the reappearance of infauna indicate a brief return to relatively oxygenated conditions after the final expansion of marine anoxia. Biometric data were obtained for the two dominant bivalve species Pseudomytiloides dubius and Bositra radiata from over 226 stratigraphic levels across the event and show that shell size is related to fluctuating seawater anoxia as recorded from Mo abundance and Mo-isotope proxy data. This presentation will show the integrated geochemical and biological data set through this oceanic anoxic event and how it compares with other anoxia periods in the Mesozoic.

Caswell, B. A., Coe, A. L. & Cohen, A. S. 2009. Journal of the Gological Society, London, 166, 859-872.

Cohen, A. S. Coe, A. L. & Kemp, D. B. 2007. Journal of the Geological Society, 164, 1093-1108.

Kemp, D. B., Coe, A. L., Cohen, A. S. & Schwark, L. 2005. Nature, 437, 396-399.

Pearce, C. R. Cohen A. S. & Coe, A. L. 2008. Geology, 36, 231-234.