GEOCHEMICAL EVIDENCE FOR PERSISTENT ANOXIA IN EASTERN PANTHALASSA DURING THE EARLY JURASSIC

The Mesozoic Oceanic Anoxic Events (OAEs) represent intervals of Earth’s history when increased organic carbon burial was associated with the increased geographic extent of marine anoxia. These OAEs are also associated with global warming and cooling, perturbations to the global carbon cycle recorded as carbon isotope excursions (CIEs), and elevated marine extinction rates. Records of the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma) from several sedimentary successions in Europe have led to differing interpretations of both the timing and extent of marine anoxia during the event. To further explore the record of anoxia during the T-OAE, we have generated geochemical records from western North America to reconstruct the redox conditions in eastern Panthalassa.

We have identified the T-OAE interval and CIE from both outcrop and drill core of the Early Jurassic Fernie Formation in the Western Canada Sedimentary Basin. Together these different sample locations represent a basinal transect, enabling us to compare the redox records at various water depths within the basin. High-resolution iron speciation records from the Fernie Formation suggest that at all studied sites anoxic water columns were present throughout the Early Jurassic. Pre-CIE data suggest the presence of both ferruginous (anoxic, iron replete) and euxinic (anoxic, sulfidic) conditions, whereas syn- and post-CIE values suggest solely euxinic conditions. These records will be further compared to those of redox sensitive trace metals in order to further differentiate changes in local and global redox during the event. For example, pre- and post-CIE Re abundances in the Fernie are high, but are significantly lower during the CIE. This pattern may represent the local enrichment of Re in the Fernie before and after the T-OAE due to the local redox conditions, and a global drawdown of Re inventory from increased extent of marine anoxia during the T-OAE. These new geochemical data suggest that local and regional oceanographic dynamics may have asserted a control on marine oxygenation during the Early Jurassic; however, there is still clear evidence for the development of widespread marine anoxia during the T-OAE interval. Further, metal drawdown might limit bioessential nutrients, result in major ecological shifts, and terminate OAEs.