The Global Record of the Toarcian Ocean Anoxic Event: Perspectives from the Eastern Panthalassic Ocean

Abstract:

Much of the debate surrounding Oceanic Anoxic Events (OAEs) concerns the regional versus global aspects of biogeochemical and environmental changes associated with it. Untangling local versus global aspects of OAEs remains a major challenge. This is particularly true for the Toarcian Oceanic Anoxic Event or T-OAE of the Early Jurassic (~182 million years ago) since few open ocean, deep-sea sedimentary records remain for this time interval. Much of the uncertainly surrounding the geochemical record of this event derives from the fact that the majority of the studied stratigraphic successions occur in Western Europe. These localities record the paleoceanographic conditions within the western Tethys Ocean and adjacent epeiric seas. To develop a greater understanding of the T-OAE we will present geochemical and biostratigraphic data from western North America (Alberta) that will enable us to access paleoceanographic changes associated with the T-OAE in the eastern Panthalassic Ocean.

The Early Jurassic stratigraphic succession in Alberta consist of carbonate ramp facies that transition into basinal fine-grained siliciclastic facies. Ammonite biostratigraphy from this succession identifies the time interval spanning the Pliensbachian and Toarcian Stages (Carlottense, Kanense and Planulata zones of western North America). Organic carbon isotope data from the studied sections show the negative excursion that is now recognized globally to occur during the T-OAE. Locally, geochemical data indicate that the distal portions of a carbonate ramp and basinal facies were anoxic during the T-OAE and the carbonate factory appears to have been drowned at the onset of the T-OAE. In addition, sulfur isotope and iron speciation data will be presented from these successions to examine further the paleoceanographic changes of the Panthalassic Ocean in response to the T-OAE. Importantly, this Panthalassic succession provides an additional perspective for understanding global biogeochemical cycles during the Early Jurassic.