Drivers of Anoxia in an Epicontinental Seaway - An Organic Geochemical Transect across the Cretaceous Interior Seaway during Oceanic Anoxic Event 2

The Cretaceous Western Interior Seaway (WIS) of North America, stretching from the Gulf of Mexico to the Arctic Ocean, is an exceptional region to investigate mechanisms associated with Oceanic Anoxic Events (OAEs). The WIS covered a broad latitudinal range, with paleo-latitudes from 30° to 75° N, and its geologic record is well-studied, with excellent age constraints and high-resolution stratigraphy. Previous workers have postulated that high-resolution anoxic trends within this dynamic, semi-restricted, epeiric seaway may be antiphase to that of the open ocean during the Cenomanian-Turonian OAE-2. Proposed mechanisms driving anoxia/oxygenation include: 1) the presence of an anti-clockwise gyre drawing in Boreal and Tethyan waters; 2) downwelling (caballing) of oxygenated surface-waters at the mixing front of Tethyan and Boreal waters during maximum transgression; 3) restriction-driven stratification and the presence of a fresh-water lid; and 4) Ekman-transport driven upwelling.

To test these hypotheses, biomarker and isotopic data were generated to understand the physical and biogeochemical evolution of the WIS during the mid-Cretaceous, including variations in organic matter provenance and preservation, water column oxygenation, stratification and terrestrial organic matter input. Low TOC and the absence of increases in redox-sensitive biomarkers indicate that conditions did not favour the classic OAE "black shale" model. Gammacerane, a proxy for water column stratification, indicates increasing stratification during OAE-2 however, and temporally at each site there is a counter-intuitive relationship between anoxia and stratification proxies. The flowering plant biomarker oleanane is highest in less-stratified environments, refuting the freshwater-enhanced salinity stratification hypothesis. These data also suggest a large, tropical river catchment in the eastern hinterland. C28-sterane abundances suggest a relatively stressed biotic environment in the WIS, with less-stressed environments dominating during OAE-2. Further work will be undertaken to investigate additional anoxia proxies, such as trace metal analysis and iron speciation, however the results generated thus far support the premise that the WIS did not experience 'typical' OAE conditions during OAE-2.