The roles of production and preservation during Oceanic Anoxic Event III: A multi-proxy record from the Western Interior Seaway

Cretaceous marine deposits are characterized by periods of enhanced organic carbon burial, known as Oceanic Anoxic Events (OAEs). A major unresolved debate about OAEs is whether changes in preservation or production are responsible for organic carbon burial. To address this question it is important to determine the relative roles of reducing bottom water conditions and surface water primary productivity at the onset of the event. Here we examine the Coniacian-Santonian event (OAE III) using high-resolution major element and trace metal records of the Niobrara Formation recovered in the USGS #1 Portland core from Cañon City Basin, Colorado. Preliminary results of redox sensitive trace metals (Re, Mo, U) and trace metals associated with organic matter (Cd, Ag) indicate that both bottom water suboxia and elevated primary productivity played a role in organic carbon burial during OAE III. The onset of organic carbon accumulation is characterized by two distinct periods of redox-sensitive trace metal enrichments indicating that initial carbon burial is associated with the development of bottom water suboxia. The second enrichment is followed by a shift in trace metal baseline values. Elevated concentrations of Cd and Ag without corresponding major enrichments in Re, Mo and U indicate the increased influence of high primary productivity. We compare trace metals records with bulk organic analyses (%TOC, C:N ratios, and organic carbon isotopes) to further elucidate the relationship between organic carbon accumulation, primary productivity, and bottom water oxygenation. Results from the Portland core are also compared to nearby cores in Colorado and Kansas to evaluate spatial variability throughout the carbon burial event, to examine differences between marginal and open seaway records, and to assess the role of proximity to terrestrial input.