TRACE METAL CHEMISTRY REVEALS MECHANISTIC DIFFERENCES FOR CARBON BURIAL DURING OAE 2 AND 3 IN THE WESTERN INTERIOR SEAWAY

During the Cretaceous, a shallow sea flooded western North America extending from the Gulf of Mexico to the Arctic Ocean. The sediments deposited in the resulting Western Interior Seaway (WIS) record multiple episodes of oxygen depletion and organic carbon-rich sedimentation, including Oceanic Anoxic Events (OAE) 2 and 3. OAE 2 represents the archetypal example of an OAE. The event was temporally short (<1 myr), near global in geographic extent, and characterized by a distinct, short-lived, positive carbon isotope excursion (>2‰). Previous work on OAE 2 has concluded that the enhanced organic carbon burial was initiated due to a combination of enhanced nutrient input from continental and volcanogenic sources and a paleocirculation pattern that was effective at transporting these nutrients to the low latitude open ocean (Du Vivier et al., 2013). By contrast, OAE 3 is far more enigmatic and its driving mechanism less well understood. With a longer duration of ~ 3 myrs, OAE 3 was globally geographically limited to the restricted basins and shallow seaways, and characterized by a muted positive carbon isotope plateau.

Trace metal enrichments are one of the most utilized proxies for reconstructing marine redox conditions, as well as changes in export productivity. Previous trace metal studies of OAE 2 have shown that redox sensitive metals, specifically Mo, exhibit no enrichments within the seaway, which is anomalous for an organic carbon burial event (Meyers et al., 2005). In comparison, Mo concentrations during OAE 3 reach levels >100 ppm, suggesting fundamentally different bottom water and sediment chemistry during the two events. Here we compare carbon data (%TOC and %CaCO₃), major element (Fe, Mn, and P) and trace metal (Cu, Mo, Ni, V, Zn) enrichments from OAE 2 and 3 in the WIS from the USGS #1 Portland core to assess the processes controlling trace metal accumulations during each organic carbon burial event. Similar to Mo, these trace metals are more enriched during OAE 3 than OAE 2. Comparison of trace metal records between the two events in the seaway suggests that while changing redox and productivity both played a role in the extended carbon burial during OAE 3, the driving mechanisms and seawater chemistry in the WIS were distinctly different from the better studied OAE 2 event.