Decoupling of Organic and Inorganic Carbon Isotope Records during the Coniacian-Santonian in the Western Interior Seaway: Implications for Carbon Cycling during OAE 3

Abstract:

The sedimentary record of the Cretaceous Western Interior Seaway is characterized by periods of enhanced organic carbon burial known as Oceanic Anoxic Events (OAEs). Compared to older OAEs, the last Cretaceous OAE, the Coniacian-Santonian OAE 3 (~86 Ma), is more enigmatic and its driving mechanisms less well understood. In contrast to the nearly global OAE 2, OAE 3 was geographically limited to restricted basins and shallow seaways. Due to significant organic carbon burial, OAEs are traditionally identified by large, rapid positive carbon isotope excursions, whereas OAE 3 is characterized by a relatively muted carbonate positive carbon “plateau” that persists for ~3 myrs, which suggests significantly different carbon burial processes or paleoceanographic conditions.

Here we present new high-resolution paired organic-inorganic carbon isotope records from the Niobrara Formation recovered in the USGS #1 Portland core from Cañon City Basin, Colorado. At the onset of OAE 3, there is a distinct decoupling of the organic and inorganic carbon isotope records. δ¹³C_carb exhibits an abrupt ~1‰ positive isotope excursion at the onset of carbon burial superimposed on a larger ~2‰ isotope trend. Coeval with this shift, is a ~3‰ negative δ¹³C_org excursion. These results indicate that during OAE 3 in the Western Interior Seaway, while significant organic carbon was being buried, organic carbon was depleted in ¹³C. The carbon isotope records will be compared to records of organic carbon, C:N ratios, and redox sensitive metals such as U and Mo to evaluate how carbon cycling changed during OAE 3.