Controls on organic carbon and molybdenum accumulation in Cretaceous marine sediments from the Cenomanian–Turonian interval including Oceanic Anoxic Event 2

This study investigates the controls on organic carbon and molybdenum (Mo) accumulation in sediments deposited within the Western Interior Seaway across the Cenomanian–Turonian boundary interval (92.6–94.1 Ma) including Oceanic Anoxic Event 2 (OAE2). Carbon fluxes to the sediment-water interface (reflecting changes in primary productivity) and bottom-water oxygen concentrations (reflecting preservation effects) are reconstructed from field data and used to constrain a benthic model that simulates the geochemistry of unconsolidated sediments as they were deposited. The results show that increased availability of reactive iron prevents Mo sequestration as thiomolybdate (MoS42–) during OAE2 (O2 ~ 105 μM) by (i) inhibiting sulfate reduction, and (ii) buffering any free sulfide that becomes available. In the post–OAE2 period (O2 ~ 50 μM), Mo accumulation is favored by a large reduction in iron flux. Importantly, this occurs in parallel with oxygenated bottom waters and high rates of aerobic carbon degradation in the surface sediments, implying that elevated Mo burial fluxes in ancient marine facies do not necessarily reflect euxinic or even anoxic conditions within the water column. Our findings suggest that both an increase in production and preservation lead to enrichment in organic carbon in the Western Interior Seaway, and caution against the use of carbonate as a proxy for primary production. More generally, the results demonstrate that a careful consideration of the coupling between iron, carbon and oxygen cycles during the early stages of diagenesis is critical for interpreting geochemical proxies in modern and ancient settings.