Carbon Cycling During Oceanic Anoxic Event 2: Compound‐Specific Carbon Isotope Evidence From the Western Interior Seaway

Abstract

AbstractThe Cenomanian‐Turonian Oceanic Anoxic Event 2 (OAE2, ∼94 Ma) was a period of widespread ocean deoxygenation and marine organic carbon burial. Increased CO2and nutrient flux before OAE2 are generally considered the main drivers of ocean anoxia and carbon burial, though some evidence has suggested multiple phases of CO2input and burial throughout the event. To test hypotheses about the nature of carbon cycle perturbations before and during OAE2, we analyzed stable carbon isotopes in marine‐derived and terrestrial‐derived biomarkers from an expanded sedimentary record of OAE2 from the western margin of the Western Interior Seaway. Biomarker carbon isotope data were used to estimate the concentration and carbon isotopic composition of aqueous and atmospheric CO2, as well as changes in marine productivity, through OAE2. While complicated by biological and environmental uncertainties, ourpCO2reconstructions generally agree with estimates from other locations and proxies around the world. High‐resolution sampling revealed several short‐lived carbon cycle perturbations before and during OAE2 that may reflect fluctuations in the global carbon cycle, including several pulses of isotopically light carbon before OAE2, during the onset of OAE2, and during the Plenus event. This study provides new constraints on carbon cycle dynamics during OAE2 that highlight the complex nature of the event and its causes.