A HIGH-RESOLUTION RECORD OF MARINE PRODUCTIVITY AND CARBON CYCLING ACROSS OCEANIC ANOXIC EVENT 2 IN THE WESTERN INTERIOR SEAWAY

Cretaceous oceanic anoxic events (OAEs) were periods of widespread oxygen-depletion and organic carbon burial that led to a global perturbation of the carbon cycle. Oceanic Anoxic Event 2 (OAE2, ~94 Ma), a major OAE of the Mesozoic, is characterized by a positive carbon isotope excursion suggesting enhanced marine primary productivity and organic carbon burial. However, ocean-atmosphere carbon cycle dynamics as well as the timescales of carbon cycle changes during OAE2 remain poorly constrained.

We provide insight into these dynamics by studying the diversity, abundance, and compound-specific carbon isotope records of marine (algae) and terrestrial (land plant) biomarkers preserved in sedimentary sequences of the Smoky Hollow #1 core (SH1, Utah) in the western margin of the Cretaceous Western Interior Seaway. High sedimentation rates at this location allow elevated sampling resolution of up to ~300 years, which results in a detailed record of changes in algal ecology, terrestrial input, and the carbon isotopic composition of both oceanic (DIC) and atmospheric (CO₂) carbon pools at the onset of OAE2. Our results indicate the occurrence of a carbon isotopic excursion (CIE) in atmospheric CO₂ that is larger in magnitude and that developed earlier (~10 ky) than marine DIC. The CIE in marine lipids matches other marginal marine sections and suggests enhanced productivity at marginal localities during late Cenomanian transgression. We discuss the role of marine productivity, water column redox, and geologic and oceanographic changes at the onset of OAE2 to explain differences in magnitude and timing of isotopic excursions in both carbon pools.