A HIGH-RESOLUTION ORGANIC GEOCHEMICAL STUDY OF OAE2 IN THE U.S. WESTERN INTERIOR SEAWAY

The Cretaceous Ocean Anoxic Event 2 (OAE2; ~94Ma) has long been studied for its association with the vast deposition of black shales, which comprise important petroleum source rocks. High accumulations of organic matter in these rocks have been related to extreme changes in the climate-ocean system: atmospheric CO₂ concentrations ~2-3 times higher than today, sea surface temperatures of up to ~35 °C, widespread ocean anoxia, altered nitrogen and carbon cycles, and the extinctions of numerous marine organisms. Molecular fossils (lipid biomarkers) in these rocks provide information about changes in plankton ecology and ocean biogeochemistry, such as an enhanced contribution of eukaryotes, cyanobacteria, and anaerobic green sulfur bacteria. However, many questions remain concerning the nature and extent of the climatic and ecological changes at the onset and throughout the OAE2 event. Using Gas Chromatography-Triple Quadrupole-Mass Spectrometry, we analyzed the lipid biomarker record from a 30-meter long interval of a drill core from the Tropic Shale in Utah, representing OAE2 in a coastal environment on the western margin of the Cretaceous Western Interior Seaway with high sediment accumulation. We provide biomarker data (aliphatic and aromatic hydrocarbons such as n-alkanes, PAHs, algal steranes, and bacterial hopanes and carotenoids) and bulk stable carbon isotopes in organics and carbonate that further detail the story of environmental, biogeochemical and ecological responses before, during and after the onset of OAE2. We will test the hypothesis that changes in ocean redox and pH resulted in turnovers in biotic communities that favored non-calcifying planktonic organisms. Finally, we discuss the implications of our study for the interpretation of future ocean dynamics under projected scenarios of climate change.