BIOMARKERS SIGNAL PRECONDITIONING OF THE NORTH ATLANTIC PRIOR TO THE CRETACEOUS OCEANIC ANOXIC EVENT (OAE 2)

The widespread deposition of organic-rich black shales during some intervals of Earth’s history, particularly during the late Cretaceous greenhouse period (100.5 - 66 Ma), is of much interest due to their impact on climate and biogeochemistry. Increased global organic matter burial is represented through positive carbon excursions, including that at the Cenomanian-Turonian Boundary, and has been attributed to widespread ocean anoxia. However, the environmental changes preceding the Cenomanian/Turonian oceanic anoxic event (OAE2) are more poorly constrained, and biomarker records remain sparse. This study reports new long-term biomarker records for water column anoxia and ecological change from Demerara Rise (Ocean Drilling Programme Leg 207, Site 1258). Ecological changes among eukaryotes and bacteria are inferred from distributions of C₂₇-C₂₉ sterane and C₂₇-C₃₅ hopane biomarkers, respectively, and these are linked to major climatic events of the Cenomanian inferred from stable organic carbon isotopes and TEX₈₆ sea surface temperatures. Our findings indicate a long-term shift in algal community before the onset of OAE 2. Crucially, this change is associated with very high abundances of the water column anoxia biomarkers lycopane and 17a(H),18a(H),21(b)-28,30-bisnorhopane (BNH), suggesting that intervals of at least regional anoxia preceded OAE2. This is associated with high levels of carbon dioxide (CO₂), and we explore whether climate change could have driven a system already poised for anoxia towards the OAE2 state.