A distinct, mid-Cretaceous black shale layer, discovered in large swaths of the global ocean during pioneering ocean drilling in the 1970s, represents the effects of severe ocean anoxia approximately 94 million years ago during an event called Oceanic Anoxic Event 2 (OAE2). This intriguing finding that past ocean basins were abruptly deprived of dissolved oxygen has spurred decades of research focused on both the triggers and paleoceanographic responses for this and other Cretaceous OAEs. This lecture will detail key research breakthroughs in the scientific understanding of Cretaceous OAEs over the nearly 50 years since their initial discovery, incorporating the most recent findings from scientific drilling in the Indian Ocean by the JOIDES Resolution research vessel. Geochemical data and sedimentological observations will be presented from cores drilled in the Mentelle Basin (offshore Australia) during International Ocean Discovery Program (IODP) Expedition 369. Observations and data from these cores provide a new understanding of the role and intensity of ocean acidification during OAE2 caused by abrupt CO2 emission from the eruptions of large igneous provinces (LIPs). The precise source of the CO2 remains unclear given uncertainties regarding the ages and eruptive histories of many submarine LIPs. Yet expeditions to recover cores from submarine LIPs, including the most recent Cretaceous IODP expedition which cored the expansive Agulhas Plateau offshore South Africa, provide rare opportunities to help determine which specific LIP so drastically influenced Earth’s climate and oceans in the mid-Cretaceous.
Marine drilling records of the Cretaceous OAEs provide some of the best geologic records of the effects of rapid carbon emission on the ocean-atmosphere system. These records highlight the apparent sensitivity of Cretaceous oceans to the volcanogenic emissions of volatiles like CO2 and SO2. However, the natural feedback mechanisms that eventually sequestered carbon to restabilize oceanographic conditions by the end of OAE2 may provide prescient models for modern, geologically-based mitigation strategies to address rising anthropogenic CO2 levels.