Long-Term Anoxia Driven by High Temperature of Equatorial Atlantic During Cenomanian Leading up to OAE 2

Abstract

Oceanic Anoxic Event (OAE) 2 occurred during the Cenomanian/Turonian and marks one of the most significant perturbations of Earth’s system of the last 250 million years. Although the burial of black shales in the marine environment under depleted (bottom) water column oxygen conditions was widespread during OAE 2, the state of anoxia in the million years leading up to OAE 2 and the correlation between anoxia and temperature are poorly constrained. Here, we report long-term biomarkers records of (a) water column anoxia, (b) water column and photic zone euxinia (PZE), and c) TEX86-based sea surface temperature (SST) from Demerara Rise (ODP Site 1258) that span the Albian to Turonian, including OAE 2. Our data suggest a depositional environment under long-term anoxic condition at Demerara Rise during the Cenomanian as indicated by elevated total organic carbon content (up to 10 wt.%) and episodic occurrence of the biomarkers lycopane and 28,30-bisnorhopane, which at present are found in the anoxic marine such as Black Sea (Wakeham et al., 1993) and Monterey Formation (Schoell et al., 1992), respectively. The extent of water column anoxia gradually intensified and peaked during OAE 2 when the presence of hopanoid thiophenes and free isorenieratane indicate euxinic conditions in the water column, at times reaching the photic zone. This long-term progression of marine de-oxygenation coincides with a gradual increase in SSTs at the site. TEX86-based SSTs indicate a long-term warming of ∼ 7 °C during the Cenomanian to maximum values of 41 °C during OAE 2. The abrupt declined in SSTs following OAE 2 during the Turonian indicates termination of long-term anoxia via re-oxygenation of water column in phase with low TOC and absence of anoxia and euxinia biomarkers. This study highlights the impact of Earth’s climate state on marine oxygenation with an increase in temperature driving the ocean’s state from hypoxia into severe anoxia and eventually euxinia, likely due to weathering feedback mechanisms related to warming and changes in the hydrological cycle.