Mid-Cretaceous marine Os isotope evidence for heterogeneous cause of oceanic anoxic events.

During the mid-Cretaceous, the Earth experienced several environmental perturbations, including an extremely warm climate and Oceanic Anoxic Events (OAEs). Submarine volcanic episodes associated with formation of large igneous provinces (LIPs) may have triggered these perturbations. The osmium isotopic ratio ( 187 Os/ 188 Os) is a suitable proxy for tracing hydrothermal activity associated with the LIPs formation, but 187 Os/ 188 Os data from the mid-Cretaceous are limited to short time intervals. Here we provide a continuous high-resolution marine 187 Os/ 188 Os record covering all mid-Cretaceous OAEs. Several OAEs (OAE1a, Wezel and Fallot events, and OAE2) correspond to unradiogenic 187 Os/ 188 Os shifts, suggesting that they were triggered by massive submarine volcanic episodes. However, minor OAEs (OAE1c and OAE1d), which do not show pronounced unradiogenic 187 Os/ 188 Os shifts, were likely caused by enhanced monsoonal activity. Because the subaerial LIPs volcanic episodes and Circum-Pacific volcanism correspond to the highest temperature and pCO 2 during the mid-Cretaceous, they may have caused the hot mid-Cretaceous climate.