A euxinic southern North Atlantic Ocean during the Cenomanian/Turonian oceanic anoxic event

During mid-Cretaceous times large amounts of organic carbon (Corg) became sequestered in `black shalesʹ, possibly due to `oceanic anoxic eventsʹ characterized by the development of an extended oxygen minimum zone (OMZ). Here, we provide the first direct evidence for an open ocean OMZ in the Cenomanian/Turonian (C/T) southern North Atlantic and in fact show that the base of the photic zone was euxinic as revealed by molecular fossils from photosynthetic green sulfur bacteria in C/T black shales. This, together with evidence for bottom water anoxia and accumulation of redox-sensitive trace metals and hydrogen-rich organic matter, indicates a continuously euxinic water column. Concurrent with the high Corg accumulation rates, which are 15–150 times greater in the southern than in the northern North Atlantic, and the low biological productivity, this suggests that preservation controlled the accumulation of Corg in C/T black shales.