Increased thermohaline stratification as a possible cause for an ocean anoxic event in the Cretaceous period

Ocean anoxic events were periods of high carbon burial that led to drawdown of atmospheric carbon dioxide, lowering of bottom-water oxygen concentrati ons and, in many cases, significant biological extinction(1-5). Most ocean anoxic events are thought to be caused by high productivity and export of c arbon from surface waters which is then preserved in organic-rich sediments , known as black shales. But the factors that triggered some of these event s remain uncertain. Here we present stable isotope data from a mid-Cretaceo us ocean anoxic event that occurred 112 Myr ago, and that point to increase d thermohaline stratification as the probable cause. Ocean anoxic event 1b is associated with an increase in surface-water temperatures and runoff tha t led to decreased bottom-water formation and elevated carbon burial in the restricted basins of the western Tethys and North Atlantic. This event is in many ways similar to that which led to the more recent Plio-Pleistocene Mediterranean sapropels, but the greater geographical extent and longer dur ation (similar to 46 kyr) of ocean anoxic event 1b suggest that processes l eading to such ocean anoxic events in the North Atlantic and western Tethys were able to act over a much larger region, and sequester far more carbon, than any of the Quaternary sapropels.