ONSET OF SEAWATER 87SR/86SR EXCURSION PRIOR TO CENOMANIAN-TURONIAN OCEANIC ANOXIC EVENT 2? NEW LATE CRETACEOUS STRONTIUM ISOTOPE CURVE FROM THE CENTRAL PACIFIC OCEAN

A seawater Sr isotopic excursion toward lower values across the Cenomanian/Turonian boundary has been interpreted to indicate an episode of mantle plume volcanism that might have ultimately triggered Oceanic Anoxic Event (OAE) 2. However, the standard Sr isotope curve is not definitive enough to confirm the exact coincidence of the onset of the Sr-87/Sr-86 excursion and the genesis of OAE 2, as revealed by the black shale deposition or positive delta C-13 excursion. New high-precision Sr-87/Sr-86 data from Upper Cretaceous chalks in a diagenetically well-constrained Pacific deep-sea section (Deep Sea Drilling Project Site 463), in conjunction with delta C-13 stratigraphy and revised planktonic foraminiferal zonation, show that the onset of the Sr-87/Sr-86 shift might have predated OAE 2, presumably by >0.6 m.y. Our results point to the possibility that the massive volcanism, and associated Cenomanian-Turonian global change might have preconditioned the climatic and oceanic regimes for OAE 2, but not have actually triggered the event.