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 87Sr/86Sr excursion and the genesis of OAE 2, as revealed by the black shale deposition or positive δ13C excursion. New high-precision 87Sr/86Sr data from Upper Cretaceous chalks in a diagenetically well-constrained Pacific deep-sea section (Deep Sea Drilling Project Site 463), in conjunction with δ13C stratigraphy and revised planktonic foraminiferal zonation, show that the onset of the 87Sr/86Sr 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.

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