Fluctuations in oxygen (δ18O) and carbon (δ13C) isotope values of benthic foraminiferal calcite from the tropical Pacific and Southern Oceans indicate rapid reversals in the dominant mode and direction of the thermohaline circulation during a 1 m.y. interval (71–70 Ma) in the Maastrichtian. At the onset of this change, benthic foraminiferal δ18O values increased and were highest in low-latitude Pacific Ocean waters, whereas benthic and planktic foraminiferal δ13C values decreased and benthic values were lowest in the Southern Ocean. Subsequently, benthic foraminiferal δ18O values in the Indo-Pacific decreased, and benthic and planktic δ13C values increased globally. These isotopic patterns suggest that cool intermediate-depth waters, derived from high-latitude regions, penetrated temporarily to the tropics. The low benthic δ13C values at the Southern Ocean sites, however, suggest that these cool waters may have been derived from high northern rather than high southern latitudes. Correlation with eustatic sea-level curves suggests that sea-level change was the most likely mechanism to change the circulation and/or source(s) of intermediate-depth waters. We thus propose that oceanic circulation during the latest Cretaceous was vigorous and that competing sources of intermediate- and deep-water formation, linked to changes in climate and sea level, may have alternated in importance.

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