Pacific circulation during the middle Miocene climate transition: Monitoring ocean overturning and the east-west hydrographic gradient

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About 13.9 million years ago, the Earth s climate cooled dramatically after an extended period of relative warmth. This key transition in Earth s climatic and biotic evolution, which marked the final stage of stepwise Cenozoic cooling, remains one of the most enigmatic episodes in Earth s Cenozoic climate history. While there is evidence that orbital forcing and atmospheric CO2 variations strongly influenced climate evolution, the role of the ocean circulation as enhancer or driver of climate change remains unclear. Our primary goals are to investigate circulation changes in the Pacific during the middle Miocene warm climate phase and subsequent global cooling. In particular, we will test the hypothesis that global cooling and ice-sheet expansion at - 13.9 Ma coincided with a fundamental re-organization in Pacific circulation and initiation of a West Pacific Warm Pool system. Our investigation will provide a synthesis of ocean chemistry proxies that will allow detailed correlation of paleoceanographic events in different regions of the Pacific and help constrain modeling studies of past and future climate. Furthermore, the early middle Miocene climate optimum represents an exceptional analogue to study the causes and consequences of global warming, in particular the potential switches in deep-water sources during intervals of rapid warming.