Miocene ocean circulation shifted expansive oxygen deficient zones to the Atlantic.

Contemporary observations indicate that dissolved oxygen concentrations are generally declining as global temperatures rise, which has broad implications for carbon cycling and the habitable ranges of marine animals. Here, we use the foraminiferal iodine redox tracer to evaluate the distributions of oxygen deficient zones (ODZs) and adjacent low oxygen water masses in the oceans during the Miocene Climatic Optimum ('MCO', 14.7-17 million years ago)-the last time that atmospheric CO2 was consistently higher than today. The Pacific lowest oxygen water masses were confined to a narrow latitudinal range of ~10°N-20°S, which is substantially contracted relative to today. In contrast, in the Atlantic, where modern ODZs are minor compared to their Pacific counterparts, our data indicate spatially expansive low oxygen distribution during the MCO. Earth System model simulations provide evidence that the Pacific-Atlantic ODZ seesaw was driven by the very different pattern of ocean circulation and nutrient transport that was induced by an open Central American Seaway. Our findings highlight the key role played by tectonics and ocean circulation, independent of warming, in setting the pattern of ODZs and hence related loci of organic burial and marine habitats.