Is the ocean losing its breath? Insights into ocean oxygenation from Cenozoic warm periods.

The continuous expansion of oxygen-deficient zones (ODZs) poses risks to marine ecosystems and societies dependent on fisheries for income and sustenance. However, the trajectory of this deoxygenation in response to 21st-century climate change remains uncertain. To gain a clearer understanding of future oxygen dynamics and processes leading to deoxygenation, we investigate the response of ODZs during Cenozoic global warming periods in the Miocene and Early Eocene, using a combination of oxygen-sensitive proxies including foraminifer-bound nitrogen isotopes (FB-15N) and iodine-to-calcium ratios in planktic foraminifer shells (I/Ca). Our findings reveal contracted, rather than expanded, tropical Pacific ODZs during all studied warm intervals. The increased oxygenation closely aligns with high-latitude warming and reduced meridional sea surface temperature gradients, indicating a climatic driver behind these observed changes. We discuss potential causes for the contraction of ODZs, including (i) diminished wind-driven equatorial upwelling and primary productivity, and/or (ii) increased deep-ocean ventilation. Finally, we compare the behaviour of Pacific vs. Indian Ocean ODZs during the Miocene and investigate potential teleconnections between these two wind-driven ODZs.