Contrasting evolution of the Arabian Sea and Pacific Ocean oxygen minimum zones during the Miocene.

Ocean oxygen minimum zones have expanded since the mid-20th century, yet their future remains uncertain. Previous studies show that the eastern tropical North Pacific was well oxygenated during the warm Miocene Climatic Optimum (17.0-14.8 Ma), suggesting better oxygenation under climatic warming. To explore whether this response was global, we reconstruct Miocene oxygenation in the second largest oxygen minimum zone, the Arabian Sea. Trace elements and nitrogen isotopes in planktonic foraminifera show that the Arabian Sea was also better oxygenated during the Miocene Climatic Optimum than today. However, deoxygenation history and establishment of a true oxygen deficient zone following the Miocene cooling lagged in the Arabian Sea, indicating the important role of regional oceanographic processes like proto-monsoon or Tethys outflow. Our study supports future projections of deoxygenation reversals in both oxygen minimum zones, but with more complexity in the Arabian Sea due to competing changes in monsoonal upwelling and influx from marginal seas.