Contraction of the eastern tropical Pacific oxygen deficient zone during Cenozoic warm periods

Dissolved oxygen (O2) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming. However, models provide an unclear picture of future ODZ change in both the near term and the long term. Studies of past climates can help explore the possible range of ODZ changes in warmer-than-modern periods. Our work over the past years has shown that water-column denitrification in the eastern tropical North Pacific was systematically reduced during the most important periods of sustained global warming of the past 65 million years, i.e. the Middle Miocene Climatic Optimum (MMCO) and the Early Eocene Climatic Optimum (EECO). Because denitrification is restricted to oxygen-poor waters, our results indicate that, contrary to mos expectations, during these periods ODZs contracted, not expanded (Auderset et al. 2022). These results suggested that that global warming may eventually cause ODZ contraction. In a later study we have confirmed these results using independent methods that show that the Eastern Tropical Pacific ODZ was fully oxygenated (>100 µmol kg−1) during the MCO (Hess et al. 2023).

Map showing the location of the Eastern Tropical Pacific Oxygen Deficient Zones (in red)

However, both the MMCO and the EECO lasted millions of years leaving some doubt about the relevance of the reconstructed changes for the ODZs under anthropogenic warming in the next centuries. In  a more recent study we investigated changes in the ODZs during the Paleocene-Eocene Thermal Maximum (PETM), i.e. the largest and fastest known preanthropogenic global warming event in the Cenozoic. Our results indicate that the tropical North Pacific ODZ contracted contracted rapidly at the onset of climate warming (Moretti et al. 2024). These changes are consistent with ocean model simulations of warming, in which a decline in biological productivity allows tropical subsurface oxygen to rise even as global ocean oxygen declines.