Expanded subsurface ocean anoxia in the Atlantic during the Paleocene-Eocene Thermal Maximum.

The ocean has experienced substantial oxygen loss over recent decades, affecting marine ecosystems and fisheries. Investigating ocean deoxygenation during hyperthermal events, such as the Paleocene-Eocene Thermal Maximum (PETM), offers insights into its future dynamics. Here, sediment cores from the South Atlantic reveal a pronounced decline in foraminifera-bound δN, concurrent with an increase in marine barite δS and enhanced ocean productivity during the PETM. These findings suggest an expansion of oxygen-deficient zones (ODZs) from suboxia to anoxia in the thermocline, with ammonium and sulfide accumulation. Model simulations indicate "ammonium-type" ODZs were driven by Southern Ocean warming and elevated productivity. Intense fixed nitrogen loss at the upper boundary of these ODZs, along with increased oceanic phosphorus inventory, likely spurred a compensatory rise in N fixation. While the Pacific might experience different oxygenation conditions during the PETM, parts of the Atlantic thermocline became anoxic, highlighting potential spatial variabilities of ocean deoxygenation under global warming.