Oxygen rise in the tropical upper ocean during the Paleocene-Eocene Thermal Maximum

The global ocean’s oxygen (O2) inventory is declining in response to
global warming, but the future of the low-oxygen tropics is uncertain. We
present new evidence for tropical oxygenation during the Paleocene-Eocene
Thermal Maximum (PETM), a warming event that serves as a geologic analogue
to anthropogenic warming. Foraminifera-bound nitrogen isotopes indicate
that the tropical North Pacific oxygen-deficient zone contracted during
the PETM. A concomitant increase in foraminifera size implies that oxygen
availability rose in the shallow subsurface throughout the tropical North
Pacific. 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. The
tropical oxygen increase may have helped avoid a mass extinction during
the PETM.