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Abstract

Oxygen is a fundamental asset for most marine ecosystems, and its distribution affects behaviour and distribution of marine life. Recently, the whole-ocean oxygen inventory has declined due to global warming, but climate models yielded contradictory predictions about the future of tropical subsurface oxygenation. Paleo-oxygen reconstructions for Miocene and Eocene warm periods provide compelling evidence for enhanced oxygenation in the East Pacific oxygen deficient zones (ODZ), but relevance of these findings for the future remains unclear, given the multi-million year timescales of these events. The Paleocene-Eocene Thermal Maximum (PETM) was a greenhouse gas-driven warming event, characterized by abrupt warming, ocean acidification and faunal turnover in land and marine ecosystems, with patterns of change akin to those predicted under future global warming scenarios. Here, we report foraminifera-bound nitrogen isotope results indicating an abrupt contraction of the North Pacific ODZ at the onset of the PETM. We complemented these findings by using foraminifera body size as a novel approach to correlate shell size of planktic and benthic microfossils to environmental oxygen partial pressure (pO2) by means of a metabolic model. The increase in tropical planktic foraminifera body size at a site distant from the ODZs suggests that oxygen availability rose in the shallow subsurface throughout the tropical North Pacific, even as the pO2 of the underlying deep ocean declined. These findings indicate that the ODZ contraction was part of a much broader tropical upper ocean rise in pO2, and that the oceanographic mechanism for pO2 rise on millennial timescales must operate beyond the regional scale of the North Pacific ODZ. We compared the reconstructed changes in water column pO2 to the outcome of a 3-dimensional Ocean General Circulation Model, and observed a consistent pattern between the vertical structure of ocean oxygenation sensitivity to warming, during which a decline in upwelling-driven biological productivity allows tropical O2 to rise in spite of a global O2 decline. The increase in tropical O2 would have offset the impact of warming on the loss of aerobic habitats of marine species, and may have helped avoid an upper ocean mass extinction.