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

Alfredo Martínez García, Climate Geochemistry Department, Max-Planck-Institute for Chemistry, Mainz, Germany

Abstract: 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 geochemical measurements
indicate 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. Because denitrification is restricted to oxygen-poor waters, our
results indicate that in these periods ODZs were contracted, not expanded. Thus, our
results suggest that that global warming may eventually cause ODZ contraction.

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