Essay by Eric Worrall
Yet another mystery for the settled science.
Climate change and ocean oxygen: Oxygen-poor zones shrank under past warm periods, scientists discover
by Princeton UniversityAUGUST 31, 2022
In the last 50 years, oxygen-deficient zones in the open ocean have increased. Scientists have attributed this development to rising global temperatures: Less oxygen dissolves in warmer water, and the tropical ocean’s layers can become more stratified.
But now, contrary to widespread expectations, an international team of scientists led by researchers from the Max Planck Institute for Chemistry and Princeton University has discovered that oxygen-deficient zones shrank during long warm periods in the past.
“We had not expected such a clear effect,” said Alexandra Auderset, first author of the new paper in the journal Nature and currently a visiting postdoctoral research fellow at Princeton University. She led the study with Alfredo Martínez-García at the Max Planck Institute for Chemistry in Mainz, as part of a longterm collaboration with Daniel Sigman’s group at Princeton University.
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It is not yet clear, however, what this means for the current expansion of the oxygen-deficient open ocean zones, said Auderset. “Unfortunately, we don’t yet know whether our finding of shrinking marine oxygen-deficient zones is applicable to the coming decades or only to the much longer term,” she said. “This is because we have to resolve whether short- or long-term processes were responsible for the change.”
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The abstract of the study;
Enhanced ocean oxygenation during Cenozoic warm periods
Published: 31 August 2022Alexandra Auderset, Simone Moretti, Björn Taphorn, Pia-Rebecca Ebner, Emma Kast, Xingchen T. Wang, Ralf Schiebel, Daniel M. Sigman, Gerald H. Haug & Alfredo Martínez-García
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 warming1,2. However, models provide an unclear picture of future ODZ change in both the near term and the long term3,4,5,6. The paleoclimate record can help explore the possible range of ODZ changes in warmer-than-modern periods. Here we use foraminifera-bound nitrogen (N) isotopes to show that water-column denitrification in the eastern tropical North Pacific was greatly reduced during 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, in these two Cenozoic periods of sustained warmth, ODZs were contracted, not expanded. ODZ contraction may have arisen from a decrease in upwelling-fuelled biological productivity in the tropical Pacific, which would have reduced oxygen demand in the subsurface. Alternatively, invigoration of deep-water ventilation by the Southern Ocean may have weakened the ocean’s ‘biological carbon pump’, which would have increased deep-ocean oxygen. The mechanism at play would have determined whether the ODZ contractions occurred in step with the warming or took centuries or millennia to develop. Thus, although our results from the Cenozoic do not necessarily apply to the near-term future, they might imply that global warming may eventually cause ODZ contraction.
I’m personally surprised by this discovery. I accepted the theory that outgassing from warmer water would slightly reduce ocean oxygen levels, though given the abundance of ocean life during previous warm periods, I didn’t think it would be a problem.
Obviously it would be funny to suggest this surprise discovery is proof the oceans are cooling, and this might actually be the explanation – the sea level has dropped 6ft since the Holocene Optimum ended 6000 years ago. Maybe our modern warm period has been too brief a blip to have impacted the ocean oxygen changes wrought by our long term Holocene cooling trend.
But I accept the Princeton explanation, that we just don’t know.
