In the past 50 years, the areas of the open oceans experiencing anoxic conditions have increased. Scientists have attributed this development to rising global temperatures: less oxygen dissolves in warmer waters, and the layers of the tropical ocean can become more layers.
But now, contrary to widespread predictions, an international team of scientists led by researchers from the Max Planck Institute for Chemistry and Princeton University has discovered that areas of hypoxia shrank during long warm periods in the past.
“We didn’t expect such an obvious effect,” said Alexandra Oderst, first author of the new paper in the magazine. temper nature He is currently a Visiting Postdoctoral Research Fellow at Princeton University. She led the study with Alfredo Martinez Garcia at the Max Planck Institute for Chemistry in Mainz, as part of a long-term collaboration with Daniel Sigman’s group at Princeton University.
Understanding these changes is important because “when oxygen becomes scarce, life has a harder time,” said Sigman, professor of geological and geophysical sciences at Duesenbury. For example, in the low-oxygen regions of the eastern Pacific and northern Indian Oceans, only specialized microbes and organisms with slow metabolisms—such as jellyfish—can survive.
The previous oxygen content of the oceans can be read in the sediments
The researchers made this discovery by studying archives of marine sediments. Drill cores can be used to determine the past Environmental conditions In a similar way to tree rings. Among other things, sediment layers provide information about the oxygen content of the sea in the past. This is due to plankton like foraminifera, which once lived in The surface of the sea Their skeletons sank to the sea floor, where they became part of the sediment.
During their life, these zooplankton absorb chemical elements such as nitrogen, the isotopic ratio of which, in turn, depends on environmental conditions: under hypoxic conditions, a process called bacterial denitrification occurs, in which bacteria convert nutrient nitrate into molecular nitrogen. These bacteria prefer to absorb light rather than heavy nitrogen isotopes, so the ratio changes in the periods when the bacteria were active in the oceans. Scientists can measure this to determine the extent of areas that were previously lacking in oxygen.
The equatorial Pacific has been well oxygenated during past warm periods
Using nitrogen isotopes from foraminifera, scientists from Mainz and Princeton have shown that denitrification of the water column in the eastern equatorial region of the North Pacific decreased dramatically during two warm phases about 16 and 50 million years ago.
“We’ve worked over decades to develop methods that allowed these results,” Sigman said. “Immediately, the results are changing our view of the relationship between climate and oxygen conditions in the ocean.”
Oderst said it’s not yet clear what this means for the current expansion of the oxygen-deficient open ocean. “Unfortunately, we don’t know yet whether our finding of shrinking anoxic marine areas is applicable to the coming decades or only in the long term,” she said. “This is because we have to determine whether short or long-term operations are responsible for the change.”
Find the reason
One of the main possibilities for the decline in hypoxic regions under warming involves a decrease in biological productivity fed by tropical surface waters. A decrease in productivity could have occurred due to weaker winds in the equatorial Pacific under a warmer climate.
In the current study, the authors also found that during the two warm Cenozoic periods—the optimal mid-Miocene climate about 16 million years ago and the early Eocene optimum about 50 million years ago—the temperature difference between high and low latitudes was much smaller than it is in this time. Both global warming and the weakening of the temperature difference at high to low latitudes were supposed to weaken the tropical winds, reducing the upwelling of deep, nutrient-rich seawater. This, in turn, would have reduced biological productivity at the surface and reduced the sinking of dead algal organic matter into the ocean depths, providing less fuel for the oxygen consumption that results in hypoxic conditions.
This chain of events can happen relatively quickly. Thus, if a similar change applies to a human Global Warming In addition, there could be a decrease in the extent of hypoxia in the open ocean in the coming decades.
Alternatively, the reason may lie in the Southern Ocean, thousands of kilometers away. During past extended warm periods, the process of water exchange between the surface waters of the Southern Ocean and the depths of the ocean may have accelerated (“deep ocean inversion”), increasing the oxygen in the ocean interior as a whole and thus shrinking the low-oxygen regions. If it is stronger driven by the Southern Ocean ocean depths The inversion was the main cause of the shrinking of the hypoxic tropics, so this effect would take more than a hundred years at the earliest.
“Perhaps both mechanisms play a role,” said Martinez Garcia, a former visiting researcher with Sigman’s research group. “The race is now to see which mechanism is most important.”
Looking into the future
“Considering our current uncertainties about the time scale of change, our findings have important implications for the future of ocean oxygen,” Sigman said. “Because the solubility of oxygen in warm water decreases, it is very likely that the surface waters of the global ocean will continue to decline, but our results indicate that the open, hypoxic areas of the oceans will eventually shrink. The end result will be a spatial ocean with a weaker difference in oxygen than It exists today, and this will affect the ecosystems of the oceans.”
In coastal waters, enhanced hypoxia can damage ecosystems and threaten human activities. However, the areas that suffer from a lack of oxygen in open perimeter Fundamental to the Earth’s chemical and biological cycle. Moreover, if its shrinkage is due to a decrease in tropical productivity, the combined changes are likely to be detrimental to the biological productivity and fisheries of the tropical ocean. Given the complex chain of effects associated with climate change, the researchers said, everything would require efforts to limit human-caused warming.
Alexandra Auderset et al, Ocean oxygenation enhancement during Cenozoic periods, temper nature (2022). DOI: 10.1038 / s41586-022-05017-0
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