In the last 50 years, oxygen-deficient zones in the open ocean have increased. This poses major problems not only for marine ecosystems, but also for coastal inhabitants and countries that rely on fisheries as a source of food and income. 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 how will this development continue, and what happened in past warm periods?
A team led by Alexandra Auderset and Alfredo Martínez-García at the Max Planck Institute for Chemistry in Mainz has shown in a recent study that, in the open ocean, oxygen-deficient zones shrank during warm periods of the past.
The past oxygen content of the oceans can be read in sediments
The researchers read this finding from marine sediment archives. Drill cores can be used to determine past environmental conditions in a similar way to tree rings. Among other things, the sediment layers provide information about the oxygen content of the sea in the past. This is due to microorganisms such as foraminifera, which once lived on the sea surface and whose skeletons sank to the sea floor where they became part of the sediment. During their lifetime, these zooplankton absorbed chemical elements such as nitrogen, whose isotope ratio in turn depended on environmental conditions. Under oxygen-deficient conditions, bacterial denitrification occurs. In this process, the nutrient nitrate is chemically reduced to molecular nitrogen (N2) by bacteria. As they prefer to absorb light isotopes from the water instead of heavy ones, the ratio of light 14N shifts to heavy 15N in periods when the bacteria were active in the oceans. This changing isotopic signal, in turn, can be used to determine the extent of earlier oxygen-deficient zones.
The tropical Pacific Ocean was well oxygenated during past warm periods
Using nitrogen isotopes from foraminifera, the scientists from Mainz and Princeton University showed that denitrification of the water column in the eastern tropical North Pacific was greatly reduced during two warm phases of the Earth’s modern era, the Cenozoic, about 16 and 50 million years ago.
“We had not expected this clear effect. From the correlation between high global temperatures and low denitrification rates, we conclude that the tropical Pacific’s oxygen-deficient zones shrank,” says Auderset about the results, which were recently published in the journal Nature.
“It’s been a decades-long campaign to develop the methods that allowed for these findings,” says Daniel Sigman, Dusenbury Professor of Geological and Geophysical Sciences, whose group collaborated in the study. “And it turns out that even these first results are altering our view of the relationship between climate and the ocean’s oxygen conditions.”
It cannot, however, yet be precisely estimated what this means for the current expansion of the oxygen-deficient open ocean zones: “Unfortunately, it remains unclear whether our finding of shrinking marine oxygen-deficient zones is applicable to the coming decades or only to the much longer term,” adds the paleoclimatologist Auderset. “This is because we don’t yet know whether short- or long-term processes were responsible for the change.”
Searching for the cause
One leading possibility for the decline in oxygen-deficient zones under warming involves a reduction in the upwelling-fueled biological productivity of tropical surface waters. A decline in productivity could have occurred because winds weakened in the equatorial Pacific under warmer climate.
In the current study, the authors also found that during the two warm periods of the Cenozoic — the mid-Miocene climate optimum about 16 million years ago and the early Eocene climate optimum about 50 million years ago — the temperature difference between high and low latitudes was much smaller than at present. Both, the global warming and the weakening of high-to-low latitude temperature difference, should have worked to weaken tropical winds, reducing the upwelling of nutrient-rich deep seawater. This, in turn, would have resulted in lower biological productivity at the surface and less sinking of dead algal organic matter into the deep ocean, providing less fuel for the oxygen consumption that produces oxygen-deficient conditions. This chain of events can occur relatively quickly. Thus, if a similar change applies to human-driven global warming as well, then there could be a decline in the extent of open ocean oxygen deficiency in the coming decades.
Alternatively, the cause may lie in the Southern Ocean, thousands of kilometers away. During past prolonged warm periods, the exchange water between Southern Ocean surface waters and the deep ocean (“deep ocean overturning”) may have accelerated, leading to higher oxygen in the ocean interior as a whole and thus shrinking the low-oxygen zones. If stronger Southern Ocean-driven deep ocean overturning was the main cause of the shrunken tropical oxygen-deficient zones, then this effect would take more than a hundred years at earliest to come into play.
“Both mechanisms probably play a role,” says Alfredo Martínez-García, “The race is now on to figure out which mechanism is most important.”