Circulating water in the oceans allows oxygen-rich water to flow from the surface to the depths – a basic requirement for life in the deep sea. It was previously assumed that the concentration of oxygen at depth mainly depends on the concentration of oxygen in the atmosphere. However, using climate models spanning 540 million years of Earth’s history, a study now shows that plate tectonics and the resulting ocean currents also play an important role in marine oxygen supply. The results show the shortcomings of previous climate models and are particularly relevant in looking at climate change.
Driven by physical processes, water circulates in the oceans and in this way enables diverse life – not only at the surface, but even at a depth of several thousand meters. This is possible because the oxygen-rich water cools from the sea surface near the poles and sinks due to the increasing density, as the oxygen is inhaled from the organisms that live there. So far, researchers have assumed that the oxygen content of deep sea water is primarily affected by how much of this gas can be absorbed by surface waters—which in turn depends on the concentration of oxygen in the atmosphere on the one hand and on the sea surface temperature on the one hand. surface water on the other hand. But this approach clearly falls short.
Climate models over 540 million years
This is now shown by research by a team led by Alexander Ball of the University of California, Riverside. Accordingly, the movement of continental plates is a hitherto neglected factor that has an important influence on ocean circulation during geological timescales. “Using a series of Earth system model experiments, we have shown how the continental transition causes profound fluctuations in the ocean’s oxygen supply,” the researchers said.
In order to make visible the influence of plate tectonics on oxygen supply in the deep sea, Buhl and colleagues created a three-dimensional climate model that simulates conditions on Earth from 540 million years ago to the present day, and in particular, takes into account ocean currents that are influenced by the distribution of the continents. In doing so, they were not only concerned with reproducing past reality, but also conducted targeted experiments to determine the influence of individual factors. In one typical experiment, they kept the concentration of oxygen in the atmosphere constant and only allowed changes in the position of the continental plates.
Ocean oxygen depends not only on the atmosphere
The result: regardless of the oxygen concentration in the atmosphere, the oxygen content in the sea continues to increase, driven by ocean currents caused by plate tectonics. But plate tectonics can not only increase the uptake of oxygen in the oceans, but also slow it down. “Millions of years ago, not long after animal life began in the ocean, the entire global ocean circulation appeared to periodically freeze,” says Paul’s colleague Andy Ridgewell. “We did not expect that the movement of the continents could stop surface water and oxygen from sinking, which could significantly affect the evolution of life on Earth.”
Since the study aims only to prove the underlying mechanism, the models and results are not suitable for making predictions of actual developments. However, they show significant weaknesses in previous climate models. “The study by Paul and colleagues demonstrates the complexity of the processes that can affect the oxygen content of the oceans – a complexity that is not adequately represented in models,” write Catherine Meissner of the University of New South Wales in Australia and Andreas Oscules of the University of New South Wales in Australia. . The Helmholtz Center for Ocean Research Kiel in a commentary accompanying the study, also published in the journal Nature.
There is an urgent need for a better understanding
According to Meissner and Oschlies, recent climate models only reflect about half of the decline in global ocean oxygen concentration observed in recent years. So far, this decrease has been attributed, among other things, to the fact that the oceans can absorb less oxygen due to higher surface temperatures as a result of climate change. However, this is not fully explained. “A better understanding of all the underlying mechanisms is urgently needed, because the underestimated phenomenon of marine oxygen depletion could cause serious problems for ocean ecosystems and human society in the near future,” say Meissner and Oschlies. New evidence provided by the study by Paul and colleagues could help.
Source: Alexander Paul (University of California, Riverside, USA) et al., Nature, doi: 10.1038/s41586-022-05018-z
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