The water circulation in the oceans allows oxygen-rich water to flow from the surface to the depths - the basic requirement for life in the deep sea. It was previously assumed that the oxygen concentration at depth depends primarily on the atmospheric oxygen concentration. However, using climate models that span 540 million years of Earth's history, a study now shows that plate tectonics and the resulting ocean currents also play a crucial role in marine oxygen supply. The results show the shortcomings of previous climate models and are particularly relevant in view of climate change.
Driven by physical processes, the water circulates in the oceans and in this way enables diverse life - not only on the surface, but even several thousand meters below. This is possible because oxygen-rich water cools off the sea surface near the poles and sinks due to the increasing density, where the oxygen is breathed by the organisms living there. Until now, researchers have assumed that the oxygen content of deep sea water is mainly influenced by how much of this gas the surface water can absorb - which in turn depends on the oxygen concentration in the atmosphere on the one hand and on the temperature of the surface water on the other. But apparently this approach falls short.
Climate models over 540 million years
This is now shown by research by a team led by Alexandre Pohl from the University of California at Riverside. Accordingly, the movement of the continental plates is a hitherto neglected factor that has had an important influence on the circulation in the oceans over geological time periods. "Using a series of Earth system model experiments, we show how the continental relocation causes profound fluctuations in the oxygen supply of the ocean," the researchers report.
In order to make the influence of plate tectonics on the oxygen supply of the deep sea visible, Pohl and his 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 the ocean currents influenced by the distribution of the continents. In doing so, they were not only concerned with reproducing past reality, but also carried out targeted experiments to determine the influence of individual factors. In one of the model experiments, they kept the atmospheric oxygen concentration constant and only allowed changes in the position of the continental plates.
Ocean oxygen not only dependent on atmosphere
The result: Regardless of the atmospheric oxygen concentration, the oxygen content of the sea still increased, driven by the ocean currents caused by plate tectonics. But plate tectonics can not only boost the oxygen uptake of the oceans, but also slow them down. "Millions of years ago, not long after animal life began in the ocean, the entire global ocean circulation seemed to periodically freeze," says Pohl's colleague Andy Ridgwell. "We didn't anticipate that the movement of the continents could stop surface water and oxygen from sinking, potentially dramatically affecting how life evolved on Earth."
Since the study only aimed to prove the basic mechanism, the models and results are not suitable for making predictions for actual developments. However, they show significant weaknesses of previous climate models. "The study by Pohl and his colleagues illustrates the complexity of the processes that can affect the oxygen content of the oceans - a complexity that is inadequately represented in models," write Katrin Meissner from the University of New South Wales in Australia and Andreas Oschlies from the Helmholtz Center for Ocean Research Kiel in a commentary accompanying the study, also published in the journal Nature.
Better understanding urgently needed
According to Meissner and Oschlies, modern climate models only reflect about half of the decline in global oxygen concentration in the oceans observed in recent years. So far, this decline has been attributed, among other things, to the fact that the oceans can absorb less oxygen due to rising surface temperatures as a result of climate change. However, it has not yet been fully explained. "A better understanding of all 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. The new evidence provided by the study by Pohl and his colleagues could help.
Source: Alexandre Pohl (University of California, Riverside, USA) et al., Nature, doi: 10.1038/s41586-022-05018-z