Continental movement is able to strangle marine oxygen.
An overlooked factor, the position of the continents, helps fill Earth’s oceans with life-sustaining oxygen. Ultimately, continental drift could have the opposite effect, killing off most deep-ocean creatures.
“Continental drift seems so slow, as if nothing drastic could come of it, but when the ocean is ready, even a seemingly small event could trigger the widespread death of marine life,” said Andy Ridgwell, a geologist at the University of California, Riverside. Ridgwell is a co-author of a new study on the forces affecting ocean oxygen.
As ocean surface water approaches the north or south pole, it becomes colder and denser and then sinks. When water sinks, it carries oxygen taken from the Earth’s atmosphere to the bottom of the ocean.
Finally, a return flow returns nutrients released from the sunken organic matter to the surface of the ocean, where it feeds plankton growth. Today’s oceans contain an incredible diversity of fish and other animals that are supported both by the uninterrupted supply of oxygen at the lowest depths and by the organic matter produced at the surface.
New research has found that this circulation of oxygen and nutrients can come to an abrupt end. Using complex computer models, scientists investigated whether the locations of continental plates affect how the ocean moves oxygen. They were surprised to find that it did.
This finding led by UC Riverside researchers is detailed in the journal Nature. It was published today (August 17, 2022).
“Many millions of years ago, not long after animal life began in the ocean, the entire global ocean circulation appeared to shut down periodically,” Ridgwell said. “We didn’t expect to find that the movement of continents could cause surface water and oxygen to stop sinking, potentially dramatically affecting the way life evolved on Earth.”
Until now, the models used to investigate the evolution of marine oxygen over the past 540 million years were relatively simple and did not take ocean circulation into account. In these models, oceanic anoxia – times when oceanic oxygen disappeared – involved a drop in atmospheric oxygen concentrations.
“Scientists previously assumed that changing oxygen levels in the ocean mostly reflected similar fluctuations in the atmosphere,” said Alexandre Pohl, first author of the study and a former UCR paleoclimate modeler, now at the Bourgogne Franche-Comté University in France.
For the first time, this study used a model in which the ocean was represented in three dimensions and in which ocean currents were taken into account. According to the results, the collapse of the global water circulation leads to a clear separation between the oxygen levels at the upper and lower depths.
This separation meant that the entire seafloor, except for shallow places near the coast, was completely deprived of oxygen for many tens of millions of years, until about 440 million years ago at the beginning of the Silurian period.
“Circulation collapse would have been a death sentence for anything that couldn’t swim closer to the surface and the vital oxygen that’s still present in the atmosphere,” Ridgwell said. Creatures of the deep include strange-looking fish, giant worms and crustaceans, squid, sponges and more.
The paper does not address if or when Earth might expect a similar event in the future. Indeed, it is difficult to identify when a collapse may occur or what triggers it. However, existing climate models confirm that increased global warming will weaken the ocean circulation, and some models even predict an eventual collapse of the circulation branch that begins in the North Atlantic.
“We will need a higher-resolution climate model to predict a mass extinction event,” Ridgwell said. “That said, we already have concerns today about the circulation of water in the North Atlantic, and there is evidence that the flow of water at depth is decreasing.”
In theory, an unusually warm summer or the erosion of a cliff could trigger a cascade of processes that change life as it appears today, Ridgwell said.
“You would think that the surface of the ocean, the bit where you can sail or sail, is where all the action is. But underneath, the ocean works tirelessly, providing life-giving oxygen to animals in the dark depths,” he said Ridgwell.
“The ocean allows life to thrive, but it can take it out again. Nothing rules that out as the continental plates continue to move.”
Reference: “Continental configuration controls ocean oxygenation during the Phanerozoic” by Alexandre Pohl, Andy Ridgwell, Richard G. Stockey, Christophe Thomazo, Andrew Keane, Emmanuelle Vennin, and Christopher R. Scotese, August 17 2022, Nature.
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