A sleeping giant could end life in the depths of the ocean

The movement of the continents is capable of strangling marine oxygen.

A previously overlooked factor – the position of the continents – helps fill Earth’s oceans with vital oxygen. The movement of the continents could ultimately have the opposite effect, killing off the majority of deep sea creatures.

“Continental drift seems so slow, as if nothing drastic could come of it, but when the ocean is primed, even a seemingly small event could trigger widespread death of marine life,” said Riverside geologist Andy Ridgwell. at the University of California. Ridgwell is co-author of a new study on the forces affecting ocean oxygen.

As water at the surface of the ocean approaches the north or south pole, it becomes colder and denser, and then sinks. When water sinks, it carries oxygen extracted from the earth’s atmosphere to the bottom of the ocean.

Eventually, a return flow brings the nutrients released from the sunken organic matter back to the ocean surface, where they fuel the growth of plankton. Today’s oceans feature an incredible diversity of fish and other animals that are sustained both by the uninterrupted supply of oxygen at lower depths and by the organic matter produced at the surface.

New research has shown that this flow of oxygen and nutrients can stop quite suddenly. Using complex computer models, scientists investigated whether the location of continental plates affects how the ocean moves oxygen. They were surprised to find that this is the case.

This discovery led by researchers based at UC Riverside is detailed in the journal Nature. It was released today (August 17, 2022).

“Several million years ago, soon after animal life began in the ocean, all global ocean circulation seemed to periodically stop,” Ridgwell said. “We did not expect to find that the movement of continents could prevent surface water and oxygen from flowing, and possibly significantly affect the way life evolved on Earth.”

Until now, the models used to study the evolution of marine oxygen over the last 540 million years were relatively simple and did not take ocean circulation into account. In these models, ocean anoxia – times when ocean oxygen disappeared – implied a drop in atmospheric oxygen concentrations.

“Scientists previously assumed that changing oxygen levels in the ocean primarily reflected similar fluctuations in the atmosphere,” said Alexandre Pohl, the study’s first author and former UCR paleoclimate modeller, now at Burgundy 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 sharp separation between oxygen levels at upper and lower depths.

This separation meant that the entire sea floor, except for shallow places near the coast, lost oxygen entirely for several tens of millions of years, until about 440 million years ago. years at the beginning of the Silurian period.

“The circulation collapse would have been a death sentence for anything that couldn’t swim closer to the surface and the life-giving oxygen still in the atmosphere,” Ridgwell said. Creatures of the deep include bizarre-looking fish, giant worms and crustaceans, squids, sponges, and more.

The document does not specify if or when Earth might expect a similar event in the future. In fact, it is difficult to identify when a collapse might occur, or what triggers it. However, existing climate models confirm that increasing global warming will weaken the ocean circulation, and some models even predict a possible collapse of the circulation branch that begins in the North Atlantic.

“We would need a higher resolution climate model to predict a mass extinction event,” Ridgwell said. “Having said that, we already have concerns about water circulation in the North Atlantic today, and there is evidence that the flow of water to depth is decreasing.”

In theory, an unusually hot summer or cliff erosion could trigger a cascade of processes that disrupt life as it appears today, Ridgwell said.

“You would think that the surface of the ocean, the part you might surf or sail on, is where all the action takes place. But below, the ocean works tirelessly, providing life-saving oxygen to animals in the dark depths,” Ridgwell said.

“The ocean allows life to flourish, but it can take it away. Nothing rules this out as the continental plates continue to move.

Reference: “The continental configuration controls the oxygenation of the oceans 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.
DOI: 10.1038/s41586-022-05018-z