Sleeping giant could end life in the deep ocean

Red Medusa

Red jellyfish found just off the bottom of the deep sea in Alaska. Credit: Hidden Ocean 2005/NOAA

Continental movement is able to limit marine oxygen.

A previously overlooked factor — the position of continents — helps fill Earth’s oceans with life-sustaining oxygen. Continental movement could eventually have the opposite effect, killing the majority of deep-sea creatures.

“Continental drift seems so slow, like nothing drastic could come out, but when the ocean is ready, even a seemingly minor event can cause the widespread death of marine life,” said University of California Riverside geologist Andy Ridgwell. Ridgwell is co-author of a new study on forces that affect oceanic oxygen.

As the water at the ocean’s surface approaches the north or south pole, it gets colder and denser and then sinks. When the water sinks, it transports oxygen from the Earth’s atmosphere to the ocean floor.

Deep Reef Fish Papahānaumokuākea Marine National Monument

Deep reef fishing at Pearl and Hermes Atoll in Papahānaumokuākea Marine National Monument near Hawaii. Credit: Greg McFall, NOAA

Ultimately, a return flow returns nutrients released from submerged organic matter back to the ocean’s surface, where it fuels plankton growth. Today’s oceans have an incredible diversity of fish and other animals that are supported by both the uninterrupted supply of oxygen to lower depths and organic matter produced at the surface.

New research has found that this circulation of oxygen and nutrients can end quite suddenly. Using complex computer models, the scientists examined whether the locations of continental plates affect how the ocean moves oxygen. They were surprised to find that it is.

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

Resting Balloon Fish

Resting balloon fish near the Florida Keys. Credit: OAR/National Submarine Research Program (NURP); University of Maine

“Many millions of years ago, not long after ocean life began, the entire global ocean circulation seemed to come to a periodic standstill,” Ridgwell said. “We didn’t expect that the movement of continents could stop surface water and oxygen from sinking, potentially dramatically impacting how life evolved on Earth.”

Until now, models used to investigate the evolution of marine oxygen over the past 540 million years have been relatively simple and have not taken into account ocean circulation. In these models, ocean anoxia – times when oceanic oxygen disappeared – implied a drop in atmospheric oxygen concentrations.

“Scientists previously assumed that changing oxygen levels in the ocean largely reflected similar fluctuations in the atmosphere,” said Alexandre Pohl, lead author of the study and former UCR paleoclimate modeler, now at Université Bourgogne Franche-Comté in France.

Diorama of ancient marine life from the Ediacaran period

Diorama of ancient marine life from the Ediacaran period on display at the Smithsonian Institution. Credit: Smithsonian

For the first time, this study used a model representing the ocean in three dimensions and taking ocean currents into account. According to the results, the collapse of the global water circulation leads to a strong separation between oxygen levels in the upper and lower depths.

This separation meant that the entire seabed, except for shallow areas close to the coast, completely lost oxygen over many tens of millions of years, until about 440 million years ago at the beginning of the Silurian.

“The circulatory collapse would have been the death penalty for anything that couldn’t swim closer to the surface and the life-giving oxygen left in the atmosphere,” Ridgwell said. Deep creatures include bizarre-looking fish, giant worms and crustaceans, squids, sponges, and more.

The paper does not elaborate on if or when Earth could expect a similar event in the future. In fact, it is difficult to determine when a collapse could occur or what is causing it. However, existing climate models confirm that increasing global warming will weaken ocean circulation, and some models even predict an eventual collapse of the circulation branch beginning in the North Atlantic.

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

In theory, an unusually warm summer or the erosion of a cliff could trigger a cascade of processes that turns life upside down as it stands today, Ridgwell said.

“You would think that the surface of the ocean, the part where you could surf or sail, is where all the action is. But below that, the ocean is tirelessly working away, supplying vital oxygen to animals in the dark depths,” Ridgwell said.

“The ocean makes life flourish, but it can also take that life away. Nothing rules out that, as continental plates keep moving.”

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.
DOI: 10.1038/s41586-022-05018-z

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