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Scientists Discover New Trigger for Mass Extinction of All Deep Ocean Life

Oxygen is one of the key ingredients for life as we know it on Earth, and its abundance or scarcity hugely influences the types of creatures that can survive in any given environment. Marine animals, for instance, are dependent on ocean currents to circulate dissolved oxygen into their habitats; whole ecosystems can get choked out if this delivery system fails.

Now, scientists led by Alexandre Pohl, a paleoclimate researcher at the Université Bourgogne Franche-Comté, have revealed the “unrecognized role” of continents in circulating oxygen through the oceans, with life-or-death consequences for marine life, according to a study published on Wednesday in Nature

The team ran a model of ocean currents spanning 540 million years, the largest timeframe ever for a study of this kind, to shed light on the impact of continental configurations in ocean oxygen circulation. The results revealed that the position of continents can cut off the oxygen supply to the deep sea, sparking mass die-offs in these habitats, which the study called “a radically different conclusion” from traditional interpretations of geological evidence.

“The last ~540 million years represent the period over which complex forms of life (i.e. life as we know it today) appeared,” said Pohl in an email. “And we know that oxygen is a linchpin for complex marine organisms (such as fish; we need to breathe, they do too). Therefore, it is important to determine how ocean oxygen evolved to understand the co-evolution of life and the environment.”

“Studies had been conducted on specific time slices,” he added, citing the Oceanic Anoxic Event 2 that starved the oceans of oxygen 95 million years ago, killing off many large Cretaceous creatures. This research “suggested a key role of the continental configuration in shaping ocean circulation, thus ocean oxygenation. We decided to extend that work to the whole Phanerozoic (last ~540 million years) to really quantify the impact of continental changes on ocean oxygenation over the period that witnessed the evolution of complex forms of marine life.”

The researchers simulated the position of the continents at 28 points in time, each one 20 million years apart, with a focus on how shifting plate tectonics affect ocean currents. While the team predicted the continents would play a role in ocean currents, and therefore oxygen circulation, they were surprised that the model presented a severe decoupling of oxygen exchange between the upper and lower portions of the ocean during the early Paleozoic period, some 540 and 460 million years ago.

“It was not unexpected that the continental configuration would impact ocean currents, thus oxygenation,” Pohl said. “The amplitude of the changes, however, was unexpected. They are big.”

Complex life first took hold in the oceans during that particular stretch of time, but geologists have also found evidence that the ocean was poorly oxygenated, or “anoxic,” presenting challenges to many species.  

“So far, it was assumed that this poor oxygenation was the result of a reduced oxygen concentration in the atmosphere,” Pohl explained. “Here we show that’s not necessarily the case: the continental configuration and associated ocean circulation make the ocean intrinsically prone to deoxygenation at that time in our model. Indeed, the deep ocean is largely anoxic in our model in the early Paleozoic even when the atmospheric oxygen concentration is maintained to its present-day level.”

“The usual rationale is that a poorly oxygenated ocean in the deep geological time implies a poorly oxygenated atmosphere,” he added. “Our results show that it’s not so simple. The ocean circulation sometimes induces a decoupling between upper-ocean and deep-ocean oxygen concentrations.”

The study is strictly focused on the deep past, so there’s no need to worry that the current position of continents on Earth will suddenly trigger a mass die-off. Pohl noted that the timescales of plate tectonics cover tens of millions of years, which is too slow to be an immediate concern on human lifespans.

The movement of continents isn’t the only thing that could kick off a similar mass die-off of marine life, however. The study is part of a growing body of research that aims to reconstruct all the complex factors at play in Earth’s biosphere, including the effects of human-driven climate change, which is depleting oxygen in some parts of the oceans on rapid timescales.

“We’d need a higher resolution climate model to predict a mass extinction event,” said Andy Ridgwell, a geologist at the University of California Riverside who co-authored the study, in a statement. “That said, we do already have concerns about water circulation in the North Atlantic today, and there is evidence that the flow of water to depth is declining.”

To that end, Pohl and his colleagues plan to delve deeper into the role played by continental changes, among others, in ocean oxygenation and marine biodiversity. 

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