Scientists Find Surprising Cause of Devastating Mass Extinctions

About 183 million years ago, Earth was rocked by apocalyptic volcanic eruptions that belched greenhouse gasses into the skies and triggered widespread extinctions around the world.

The exact origins of this tumultuous era, which is known as the Early Toarcian oceanic anoxic event (T-OAE), have been shrouded in mystery for years. Now, scientists led by Micha Ruhl, an assistant professor in sedimentology at Trinity College Dublin, have presented “an as-yet unexplored” cause of the T-OAE—a slowdown in the motion of continents—according to a study published on Friday in Science Advances.

This change may have enabled roiling underground magma to burst out onto the surface, creating huge expanses of volcanic rock, called large igneous provinces (LIPs), that are still etched into the landscapes of Southern Africa and Antarctica. 

It’s an elegant and novel explanation for this era of mass disruption in the Jurassic, when dinosaurs roamed the planet and the supercontinent Pangaea was entering its twilight years. But Ruhl and his colleagues also suggest that this reduction in continental velocity has broader implications because “the mechanism is consistent with the timing of several of the largest LIP volcanic events throughout Earth history and, thus, the timing of many of Earth’s past global climate change and mass extinction events,” according to the study.

“We started looking at other major volcanic events, because there have been quite a few in the past, and we observed that the same thing is happening for many of these volcanic events, so the model might be applicable to quite a large part of Earth’s history,” Ruhl told Motherboard. 

“Conceptually, it is a very simple model,” he said. “Everyone has said, ‘that makes a lot of sense—why did no one else think of this?’ We had the same thought initially” but the team nonetheless found that “it appears to be a new model.”

It has long been known that the cataclysmic volcanic activity that creates LIPs is driven by plumes of magma that push up onto Earth’s surface from the mantle. Ruhl and his colleagues have now discovered that many of these periods eerily coincide with the velocities with which continental plates move across our planet. 

Continents today are shifting at a rate of several centimeters per year, and they were also moving at a relatively quick clip, geologically speaking, millions of years before the T-OAE.

The researchers studied incredibly well-preserved sediments from the period preserved in a one-mile-deep borehole in Wales, which span the T-OAE, and almost serendipitously connected the data to models of plate motion before and after the enormous event. They linked a slowing of the northward trend of continental drift that led up the T-OAE, followed by a cessation of movement, to the timeline of huge volcanic eruptions. 

The process is somewhat similar to holding up a piece of paper above a lit candle, Ruhl said. If the paper moves over the flame quickly, it may be singed, but if the paper moves slowly, the candle can burn a hole through it. Similarly, slow continental movement exposes the plates to searing magma for a longer period, enabling mantle plumes to bore through the bottom of the landscapes and blast onto the surface.

The team showed that slow or paused continents overlapped with the most severe mass extinction event of all time, known as the Great Dying, which killed off more than 90 percent of life on Earth 250 million years ago, in addition to several other huge die-offs over the past 500 million years. 

In addition to rewriting our view of the past, the new findings can also help us anticipate the future of our world in the era of human-driven climate change. Though volcanic activity and human fossil fuel use are very different processes, they both emit huge amounts of carbon dioxide and other warming gasses into the atmosphere over relatively short timespans. Studying how these spikes in atmospheric greenhouse gasses affected Earth’s habitats, and the diverse lifeforms that lived in them, can give us some idea what to expect as global temperatures rise in the decades and centuries to come.

“The relationship between volcanism and environmental and climatic perturbations is really important, because we know that volcanoes release large amounts of carbon and greenhouse gasses into the atmosphere,” Ruhl said. “Understanding that temporal link is really quite crucial because it allows us to say something about how much carbon is being released in the first place and what it meant in terms of degrees of temperature increase in ocean surface waters or degrees of reduction in pH, and so on.”

“These kinds of studies allow us to understand how much carbon is being released and, more importantly, the timescale or the rates of carbon release per year, and what that means for the planet and the Earth system as a whole,” he concluded.