What Killed Dinosaurs and Other Life on Earth? – Astrobiology

Determining what killed the dinosaurs 66 million years ago at the end of the Cretaceous has long been a subject of debate as scientists have sought to determine what caused the five mass extinction events that transformed life on planet Earth in one geological instant. Some scientists argue that comets or asteroids crashing into Earth were the most likely causes of mass destruction, while others argue that large volcanic eruptions were the cause. A new Dartmouth-led study published in the Proceedings of the National Academy of Sciences (PNAS) reports that volcanic activity appears to have been the main reason for the mass extinction.

The results provide the most compelling quantitative evidence yet that the link between large volcanic eruptions and mass turnover of species is not simply a matter of chance.

Four of the five mass extinctions coincide with a type of volcanic eruption called flood basalt, the researchers say. These eruptions flood vast areas – even an entire continent – with lava in the geological blink of an eye of a million years. As evidence, they leave huge fingerprints — vast regions of step-like igneous rock (solidified by the erupted lava) that geologists call “great igneous provinces.”

To be considered “large,” a large magmatic province must contain at least 100,000 cubic kilometers of magma. For context, the 1980 Mount St. Helens eruption involved less than a cubic kilometer of magma. The researchers say most of the volcanoes pictured in the study erupted on the order of a million times more lava.

The team relied on three established datasets on geologic timescale, paleobiology, and large magmatic provinces to study the temporal relationship between mass extinction events and large magmatic provinces.

“The large step-like areas of igneous rock from these large volcanic eruptions appear to coincide in time with mass extinctions and other significant climatic and environmental events,” says lead author Theodore Green ’21, who conducted this research as part of the Senior Fellowship program at Dartmouth and is now PhD student at Princeton.

In fact, about 252 million years ago, a series of eruptions in present-day Siberia triggered the most devastating mass extinction event, releasing a gigantic burst of carbon dioxide into the atmosphere and suffocating almost all life. Witnessed by the Siberian Traps, a large region of volcanic rock roughly the size of Australia.

Volcanic eruptions also rocked the Indian subcontinent around the time of the great dinosaur extinction, creating what is now known as the Deccan Plateau. Similar to the asteroid impact, this would have had far-reaching global effects, blanketing the atmosphere with dust and noxious fumes, choking out dinosaurs and other life, and altering the climate for long periods of time.

On the other hand, the researchers say, theories in favor of annihilation from asteroid impact depend on the Chicxulub impactor, a space rock that crashed on Mexico’s Yucatan Peninsula around the same time the dinosaurs went extinct.

“All other theories that tried to explain what killed the dinosaurs, including volcanism, were crushed when the Chicxulub impact crater was discovered,” says co-author Brenhin Keller, assistant professor of geosciences at Dartmouth. But despite decades of exploration, there is very little evidence of similar impact events coinciding with the other mass extinctions, he points out.

At Dartmouth, Green set out to find a way to quantify the apparent link between eruptions and extinctions, and to test whether the coincidence was just coincidence or whether there was evidence of a causal link between the two. Working with Keller and co-author Paul Renne, Professor-in-Residence of Earth and Planetary Sciences at the University of California, Berkeley and Director of the Berkeley Geochronology Center, Green recruited the Dartmouth Discovery Cluster’s supercomputers to crack the numbers.

Researchers compared the best available estimates of flood basalt eruptions to periods of drastic species extinctions on the geologic timescale, including but not limited to the five mass extinctions. To prove the timing was more than a coincidence, they examined whether the eruptions would match a randomly generated pattern just as well, and repeated the exercise with 100 million such patterns. They found that the coincidence with extinction times was far greater than chance.

“While it’s difficult to determine whether a specific volcanic eruption caused a specific mass extinction, our findings make it difficult to ignore the role of volcanism in the extinction,” says Keller. If a causal link were found between volcanic flood basalts and mass extinctions, scientists expect that larger eruptions would lead to more severe extinctions, but such a correlation has not been observed.

Rather than considering the absolute magnitude of the eruptions, the research team ranked the volcanic events by the rate at which they spewed lava. They found that the volcanic events with the highest eruption rates actually caused the most destruction and produced more severe extinctions to the point of mass extinctions.

“Our results suggest that there would have been, in all probability, a mass extinction event at the Cretaceous-Tertiary boundary of substantial magnitude, whether there was an impact or not, which can now be shown more quantitatively,” says Renne. “The fact that there was an impact undoubtedly made matters worse.”

The researchers also performed the numbers for asteroids. Coincidences of impacts with periods of species change were significantly weaker and worsened dramatically when the Chicxulub impactor was excluded, suggesting that other smaller known impactors did not cause significant extinctions.

The eruption rate of the Deccan Traps in India suggests the stage was set for widespread extinctions even without the asteroid, Green says. The impact was the double whammy that loudly rang the death knell for the dinosaurs, he adds.

Flood basalt outbursts are not common in the geologic record, Green says. The last of comparable, but much smaller scale, occurred about 16 million years ago in the Pacific Northwest.

“While the total amount of carbon dioxide released into the atmosphere by modern climate change is still very much smaller than the amount emitted by a large volcanic province, fortunately we are emitting very rapidly,” says Keller, “and that is a reason to be concerned.” Green says the carbon dioxide emissions are uncomfortably similar to the rate of the polluting flood basalts they study. That puts climate change in the context of historical environmental disasters, he says.

Green ([email protected]) and Keller ([email protected]) are available to comment on the study.

astrobiology