Determining what caused the mass extinctions and killed the dinosaurs 66 million years ago at the end of the Cretaceous period has long been a matter of debate, as scientists set out to determine what caused the five events that reshaped life on the planet Earth in a geological instant.
Some scientists argue that comets or asteroids that crashed into Earth were the most likely agents of mass destruction, while others argue that large volcanic eruptions were the cause. A new study by Dartmouth published in the Proceedings of the National Academy of Sciences (PNAS) reports that volcanic activity appears to have been the key factor in mass extinctions.
The results so far provide the most compelling quantitative evidence that the link between major volcanic eruptions and massive species turnover is not simply a matter of chance. Four of the five mass extinctions are contemporary with a type of volcanic effusion called alluvial basalt, the researchers say.
These eruptions inundate vast areas, even an entire continent, with lava in the blink of an eye, only a million years old. They leave behind giant fingerprints as evidence: vast regions of stepped igneous rock (solidified by erupted lava) that geologists call “Great igneous provinces”.
To be considered “great”, a large igneous province must contain at least 100,000 cubic kilometers of magma. For context, the 1980 eruption of Mount Sant’Elena involved less than one cubic kilometer of magma. The researchers say that most of the volcanoes represented in the study erupted on the order of a million times more lava of that.
The team drew on three consolidated datasets on geological, paleobiological and large igneous time scales to examine the temporal connection between mass extinction and large igneous provinces.
“The large stepped areas of igneous rock of these large volcanic eruptions seem to align over 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 a graduate student at Princeton.
Indeed, a series of eruptions in present-day Siberia triggered the most destructive of mass extinctions some 252 million years ago, releasing a giant pulse of carbon dioxide into the atmosphere and nearly choking all life. Witness the Siberian traps, a vast region of large volcanic rock about the same as Australia.
Mass extinctions and temperatures
Volcanic eruptions also rocked the Indian subcontinent around the time the great dinosaurs died, creating what is now known as the Deccan Plateau. This, much like an asteroid attack, would have far-reaching global effects, blanketing the atmosphere in toxic dust and fumes, asphyxiating dinosaurs and other life forms as well as altering the climate over long time scales.
On the other hand, the researchers argue, theories in favor of annihilation by impact of an asteroid are based on the Chicxulub impactor, a space rock that crashed on Mexico’s Yucatan Peninsula around the same time the dinosaurs went extinct.
“All the other theories that attempted 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 earth sciences at Dartmouth. But there is very little evidence of similar impact events coinciding with other mass extinctions despite decades of exploration, he points out.
In Dartmouth, Green set out to find a way to quantify the apparent link between eruptions and extinctions and see if the coincidence was just a coincidence or if there was evidence of a causal relationship between the two. Working with Keller and co-author Paul Renne, resident professor of earth and planetary sciences at the University of California, Berkeley and director of the Berkeley Geochronology Center, Green recruited supercomputers from the Dartmouth Discovery Cluster to crunch the numbers.
The researchers compared the best available estimates of alluvial basalt eruptions with periods of drastic killing of species in the geological time scale, including but not limited to the five mass extinctions.
To show that timing was more of a random possibility, they examined whether the eruptions would line up just as well with a randomly generated pattern and repeated the exercise with 100 million similar patterns. They found that the deal with extinction periods was far greater than chance.
“Although it is difficult to determine whether a particular volcanic explosion caused a particular mass extinction, our results make it difficult to ignore the role of volcanism in extinction.”says Keller. If a causal link were found between volcanic basalts and mass extinctions, scientists expect larger eruptions would result in larger extinctions, but no such correlation has been observed.
Rather than considering the sheer magnitude of the eruptions, the research team sorted the volcanic events by speed with which they erupted lava. They found that volcanic events with the highest eruptive rates actually caused the most destruction, resulting in more severe extinctions up to mass extinctions.
“Our results indicate that in all likelihood there would have been a mass extinction at the Cretaceous Tertiary boundary of some significant magnitude, regardless of whether there was an impact or not, which can now be shown more quantitatively.”says Renne. “The fact that there has been an impact has undoubtedly made things worse”.
The researchers also calculated the numbers for the asteroids. The coincidence of the impacts with the replacement periods of the species was significantly weaker and worsened dramatically when the Chicxulub impactor was not considered, suggesting that other known smaller impactors did not cause significant extinctions.
The eruption rate of the Deccan Traps in India suggests that the ground was ready for widespread extinction even without the asteroid, Green says. The impact was the double whammy that rang the death knell for the dinosaurs, she adds.
Flood basalt eruptions are not common in the geological record, Green says. The last one of comparable but significantly smaller scale occurred about 16 million years ago in the Pacific Northwest.
“Although the total amount of carbon dioxide released into the atmosphere during modern climate change is still much less than the amount emitted by a large igneous province, thankfully”says Keller, “We are releasing it very quickly, which is why we should be very worried”.
Green says the carbon dioxide emissions are uncomfortably similar to the rate of alluvial basalts that have an environmental impact they have studied. This places climate change in the context of historical periods of environmental catastrophe, he says.
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