Determining why the dinosaurs killed 66 million years ago at the end of the Cretaceous period has long been a topic of debate, as scientists set out to determine the cause of the five mass extinctions that reshaped life on planet Earth in a geological moment. Some scientists argue that comets or asteroids that collided with Earth were most likely agents of mass destruction, while others argue that large volcanic eruptions were the cause. A new study led by Dartmouth published in Proceedings of the National Academy of Sciences (PNAS) Reports indicate that volcanic activity appears to have been the main driver of the mass extinction.
The results provide the most compelling quantitative evidence to date that the relationship between major volcanic eruptions and wholesale species rotation is not just a matter of chance.
The researchers say that four of the five mass extinctions coincided with a type of volcanic flow called a basalt flood. These eruptions are engulfing vast regions – even an entire continent – with lava in the blink of a geological eye, only a million years old. They leave behind giant fingerprints as evidence – vast regions of step-like igneous rock (solidified by exploding lava) that geologists call “big igneous provinces.”
To be counted as “large,” a large igneous county must contain at least 100,000 cubic kilometers of magma. For context, the 1980 eruption of Mount Saint Helens involved less than one cubic kilometer of magma. The researchers say that most of the volcanoes represented in the study erupted a million times from lava.
The team relied on three well-established geological timescale datasets, paleobiology, and large igneous provinces to examine the temporal relationship between mass extinctions and large igneous provinces.
Says lead author Theodore Green21, who conducted this research as part of the Dartmouth Senior Fellowship Program team and is now a graduate student at Princeton University.
In fact, a series of volcanic eruptions in present-day Siberia triggered the most devastating mass extinction event about 252 million years ago, releasing a giant pulse of carbon dioxide into the atmosphere and stifling nearly all life. Witness this is the Siberian Traps, a large area of volcanic rock roughly the size of Australia.
Volcanic eruptions also rocked the Indian subcontinent around the time of the great dinosaur death, giving rise to what is known today as the Deccan Plateau. This, just like the asteroid strike, would have had far-reaching global effects, blanketing the atmosphere with dust and toxic fumes, suffocating dinosaurs and other life as well as changing the climate on long time scales.
On the other hand, the researchers say, theories in favor of annihilation by an asteroid impact hinge on Chicxulub, a space rock that fell in Mexico’s Yucatan Peninsula around the same time as the dinosaurs became extinct.
“All other theories that have tried to explain why the dinosaurs were killed, including volcanic activity, spread when the Chickxulub impact crater was discovered,” says co-author Brenhin Keller, associate professor of Earth sciences at Dartmouth. But there is very little evidence for similar impact events coinciding with other mass extinctions despite decades of exploration, he points out.
At Dartmouth, Green set out to find a way to determine the apparent link between eruptions and extinctions and to test whether chance was just a coincidence or whether there was evidence of a causal relationship between the two. Working with Keeler and co-author Paul Raine, resident professor of Earth and planetary sciences at the University of California, Berkeley and director of the Berkeley Center for Earth Science, Green enlisted the Dartmouth Discovery Cluster supercomputers to solve the numbers.
The researchers compared the best available estimates of basalt flood volcanic eruptions with violent species killing periods in the geologic time scale, including but not limited to the five mass extinctions. To demonstrate that timing was more than just random chance, they examined whether the explosions would also line up with a randomly generated pattern and repeated the exercise with 100 million such patterns. They found that the agreement with periods of extinction was much greater than random chance.
“While it is difficult to determine whether a particular volcanic eruption caused a particular mass extinction, our results make it difficult to ignore the role of volcanoes in the extinction,” says Keeler. If a causal relationship is found between volcanic basalt and the mass extinction, scientists expect that larger eruptions will lead to more severe extinctions, but no such association has been observed.
Rather than looking at the absolute size of the eruptions, the research team ordered the volcanic events at the rate of lava releasing. They found that volcanic events with the highest eruption rates actually caused the most devastation, leading to more severe extinctions up to mass extinctions.
“Our results indicate that in all likelihood there was a mass extinction on the Cretaceous boundary of a significant degree, regardless of whether or not there was an impact, which can now be shown quantitatively,” says Wren. “The fact that there was an effect made things worse without a doubt.”
The researchers also ran asteroid numbers. The coincidence of impacts with periods of species rotation was significantly weaker, and was significantly exacerbated when the Chicxulub ramming was not considered, indicating that other known smaller forcings did not cause major extinctions.
Green says the rate of eruptions of the Deccan Traps in India suggests the stage is set for large-scale extinction even without the asteroid. The effect, he adds, was the double whammy that sounded the death knell of the dinosaurs.
Greene says flood basalt eruptions are not uncommon in the geological record. The last one of the comparative but significantly smaller scale occurred about 16 million years ago in the Pacific Northwest.
“While the total amount of carbon dioxide released into the atmosphere under modern climate change is still much less than the amount released from a large pyrotechnic province, fortunately we are emitting it very quickly, which is cause for concern,” says Keeler. Greene says the CO2 emissions are alarmingly similar to the ecological flood basalt rate they studied. He adds that this places climate change in historical periods of an environmental catastrophe.