Everyone knows an asteroid strike wiped out the dinosaurs, right? Lots of evidence shows that the Chicxulub impact event had terrible consequences for the dinosaurs. But the picture is a little more complicated than that. Extreme volcanic activity may have contributed to the extinction.
At the end of the Cretaceous period, about 66 million years ago, the dinosaurs went extinct. And not just the dinosaurs; about 75% of all plants and animals went extinct. Avian dinosaurs survived.
At the same time, a massive asteroid or comet slammed into Earth’s Yucatan peninsula, in present-day Mexico. Called the Chicxulub impact event, it clouded the atmosphere with water vapour and with particularly long-lasting dust, due to the structure and type of rock at the impact site.
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It’s been easy to conclude that Chicxulub caused the extinction. There’s lots of evidence beyond the impact site itself.
First of all, the extinction itself is called the Cretaceous-Paleogene extinction (K-Pg extinction), because it marked the end of the Cretaceous era and the beginning of the Paleogene period. (It’s also called the Cretaceous-Tertiary extinction (K-T extinction.)
The geologic record contains a sediment layer from 66 million years ago called the K-Pg boundary. The K-Pg boundary is present in both marine and terrestrial rocks around the world. It contains lots of the metal iridium, which is rare on Earth, but abundant in asteroids. The conclusion is that the Chicxulub impact spread iridium into the atmosphere globally, and its now preserved in the geology of Earth, a kind of smoking gun for the impact event.
But there’s growing evidence that volcanic eruptions contributed to the mass extinction 66 million years ago, and that evidence comes from a rock formation called the Deccan Traps. A new study strengthens the link between the K-Pg Extinction and the volcanic activity that caused the Deccan Traps.
The Deccan Traps is a region in India known as a large igneous province. It’s one of the largest volcanic features on Earth. The Traps are multiple layers of basaltic lava, and all together they’re more than 2 km (1.2 miles) thick. The Deccan Traps cover an area of 500,000 km. sq. (200,000 mi. sq.), though at one time they covered as much as 1.5 million sq. km. (600,000 mi. sq.) The volume of the lava is 1 million cubic km (200,000 cubic miles.)
The name “Traps” comes from the Swedish word “trappa” which means stairs. It refers to the step-like structure in the region’s landscape.
There’s more to these Traps than rock though. The amount of volcanic activity required to create the Deccan Traps would have polluted the atmosphere with poisonous gases. Now two geoscientists from Princeton University have drawn a stronger link between the Deccan Traps and the K-Pg Extinction with the first high-resolution timeline for the eruptions that created India’s Deccan Traps. Their research appears in the February 22nd issue of Science.
The two scientists are Blair Schoene and Gerta Keller, both of Princeton University. They led an international team for this study, which tried to date the different layers of the Deccan Traps more precisely than ever before.
“Everyone has heard that the dinosaurs died from an asteroid hitting the Earth,” said Schoene, an associate professor of geosciences. “What many people don’t realize is that there have been many other mass extinctions in the last 500 million years, and many of them coincide with large volcanic outpourings” from the massive volcanoes known as flood basalts or large igneous provinces.
This is not the first time that the traps have been implicated in the K-Pg extinction. But the precision in this new study drives the point home.
The effort to date geological formations is called geochronology. Geochronology uses inherent traits of the rocks themselves to find their age, usually relying on isotope ratios and radioactive decay to do so.
The most famous geochronology technique, usually called carbon dating, uses the decay rate of radioactive carbon-14 to find fossils ages. But carbon dating only works for living tissue that’s a few thousand years old, making it useful for archaeology but not for 66-million-year-old basalt.
For rocks from around the time of the mass extinction, geoscientists have a few choices of naturally radioactive materials. Uranium-lead geochronology yields very precise ages, but minerals containing uranium rarely occur in basalt, the rock that makes up the massive lava flows at the Deccan Traps. Uranium-bearing zircon is another method of dating ancient rock, but it’s more often found in explosive eruptions from Mount St. Helens-type volcanoes, which have a more silica-rich chemistry.
With these dating restrictions in mind, the team of scientists were cautious about predicting success. They never anticipated that their first trip to the Deccan Traps would yield the results they did.
“I don’t think that any of us anticipated that our first trip to the Deccan Traps would lead to the type of dataset that we were able to produce,” said Mike Eddy of the Class of 2011, now a postdoctoral research associate in geosciences and a co-author on the Science paper.
But they got lucky.
In their first few days at the Deccan Traps, the scientists gathered samples of what are called coarse-grained basalts. Basalt is the most common type of volcanic rock on Earth. They were looking for samples containing uranium-bearing minerals, because the radioactive decay of uranium is one benchmark method of geochronology. At first they found none, because such rocks are rare in formations like the Deccan Traps and more common in volcanic ash.
But after a few days, they found the type of silica-rich rocks they were looking for.
“During our first week in India, we found a high-silica ash bed between two basalt flows, and it got our gears turning,” Eddy said. The researchers knew that silica-rich volcanic ash could easily contain the tiny zircon crystals that preserve radioactive uranium. “The real breakthrough came a day or two later, when Blair realized that the fossil soils may have also collected this type of ash in small amounts,” Eddy said.
So the team changed their focus. Instead, they searched for the ash deposits between the basalt flows, looking for the radioactive uranium inside the zircon, contained in the ash. The uranium makes the zircon, and hence the ash layers, easy to date. By dating the ash layers above and below a lava flow, they could more accurately date the lava itself, and the date of eruption.
The team spent three field seasons at the Deccan province, and sent samples from 141 sites back to the lab at Princeton. 24 of the samples held what the team needed: uranium-bearing zircon crystals. Analysis of the samples showed that the Deccan Traps were created by four distinct eruption pulses. And each of those pulses spelled doom for the dinosaurs, and for most of the other life forms on Earth at the time.
Each time a volcano erupts, it changes the atmosphere. Enormous amounts of sulfur and carbon dioxide are spewed into the atmosphere from their long-term sequestration inside the rocks. The sulfur has a short-term cooling effect on the atmosphere, while the carbon dioxide has a long-term warming effect. The two combined can lead to wild climate swings.
“These can lead to climate swings between warm and cold periods that make it really hard for life on Earth,” said Schoene. But to know more clearly how these eruptions would have effected life on Earth, they had to get the timing right. A mass quantity of CO2 will have a very different effect if injected into the atmosphere in one hundred years than if it took a million years to be injected.
Of the four eruption pulses that the scientists identified, two of them took place before the mass extinctions. The second of those two began only tens of thousands of years before the Chicxulub impact, which is almost at the same instant in geological terms. “The first two pulses … correspond with a period of time where climate fluctuated from cold to hot to cold again, and many scientists think this indicated an initial disruption to the climate that may have contributed to the mass extinction event,” said Schoene. “Our data show that maybe the second pulse could have played an important role in the extinction itself because it happened right before it.”
“Deccan volcanism is the most likely cause of the dinosaur mass extinction,” said Gerta Keller. “The Chicxulub impact may have contributed to their demise, though the timing and environmental effects of this impact still remain to be determined.”
Another recent study published in the same issue of Science used a different method to date the Deccan Traps and came up with different dates. This study concluded that there weren’t four distinct volcanic pulses that the Princeton study identified, and that greater than 90% of the Deccan Traps volume erupted in less than 1 million years. It also concluded that about 75% of it happened after the K-Pg extinction, and that the climate change at the end of the Cretaceous coincided with the smallest phases of eruptions at the Deccan Traps. If so, the Deccan Trap volcanism couldn’t have caused the extinction.
Other scientists, aware of the nature of the impact versus volcano debate that this study is a part of, are more circumspect in their conclusions.
“In general, I think this paper is significant and interesting,” said Pincelli Hull, an assistant professor of geology and geophysics at Yale who was not involved in this research and who has argued against the role of the volcano in the mass extinction. “The paper is a huge advance in timing the [Deccan Traps] eruptions, but how that relates to the timing of outgassing is still a major question that needs to be resolved to figure out exactly what the relevant roles of volcanism and impact were.”
It’s seldom that one study in a long line of studies puts a scientific debate to bed, and this one’s no different. Science progresses as scientists get better at measuring things, and at thinking about them. This won’t be the end of the debate.
It may be that it took a one-two punch to cause the K-Pg Extinction. The dinosaurs might have been knocked down by the Chicxulub impact and on their way back up, then knocked down for good by volcanoes. Or it may be even more complex than that.
A 2016 study shows that dinosaur species were already going extinct millions of years before the K-Pg extinction, and that new species weren’t appearing to replace them. At the same time, mammal species were becoming more varied, and maybe better positioned to adapt to the changes caused by volcanic activity and the Chicxulub impact. Maybe evolutionarily, the dinosaurs had run their course, and the impact and the volcanoes were just the exclamation point.
And maybe we’ll never know for sure.
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