At the beginning of the Mesozoic, some 66 million years ago, two of the greatest disasters of prehistoric times struck almost simultaneously. We have all become familiar with the cosmic collision that sent the non-avian dinosaurs, ending their 135 million-year run as overlords of the world. But over the past four decades, as the story of the asteroid settled into the collective conscience, a vigorous debate has raged as scientists continued to reinforce another character in the story: volcanism
Despite being the most studied of the “Big Five ”mass extinctionsThe Cretaceous Paleogene – which, besides dinosaurs, has wiped out three-quarters of all species – still has its secrets. The case is confused, with two suspects confirmed at the scene of the crime. "I don't want to be too morbid," says paleontologist Stephen Brusatte of the University of Edinburgh, "but a body disappears, and you know both John Wayne Gacy and Jeffrey Dahmer were around the day before." Which murderer is to blame? Did they act together?
The impact crater and its debris coincide more or less perfectly with the extinction. Around the world, a thin layer of iridium – a chemical element rare on Earth but common in alien objects – buries dinosaurs like a coffin lid. Meanwhile, the ancient lava flows from the Deccan Traps in India, one of the greatest volcanic provinces of all time, spans the end of the Cretaceous with several hundred thousand years on either side.
In the late 20th century, the asteroid theory became a virtually uncontroversial fact, and it is still the more widely accepted culprit. But today, many researchers are seriously considering the possibility that the monstrous eruptions may have helped by heating the planet to a deadly degree, and many of the recent extinction literature tries to assign the right debt levels. "Fewer and fewer people will argue that volcanism was irrelevant," said Paul Renne, a geophysicist at the University of California, Berkeley. "The question now is: what is the balance between debt?"
The time-honored explanation for the dinosaurs' disappearance describes perhaps the worst day in our planet's history – a six-mile-wide space rock that was smashed into the ocean off the coast of the Yucatan peninsula with such cruelty that it destroyed debris. the sun was blocking. and caused a global nuclear winter. Locally, it caused voracious fires and steered it a tsunami about 200 miles inland from the Gulf of Mexico.
Renne was a graduate student at the time the physicist Luis Alvarez first proposed this hypothesis“I thought it was a bunch of bulls…, to be honest,” he recalls. "But gradually the impact theory was introduced and it became a dogma."
All along, however, evidence has been accumulating for the role of eruptions in other major die-offs. In the 1990s, the two mass extinctions prior to the Cretaceous Period (de Perm and Trias) were linked to volcanic activity, characterized by layers of flood basalt, much like those in India. These eruptions released massive amounts of carbon dioxide, which heats the planet and acidifies the oceans, as well as sulfur dioxide, which, on the other hand, filters sunlight and cools the planet. In fact, the Permian was more deadly than the Cretaceous, killing 95 percent of marine species. “In light of that,” says Renne, “people started to think,“ Gosh, if flood basalt alone is able to do this, maybe we should rethink the Deccan Traps. "
In contrast, asteroid destruction has no precedent, leaving some experts unconvinced. Gerta Keller, a Princeton geologist and paleontologist, has drawn attention for being particularly hostile to the theory. She has compared it to a fairytale, saying “it has all aspects of a very beautiful story. It just isn't true. "
Some have tried to earn credit for both disasters. Greg Wilson, a paleobiologist at the University of Washington and curator of vertebrate paleontology at the Burke Museum, considers extinction a one-two punch: global warming – and perhaps cooling – could destabilized the biosphere, making it more sensitive to the devastation of the incoming asteroid and less able to rebound in its aftermath. "I think the impact in itself would have had an effect on ecosystems," says Wilson, "but the severity was amplified by … climate change related to volcanism."
Refine the record
Wilson's view is based in part on the fact that dinosaurs appear to have declined towards the end of the Cretaceous, indicating some degree of ecological disruption. In Hell Creek, the fertile Montana site where he often works, their diversity is declining in the run-up to the impact. "It seems to me there's growing evidence that it's more subtle than just an asteroid hitting Earth … and everything else before that was great," says Wilson.
Others disagree. "There is no reason to believe that the dinosaurs were really in trouble," says Brusatte. And, he adds, "you never want to overlook the fossil record" – these sparse remains may not reveal the whole picture. In recent years a number of other evidences, such as climate and ecological modeling, have implicated the asteroid as the main driver, although some researchers acknowledge that eruptions could have played a minor role and may have slowed life's recovery. A paper published early last year described the extinction as "impact with a hint of volcanism."
That sums up Brusatte & # 39; s own opinion. Again, fossils can only say so much. But, he says, “It's really just a very striking and very suggestive thing that you never find dinosaur bones above the iridium. Never. "Getting back to the forensic metaphor," Sometimes we just have to look at the most visceral evidence. If there's a crime scene and there's a business card with the killer's name on it, don't overthink it – that is the murderer. "
That said, he and Wilson agree that the fossil record is vastly under-sampled. Western North America is the only region that has been studied in depth, and even there, Wilson still spends his summers filling the gaps at Hell Creek, where collecting began in the early 1970s. Those extinction patterns, which are so close to the impact site, may not reflect what happened on the other side of the planet. Some paleontologists hope to investigate fossils in India – “ground zero of volcanism and antipodal of impact,” as Wilson put it. Other promising sites are unknown in the Pyrenees and South America.
A better idea of when exactly the eruptions occurred could also resolve some uncertainty and suggest unexpected possibilities. When Renne dated the layers of the Deccan Traps, he realized that around the time of impact, they began to release larger, thicker lava flows. In 2015, he and his colleagues speculated that the asteroid may have been activated earth-shaking earthquakes that unleashed this powerful flow of volcanism, like a giant hand shaking an earth-sized bottle of coke to the point of explosion. "Or beer," says Renne, "if that's your preference."
After 40 years, the nuances of the Cretaceous-Paleogene disaster remain elusive. But with a global perspective on fossils, an improved timeline, and new methods to model the likely effects of impact and volcanism, future scientists may be able to come up with more definitive answers. “When all those things come together, we'll have an even better understanding of extinction,” Brusatte says. And there can be surprises there. I am always open to my mind. "