On the last day of the Cretaceous period, a 7.5-mile-wide asteroid smashed into the Yucatan peninsula in Mexico and changed the arc of life on Earth. Sixty-six million years later, scientists have used supercomputers to recreate the devastation of that infamous impact in unprecedented detail. The models are helping to solve an ancient mystery about what killed all dinosaurs except birds, and offer a new insight into how life on Earth responds to rapid environmental change.
Scientists already knew that the extraterrestrial impactor blew a crater about 120 miles wide in the Earth’s crust, striking at the right place, and at the right angle, to spew colossal amounts of refrigerant gases and soot into the upper atmosphere. The skies blackened as huge tsunamis pierced the oceans and forest fires spread for hundreds of miles around. In a matter of years, temperatures plummeted over 60 degrees Fahrenheit, plunging the world into a prolonged period of cold known as a winter shock that scientists believe killed more than three-quarters of life on Earth. (Learn more about how dinosaurs went extinct.)
The event was like “Dante’s Hell on Earth, “says Alfio Alessandro Chiarenza, associate researcher at University College London, and leader of a new study published today in the procedures of the National Academy of Sciences.
Around the same time that the asteroid hit, known as the Chicxulub impact, a large volcanic complex in what is now southern India was erupting, releasing more than 200,000 cubic miles of lava and pumping climate-altering gases into the heaven. While most scientists agree that the asteroid triggered the extinction event, researchers have long wondered if these volcanoes, called Deccan traps, also contributed to the devastation of life.
In the new study, Chiarenza and her colleagues use models to recreate Earth’s ancient climate, adjusting the variables for various apocalyptic scenarios. These simulations reveal that only the asteroid made the planet uninhabitable for all dinosaurs other than birds. Perhaps counterintuitively, the Deccan Trap volcanoes may have made Earth plus hospitable, no less.
“This is, I think, a nail in the coffin [for the hypothesis that] Deccan traps led to mass extinction, “says paleontologist Anjali Goswami, research leader at the London Museum of Natural History, who was not involved in the study.
Extinct
Named for the two geological periods on both sides of the event, the mass extinction of the Cretaceous-Paleogene occurred with remarkable speed. Intense cold, constant darkness, forest fires, tsunamis, unbearable heat in the impact area and eventual acid rain destroyed the planet. The sudden destruction caused by the extinction event presents an opportunity for today’s scientists to study how life can respond to rapid and severe stresses.
“It gives us an idea of what organisms do when someone pulls the rug out from under them,” says Yale University paleontologist Pincelli Hull, an expert in dinosaur extinction.
However, to really understand how the mass extinction of dinosaurs developed, scientists must agree on what exactly caused it.
Over the past decade, geologists have confirmed that Deccan traps erupted in multiple pulses for 700,000 years, a period that overlaps with the Chicxulub impact. Because volcanoes were erupting during the extinction event, scientists have wondered if they played a role in killing the animals. Two of the five largest mass extinctions were caused by strong volcanic carbon dioxide warming, including the worst of all: the Permian-Triassic mass extinction 252 million years ago, caused by ancient eruptions in what is now Siberia, eliminated 96 percent of marine life. and approximately three out of every four species on land.
Deccan traps could have affected life 66 million years ago in two main ways. On shorter time scales, the sulfur dioxide released by volcanoes could have cooled the planet and fueled acid rain, disarming Earth’s oceans and broader chemical cycles. Over time, the large amount of CO2 released in the eruptions could have led to constant warming, which could stress global ecosystems.
Two efforts published last year that attempted to date the largest pulse of Deccan trap eruptions disagree in tens of thousands of years, a difference between the largest eruptions occurring before the asteroid hit, when they could have affected the deaths, or a short time later, when they would have played no role in the extinction.
To test disasters, Chiarenza and her colleague Alexander Farnsworth, a climatologist at the University of Bristol, built computer models of Earth’s climate as it was 66 million years ago. They ran 14 different scenarios that included asteroid impact, Deccan traps, and the two events combined. The simulations assumed CO2 levels between 560 and 1,680 parts per million, up to four times more than today. The scientists also dimmed virtual sunlight by 5 to 20 percent from pre-impact levels.
In some of the simulations, Chiarenza and Farnsworth also modeled the short-term cooling effects of the Chicxulub impact by injecting a hundred times more ash and aerosols than the eruption of Mount Pinatubo that shook the Philippines in 1991. To trace how disasters affected the dinosaurs, Chiarenza mapped discover the most probable habitats of ancient animals with another computer model based on ancient climate data and locations of dinosaur fossils.
All models showed that Deccan traps could not have killed the dinosaurs. The long-term warming caused by volcanoes would not have eliminated the dinosaurs; in any case, it expanded the area of land where they could comfortably inhabit. Even the most extreme mitigation scenario of the Deccan traps did not annihilate the ecological niche of the dinosaurs, the new study shows.
The asteroid impact scenarios, however, were downright horrible. In some, the average temperature of the earth plummeted from more than 68 ° F (20 ° C) to well below zero, and precipitation decreased between 85 percent and 95 percent. When Chicxulub’s virtual impact dimmed sunlight by 15 percent or more, no habitat anywhere on Earth could support non-avian dinosaurs. (Learn more about how birds survived mass extinction.)
Looking at the new data, “it becomes quite reasonable why some things went extinct,” says Goswami. “Actually, it becomes surprising that everything hasn’t.”
Bouncing
The research team’s models also revealed something unexpected: Deccan traps may have helped life recover, as CO2 emissions from volcanoes lessened the impact of winter severity.
“That’s a great plot twist,” says Hull. “I don’t think anyone was thinking about volcanism making the impact less bad. That’s really amazing. “
Recent work suggests that Deccan traps likely exploded in a trickle of activity for hundreds of thousands of years, rather than dealing a major global blow to ecosystems. Last November, a group led by Hull discovered that Earth’s oceans rapidly acidified in the tens of thousands of years after Chicxulub, likely due to post-impact acid rain, but the pH levels of the oceans remained. stable for the approximately 100,000 years before impact, even while the Deccan traps were already erupting.
In a follow-up study, Hull showed that in the 300,000 years before impact, global temperatures gradually increased and decreased by approximately 4 degrees Fahrenheit, indicating an increase and decrease in CO2 levels, but nothing so extreme that it would threaten to the dinosaurs.
There are also signs away from the lava flows in India that hint at the role of Deccan traps in facilitating the recovery of life. Last October, a group led by Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science, unveiled several sites in the Rocky Mountains that record North America’s post-asteroid flora and fauna. Lyson’s team discovered that for 100,000 years after the impact, ecosystems did not have many types of life, but then, the diversity of mammals and plants flourished in pulses that correlate with periods of mild warming, consistent with jets of CO2 that Deccan traps could have issued.
Recent studies, Lyson says, have led to a captivating idea: “Deccan as the creator, versus Deccan as the destroyer.”
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