Scientific ideas are always open to question and to new lines of evidence, so although many observations are consistent with the asteroid hypothesis, the investigation continues. So far, the evidence supports the idea that a giant asteroid struck Earth at the end of the Cretaceous — but did it actually cause most of the extinctions at that time? Some observations point to additional explanations. Further research (much of it spurred by the asteroid hypothesis) has
revealed the end of the Cretaceous to be a chaotic time on Earth, even ignoring the issue of a massive asteroid collision. Volcanic activity peaked, producing lava flows that now cover about 200,000 square miles of India; major climate change was underway with general cooling punctuated by at least one intense period of global warming; sea level dropped and continents shifted with tectonic movements. With all this change going on, ecosystems were surely disrupted. These factors could certainly
have played a role in triggering the mass extinction — but did they?
In short, the evidence points to several potential culprits for the mass extinction. Which is the true cause? Well, perhaps they all are.
Just as the extinction of an endangered species today may be traced to many contributing factors (global warming, habitat destruction, an invasive predator, etc.), the KT mass extinction may have been triggered by several different agents (e.g., volcanism and an asteroid impact, with a bit of climate change thrown into the mix). If this is indeed the case and multiple causes were in play, teasing them apart will require a more integrative approach, exploring the relationships between abiotic factors (like asteroid impacts and sea level change) and extinction: which groups survived the mass extinction and which did not? Birds, for example, survived the extinction, but all other dinosaurs went extinct. What does this tell us about the cause of the extinction? Are there different patterns of extinction in different ecosystems or different parts of the world? Do these differences point to separate causal mechanisms?
A VOLCANIC EPILOGUE
Posted January 2018
Research into the causes of the KT mass extinction has indeed continued since the original publication of this module. In particular, since volcanic activity has been implicated in several other mass extinctions, scientists have wondered about the potential interrelationships between the volcanism in India (known as the Deccan Traps), the Chicxulub meteor impact, and the mass extinction. Recent evidence, in the form of ever more precise dates on KT rocks, shows that the Deccan eruptions began before the meteor impact, but more than doubled their flow rate immediately afterwards. Modeling and several other lines of evidence suggest that the seismic energy produced by the meteor impact could have triggered eruptions and been the cause of increased volcanism half way around the world — so the fact that this large volcanic province was produced at the same time that a massive meteor struck Earth was probably not a coincidence! This hypothesis explains much of what we know was occurring on Earth at the time. The initial Deccan volcanism would have produced greenhouse gases, leading to global warming. Then an asteroid impact caused a huge disruption to Earth’s ecosystems directly, as well as a surge in volcanic activity, which likely further exacerbated the environmental damage caused by the impact. Evidence still points to the meteor as the main cause of the KT mass extinction, but as additional data are uncovered, our understanding of this tumultuous moment in Earth history becomes ever sharper. To keep up to date on the latest research on this topic, check in with the scientists themselves.
Although any T. Rex–enthralled kid will tell you that a gigantic asteroid wiped the dinosaurs off the planet, scientists have always regarded this impact theory as a hypothesis subject to revision based on further evidence gathered from around the globe. Other possible causes, such as volcanism and smaller, multiple asteroid strikes, never actually went away, and over the years researchers raised important points that did not fully jibe with a history-changing celestial impact near the Yucatan peninsula one awful day some 65.5 million years ago.
A group of 41 researchers have pored over the evidence and decided that—in accordance with the original postulate put forth 30 years ago by a team led by father and son researchers Luis and Walter Alvarez—it was, indeed, a massive asteroid that slammed into Earth, creating Chicxulub Crater on Mexico's Gulf Coast, that killed off many of the species on the planet, including the non-avian dinosaurs.
The review, published online March 4 in Science, evaluated the whole picture, according to Kirk Johnson of the Research and Collections Division at the Denver Museum of Nature and Science and co-author of the paper. And that meant assessing the other theories that have been put forth about what spelled death for the dinosaurs.
Fiery failures
The researchers dismiss the theory that the volcanism that produced the great Deccan Trap formation in western India at the end of the Cretaceous period might have produced enough sulfur and carbon dioxide to initiate a
massive shift in climate. They note that pinpointing the times when the heavy volcanism occurred is sketchy, and it likely kicked off some 400,000 years before the extinction event. In fact, as Johnson noted in a March 3 conference call with reporters, the
emissions from these volcanoes likely warmed the planet slightly, actually making life easier for many animals and encouraging diversification and dispersion over wider geographical areas.
Some scientists have pointed to multiple layers of impact residue as evidence that there was more than one asteroid involved in generating the extinction. This theory did not seem to measure up, either. Johnson says they see "no evidence for multiple impacts," and sites that had turned up these various layers were so close to Chicxulub itself that the chaotic event likely churned the layers into different locations in the sediment.
An assertion that the impact occurred hundreds of thousands of years before the extinctions also failed to hold water with the researchers. Evidence of Cretaceous period shells on top of the impact crater are likely not a sign that the animals persisted after the impact, but rather that they got "washed into the hole," Johnson noted.
Global
ground zero
The researchers assessed reports from some 350 sites all over the globe that had evidence of the impact—whether it was a dusting of iridium (an element much more common in extraterrestrial objects) or bits of shocked quartz—and could be traced back to the Chicxulub location. In some areas near the crater, the layer was 80 meters thick, pointing to one single devastating day for life on the planet.
"That's the single best explanation for the extinction of so many groups," says Neil Landman, a curator at the American Museum of Natural History in New York City and was not involved in the review, about the single impact theory.
"We've examined sites around the world," he notes of his study of ammonoids, which are shelled cephalopods that went extinct after the Cretaceous. And from the work he and his colleagues have done, he says, the evidence for the Chicxulub asteroid impact is the most consistent. "I'm very comfortable with this explanation."
A massive blow
Based on the size of material from rocky shrapnel and the crater diameter, researchers have estimated the dino-demolishing object to be some 10 kilometers across. And when it struck—at about 20 kilometers per second—it created an instant crater
about 100 kilometers wide and 25 to 30 kilometers deep "almost piercing the crust of the Earth," Johnson noted. The final crater that formed after the initial impact was about 180 kilometers across and two kilometers deep, which is still close to the depth of the Grand Canyon, Johnson pointed out.
The impact spewed rock so high, some of it likely was shot into orbit, whereas other pieces entered the upper atmosphere, reheating as they fell back to the ground. The jolt would have spurred massive earthquakes—some surpassing magnitude 11—tsunamis and landslides. While examining ammonoid fossils in southeastern Missouri, Landman says, he found a shallow water site that was "just immediately covered over by a jumble of stuff," he says. "I think what we're seeing is a tsunami," which might have reached as far from the Yucatan impact site as southern Illinois.
Perhaps most devastating, however, the crash would have caused acid rain and darkness, as particulate matter blocked sunlight, prohibiting photosynthesis in both land and water ecosystems, effectively shutting down large swaths of the food chain. Directly after the extinction event, ferns (which reproduce from spores) proliferated and species that depended on detritus seemed to survive.
From Landman's study of ammonoids, he points out that even for groups that eventually went extinct after the collision (producing the so-called K–T boundary in the fossil record), the asteroid's impact did not mean sudden eradication. "There seems to be some suggestion of some survival for awhile after the event," he says. Fossils found above the iridium layer show that ammonoids might have survived "for tens to possibly hundreds of years afterward" perhaps because "things in the marine realm were a little more insulated," he explains.
Although these estimates might seem rough for such a dramatic event, revealing details on the resolution of years and months "was unimaginable" in decades past, he says. "It's one of the best studied intervals of the geologic record," he notes. And all of this attention has led to increasing nuance in the timeline.
"This is not geologic time—this is instant time," Johnson said, acknowledging that it is a very tricky task to pin down a single event from 65.5 million years ago. But, judging from the chemical, geochemical and geochronological evidence, he said, "The Chicxulub Crater really is the culprit."