An artist's rendering of the moment of impact when an enormous space rock struck the Yucatán peninsula at the end of the Cretaceous Period.
Thirty years ago, Luis Alvarez, Jan Smit and their coworkers suggested a large meteorite slammed into Earth 65 million years ago and caused one of the most severe mass extinctions in Earth's history, ending the age of the dinosaurs. In 1991, a more than 200-kilometer-wide impact crater was discovered in Yucatan, Mexico, that coincided with the extinctions. Since then, the impact hypothesis has gained overwhelming acceptance within the scientific community.
Still in recent years, a few scientists have challenged this hypothesis. To address their claims, a panel of 41 experts from Europe, the U.S., Mexico, Canada and Japan provide new data from the analysis of ocean drilling and continental sites and re-analyze the relevant literature in the field, including the most recent research. The team is not only multinational, but also multidisciplinary, including experts on impact modeling, tsunamis, radioactive dating, geology, paleontology, sedimentology, microbiology, ecology, petrology and geochemistry. In a review paper in the March 5 edition of the journal Science, they find that alternative hypotheses are inadequate to explain the abrupt mass extinction and that the impact hypothesis has grown stronger than ever.
The fossil record clearly shows a mass extinction event across the planet at about 65.5 million years ago. Because this change is so dramatic, geologists use it to define the end of the Cretaceous period and the start of the Paleogene period (formerly called the Tertiary period). They refer to the time of the extinctions as the K-Pg boundary.
Some scientists have suggested that the Chicxulub ("chik-shoo-loob") impact in Mexico happened 300,000 years before the K-Pg boundary and therefore, came too early to have been the major cause of extinctions.
They point to deposits at sites around the Gulf of Mexico with a layer of tiny glass-like blobs of melted impact material that, according to their interpretation, was deposited at about 300,000 years before the K-Pg boundary mass extinction. As an alternative, they suggest the Deccan Traps — unusually active volcanoes in what is now India — led to global cooling and acid rain, and were the major cause of mass extinction, not the Chicxulub impact in Mexico.
However, the study's authors find that what appears to be a series of layers neatly laid down over 300,000 years near the impact site were actually violently churned and then dumped in a thick pile in a very short time. Models suggest the impact at Chicxulub was a million times more energetic than the largest nuclear bomb ever tested. An impact of this size would eject material at high velocity around the world, cause earthquakes of magnitude >10, continental shelf collapse, landslides, gravity flows, mass wasting and tsunamis and produce a relatively thick and complex sequence of deposits close to Chicxulub.
"If we are to unravel the sequence of events across the K-Pg boundary, perhaps the last place in the world we should look is close to the Chicxulub impact site, where the sedimentary deposits will be most disturbed," write the authors.
In addition, the authors note, as you go farther from the impact site, these layers become thinner and the amount of ejected material decreases until it becomes one layer that can be found globally exactly at the K-Pg boundary coincident with the mass extinction. Moreover, the ejecta within the global K-Pg layer is compositionally linked to the specific sediments and crystalline rocks at Chicxulub.
The authors find that despite evidence for relatively active volcanism in India, marine and terrestrial ecosystems showed only minor changes within the 500,000 years before the K-Pg boundary. Then, precisely at the boundary, there was an abrupt and major decrease in productivity (a measure of the sheer mass of living things) and species diversity.
The Deccan hypothesis is further weakened by a review of models of atmospheric chemistry. Although significant volumes of sulfur may be emitted during each volcanic eruption and form aerosols in the stratosphere, these sulfur aerosols fall out rapidly and any adverse environmental effects are apparently only short-lasting. In comparison, during the Chicxulub impact, much larger volumes of sulfur, dust and soot were released in a much shorter time, leading to extreme environmental perturbations (such as darkening or cooling).
"Combining all available data from different science disciplines led us to conclude that a large asteroid impact 65 million years ago in modern-day Mexico was the major cause of the mass extinctions," says Peter Schulte, assistant professor at the University of Erlangen in Germany and lead author of the review paper.
Far from Chicxulub, the geologic record clearly shows a single large meteorite hit the Earth exactly at the K-Pg boundary. Thickening of the K-Pg boundary layer towards Chicxulub shows Chicxulub was the impact site. The significant changes in Earth's ecosystems all occur precisely at this boundary and thus, say the authors, a large asteroid impact into the sulfate-rich sediments at Chicxulub remains the most plausible cause for the K-Pg boundary mass extinction.
Several mechanisms have been proposed to explain why the impact was so deadly. In February 2008, Sean Gulick and Gail Christeson, research scientists at The University of Texas at Austin's Institute for Geophysics, and their colleagues published a study in the journal Nature Geoscience finding that the asteroid landed in deeper water than previously assumed and therefore released more water vapor and sulfate aerosols into the atmosphere. Gulick, a co-author of the new review paper in Science, said this could have made the impact deadlier in two ways: by altering climate (sulfate aerosols in the upper atmosphere can have a cooling effect) and by generating acid rain (water vapor can help to flush the lower atmosphere of sulfate aerosols, causing acid rain). That finding and many others strengthen the case for the impact hypothesis.
CHICXULUB IMPACT QUICK FACTS:
• The asteroid was about 10 kilometers (6 miles) wide. In the UK, the city of Bristol, the Isle of Wight and Jersey are all about that size.
• The asteroid was about 10,000 times more massive than the total mass of the human world population. In other words, the asteroid was about 3x10^15 kilograms (or about 3x10^12 tonnes).
• At impact, the asteroid is estimated to have been traveling at 20 kilometers per second (44,640 miles per hour), roughly 10 times the speed of a rifle bullet.
• The impact released about a billion times more energy than the atomic bombs dropped on Hiroshima and a million times larger than the largest nuclear bomb ever tested.
• The initial impact crater was about 100 kilometers (60 miles) wide and 30 kilometers (18 miles) deep. After the dust cleared, the crater was about 180 kilometers (110 miles) wide and 2 kilometers (1 mile) deep.
• Impacts of this size on Earth are thought to happen on average about once every hundred million years.
• The impact released earthquakes on the order of magnitude 11. That's more than 100 times more powerful than the magnitude 8.8 Chilean earthquake on Feb. 27, 2010.
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