Sixty-five million years ago, life on Earth suffered one of the worst mass extinctions of all time. It was an event that killed creatures across the spectrum of life's diversity, from tiny marine invertebrates to the largest dinosaurs, but what could have caused it?
A number of hypotheses have been forwarded over the years, most of which have focused on dinosaurs. It would take an entire book to discuss them all. Depending on who you ask, the non-avian dinosaurs succumbed to disease, nest-raiding mammals, hungry-hungry caterpillars, or simply became too big to survive, but over the past three decades most paleontologists have agreed that the impact of an asteroid in what is now the Yucatan Peninsula played a major role in the end-Cretaceous extinction. A collaboration by more than 40 scientists published last week in the journal Science reaffirms this hypothesis.
The end of the Cretaceous was a time marked by catastrophic geological events. Not only did a chunk of extraterrestrial rock strike the earth, but, prior to the impact, a group of volcanoes in India known as the Deccan Traps were undergoing massive eruptions. These events have been well established through geological evidence, but the question is what roles they might have played in the extinction of so many kinds of organisms at the end of the period. This is what the international team behind the Science paper wanted to determine.
After looking at a variety of sites recording the end of the Cretaceous and the beginning of the next period, the Paleogene, the scientists determined that the asteroid impact occurred at the boundary between the two (and not hundreds of thousands of years earlier, as some geologists have recently proposed). This is important because the timing of the geological event must be tied to the record of species extinction seen in the fossil record, and the present study suggests that the impact and the extinctions are closely associated. The consequences of the eruptions of the Deccan Traps should not be ignored, but it appears that they do not fit the pattern of mass extinction as well as the asteroid impact.
But what exactly happened that resulted in the deaths of so many kinds of organisms? According to the authors, the initial impact would have triggered massive earthquakes in the region and sent enormous tidal waves to the shore. From a distance it would have looked like a bomb going off, with the impact throwing a mixture of scalding air, material from the asteroid and fragments of the Earth's crust. The matter exploded with so much force that some probably escaped into space. Some of this material landed in parts of the globe far from the center of impact, and while they were not hot enough to start forest fires (as was previously thought) the bits and pieces could have heated things up in habitats all around the world.
Yet some of the most devastating effects of the impact would not be felt immediately. Among the rocky slurry cast up into the atmosphere were soot and sulfur gases which, by current estimates, could have cooled the global climate by up to 10 degrees Celsius (18 degrees Fahrenheit) for several decades, and some of that atmospheric sulfur precipitated back down in the form of acid rain. Indeed, the most significant part of the event was not the shock of impact but the fact that the asteroid struck an area rich in sulfur and other materials that, once thrown up into the atmosphere, drastically changed the global climate and ecology. Had the asteroid struck somewhere else on the planet the consequences could have been very different for life on earth.
This does not mean that the details of the end-Cretaceous extinction have been all wrapped up, however. Most of what we know about the extinction comes from North America, but we still don't know very much about what was going on elsewhere in the world. To draw an analogy with forensics, scientists have identified the weapon used in the massacre, but doing so is only a small part of fully understanding what happened.
Schulte, P., Alegret, L., Arenillas, I., Arz, J., Barton, P., Bown, P., Bralower, T., Christeson, G., Claeys, P., Cockell, C., Collins, G., Deutsch, A., Goldin, T., Goto, K., Grajales-Nishimura, J., Grieve, R., Gulick, S., Johnson, K., Kiessling, W., Koeberl, C., Kring, D., MacLeod, K., Matsui, T., Melosh, J., Montanari, A., Morgan, J., Neal, C., Nichols, D., Norris, R., Pierazzo, E., Ravizza, G., Rebolledo-Vieyra, M., Reimold, W., Robin, E., Salge, T., Speijer, R., Sweet, A., Urrutia-Fucugauchi, J., Vajda, V., Whalen, M., & Willumsen, P. (2010). The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary Science, 327 (5970), 1214-1218 DOI: 10.1126/science.1177265