A group of 41 scientists from around the world has concluded that the mass extinction that included the last dinosaurs among its victims was in all likelihood the result of a single cataclysmic asteroid strike—a conclusion bolstered by a telling geological core sample plucked from beneath the Atlantic Ocean sea floor in 2007 by an MU geologist.
That sample, described three years ago by MU associate professor of geological sciences Ken MacLeod and the crew of the research ship JOIDES Resolution, added clarity to the sometimes muddled geological record at ground zero of the great asteroid’s impact, the 110-mile wide Chicxulub crater in Mexico’s Yucatan peninsula. Illumination published a full account of MacLeod’s research in our Spring 2007 edition.
The new validation of the single-impact theory—so called to distinguish it from theories suggesting multiple asteroid strikes were responsible for the extinction event—was based on a comprehensive review of hundreds of previous investigations, MacLeod’s among them. It appeared, with MacLeod as a co-author, in the March 5 edition of the journal Science.
Single-impact advocates had long argued that the unique occurrence of high concentrations of the element iridium, rare minerals and “melt glass” precisely at the 65.5 million-year-old “K/T Boundary” conclusively pointed to a single event as the cause for the extinctions. The K/T Boundary is a distinctive break in the geological record separating the Cretaceous Period (K), when dinosaurs are plentiful in the fossil record, from the Tertiary Period (T), when, suddenly, they are not.
The single-impact theory has long had its doubters, and debate surrounding the issue has, at times, been public and testy. Scientists who dismiss the theory acknowledge that the iridium at the boundary was likely left by an unwelcome visitor from space. But this evidence, they argued, did not disprove their contention that only a series of strikes over time, coupled with other environmental changes, would have been sufficient to cause a mass extinction of such magnitude. Samples taken from the Chicxulub crater were too ambiguous, critics added, to confirm that only a single asteroid brought down the dinosaurs.
MacLeod’s 2007 finding didn’t change the doubters’ minds, but it did give them pause. His core sample, a tube-shaped plug of stratified rock and sediment pulled from deep beneath the seabed, showed conclusively that the iridium and other exotic materials were present only at the boundary—not in any of the surrounding strata. Because it was unearthed at a site sufficiently far from the asteroid’s impact, it was untainted by blast effects that made samples from near Chicxulub open to dispute.
These blast effects were no trivial matter. When it hit the earth, according to a computer model run by the Los Alamos National Laboratory, the impact would likely have equaled the force of 300 million nuclear weapons. “Immediate effects would include an eardrum-puncturing sonic boom, intense blinding light, severe radiation burns, a crushing blast wave, lethal balls of hot glass, winds with speeds of hundreds of kilometers per hour, and flash fires.” Soon thereafter, the report continues, tsunamis, some perhaps 1,000 feet high, would have drowned huge swaths of land, even as the dry land burned. Within hours, vast clouds of dust and soot would have blocked out the sun, plunging the entire planet into darkness and cold.
“Most geologic changes are too slow to be easily detected, and even very large events, like the recent earthquakes, have only regional consequences,” MacLeod says. “This event created the boundary between the age of dinosaurs and the age of mammals, but it played out in a biological instant. The event was an experiment done in the past that we wouldn’t want to have happen in the present.”