Lab Researchers Find Impact
Site Leading to Dinosaurs' Demise
By Karre Marino

The Earth's dinosaurs may have been on the decline some 65 million years ago, but according to a team of JPL researchers, it was an asteroid that struck a geologically unique area in Mexico that ultimately did them in.

"We believe that an asteroid, 10 to 30 kilometers (about six to 18 miles) in diameter, impacted a sulfur-rich site in a region of the Yucatan Peninsula," according to Adriana Ocampo, planetary geologist in the Space and Earth Science Division 32.

A paper detailing the results of the researchers' findings was published this month in the journal Earth and Planetary Sciences Letters and was co-authored by Ocampo; Kevin Baines, also in Division 32; Kevin Pope of Geo Eco Arc Research in La Canada; and Boris Ivanov of Moscow's Russian Academy of Science.

"Several lines of evidence have prompted the scientific community to believe that this crater--called Chicxulub--(which means tail of the devil in Maya) caused the extinction of more than 50 percent of the Earth's species," Ocampo added.

That theory was first aired in 1980, when University of California, Berkeley, geology professor Walter Alvarez and his colleagues proposed that dinosaurs disappeared due to a large impact. The main evidence was the high concentration of iridium found in the clay layer in Italy in the Cretaceous/Tertiary boundary, which marks the time transition between these two geological periods. Iridium, an element rare on Earth, is found in high concentrations in asteroids and comets, and in rocks that date to the mass extinction.

The scientific community found this notion to be highly controversial, Ocampo explained. "It was hard for people to conceive that the Earth had been so catastrophically transformed by the impacts of an asteroid or comet," she said.

In order to be convinced, scientists had to find the actual impact site.

It took them a decade to do so. In 1989, Pope and Charles Duller, of NASA's Ames Research Center, discovered a 170- kilometer-diameter (105-mile) semi-circle of sinkholes at Chicxulub, Mexico. After Ocampo studied the gravity, magnetic and stratigraphic data and correlated them with the sinkholes, she recognized that the area had the classic characteristics of an impact crater.

These results were published in 1991, the same year Ocampo and Pope discovered an unusual deposit of large boulders at the Cretaceous/Tertiary boundary in Belize in Central America, 360 kilometers (223 miles) south of the Chicxulub crater.

The boulders, some the size of a Volkswagen bug, were spewed out of the crater upon impact, and flew 320 kilometers (200 miles) to land in Belize, she said. Fragments of glass, created by the melting of rock upon impact, were found with the boulders. Spherical fragments known as tektites, which were formed as the molten glass flew through the air and cooled, were also found.

"These tektites have been found in Haiti, Mexico, Texas and Alabama," Ocampo noted, "but the large boulders are only known in Belize, because it's so close to the crater."

Along with the tektites, spherical pieces of calcium carbonate, some with an unusual radial structure, were found. The formation of these "spherules" remains a mystery, but the scientists speculate that they may have formed from the residue of vaporized carbonates and sulfates.

Another important find in the boulder deposit was limestone with fossils dating to the early part of the Cretaceous, when the Yucatan platform first appeared. "Fossils of this age don't belong in northern Belize," Ocampo observed. "Early Cretaceous fossils are known from deep down in the platform, recorded in the drilling records of a Mexican petroleum company."

The scientists suggest that the limestone found in Belize was excavated by the impact, which probably blew a hole more than 15 kilometers (nine miles) deep in the Yucatan platform.

Results of the Belize research by Ocampo, Pope and Alfred Fischer of the University of Southern California are scheduled to appear with other works in a special paper of the Geological Society of America, detailing recent research on major catastrophes in Earth's history.

Now that the crater had been found, "The real challenge was to show how it killed the dinosaurs," Pope said.

In studying the site and modeling the resulting changes in the biosphere, the scientists believed that what proved lethal to life on Earth was where the asteroid hit.

"The target area was rich in salts and sulfur, because the Gulf of Mexico was cut off from the sea for much of the Cretaceous. The evaporites produced by the evaporation of sea water were rich in sulfur," Ocampo explained.

Had the celestial body impacted somewhere else--in the Sierra Nevada, for instance--"The extinctions may not have occurred," she maintained.

"The impact created a melange of sulfuric acid, dust and soot that exploded from the crater to the highest levels of the atmosphere, distributing the materials worldwide," Ocampo added. "Initially, the Earth experienced a total blackout due to soot and debris wafting into the atmosphere as a result of the impact. Photosynthesis was shut down for approximately six months, after which the sky partially cleared."

From related work on the sulfuric-rich atmosphere of Venus and a sophisticated computer model of the impact, the team determined that "Sulfuric acid clouds, such as those that perpetually cover Venus, blanketed the Earth for more than a decade," Baines said. "The shielding effect of these high-altitude chemical hazes cooled the surface to near freezing across the face of the planet."

The researchers concluded that winter-like conditions lasted long enough to cool the ocean, as well as the atmosphere, even in formerly balmy tropical seas on the other side of the globe.

The scientists speculate that after the sulfuric acid cloud precipitated, dousing the Earth with acid rain, a greenhouse effect may have taken over, caused by the carbon dioxide also released by the impact. Nevertheless, it was the duration and worldwide extent of the cold period that caused such devastating effects and ended the era of the dinosaurs.

This research was funded by the NASA Exobiology Program in the Solar System Exploration Division, and by the Planetary Society in Pasadena.