Sulfur-Rich Impact Site May Have Lead to Dinosaur's Demise

[Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>]

>From the "JPL Universe"
December 16, 1994

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.
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