May 6, 2009 11:05 AM
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Princeton geoscientist Gerta Keller has new evidence to support her alternative theory that volcanoes, not meteorites, wiped out the dinosaurs. Indeed, the evidence is so compelling that we might be dropping the whole "alternative" part.
Keller is one of several co-authors on a new paper in The Journal of the Geological Society of London that lays out the startling new evidence. Previous studies of rock formations in India, Mexico, and the United States had first prompted Keller to conclude there was a discernible period between the massive meteorite impact that has been advanced as the killer of the dinosaurs, and their actual final extinction.
This extinction, which is known to have occurred 65 million years ago, marked one of the most drastic biological upheavals this planet has ever seen. Signaling the end of the age of reptiles (the Mesozoic Era) and the beginning of the age of mammals (the Cenozoic Era), this boundary between the Cretaceous (abbreviated "K") and Tertiary ("T") periods is known in scientific literature as the K-T Boundary. The K-T Boundary can be observed geologically through the vastly different plant and animal species found on either side of the divide.
Another potential sign of the K-T Boundary is a thin layer of clay rich in iridium, an element found far more often in asteroids or comets than on Earth. This connection to space strengthened the theory, put forth in the eighties, that an object from space was responsible for the mass extinction of 65 million years ago, even singling out the Chicxulub Crater in the northern Yucatan peninsula of Mexico.
But the new paper provides a wealth of new biotic evidence, or fossil remains of animals and plants, that throws a monkey wrench in this theory by demonstrating there was extensive continuity of species at periods before and after the established time of the asteroid impact. Indeed, one key area of study was Chicxulub itself, where Keller and her students found Cretaceous period one-celled organisms in layers spatially above and thus chronologically after the Chicxulub impact. Using the remains of these populations as a guide, Keller estimated Cretaceous organisms endured for some 300,000 years after the Chicxulub impact.
The picture is complicated by the fact that there were several traumatic geological events around 65 million years ago in the Mexico and Texas areas alone:
Over the years, Keller's group has amassed evidence for as many as four major events widely separated in time in that area of Mexico as well as in Texas. The oldest of the four events is the Chicxulub impact, seen by the fallout of glass beads. The second is about 150,000 years later and seen in a layer of sandstone with Chicxulub impact glass beads that were transported from shallow shore areas into deep waters during a sea level fall and was commonly interpreted as a tsunami generated by the Chicxulub impact. About 100,000 to 150,000 years later, the third event struck at the time of the K-T boundary with its iridium layer and mass extinction. This event may represent a second large impact or massive volcanism. The fourth event is possibly a smaller impact as evidenced by another iridium layer about 100,000 years after the mass extinction.
Proponents of the Chicxulub impact theory have argued the apparent 300,000 years separating the impact and the extinction at the K-T Boundary are a trick of geological disturbance, a result of tsunamis and earthquakes caused by the impact reshuffling sediments and making the geological record difficult to read. Keller disputes this, saying her latest findings at the El Peñon in Mexico show no evidence of geological disturbance.
What her findings do show, however, is unprecedented species continuity from before and after the Chicxulub impact:
Also at El Peñon, the researchers found 52 species present in sediments below the impact layer and counted all 52 still present in the layer above it, indicating that the impact has not had the devasting biotic effect on species diversity as has been suggested. "Not a single species went extinct as a result of the Chicxulub impact," Keller said.In contrast, she noted, at a nearby site known as La Sierrita where the K-T boundary, iridium anomaly and mass extinction are recorded, 31 out of 44 species disappeared from the fossil record at the K-T boundary.
So if an asteroid didn't kill the dinosaurs, what did? Keller suspects it might well be volcanoes:
Keller suggests that the massive volcanic eruptions at the Deccan Traps in India may be responsible for the extinction, releasing massive amounts of dust and gases that could have blocked sunlight, altered climate and caused acid rain. The fact that the Chicxulub impact seems to have had no effect on biota, she said, despite its 6-mile-in-diameter size, indicates that even large asteroid impacts may not be as deadly as imagined.
Keller readily admits there will never be consensus on the question of what caused the K-T extinction event, but she considers this an impractical benchmark anyway and not something worth waiting for as she prepares to investigate the Deccan Traps. Instead, she will simply continue to build on the evidence she has already found, hoping to put together an ever more convincing picture of just what happened 65 million years ago.
Send an email to Alasdair Wilkins, the author of this post, at alasdair@io9.com.
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