Fatal impact. Geologists now think they know why most denizens of streams and lakes survived the Chicxulub impact (shown here in an artist's rendition).

Why We Still Have Turtles

The meteorite that plowed into Earth 65 million years ago created all sorts of havoc: It threw up dust that darkened the planet for months, loaded the atmosphere with carbon dioxide, obliterated the protective ozone layer, sparked massive wildfires, triggered tsunamis, and drenched the lands with acid rain. More than two-thirds of all living species--including the dinosaurs--succumbed to the catastrophe. Mysteriously, freshwater species escaped unscathed, even though acid rain should have hit them the hardest. Now scientists may have found an answer to this riddle.

The 15-kilometer-wide meteor probably contained a substantial amount of sulfur minerals and happened to hit a sulfur-rich part of the Yucatan Peninsula, heaving tons of sulfur into the atmosphere which reacted with water to produce acid rain. In modern environments, animals living in rivers and lakes are often the most affected by this type of pollution. But 90% of freshwater species, such as crocodiles and turtles, somehow survived.

How did they manage? It's possible that the impact didn't generate enough acid rain to kill the freshwater animals, but several lines of evidence that severe acid rain did fall on land argue against this explanation. Now, geochemists Teruyuki Maruoka and Christian Koeberl of the University of Vienna in Austria have come up with another idea, which they outline in the June issue of Geology. They believe the crocodiles owe their survival to the same impact event that doomed T. rex. The sediments on the Yucatan were not only sulfur-rich, they were also full of calcium and silica. According to Maruoka and Koeberl, the calcium and silica were vaporized by the impact and then crystallized as larnite, a substance that's extremely efficient at neutralizing acid in water. Their calculations suggest that the larnite could have consumed enough of the acid in freshwater ecosystems after the impact to restore the water to safe levels within a couple of hours of the impact.

The larnite would have been consumed in the neutralization process leaving little or no trace behind at the Chicxulub crater on the Yucatan, but larnite has been reported at the Haughton crater in Canada. David Kring of the University of Arizona in Tucson says the next step is to hunt for larnite that might have been preserved in impact sediments that settled on the ocean floor. "This work is important because this is the kind of impact event that can happen again," says Kring. Scientists estimate that impacts large enough to cause severe environmental damage happen once or twice per million years. "Understanding what happened 65 million years ago will help us better assess the hazards of future impacts."

Related sites
Information on the Chixculub impact event
Teruyuki Maruoka's research
Institute for Geochemistry at the University of Vienna
NASA fact sheet on impact hazards