The Sciences

The Year in Science: Earth 1997

Craters Without Impact

By Carl ZimmerJan 1, 1998 6:00 AM


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In the far northern reaches of the Siberian tundra is an enigmatic place called Popigai. The high cliffs along the rivers there are made of rock that shows signs of once having been completely melted, and satellite images reveal that the tundra actually forms a giant ring-shaped depression 60 miles across—which suggests that Popigai is a vast meteorite crater. Last July a team of Canadian and Russian scientists announced that they had determined when the meteorite hit: 35.7 million years ago, give or take 200,000 years. They calculated that date from the amount of radioactive argon that had decayed in the rocks since they resolidified after the impact. Remarkably, in 1995 other researchers had pinned the age of a 50-mile-wide crater now buried in the Chesapeake Bay to almost exactly the same time.

These two impacts—the two biggest in the past 65 million years, and among the biggest of all time—struck Earth with a sudden double punch that might even have been simultaneous. Impacts this size are so rare that the timing was almost certainly no coincidence; perhaps a pair of gravitationally bound asteroids happened to cross Earth’s path. Both impacts seem to have made themselves felt around the world: the Popigai impact was most likely responsible for layers of debris that were dug up in the 1980s in Italy, while the Chesapeake crater is probably responsible for bits of quartz scattered from Georgia to Barbados.

The most famous impact of all is, of course, the one that occurred 65 million years ago at the end of the Cretaceous Period, scooping out a 125-mile-wide crater off the Yucatán coast. Most researchers now agree it wiped out the dinosaurs and many other forms of life. You’d think, then, that the combined blast of Popigai and Chesapeake would have had a similarly huge effect, and around 35 million years ago there were indeed some radical changes going on. Algae, crustaceans, and mollusks were going extinct in large numbers, while primitive whales were replaced by modern groups. On land, dense forests gave way to more open habitats, and early hoofed mammals and primates were supplanted by new forms. In the 1980s, when geologists in Italy first found layers of impact debris that seemed to coincide with these extinctions, some researchers thought they had found another smoking extraterrestrial gun.

Since then, however, paleontologists have shown that there were actually two peaks of extinction, one 37 million years ago and the other 33 million years ago. Neither coincides with the Popigai-Chesapeake impacts. The one at 37 million is way too early—nothing has hit Earth yet—and the one at 33 million is 2 million years too late, says Donald Prothero, a paleontologist at Occidental College in Los Angeles. In fact, when the impacts occurred 35.7 million years ago, nothing happened. The sizes of the Popigai crater and the Chesapeake crater are both pretty impressive, says Prothero, but the animals didn’t give a damn. They walked right through it.

According to Prothero, these extinctions were most likely caused by long-term global cooling and changes in the ocean circulation brought about by continental drift. The new dates on the impacts, he argues, call into question the Cretaceous-inspired tendency to link impacts with mass extinctions in general. It couldn’t be more obvious. This is one case of major impacts that had no effect, while major extinctions were occurring. Most of the so-called correlations between impacts and extinctions have been pretty frayed. If you just step back, you can see this bandwagon to blame everything on impacts was premature.

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