Cretaceous extinctions: the strikes add up.

Ever since geologists identified a huge crater buried beneath the Yucatan Peninsula two years ago, growing numbers of researchers have accepted the idea that a huge asteroid crashed there 65 million years ago, killing off a large fraction of Earth's existing life, including the last surviving dinosaurs. New evidence indicates a second object splashed into the Pacific Ocean at nearly the same time, magnifying the catastrophe that marks the boundary between Earth's Cretaceous (K) and Tertiary (T) periods.

As if two strikes were not enough, a separate research report suggests that the impacts sparked planet-wide blazes that burned perhaps a quarter of all vegetation on the continents.

Eric Robin and his colleagues from the Centre des Faibles Radioactivites in Gifsur-Yvette, France, concluded that an asteroid hit the Pacific after they analyzed tiny particles found in seafloor sediments collected in the northwestern part of that ocean. Dating from the time of the K-T boundary, the millimeter-wide grains sorted into two types: rounded "spherules" similar to those found elsewhere around the world, and unusual irregularly shaped fragments.

Robin thinks both types came from the same source, but the irregular grains are particularly important because they appear to be ancient versions of the minimeteorites that continually rain down on Earth's surface. The spherules and irregular fragments also contain high concentrations of iridium, an element rare in Earth's crust but concentrated within meteorites and comets, Robin and his colleagues report in the June 17 NATURE.

Because the K-T boundary sediments in the Pacific contain vast numbers of these particles, the researchers say the deposit could not have formed from the normally slow buildup of micrometeorites. They suggest the ancient grains must have formed when a large object hit Earth at the K-T time, breaking up into a spray of molten and partly molten drops that cooled as they fell into the ocean.

The particles could not have come from the Yucatan impact, however, because the irregular fragments have a delicate structure that could not have survived the force required to loft them from the Gulf of Mexico to the far Pacific, says Robin. He suggests the fragments and spherules came from a closer crash in deep water, which would have allowed meteorite material to survive the impact. A strike in the Pacific would also explain why K-T spherules found in that ocean have a distinctive composition.

"In the Pacific, in the Caribbean, and in Europe there are [particles] with very different compositions. So you have to explain all these particles. With a single impact, it's rather difficult. With multiple objects, it's easy," Robin says.

Judging from the distribution of spherules in the Pacific, he and his colleagues estimate the impacting body had a diameter of 2 kilometers. Researchers have estimated that the body responsible for the main K-T impact must have measured at least 10 kilometers across, which is large enough to have carved out the 180kilometer-wide Yucatan structure, named the Chicxulub crater.

A 35-kilometer-wide crater in Iowa also dates from the time of the K-T boundary, suggesting an extraterrestrial object hit there as well. Earlier this year, researchers proposed that a series of comets could have caused the multiple impacts (SN: 4/3/93, p.212).

Jan Smit of the Free University of Amsterdam, who has studied spherules from the K-T time, says Robins group "has something good by the tail, but it's too early to say whether they are right or wrong." Contrary to Robins assertions, it may be that such particles formed during the Chicxulub crash could have survived the trip to the Pacific, Smit says.

Researchers have theorized that a large impact would have spawned a range of catastrophic consequences, including prolonged darkness, global warming, and extremely acidic rainfall. Some evidence collected in recent years suggests global wildfires also followed the impact. In the June GEOLOGY, Linda C. Ivany and Ross J. Salawitch of Harvard University report finding hints of such massive conflagrations recorded in seafloor sediments from around the world.

The two researchers investigated the shells of one-celled organisms that contain information about the relative abundance of two carbon isotopes -- one light and one heavy -- in the ocean. Scientists have long known that the balance of the two isotopes shifted dramatically at the K-T boundary time, indicating that nearly all plankton died out in the top layer of the ocean. But lvany and Salawitch say that event can account for only part of the isotope shift. To explain the rest, they propose that wildfires burned 25 percent of the vegetation on land, filling the air with isotopically light carbon that dissolved into the ocean.