Cretaceous Extinctions: The Strikes Add Up

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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. …