In the March 9 issue of the journal Science, a team of geochemists reports that life was indeed virtually wiped out for a period of time, but then reappeared just as abruptly only 10,000 years after the initial collision.

Further, the evidence shows that the extinctions 65 million years ago, which mark the geologic time known as the Cretaceous-Tertiary (K-T) boundary, were most likely caused by a single catastrophic impact.

"There's been a longstanding debate whether the mass extinctions at the K-T boundary were caused by a single impact or maybe a swarm of millions of comets," says lead author Sujoy Mukhopadhyay, a graduate student at Caltech. "In addition, figuring out the duration of the extinction event and how long it took life to recover has been a difficult problem."

To address both questions, Mukhopadhyay and his colleagues measured the amount of cosmic dust in the sediments of an ancient sea bed which is now exposed on land about 100 miles north of Rome.

In particular they focused on a two-centimeter-thick clay deposit that previously had been dated to about 65 million years ago. The base of this clay deposit corresponds to the date of the extinction event.

The clay deposit lies above a layer of limestone sediments, which are essentially the skeletons of microscopic sea life that settled at the bottom of the ancient sea. The limestone deposit also contains a certain percentage of clay particles, which result from erosion on the continents.

Finally, mixed in the sediments is extraterrestrial dust that landed in Earth's oceans and then settled out. This dust carries a high concentration of helium-3 (3He), a rare isotope of helium that is depleted on Earth but highly enriched in cosmic matter.

The lower limestone layer abruptly ends at roughly 65 million years, since the organisms in the ocean were suddenly wiped out by the impact event.

Thus, the layer immediately above the limestone contains nothing but the clay deposits and extraterrestrial dust that continued to settle at the bottom of the ancient sea.

Immediately above the two-centimeter clay deposit is another layer of limestone deposits from microorganisms of the sea that eventually rebounded after the catastrophe.

In this study, the researchers measured the amount of 3He in the sediments to learn about the K-T extinction. They reasoned that a gigantic impact would not change the amount of 3He in the clay deposit.

This is because large impacting bodies are mostly vaporized upon impact and release all their helium into the atmosphere. Because helium is a light element, it is not bound to Earth and tends to drift away into space. Therefore, even if a huge amount were brought to Earth by a large impact, the 3He would soon disappear and not show up in the sedimentary layers.

In contrast, 3He brought to Earth by extraterrestrial dust tends to stay trapped in the dust and not be lost to space, says Kenneth Farley, professor of geochemistry at Caltech and coauthor of the paper. So 3He found in the limestone and the clay deposits came from space in the form of dust.

Based on the 3He record obtained from the limestones, the researchers eliminated the possibility that a string of comets had caused the K-T extinctions. Comets are inherently dusty, so a string of them hitting Earth would have brought along a huge amount of new dust, thereby increasing the amount of 3He in the lower limestone deposit.

But the Italian sediment showed a steady concentration of 3He until the time of the impact, eliminating the possibility of a comet swarm. In fact, the researchers found no evidence for periodic comet showers, which have been suggested as the cause of mass extinction events on Earth.

Mukhopadhyay and his colleagues reason that because the "rain-rate" of the extraterrestrial dust from space did not change across the K-T boundary, the 3He concentration in the clay is proportional to the total depositional time of the clay. "It's been difficult to measure the time it took for this two-centimeter clay layer to be deposited," says Farley.

The researchers conclude that the two-centimeter clay layer was deposited in approximately 10,000 years. Then, very quickly, the tiny creatures that create limestone deposits reemerged and again began leaving their corpses on the ocean bed. The implication is that life can get started again very quickly, Farley says.

Thus the study answers two major questions about the event that led to the extinction of the dinosaurs, says Mukhopadhyay. In addition to Mukhopadhyay and Farley, the paper is also authored by Alessandro Montanari of the Geological Observatory in Apiro, Italy.

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