updated 10/11/2006 7:14:11 PM ET 2006-10-11T23:14:11

A land bridge between Alaska and Siberia flooded to make the Bering Strait 11,000 years ago, more than 1,000 years earlier than previously thought, U.S. researchers reported on Wednesday.

This would have closed off human migration by foot across the bridge 1,000 years earlier, too, the researchers said.

A team at the Woods Hole Oceanographic Institution in Massachusetts, the Scripps Institution of Oceanography in San Diego and the University of Massachusetts found places on the ocean floor where sediment deposits were deep enough to act as a kind of geologic clock.

Most sediment cores collected from the floor of the Arctic Ocean have been taken from places where sediment has accumulated only about a centimeter, or less than half an inch — not enough to calculate periods of just 1,000 years.

But writing in Geology magazine, Lloyd Keigwin of Woods Hole and colleagues said they examined samples from new core sites north and west of Alaska in the Chukchi Sea.

This area covers part of the continental shelf exposed when sea level fell during the last Ice Age, about 20,000 years ago.

"Although we have only a few cores, this is the first evidence of flooding of the Chukchi Sea by 11,000 years ago, at least 1,000 years before previously thought," Keigwin said in a statement.

"The new data are also consistent with data from other recent studies, and show potential for developing ocean and climate histories of this region."

The researchers sampled the cores to identify skeletons of animals, known as foraminifera, that can be traced to specific water and atmospheric temperatures. The samples were also radiocarbon dated.

For decades most scientists believed that the first people to settle in the Americas were the Clovis people, and that they came via the Bering land bridge between 11,000 and 12,000 years ago.

But recent evidence has suggested that humans came much earlier.

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