Study confirms Bering land bridge flooded later than previously believed

A new study confirms the Bering land bridge that carried ancient wanderers from Asia into North America was not inundated by rising seas until about 11,000 years ago, according to a University of Colorado at Boulder researcher.

The study also shows the Bering land bridge vegetation at the time consisted primarily of tundra plants and shrubs and was unsuitable for long-term habitation by large grazing mammals, said CU-Boulder researcher Scott Elias. Because of the abundance of mammoth and other large animal fossils from adjacent areas, researchers had thought the environment of the land bridge was an arid grassland similar to the steppe region of northern Asia today, said Elias.

A paper by Elias and Susan Short of CU-Boulder's Institute of Arctic and Alpine Research, C. Hans Nelson of the U.S. Geological Survey in Menlo Park, Calif., and Hilary Birks of the Botanical Institute in Bergen, Norway, was published in the July 4 issue of Nature. The newest dates for the inundation of the land bridge corroborate radiocarbon dates calculated by Elias and colleagues in 1992 using seafloor sediment cores.

The Bering land bridge surfaced during Earth's ice ages when sea level in the Bering Sea and Chukchi Sea dropped by 300 feet or more due to a buildup of glacial ice. During the most recent ice age that ended about 10,000 years ago, the land bridge covered 580,000 square miles -- an area roughly twice the size of Texas, Elias said.

Elias and his colleagues analyzed 20 ocean-core samples obtained by U.S. Geological Survey researchers from the shallows of the Bering and Chukchi seas for the study. Taken in the 1970s and 1980s and stored at a USGS facility in Menlo Park, the cores contain layers of organic peat and silt that harbor plant, pollen and insect fossils ranging from about 4,000 years old to roughly 50,000 years old.

"The exciting thing here is that we were able to sample an ancient landscape that no longer exists," said Elias. "We don't have to guess any more as to what the Bering land bridge was like back then."

Plant seeds and stems and insect body parts were radiocarbon dated using an accelerator mass spectrometer at the University of Arizona that accelerates carbon atoms at near the speed of light. The instrument allowed researchers to count the ratio of carbon 14 atoms to carbon 12 atoms from the samples and arrive at the estimated dates for the most recent inundation of the land bridge.

"This study indicates that early people were free to move across the land bridge until about 10,500 years ago, right up to the beginning of the Holocene Period," said Elias. Several Paleo Indian sites in the Nenana Valley of central Alaska that date to about 12,000 years ago are considered the earliest reliable dates for the human occupation of North America, Elias said.

These new data confirm that the people who spread from Beringia to North America about 14,000 years ago came from a stock able to gain a livelihood from the meager resources of tundra, or perhaps from the sea coast," wrote Paul Colinvaux in an accompanying News and Views article in the July 4 issue of Nature. Colinvaux is a researcher at the Smithsonian Tropical Research Institute in Panama.

Although mammoth, horse and bison fossils unearthed in northern Alaska near the ancient land bridge had indicated it may once have supported a large number of grazing mammals, the new study shows the animals probably migrated through the area rather than living there. "This new evidence tells us the big mammals probably did not linger on the land bridge," said Elias.

Pollen samples from the peat cores dating from 14,000 years ago to 11,000 years ago match pollen from modern Alaskan tundra, which is characterized by clumps of sedges, low-lying willows, dwarf birch shrubs and grasses, he said.

The previous dates for the inundation of the land bridge -- about 14,400 years ago -- came from a 25-year-old radiocarbon sample that now appears to have been contaminated with coal deposits washed into the Bering and Chukchi seas, he said.

The study also indicates summer temperatures on the land bridge were about 8 to 11 degrees F warmer 11,000 years ago than they are today, said Elias. Since virtually no evolution has occurred in beetles over the past 700,000 years, scientists can compare today's beetles -- which are sensitive indicators of climate change -- with fossil beetles from ancient sites to reliably estimate past temperatures.

Courtesy of the University of Colorado

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