The Bering Strait and The Land Bridge

The Bering Strait links the Arctic Ocean with the Bering Sea and separates the continents of Asia and North America at their closest point. The strait averages 98 to 164 ft (30 to 50 m) in depth and at its narrowest is about 53 mi (85 km) wide. There are numerous islands in the strait, including the two Diomede Islands (about 6 sq mi [16 sq km]), and to the south of the strait lies St. Lawrence Island (about 1,000 sq mi). The U.S.-Russian boundary extends through the strait.

Some of the Bering Sea water passes through the strait into the Arctic Ocean, but most of it returns to the Pacific. In winter the region is subject to severe storms and the sea is covered by ice fields averaging 4 to 5 ft thick. In mid-summer drift ice remains in the Bering Strait. The strait is named after Vitus Bering, a Danish captain, who sailed into the strait in 1728.

Some 20,000 to 25,000 years ago, towards the end of the Pleistocene (Ice Ages), monumental continental glaciers forming in the earth's northern hemisphere (especially in Canada and Greenland) locked up so much water that the world's ocean levels were more than 300 feet lower than today. In the region of the Bering Strait, this drop in sea levels exposed a massive unglaciated tract known the Bering Land Bridge. This bridge joined northeast Asia to modern Alaska and formed part of a much larger province called Beringa. When fully exposed Beringia was over 1000 miles wide. Many scientists presume that it supported a tundra vegetation where Arctic fauna, particularly the caribou, flourished.

For most of the 20th century, archaeologists and anthropologists assumed that the forbears of the Native Americans moved across this land bridge following the game animals. However, towards the end of the 20th century, additional routes by which human beings first reached the Americas were being proposed: following the southern coastline of Beringia and Alaska, then southward along the Pacific coast all the way to the southernmost tip of South America, exploiting the rich marine fauna (shellfish, fish, sea mammals, seaweed) along the way. This coastal route could have been by foot along the beach, or by boat, drifting eastward then southward with the Japanese current. It's also possible that the first humans into the Americas exploited both coastal and interior resources.

Below is an article by Don Alan Hall which appeared in the MAMMOTH TRUMPET VOL 12, NO. 2 (1997) which discusses new findings concerning when the Bering Land Bridge was open, what the environmental conditions on the Bridge were like, and what that meant in terms of suitability for human subsistence and use.


Bering Land Bridge Was Open Until After 11,000 Years Ago - Scrub Tundra Grew in Lowland Beringia, Not 'Mammoth Steppe'

Just a few decades ago, Beringia, the land linking North America and Asia during glacial times, was a hypothetical concept. But as evidence mounted of biological connections between Siberia and Alaska, and knowledge of changing sea levels came to light, the notion of a land bridge allowing free passage of animals and humans became universally accepted. Until recently, the connection was thought to have been severed by rising sea levels about 14,000 years ago.

Now, thanks to research by Scott A. Elias and his colleagues, we know the link remained in place until sometime after 11,000 years ago, probably being flooded about 10,500 years B.P. The event cut the Americas off from Eurasia for the first time in many thousands of years. When the level of the Pacific Ocean rose to within about 40 meters of its present level, it spilled across into the Arctic Ocean.

Dr. Elias, of the University of Colorado's Institute of Arctic and Alpine Research, is able to tell us much about the lowland that once constituted the land bridge because of careful research employing cores previously collected by the U.S. Geological Survey from the floor of the Chukchi and Bering seas. Not only did the work reveal that the land bridge was open to passage long after scientists thought it had been closed, the research provided new insights into the environment before the continents were separated. Elias found that land bridge lowland had been covered with shrub tundra that would not have provided much food for grazing animals; he found no evidence of the steppe tundra that some believe must have been plentiful to nourish the great Pleistocene bison and mammoth herds with its grasses.

However, it is clear that people and animals could have freely crossed the land bridge until after 11,000 years B.P. Elias can describe the paleoenvironment of the land bridge because of research he and his colleagues have done on pollen and the remains of insects and plants. These environmental markers became preserved in shallow pools in the Beringian lowland, and when melting glaciers raised sea levels, the fossils were protected from the flood.

The research had its origins more than a decade ago when the U.S. Geological Survey sailed the U.S. research ship Discovery in the Bering and Chukchi seas to gather geologic and geophysical information. The project included the taking of dozens cores from the sea floor. These cores, about 10 cm in diameter, ranged up to nine meters in length. After the voyage, the USGS stored the cores, sealed in plastic tubes, under refrigeration at Palo Alto and Redwood City, Calif.

Elias began his research by looking through the core logs from USGS cruises in the Bering and Chukchi seas. The goal, of course, was to find cores that would yield information about the most-recent terrestrial deposits-- layers that could be expected to contain plant and insect remains.

"We picked cores that had organic lenses near their tops," Elias said. "We knew that these organic layers would represent the land bridge." The topmost core material represented Holocene marine sand, deposited after the Pacific Ocean had flooded northward. Below that were sediments from the land bridge. Below the land-bridge sediments the cores revealed an as-yet-unexplained gap in the geologic record--the next sediments are Cretaceous in age.

Elias and his colleagues then went to the USGS storage rooms to look up their chosen cores. It wasn't quite like looking up a reference in the library, because about half of the cores were stored in a refrigerated trailer near the bay at Redwood City. During the Loma Prieta earthquake a few years previously, all the cores in the trailer had fallen from their shelves and they remained in a jumble on the floor. "We had to unload literally the entire contents of the trailer--hundreds of cores--out into the parking lot in order to find the ones we were interested in."

They dug the organic lenses out of the cores and took them back to Boulder to extract the fossils. "The organic deposits were easy to differentiate because they are nearly black, whereas the rest of the sediments are nearly sterile marine sands and clays, tan to gray in color."

Accurately dating the deposits was a high priority of the research team. Previous land-bridge closure dates had been obtained from radiocarbon analysis of bulk samples, and there was a suspicion of contamination by coal. Elias and his colleagues wet-screened the samples to remove any such material, and chose individual macrofossils for radiocarbon dating by accelerator mass spectrometry. Elias found that samples of cores taken in the Chukchi Sea could provide the most accurate dating of the submergence of the land bridge.

AMS dates from screened peat from cores taken north of Alaska's Cape Lisburne were 11,330 +/- 70 years B.P. (Beta 43952) and 11,000 +/- 60 (Beta 43953). Analysis of the cores, then, indicated that the sea covered the land bridge after 11,000 B.P.; probably by 10,500 B.P.

Elias' insect analysis is done the same way archaeologists do faunal analysis of larger animals. "I compare my fossils with modern specimens in museum collections." There are two principal differences between analyzing remains of beetles and remains of mammals--evolution and scale.

"As far as we can tell, there has been no evolution of species and no extinction of species during most, if not all, of the Quaternary," he says. "So 999 out of 1,000 insect fossil specimens have living counterparts in their own species with which to compare ecological requirements and modern distribution patterns."

Elias finds insects' small size an advantage. "Insects are far easier to work with than mammals. I have more than 10,000 fossil specimens and 5,000 modern specimens in cabinets in my office, whereas a fossil- and modern-mammal collection of this number of specimens might take a gymnasium to house. I use a low-power stereo-binocular microscope to identify my specimens, but for picture taking, I prefer to use the scanning electron microscope because the images of three-dimensional specimens are sharper."

The research went back in time far beyond the final submergence of the land bridge. Elias says that fossil samples fell into three age classes--more than 40,000 years old, 20,000-14,000 years old, and 14,000-11,000 years old. Generally, the oldest period represented an environment of birch-heath-grass tundra with a few shrubs, while the middle period was tundra with fewer heaths. The latest period again was dominated by birch-heath-grass tundra--moderately moist--and there were small ponds choked with aquatic plants. By 12,000 years B.P. summer temperatures were as warm as they are now and by 11,000 B.P. summers were warmer than Alaska's north slope now experiences.

Elias and paleobotanist Susan K. Short (INSTARR) and Hilary H. Birks (University of Bergen Norway) found no evidence for steppe-tundra vegetation at any period. R. Dale Guthrie of the University of Alaska's Institute of Arctic Biology has hypothesized that Beringia was a vast steppe covered with grasses and sagebrush. Dr. Guthrie, an authority on large Pleistocene mammals, argues that Alaska's plentiful fossil record means that Beringia must have offered highly productive grazing to have allowed bison, mammoths, and other grazers to reach such giant size. Elias and his colleagues, however, have not found evidence of extensive expanses of that steppe habitat, nor had earlier work by palynologists. Is Guthrie wrong?

"I think that we could easily both be right," said Elias. "The land bridge may have been a narrow waist of mesic tundra surrounded by steppe-tundra landscapes on either side." He notes that the land bridge probably could not have sustained many big grass eaters. "I don't think that there was much for megafaunal mammals to graze on out there on the land bridge, but they may have migrated across the narrowest part of the Bering Strait region in just a few days."

"Based on our evidence, grazing animals and their human hunters probably spent little time on the land bridge. Shrub tundra offers too few food resources for the animals."

As far as the timing of human migration across the continental link, Elias doesn't believe evidence that the land bridge was usable until after 11,000 years B.P. has anything to say about when the first people reached North America. "Certainly the new dates mean that the land bridge was available to people until Clovis time, but the land bridge had been open for many thousands of years before then, as well. Furthermore, you can still walk from Siberia to Alaska across the Bering Strait in winter." Because the water is so shallow, he says, it often freezes completely; Inuit people do travel between Alaska and Siberia to visit friends and relatives.

Elias would like to broaden his research by examining deposits from other areas. "First I'd like to get cores from the southern part of the Bering shelf--the south sector of the land bridge." Deposits from cores taken in waters south of the Bering Strait yielded older dates, possibly because the stormier sea caused erosion of the most recent terrestrial deposits. "Second, I'd like to get my hands on cores from the Russian side of the Bering and Chukchi seas."

He says Russian scientists took cores there about the same time the USGS was coring on the Alaskan side. "I have recently sent out an e-mail to Russian colleagues in Moscow, trying to find out where these cores are being kept and whether I could get permission to go sample from them. I hope that the cores haven't been lost or thrown out, but with the near-collapse of the Russian science infrastructure, I am worried about the fate of these cores."

-- Don Alan Hall

Copyright © 1997 Mammoth Trumpet


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