Beringia

thumb|upright=1.6|alt=Image of the Bering land bridge being inundated with rising sea level across time|Beringia sea levels (blues) and land elevations (browns) measured in metres from 21,000 years ago to present

Beringia is a prehistoric geographical region, defined as the land and maritime area bounded on the west by the Lena River in Russia; on the east by the Mackenzie River in Canada; on the north by 72° north latitude in the Chukchi Sea; and on the south by the tip of the Kamchatka Peninsula. It includes the Chukchi Sea, the Bering Sea, the Bering Strait, the Chukchi and Kamchatka peninsulas in Russia as well as Alaska in the United States and Yukon in Canada.

The area includes land lying on the North American Plate and Siberian land east of the Chersky Range. At various times, it formed a land bridge referred to as the Bering land bridge or the Bering Strait land bridge that was up to wide at its greatest extent and which covered an area as large as British Columbia and Alberta together, totaling about , allowing biological dispersal to occur between Asia and North America. Today, the only land that is visible from the central part of the Bering land bridge are the Diomede Islands, the Pribilof Islands of St. Paul and St. George, St. Lawrence Island, St. Matthew Island, and King Island. This would have occurred as the American glaciers blocking the way southward melted but before the bridge was covered by the sea about 11,000 YBP. In 1937, Eric Hultén proposed that around the Aleutians and the Bering Strait region were tundra plants that had originally dispersed from a now-submerged plain between Alaska and Chukotka, which he named Beringia after Vitus Bering who had sailed into the strait in 1728. The distribution of plants in the genera Erythranthe and Pinus are good examples of this, as very similar genera members are found in Asia and the Americas.

American arctic geologist David Hopkins redefined Beringia to include portions of Alaska and Northeast Asia. Beringia was later regarded as extending from the Verkhoyansk Mountains in the west to the Mackenzie River in the east. Today, the average water depth of the Bering Strait is ; therefore the land bridge opened when the sea level was more than below the current level. A reconstruction of the sea level history of the region indicated that a seaway existed from  YBP, a land bridge from  YBP, an intermittent connection from  YBP, and a land bridge from  YBP, followed by a Holocene sea level rise that reopened the strait. Post-glacial rebound has continued to raise some sections of the coast.

thumb|The [[Last Glacial Period caused a much lower global sea level|upright=1.3]]

During the last glacial period, enough of the Earth's water became frozen in the great ice sheets covering North America and Europe to cause a drop in sea levels. For thousands of years the sea floors of many interglacial shallow seas were exposed, including those of the Bering Strait, the Chukchi Sea to the north, and the Bering Sea to the south.

Refugium

thumb|upright=1.35|Beringia precipitation 22,000 years ago

The last glacial period, commonly (and inaccurately) referred to as the "Ice Age", spanned 125,000

In the Late Pleistocene, Beringia was a mosaic of biological communities. Commencing from  YBP (marine isotope stage [MIS] 3), steppe–tundra vegetation dominated large parts of Beringia with a rich diversity of grasses and herbs. There were patches of shrub tundra with isolated refugia of larch (Larix) and spruce (Picea) forests with birch (Betula) and alder (Alnus) trees. It has been proposed that the largest and most diverse megafaunal community residing in Beringia at this time could only have been sustained in a highly diverse and productive environment.

thumb|upright=1.3|Duration of snow cover in days, East Beringia, 20000 years ago. Chelsa Trace 21ka variable bio/scd 200.

Analysis at Chukotka on the Siberian edge of the land bridge indicated that from  YBP (MIS 3 to MIS 2) the environment was wetter and colder than the steppe–tundra to the east and west, with warming in parts of Beringia from  YBP. These changes provided the most likely explanation for mammal migrations after  YBP, as the warming provided increased forage for browsers and mixed feeders. At the beginning of the Holocene, some mesic habitat-adapted species left the refugium and spread westward into what had become tundra-vegetated northern Asia and eastward into northern North America. The Yukon corridor opened between the receding ice sheets  YBP, and this once again allowed gene flow between Eurasia and continental North America until the land bridge was finally closed by rising sea levels  YBP. During the Holocene, many mesic-adapted species left the refugium and spread eastward and westward, while at the same time the forest-adapted species spread with the forests up from the south. The arid-adapted species were reduced to minor habitats or became extinct.

Grey wolves suffered a species-wide population bottleneck (reduction) approximately 25,000 YBP during the Last Glacial Maximum. This was followed by a single population of modern wolves expanding out of their Beringia refuge to repopulate the wolf's former range, replacing the remaining Pleistocene wolf populations across Eurasia and North America.

Beringian Gap

The existence of fauna endemic to the respective Siberian and North American portions of Beringia has led to the 'Beringian Gap' hypothesis, wherein an unconfirmed geographic factor blocked migration across the land bridge when it emerged. Beringia did not block the movement of most dry steppe-adapted large species such as saiga antelope, woolly mammoth, and caballid horses. However, ground sloth environmental DNA has potentially been recovered from Siberia.

Human habitation and migration

Around 3,000 years ago, the progenitors of the Yupik peoples settled along both sides of the straits. In 2012, the governments of Russia and the United States announced a plan to formally establish "a transboundary area of shared Beringian heritage.” Among other things this agreement would establish close ties between the Bering Land Bridge National Preserve and the Cape Krusenstern National Monument in the United States and Beringia National Park in Russia.

Previous connections

Biogeographical evidence demonstrates previous connections between North America and Asia. Saurolophus was found in both Mongolia and western North America. Relatives of Troodon, Triceratops, and Tyrannosaurus rex all came from Asia.

The earliest Canis lupus specimen was a fossil tooth discovered at Old Crow, Yukon, Canada. The specimen was found in sediment dated 1 million YBP, Twenty million years ago, evidence in North America shows the last natural interchange of mammalian species. Some, like the ancient saber-toothed cats, have a recurring geographical range: Europe, Africa, Asia, and North America.

File:land_bridge_world_map.svg|Notable past (purple) and current (orange) land bridges on a bathymetric equirectangular projection centered on

</gallery>

References

External links

CBC News: New map of Beringia 'opens your imagination' to what landscape looked like 18,000 years ago
Shared Beringian Heritage Program
International National Park in the Bering Strait
Bering Land Bridge National Preserve
D.K. Jordan, "Prehistoric Beringia";
Paleoenvironmental atlas of Beringia: includes animation showing the gradual disappearance of the Bering land bridge
Yukon Beringia Interpretive Centre
Paleoenvironments and Glaciation in Beringia
Study suggests 20000 year hiatus in Beringia
The Fertile Shore