During the Pleistocene, a series of ice ages, or glaciations, affected most of the high-latitude regions in the Northern Hemisphere. When these glaciations occurred, enormous quantities of water became frozen into continental-sized ice sheets, causing the global sea level to drop significantly. For instance, during the last glaciation, the global sea level dropped by about 120 m (almost 400 ft). The continental shelf regions of the Bering and Chukchi seas between Siberia and Alaska are relatively shallow, only about 30-110 m (100-360 ft) below modern sea level. When the sea level dropped during glacial periods, the shelf regions between Siberia and Alaska became dry land, forming a land bridge between western Alaska and eastern Siberia. This land bridge was not a narrow isthmus between the two continents: from north to south, the land bridge extended more than 1000 km (600 miles).
The lowlands of Beringia, however, were not buried by glacial ice in the Pleistocene. This is quite remarkable, because nearly all other high-latitude lands were repeatedly buried by glacial ice during the Pleistocene. Lowland Beringia remained unglaciated because the land bridge cut off circulation between the North Pacific and Arctic oceans, which, in turn, greatly diminished the landward flow of relatively warm, moist air masses from the North Pacific. So Pleistocene ice sheets did not form in Beringia because it was too dry. There were Pleistocene glaciers in Beringia, but only in high mountains. Because of this, Beringia formed a unique refuge for cold-adapted plants and animals.
During each Pleistocene glaciation, temperatures dropped in Beringia, aridity increased, and conifer forests gave way to steppe-tundra (see Polar Steppe). This vegetation covered the Beringian lowlands, and was a mixture of plants that today are found in steppe (dry grassland) regions, such as the steppes of Central Asia, and tundra plants that grow today in Arctic regions. In spite of being in such high latitudes, the steppe-tundra was a highly productive ecosystem. Unlike the modern-day Arctic, where soils are often very wet and the active layer over permafrost is shallow, the soils of the Pleistocene steppe-tundra were drier, and thawed more deeply in the summer. Steppetundra soils yielded their nutrients more readily to plants. The combination of steppe and tundra plant species formed a rich mosaic of vegetation, supporting an abundant, diverse mammal fauna. The only modern ecosystem that has as large a variety of grassland animals is the African Savannah. The large mammal fauna of Beringia included woolly mammoth, woolly
rhinoceros, saiga antelope, giant sloths, one or more species of Pleistocene horses, large-horned bison, camels, and two species of Pleistocene muskox. Predators included Pleistocene lion, saber-toothed and scimitar cats, and the giant short-faced bear. These species all became extinct at the end of the Pleistocene, but they shared the land with species that are still living, such as moose, modern muskox, Dall's sheep, and caribou. Human beings were the last large mammal species to enter Eastern Beringia (Alaska and the Yukon Territory) before the land bridge was flooded by sea water, about 12,000 years ago.
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