The area includes land lying on theNorth American Plate and Siberian land east of theChersky Range. At various times, it formed aland bridge referred to as theBering land bridge or theBering Strait land bridge that was up to 1,000 km (620 mi) wide at its greatest extent and which covered an area as large asBritish Columbia andAlberta together,[2] totaling about 1.6 million km2 (620,000 sq mi), 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 theDiomede Islands, thePribilof Islands of St. Paul and St. George,St. Lawrence Island,St. Matthew Island, andKing Island.[1]
It is believed that a small human population of at most a few thousand arrived in Beringia from eastern Siberia during theLast Glacial Maximum before expanding into thesettlement of the Americas sometime before 23,000 and 21,000 years before present (YBP).[3] This would have occurred as the American glaciers blocking the way southward melted[4][5][6][7][8] but before the bridge was covered by the sea about 11,000 YBP.[9][10]
The remains ofLate Pleistocene mammals that had been discovered on theAleutians and islands in theBering Sea at the close of the 19th century indicated that a past land connection might lie beneath the shallow waters betweenAlaska andChukotka. The underlying mechanism was first thought to beplate tectonics, but by 1930 changes in the ice mass balance which led to global sea-level fluctuations were viewed as the cause of the Bering land bridge.[11][12] In 1937,Eric Hultén proposed that around the Aleutians and theBering Strait region were tundra plants that had originally dispersed from a now-submerged plain between Alaska and Chukotka, which he named Beringia afterVitus Bering who had sailed into the strait in 1728.[13][12][14] The distribution of plants in the generaErythranthe andPinus are good examples of this, as very similar genera members are found in Asia and the Americas.[15][16]
During thelast glacial period, enough of the Earth's water became frozen in the greatice sheets coveringNorth America andEurope to cause a drop in sea levels. For thousands of years the sea floors of manyinterglacial shallow seas were exposed, including those of the Bering Strait, the Chukchi Sea to the north, and the Bering Sea to the south.
The last glacial period, commonly (and inaccurately) referred to as the "Ice Age", spanned 125,000[23]–14,500YBP[24] and was the most recentglacial period within thecurrent ice age, which occurred during the last years of the Pleistocene epoch.[23] The Ice Age reached its peak during theLast Glacial Maximum, when ice sheets began advancing from 33,000YBP and reached their maximum limits 26,500YBP. Deglaciation commenced in the Northern Hemisphere approximately 19,000YBP and in Antarctica approximately 14,500 yearsYBP, and the bridge was finally inundated around 11,000 YBP.[10] These dates are consistent with evidence that glacial meltwater was the primary source for an abrupt rise in sea level 14,500YBP.[24] The fossil evidence from many continents points to theextinction of large animals near the end of the last glaciation.[25]
During the Ice Age a vast, cold and drymammoth steppe stretched from thearctic islands southwards to China, and from theIberian Peninsula eastwards across Eurasia and over the Bering land bridge into Alaska and the Yukon where it was blocked by theWisconsin glaciation. Therefore, the flora and fauna of Beringia were more related to those of Eurasia rather than North America. Beringia received more moisture and intermittent maritime cloud cover from the north Pacific Ocean than the rest of the mammoth steppe, including the dry environments on either side of it. This moisture supported a shrub-tundra habitat that provided anecological refugium for plants and animals.[26][27] In eastern Beringia 35,000 YBP, the northern arctic areas experienced temperatures 1.5 °C (2.7 °F) degrees warmer than today, but the southern sub-arctic regions were 2 °C (4 °F) degrees cooler. During the Last Glacial Maximum 22,000 YBP the average summer temperature was 3–5 °C (5–9 °F) degrees cooler than today, with variations of 2.9 °C (5.2 °F) degrees cooler on theSeward Peninsula to 7.5 °C (13.5 °F) cooler in the Yukon.[28] In the driest and coldest periods of the Late Pleistocene—and possibly during the entire Pleistocene—moisture occurred along a north–south gradient with the south receiving the most cloud cover and moisture due to the air-flow from the North Pacific.[27] During the ice ages, Beringia, like most ofSiberia and all ofNorth andNortheast China, was notglaciated becausesnowfall was very light.[29]
In the Late Pleistocene, Beringia was a mosaic of biological communities.[30][26][31] Commencing fromc. 57,000 YBP (marine isotope stage [MIS] 3), steppe–tundra vegetation dominated large parts of Beringia with a rich diversity of grasses and herbs.[30][26][32] There were patches of shrub tundra with isolated refugia oflarch (Larix) andspruce (Picea) forests withbirch (Betula) andalder (Alnus) trees.[30][31][32][33] It has been proposed that the largest and most diversemegafaunal community residing in Beringia at this time could only have been sustained in a highly diverse and productive environment.[34]
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 fromc. 57,000 – c. 15,000 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 fromc. 15,000 YBP.[35] These changes provided the most likely explanation for mammal migrations afterc. 15,000 YBP, as the warming provided increased forage for browsers and mixed feeders.[36] At the beginning of the Holocene, somemesic habitat-adapted species left the refugium and spread westward into what had become tundra-vegetated northern Asia and eastward into northern North America.[27]
Beringia, 8000 years ago
The latest emergence of the land bridge wasc. 70,000 years ago. However, fromc. 24,000 – c. 13,000 YBP theLaurentide ice sheet fused with theCordilleran ice sheet; this event allowed gene flow between Beringia (and Eurasia) and continental North America.[37][38][39] The Yukon corridor opened between the receding ice sheetsc. 13,000 YBP, and this once again allowed gene flow between Eurasia and continental North America until the land bridge was finally closed by rising sea levelsc. 10,000 YBP.[40] 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.[27]
The ecosystem of Beringia changed as the changing climate affected the environment, and thus determined which plants and animals were able to survive. The land mass could be a barrier as well as a bridge: during colder periods, glaciers advanced and precipitation levels dropped. During warmer intervals, clouds, rain and snow altered soils and drainage patterns. Fossil remains show that spruce, birch and poplar once grew beyond their northernmost range today, indicating that there were periods when the climate was warmer and wetter. The environmental conditions were not homogenous in Beringia. Recentstable isotope studies ofwoolly mammoth bone collagen demonstrate that western Beringia (Siberia) was colder and drier than eastern Beringia (Alaska andYukon), which was more ecologically diverse.[41]
Grey wolves suffered a species-widepopulation 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 remainingPleistocene wolf populations across Eurasia and North America.[42][43][44] The extinct pine speciesPinus matthewsii has been described from Pliocene sediments in the Yukon areas of the refugium.[45]
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 assaiga antelope, woolly mammoth, and caballid horses.[27] Notable restricted fauna include thewoolly rhino in Siberia (which went no further east than theAnadyr River), andArctodus simus,American badger, Americankiang-like equids,Bootherium andCamelops in North America, with the existence ofHomotherium being disputed in Late Pleistocene Siberia. The lack ofmastodon andMegalonyx has been attributed to their inhabitation of Alaska and the Yukon being limited to interglacials.[46][47][48] However, ground sloth eDNA has potentially been recovered from Siberia.[49]
Around 3,000 years ago, the progenitors of theYupik peoples settled along both sides of the straits.[62] 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 theBering Land Bridge National Preserve and theCape Krusenstern National Monument in the United States andBeringia National Park in Russia.[63]
Map shows the connection between North America and Asia during theLate Cretaceous period (~80Ma).[64]
Biogeographical evidence demonstrates previous connections between North America and Asia.[64] Similar dinosaur fossils occur both in Asia and in North America.[65]Saurolophus was found in both Mongolia and western North America.[66] Relatives ofTroodon,Triceratops, andTyrannosaurus rex all came from Asia.[67][68]
The earliestCanis lupus specimen was a fossil tooth discovered atOld Crow, Yukon, Canada. The specimen was found in sediment dated 1 million YBP,[69] however the geological attribution of this sediment is questioned.[69][70] Slightly younger specimens were discovered at Cripple Creek Sump,Fairbanks, Alaska, in strata dated 810,000 YBP. Both discoveries point to the origin of these wolves in eastern Beringia during theMiddle Pleistocene.[69]
Fossil evidence also indicates an exchange of primates and plants between North America and Asia around 55.8 million years ago.[64][71][72] 20 million years ago, evidence in North America shows the last natural interchange of mammalian species. Some, like the ancientsaber-toothed cats, have a recurring geographical range: Europe, Africa, Asia, and North America.[64] The pattern of bidirectional flow of biota has been asymmetric, with more plants, animals, and fungi generally migrating from Asia to North America than vice versa throughout the Cenozoic.[72]
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