Preliminary Results, Part C: Western Alaska

Figure 29. Map of western Alaska and eastern Siberia, showing present-day shorelines. During winter and spring, sea ice covers the Arctic Ocean, Chukchi Sea and the northern Bering Sea.		Figure 30. Map of the Bering land bridge during the late Wisconsin glaciation, when global sea level dropped to about 120 meters or more below its present position. This exposure of huge areas of the shallow continental shelf provided a broad land connection between northeastern Asia and unglaciated parts of Alaska and Yukon. This land bridge allowed many plants and animals to spread between the Asian and American continents.

Figure 31. Aerial view of St. Michael Island in Norton Sound, northeastern Bering Sea. This volcanic island is pierced by numerous volcanic craters (maars) that are now occupied by lakes. Sediment cores from these lakes provide a history of Bering land bridge vegetation and climates spanning the past 30,000 years or more.

Our investigations in western Alaska have focused on pollen records obtained from St. Michael Island in Norton Sound, (Ager, 2002; 2003; Muhs and others, 2003); Seward Peninsula (Ager, 2002; Ager and Muhs, in progress), and from the Bering and Chukchi Seas (Ager, 2002; Ager and Phillips, in progress). These investigations are focused on reconstructing the environments and climates on the former Bering land bridge during the Wisconsin glacial interval, and on the development of modern vegetation and ecosystems during the past ca. 14,000 radiocarbon years. These investigations contribute to understanding of the environmental conditions faced by humans during the colonization of North America at least 12,000 years ago, and probably earlier. Accurate reconstructions of the environments on and adjacent to the Bering land bridge are important for understanding past climates and resources available to humans and wild mammals populations.

Figure 32. Late winter coring operations through thick lake ice, Zagoskin Lake, St. Michael Island. A 15.2 meter long sediment core was recovered from this lake. Radiocarbon dates from this core indicate that the pollen record from Zagoskin Lake spans the past 30,000 years or more. Jerry Austin photo.

During the last major glacial interval (the Wisconsin glaciation), global sea level dropped substantially because so much water was stored on land in the form of glacier ice and snowfields. During the maximum ice extent of the last major glacial interval (the late Wisconsin, ca. 27,000-10,000 radiocarbon years ago), sea level dropped about 120 meters or more below present sea level. This exposed large expanses of continental shelves in many parts of the world. The shallow shelf between western Alaska and Chukotka (northeastern Asia) was exposed as land, creating a 1600-km-wide land bridge (from north to south). Although much of southern Alaska and southern Yukon was covered by glacial ice during the last ice age, large areas remained free of glaciers because of aridity. Interestingly, unglaciated parts of Alaska and Yukon were really an extension of Asia, rather than of North America. This is because the Laurentide and Cordilleran ice sheets in Canada isolated Alaska and Yukon from the rest of North America (Fig. 30).

This broad land connection, with an area of about 1.4 million square kilometers, provided a route by which animals and plants could migrate between northeastern Asia and northwestern North America. Extensive additional lands were exposed north of Siberia. Many mammal species that evolved in Eurasia wandered into North America via the Bering land bridge during the Pleistocene (and earlier); fewer North American mammal species entered Eurasia via the land bridge, probably because of glacial ice barriers that developed numerous times across much of northern North America. In Alaska and Yukon, ice age mammal populations included bison, mammoths, camels, caribou, several kinds of horses and musk-oxen, wolves, brown bears, short-faced bears, American lions, badgers, saiga antelope, dall sheep, and black-footed ferrets. Most of these species have also been found in fossil assemblages of northeastern Asia. This mammal assemblage suggests a non-forested or minimally forested landscape with a vegetation cover that may have been more steppe-like in character than many modern tundra communities in Alaska, Yukon and northeastern Asia. Elimination of most trees may have resulted from aridity, and/or summers too cool for the survival of trees in much of Beringia. Beringia is defined as the region extending from the Kolyma River in Siberia, eastward across the Bering land bridge to unglaciated interior and northern Alaska, and Yukon. In general, trees require a July mean temperature of about 10-12 degrees C in order to survive and reproduce. Some high latitude conifer populations may be able to survive under somewhat cooler mean July temperatures, perhaps as cool as 8 degrees C.

Figure 33. During the last major glaciation (Wisconsin glaciation), many species of cold-climate adapted mammals occupied the Bering land bridge and adjacent parts of Siberia-Chukotka, Alaska, and Yukon (Beringia). Bison, caribou, woolly mammoths, and horses were the most abundant large mammals. Several species of horses wandered these landscapes. This photo shows a Przewalski's horse (reintroduced into Mongolia). This horse may be similar to some of the wild horses that roamed across northern Eurasia and Beringia during the last ice age. This photograph was taken in Mongolia by wildlife conservation biologist Richard Reading.

Figure 34. The same species of elk, or wapiti, is found today in Mongolia and North America. Wapiti crossed the land bridge into Alaska from Asia during one or more Pleistocene ice ages. Richard Reading photo.		Figure 35. Saiga antelope, sheep-sized grazing mammals, once roamed across Eurasia from the British Isles and the land bridge to northwest Canada during the last ice age. They disappeared from Alaska and NW Canada at the end of the ice age. These animals are adapted to relatively flat, well-drained terrain, can tolerate cold winters, and can graze on sparse herbaceous vegetation. They survive in central Asia today. Richard Reading photo.

It has long been thought that the Bering land bridge was the logical route by which humans first entered North America from northeastern Asia, perhaps about 12,000 years ago. The assumption was that the first humans to enter North America were big game hunters following game such as bison, mammoth, caribou, and horses. More recently, evidence from an archeological site in southern South America has been dated to at least 12,500 radiocarbon years ago (Dixon, 1999. Bones, Boats & Bison: Archeology and the First Colonization of Western North America. University of New Mexico Press, Albuquerque). In order for humans to reach southern South America that early, or earlier, they may have followed the west coast of North America, using boats and utilizing marine food resources rather than depending on large terrestrial mammals as their primary food source. Archeological sites found in southeastern Alaska, the Queen Charlotte Islands of northern British Columbia, and other sites farther south suggest humans with marine adaptations were present at least 10,000 radiocarbon years ago. The search for older west coast archeological sites is under way, but many potential sites are now deep under sea water. Melting ice sheets during deglaciation produced enormous volumes of water that caused sea level to rise to near present levels by about 5000 years ago. This will make it more challenging to locate coastal archeological sites older than about 10,000 years.

A recent discovery of an archeological site (the Yana site) in Siberia, north of 70 degrees N latitude, with radiocarbon dates around 28,000 years old suggests that some human populations had managed to develop technologies and adapt to high latitude conditions not long before the onset of full glacial climatic conditions (Pitulko, V.V. and others, 2004, The Yana RHS Site: Humans in the Arctic before the last glacial maximum: Science, v. 303, p. 52-56). This discovery raises the possibility of human entry into the Americas, via the Bering land bridge or along the North Pacific coast, prior to the onset of the late Wisconsin glacial interval (pre-26,000 years ago). Finding such ancient archeological sites, if they exist, will pose a great challenge to archeologists in the coming decades.

Our research in western Alaska (including sediment cores from northeastern Bering Sea and northeastern Chukchi Sea) provide pollen evidence and radiocarbon dates useful for reconstructing the past environmental conditions on and adjacent to the Bering land bridge. Results obtained so far (e.g., Ager, 2002; 2003; Muhs and others, 2003) suggest that the land bridge vegetation was predominantly graminoid (grasses, sedges, rushes) herbaceous tundra with some steppe-like or disturbance characteristics. This vegetation included a fairly rich assemblage of tundra plants, but peat forming plants were not conspicuous elements of the vegetation. Some shrubs were present, most notably willows. Dwarf birch shrubs and heaths were also present in the landscape, but in small amounts and they may have been found only in restricted, mesic habitats. The widespread grassy tundra vegetation probably formed a thin, discontinuous turf overlying silty loess-derived soils, in contrast with the peaty, wet, cold, acidic soils that characterize most lowland tundra communities in Alaska and Yukon today.

Figure 36. During Pleistocene ice ages, winters were severe in Beringia, but thin snow cover allowed grazing mammals to find sufficient winter forage. Dominant "ice age" grasses and sedges (e.g., Kobresia) retained enough nutrients in their above ground tissues to allow survival of grazing animals through the winter. Ice-age climate was relatively arid, and it is likely that airborne dust (loess) from glacial outwash plains contributed abundant nutrients to soils. Vegetation was grassy, with few peat-forming plants. Willows were the most abundant shrubs in the landscape. T. Ager photo.

Figure 37. Modern tundra communities in Alaska, Yukon, and Chukotka often include abundant cottongrass sedge tussocks (Eriophorum), as shown in this photo with dwarf birch (Betula nana) growing between tussocks. Modern tundra also often includes peat forming mosses, and low growing heaths (e.g., Vaccinium, Ledum, Empetrum). The shift from ice-age grassy vegetation to sedge-moss, low-shrub tundra coincides with the time of extinction of many grazing mammal species in Beringia (mammoth, horses, steppe bison, saiga antelope, helmeted musk oxen) and some of their predators (short-faced bear and American lion). Climate change, vegetation change, and hunting pressure from humans may all have contributed to the extinction of many mammal species in Beringia. T. Ager photo.

Pollen evidence from western Alaska shows that during the late, waning stages of the Wisconsin glaciation, increases in moisture and probably warmer temperatures may account for a shift from graminoid-herb tundra to mesic shrub-herb tundra between about 14,000-13,000 radiocarbon years ago. Poplar trees spread widely across much of Alaska about 11,000-10,000 years ago. Alder shrubs, which may have survived in north-central Beringia during the ice age, spread across western Alaska by about 8000-7000 years ago. Boreal forest vegetation dominated by white spruce and black spruce spread from interior Alaska to western Alaska by about the middle of the Holocene (ca. 5000 years ago). Flooding of the remaining areas of the Bering land bridge by rising seas was nearly complete by about 5000 years ago. The expansion of the Bering and Chukchi Seas increased the influence of maritime climates and contributed to the cool, moist coastal zone around the shores of the Chukchi and Bering Seas that we have today. This coastal zone supports arctic tundra communities today in the region.

Figure 38. Map of central Beringia during deglaciation, when melting of glacier ice caused sea level to rise, gradually flooding the land bridge. In this map, much of the land bridge was flooded when sea level rose to 40 m below its present position. At this stage of flooding, the land connection between northeastern Asia and Alaska was severed near Bering Strait. Compare this map with full glacial conditions and lowest sea level position in Fig. 30. The approximate shoreline positions shown on this map would have developed by about 11,000-10,500 radiocarbon years ago.

More Information

Postglacial Flooding of the Bering Land Bridge: A Geospatial Animation. By William Manley, 2002.

Paleoenvironmental Atlas of Beringia. By M.L. Duvall, T.A.Ager, P.M. Anderson, P.J. Bartlein, N.H. Bigelow, J. Brigham-Grette, L.B. Brubaker, L.C. Cwynar, M.E. Edwards, W.R. Eisner, S.A. Elias, B.P. Finney, O. Y. Glushkova, F.S. Hu, D.S. Kaufman, A.V. Lohzkin, C.J. Mock, M.A. Trumpe, and R.S. Webb, 1999.

Alaska PaleoGlacier Atlas: A Geospatial Compilation of Pleistocene Glacier Extents. By William Manley and Darrell Kaufman, with contributions by T.A. Ager, Y. Axford, N. Belascio, J. E. Beget, J. P. Briner, P. Carrara, T.D. Hamilton, R.D. Reger, H.R. Schmoll, R.M. Thorson, C.F. Waythomas, F.R. Weber, and F.H. Wilson, 2002.

Preliminary Results for the project in:

• Southeastern Alaska

• South-central Alaska

Return to Ecosystem and Climate History of Alaska main page.

Accessibility FOIA Privacy Policies and Notices

U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/research/alaska/alaskaC.html
Page Contact Information: ESP Web Team
Page Last Modified: Tue 14-Feb-2006 15:03:34 MST