Preliminary Results, Part C: Western Alaska
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.
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.
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.
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.
More InformationPostglacial 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:Return to Ecosystem and Climate History of Alaska main page.
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