POSTGLACIAL VEGETATION AND CLIMATE-CHANGE IN THE NORTHERN BRISTOL BAY-REGION, SOUTHWESTERN ALASKA

Pollen analysis of sediment cores from Grandfather and Ongi-vinuk Lake s reveals a record of postglacial vegetation and climate change in the northern Bristol Bay region. The chronology is based on six conventio nal C-14 dates of bulk organic matter from the Grandfather core. A mes ic herb tundra dominated the landscape 13,000-9800 yr B.P. Betula shru bs probably first appeared in the region 11,300 yr B.P. but were restr icted to favorable microhabitats until 9800 yr B.P. The later establis hment of Betula shrubs and relatively low Betula pollen abundance in t hese records compared to other areas of eastern Beringia suggest that postglacial warming in southwestern Alaska was dampened by regional cl imatic controls, possibly low sea-surface temperatures of the North Pa cific Ocean. Between 10,800 and 9800 yr B.P., diminished Betula shrub cover, along with decreased aquatic productivity as recorded by Pedias trum cell nets and biogenic silica, suggest a brief reversion to colde r and drier climatic conditions possibly associated with the Younger D ryas event. Around 9800 yr B.P., Betula shrub tundra and meadow commun ities expanded, probably in response to increased temperature and prec ipitation. Alnus arrived and formed extensive thickets within the regi on ca. 7400 yr B.P. The establishment of the modern boreal forest-tund ra ecotone is marked by the arrival of Picea glauca at Grandfather Lak e ca. 4000 yr B.P. and the subsequent increase to present population d ensities ca. 2000 yr B.P. The unique features of these pollen records emphasize the spatial complexity of late Quaternary vegetation and cli mate history in eastern Beringia. (C) 1995 University of Washington.