Postglacial Vegetation and Climate Change in the Northern Bristol Bay Region, Southwestern Alaska

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Abstract
Pollen analysis of sediment cores from Grandfather and Ongivinuk Lakes reveals a record of postglacial vegetation and climate change in the northern Bristol Bay region. The chronology is based on six conventional14C dates of bulk organic matter from the Grandfather core. A mesic herb tundra dominated the landscape 13,000-9800 yr B.P.Betulashrubs probably first appeared in the region 11,300 yr B.P. but were restricted to favorable microhabitats until 9800 yr B.P. The later establishment ofBetulashrubs and relatively lowBetulapollen abundance in these records compared to other areas of eastern Beringia suggest that postglacial warming in southwestern Alaska was dampened by regional climatic controls, possibly low sea-surface temperatures of the North Pacific Ocean. Between 10,800 and 9800 yr B.P., diminishedBetulashrub cover, along with decreased aquatic productivity as recorded byPediastrumcell nets and biogenic silica, suggest a brief reversion to colder and drier climatic conditions possibly associated with the Younger Dryas event. Around 9800 yr B.P.,Betulashrub tundra and meadow communities expanded, probably in response to increased temperature and precipitation.Alnusarrived and formed extensive thickets within the region ca. 7400 yr B.P. The establishment of the modern boreal forest-tundra ecotone is marked by the arrival ofPicea glaucaat Grandfather Lake ca. 4000 yr B.P. and the subsequent increase to present population densities ca. 2000 yr B.P. The unique features of these pollen records emphasize the spatial complexity of late Quaternary vegetation and climate history in eastern Beringia.