Postglacial vegetation and climate change in the northern bristol bay region, southwestern alaska

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 conventional ¹⁴C dates of bulk organic matter from the Grandfather core. A mesic herb tundra dominated the landscape 13,000-9800 yr B.P. Betula shrubs 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 of Betula shrubs and relatively low Betula pollen 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., diminished Betula shrub cover, along with decreased aquatic productivity as recorded by Pediastrum cell 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., Betula shrub tundra and meadow communities expanded, probably in response to increased temperature and precipitation. Alnus arrived 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 of Picea glauca at 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.