A multiproxy record of climate and vegetation change in Alaska's Copper River Basin following deglaciation
Canyon Lake is a 0.4 km2 lake located in the upper reaches of the Copper River Basin, Alaska. At 756 m a.s.l., Canyon Lake lies within the area inundated by glacial Lake Atna during the Last Glacial Maximum. A 255 cm core collected from Canyon Lake contains a sedimentological contact between gray, laminated silt and overlying gyttja. This contact represents the draining of Lake Atna and establishment of a proto-Canyon Lake. Pollen, plant macrofossils, δ13C and other geochemical proxies provide a record of climate and vegetation during the transition from a large glacial lake to a vegetated basin. The dominance of Betula and herb pollen throughout the glaciolacustrine silt indicates persistent shrub tundra in the region during late-glacial times. The glaciolacustrine silt contains only 0-2% Corg, indicating low primary productivity in Lake Atna. However, a significant drop in the C:N and an increase in δ13C is interpreted as an episode of relatively high primary productivity in the glacial lake. Cenococcum sclerotia indicate high levels of soil erosion immediately below the glaciolacustrine silt-gyttja contact. Gyttja immediately above the contact contains abundant macrofossil evidence of aquatic to emergent aquatic plants and freshwater bryozoans. Graminoid fragments and moss provide additional evidence of ponding on the post-Atna landscape. This change is accompanied by an increase in Corg and C:N indicating an increase in terrestrial organic matter. Betula and herbaceous pollen remain dominant until the onset of Picea pollen approximately 15 cm above the contact that records the establishment of boreal forest in the region.