Boreal ecosystem development in the northwestern Alaska range since 11,000 yr BP

Analyses of pollen, plant macrofossils, macroscopic charcoal, mollusks, magnetic susceptibility, and geochemical content of a sediment core from Farewell Lake yield a 11,000-yr record of terrestrial and aquatic ecosystem changes in the northwestern foothills of the Alaska Range. Between 11,000 and 8500 qr B.P., the regional landscape was dominated by a Betula shrub tundra, in which Populus-Salix communities were common. Abundant charcoal in sediments indicates that fires were common in the lake catchment during this period, and high mineral accumulation rates, allogenic elemental content, and magnetic susceptibility suggest intense soil erosion. In addition, mollusks, pollen and macrofossils of aquatic macrophytes, and biogenic silica provide evidence that the lake was substantially shallower and more productive 11,000-8500 yr B.P. than later. Low lake level and high aquatic productivity might have been caused by warm and dry summers associated with early postglacial insolation maximum in northern high latitudes. About 8000 yr B.P., Picea glauca arrived within the lake catchment, forming a forest tundra association until ca. 6000 yr B.P. Alnus shrub thickets established in the region ca. 6500 yr B.P., and Betula papyrifera arrived ca. 6000 yr B.P. Closed P. glauca forests developed ca. 6000 yr B.P. Picea mariana became important subsequently and replaced P. glauca as the dominant tree species in the region ca. 4000 yr B.P. An increase in authigenic Fe/Mn ratios suggests that the development of waterlogged soils accompanied this vegetation change. Fires increased in importance at this time and might have accelerated soil erosion. The establishment of P. mariana forests probably reflected complex responses of forest ecosystems to the onset of cooler and wetter climate conditions during the late Holocene. (C) 1996 University of Washington.