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Hu, F.S., 1994

An ecosystem approach to the study of late Quaternary environmental change in southwestern Alaska

Bibliographic Reference

Hu, F.S., 1994, An ecosystem approach to the study of late Quaternary environmental change in southwestern Alaska: University of Washington, Seattle, Ph.D. dissertation, 123 p., illust.


Sediment cores from four lakes in southwestern Alaska were analyzed for a suite of proxy environmental indicators to infer ecosystem and climate changes during the late Quaternary. Pollen, plant-macrofossil, macroscopic-charcoal, and geochemical records from Wien and Farewell Lakes provide information on vegetation changes and associated ecosystem processes. Betula shrub tundra dominated the regional landscape 12,000 - 9,500 BP, but Populus-Salix communities were prominent ca. 11,000 - 9,500 BP. At Wien, the marked increases in sediment organic content and authigenic concentrations of Fe, Mn and Al during the Populus-Salix period suggest humic buildup and stabilization of catchment soils. Picea glauca formed open woodlands with B. papyrifera 9500-7500 BP. Between 8500 and 7500 BP, P. glauca populations declined in response to a climatic cooling. This species continued to spread southwestward during the cooling episode, forming a forest tundra association at Farewell 8000-6000 BP. Modern P. mariana forests established 6500 BP at Wien and 4000 BP at Farewell. Stratigraphic patterns of macroscopic charcoal from both sites indicate that the widespread establishment of P. mariana stands was associated with increased fire occurrence. An increase in authigenic Fe/Mn ratios at Farewell suggests that this vegetation change was also accompanied by the development of waterlogged soils. These ecosystem changes probably reflected complex responses to the onset of cooler and wetter climate conditions. Pollen analysis of sediment cores from Grandfather and Ongivinuk Lakes reveals a 13,000-year record of vegetation and climatic change in the northern Bristol Bay region. A mesic herb tundra dominated the landscape 13,000-9800 BP. Betula shrubs first appeared 11,300 BP but were probably restricted to favorable microhabitats until 9800 BP. The later establishment of widespread Betula shrubs and relatively low Betula pollen percentages in these records compared to other areas of eastern Beringia suggest that the postglacial warming trend was dampened by regional climatic controls in southwestern Alaska. Diminished Betula shrub cover 10,800-9800 BP suggests that the northern Bristol Bay region experienced a climatic reversal during the glacial-interglacial transition. Alnus arrived and formed extensive thickets 7400 BP. The establishment of boreal forest-tundra ecotone was marked by the arrival of P. glauca 4000 BP.

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