CGRG Bibliography of Canadian Geomorphology
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Author : Anderson, L.; Abbott, M.B.; Finney, B.P.; Edwards, M.E .; Witthaus, L. ; Franklin-Smith, L.; and Tsukernik, M.
Date : 2006.
Title : Holocene paleoclimate and ecosystem change in the Alaska and Yukon sub-Arctic from lake sediment records.
Publication : 36th International Arctic Workshop. March 16-19, 2006. Institute of Arctic and Alpine Research, University of Colorado at Boulder, Colorado, U.S.A.
Issue :
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Abstract
High latitude sub-Arctic regions are among the most sensitive to climate change. We are conducting multiple climate-proxy sedimentary analyses of lake sediments on an expanding network of sites in the semi-arid interior region of Alaska and southwest Yukon Territory (60º - 63ºN). Our goals are to improve the detail and resolution of the regional climatic history and to test hypotheses concerning shifts in regional precipitation-evaporation balance, atmospheric circulation patterns and ecosystem response. In the semi-arid regions of the sub-Arctic, precipitation and evaporation are critical in regulating many important ecological and biogeochemical processes, such as boreal forest dynamics and trace greenhouse gas fluxes that are controlled in part by soil moisture. We have used a variety of paleoclimatic and palaeoenvironmental proxies to document changes on millennial to multi-annual time scales. Lake level records provide a direct means to determine regional effective moisture because they reflect precipitation balance and/or the water table. Sediment cores taken from a depth transect at several hydrologically closed basin sites (Birch Lake, Jan Lake, Marcella Lake, Wrong Lake, Jackfish Lake) record lake level variations at millennial time scales. Each site indicates relatively rapid increases in effective moisture during the early Holocene that may have been significant for the regional establishment of spruce forest. Marcella Lake, a closed basin that remained sensitive during the late Holocene, had higher-than-modern lake levels between ~4,000 and 2,000 cal BP which coincide with decreasing summer solar insolation and the onset of late Holocene glacial activity. Lake levels lowered after ~2000 cal BP and Little Ice Age glacial advances occurred under drier conditions than preceding millennia. Oxygen isotope compositions of authigenic lake sediment carbonate are related to climate because d 18OCa is a function of (1) lake water temperature and (2) lake-water d 18O. In the southwest Yukon, temperature is an insignificant control relative to evaporation and 18O-depletion caused by vapor transport across the St. Elias and Coast Mountain massif. Instead, lake water d 18O is controlled primarily by lake hydrology and atmospheric circulation patterns. Recent changes in d 18OCa at hydrologically open sites, which are insensitive to evaporation, correspond to changes in the North Pacific Index (NPI), a measure of the intensity and position of the Aleutian Low (AL). Thus, they provide records of changes in North Pacific atmospheric circulation (Jellybean Lake, Squanga Lake, Seven Mile Lake). Similar oxygen isotope data from hydrologically closed lakes, sensitive to evaporation, record changes in rates of input and evaporation (Marcella Lake, Scout Lake, Como Lake). Prominent late Holocene AL variations and coinciding effective moisture shifts lead us to hypothesize two regional climatic patterns (1) simultaneous increases in moisture at coastal high-elevation and low-leeward locations or, (2) wet conditions at coastal high-elevations coinciding with dry conditions at low-leeward locations. These results highlight the spatially heterogeneous response to regional climate change caused by the combined effects of circulation shifts over the Gulf of Alaska superimposed on regional topography. Detailed pollen and charcoal studies (Marcella Lake, Dragonfly Pond, Little Harding Lake) are ongoing and will document vegetation changes and fire frequency patterns, providing insights into forest ecosystem dynamics in response to Holocene climate change. Other ongoing work includes a high-resolution oxygen isotope study of the last 2000 years from an evaporation-sensitive lake (Scout Lake), expected to improve our late Holocene effective moisture reconstructions. A detailed record of the d 18OCa from an evaporation-insensitive site (Squanga Lake) appears to provide a record of early Holocene AL variability during the periods when solar insolation at 60°N reached a maximum, sea levels were rapidly rising and spruce forests were being established (see Witthaus et al., this volume). In addition to new site exploration, future work includes detailed studies of recent synoptic climate patterns and storm tracks and corresponding oxygen isotopes in precipitation to test further our atmospheric circulation hypotheses in increasing detail.
Bibliography of Canadian Geomorphology