HOLOCENE PALEOCLIMATE FROM CLOSED BASIN MARL LAKES IN THE YUKON TERRITORY, CANADA

Closed basin lakes in semi-arid environments may be highly sensitive to changes in regional moisture balance if other non-climatic hydrological factors controlling groundwater activity are minimal. In such an environment, lake-levels rise and fall according to changing effective moisture. Furthermore, small, closed basin lake-waters enrich and deplete in heavy oxygen isotopes in response to regional moisture balance, changing source water, and/or changing circulation patterns. Smaller lakes are sensitive to regional climate change because a smaller water mass responds rapidly. If sedimentation rates are also sufficiently rapid, then sedimentary material recording the lake response is preserved. The thick Late-Pleistocene mantle of carbonaceous tills and glaciofluvial outwash covering the surface of the southern Yukon Territory is pockmarked with thousands of closed basin kettle lakes, many of which have subsequently developed relatively thick Holocene marl sedimentary sequences. We are using a series of such sites to develop a detailed Holocene paleoclimate history by inferring lake-level fluctuations from sedimentary records and by stable isotope analyses of carbonate sediment that was precipitated within the lake waters.

Sediment cores from deep to shallow water transects in four closed basin lakes from the semi-arid southern and central Yukon Territory have been analyzed for bulk sedimentary characteristics following detailed core description and core photography. Chronologies are being established using radiocarbon dating of terrestrial macrofossil material and 210-Lead analyses of the uppermost sediments. Bulk C/N ratios and carbon isotope analyses of organic matter and oxygen isotope analyses of authigenic carbonate from selected cores are in progress. Results to date indicate that the lakes filled rapidly and experienced large lake-level fluctuations (greater than 3 m) during the early to middle Holocene. Preliminary isotope analyses suggest that lake-water isotopic composition responded coherently with late Holocene glacial advances in the nearby St. Elias Range and that conditions were at times wetter than present during the early Holocene. These results are raising testable hypothesis regarding trends and rapid changes of circulation patterns and/or moisture sources.