HR: 1340h
AN: PP13B-1282    [Abstracts]
TI: The Oxygen-isotope Compositions of Holocene Lake Water Based on Study of Ostracodes from Four Sediment Cores, Lake Superior
AU: * Mae, A
EM: amae@uwo.ca
AF: The University of Western Ontario, 1151 Richmond St., London, ON N6A5B7, Canada
AU: Longstaffe, F J
EM: flongsta@uwo.ca
AF: The University of Western Ontario, 1151 Richmond St., London, ON N6A5B7, Canada
AU: Crowe, A S
EM: Allan.Crowe@ec.gc.ca
AF: National Water Research Institute Environment Canada, 867 Lakeshore Road, P.O. Box 5050, Burlington, ON L7R4A6, Canada
AB: An 18m-long piston cores were obtained from the Duluth Basin, Thunder Bay Trough, Caribou Basin, and Ile Parisienne Basin in Lake Superior. The valves of ostracodes, Candona subtriangulata, were collected from 10 cm intervals, and analyzed for their oxygen isotope compositions. Using the oxygen isotope compositions of the valves and a lake bottom temperature of 4 °C, we have calculated the oxygen isotope compositions of water in early Lake Superior. From 18 to 7 m (Duluth Basin), 12 m (Thunder Bay Trough), and 17 m (Ile Parisienne Basin) depth, the oxygen- isotope values of the water decrease from -23 to -28 ‰. This suggests that the lake water at this time was dominated by glacial meltwater. Isotopically depleted meltwater amount in the lake was likely increased by ice melting in the basin and near Lake Agassiz, causing lower oxygen-isotope values of lake water. Then the water isotopic values increase to -23, -21, and -15 ‰ at 6 m, 10 m, and 15 m (Duluth Basin, Thunder Bay Trough, Ile Parisienne Basin, respectively) around 8,000 ¹⁴C years BP. probably due to decreasing of glacial meltwater supply after glaciers retreated from the basin. The variation in the values may be resulted from the degree of stratification of Lake Superior water depending on the water depths. From 6 m (Duluth Basin), 10 m (Thunder Bay Trough), and 15 m (Ile Parisienne Basin) to 0 m depth, no ostracodes were found in the sediments, suggesting they are postglacial sediments that ostracodes were not preserved in because of the low sedimentation rate and no delivery of calcareous till eroded from Paleozoic rocks. From the 18 to 6 m, the oxygen-isotope compositions of the lake water calculated from ostracodes from Caribou Basin sediment core varied little (-26 to -24 ‰). Because the sediment is much older than those from other three basins, it recorded lake water of glacial time, which may have fairly constant isotope values. From 6 to 3 m depth, then, the values decrease to -27 ‰, which shows the same trend with other cores. From 3 to 0 m depth, sediments from Caribou Basin were lost when sampling. The oxygen-isotope compositions of ostracodes are a very useful proxy to assess glacial meltwater amount and its effect to the lake water.
DE: 0458 Limnology (1845, 4239, 4942)
DE: 0746 Lakes (9345)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
DE: 1105 Quaternary geochronology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting