Paper No. 4
Presentation Time: 2:20 PM
EVOLUTION OF LATE GLACIAL LAKE AGASSIZ AS SHOWN BY ITS OFFSHORE SEDIMENTARY RECORD BENEATH NORTHERN LAKE WINNIPEG, MANITOBA, CANADA
LEWIS, C.F.M.1, TODD, B.J.
1, ANDERSON, T.W.
2, KING, J.W.
3, MATILE, G.L.D.
4, RODRIGUES, C.G.
5 and THORLEIFSON, L.H.
6, (1)Geological Survey of Canada Atlantic, Natural Resources Canada, Bedford Institute of Oceanography, Box 1006, Dartmouth, NS B2Y 4A2, Canada, (2)Geological Survey of Canada, Natural Resources Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada, (3)Graduate School of Oceanography, University of Rhode Island, Bay Campus, South Ferry Road, Narragansett, RI 02882, (4)Manitoba Geological Survey, Manitoba Science, Technology, Energy, and Mines, 1395 Ellice Ave, Winnipeg, MB R3G-3P2, Canada, (5)Faculty of Science, University of Windsor, Windsor, ON N9B 3P4, Canada, (6)Minnesota Geological Survey, Univ of Minnesota, 2642 University Avenue, St. Paul, MN 55114, Michael.Lewis@NRCan-RNCan.gc.ca
Cores of rhythmically laminated silt and clay (varves) over a glacial diamict provide a record of late phases of glacial Lake Agassiz in the northeastern Lake Winnipeg region after about 10.36 cal (9.2) ka when the site was deglaciated, until ~8.49 cal (7.7) ka. The inferred drainage of the lake during this ~1875-year period, an interval estimated from varve counts and sedimentation rates, was first to Lake Superior basin, then to Ottawa and St. Lawrence valleys after Lake Agassiz had merged with Lake Ojibway. Sediment chronology was evaluated from counts of rhythmites (shown to be annual deposits by their seasonal pollen content), AMS C
14 ages on a spruce needle and ostracod fossils, and recognition of paleomagnetic-inclination events. Changing trends in varve thickness, bulk geochemistry, ostracod paleoecology and isotopic composition indicate the evolution of this sector of Lake Agassiz from a deep oligotrophic glacial lake to a shallow productive water body.
1) Initially, for 600 years the lake floor accumulated silt and clay couplets ranging 0.2-1.2 cm thick with an upward increasing Al trace element composition, representative of the underlying metamorphic Precambrian bedrock. A sparse concentration of benthic ostracod fossils (Candona subtriangulata), increasingly depleted in δ18O (-17.5 to -19.5 ‰ pdb), indicates the bottom waters were increasingly supplied by meltwater. 2) This trend was interrupted by 100 years of significantly greater meltwater influx which resulted in increased (~10x) varve thickness, a spike of more depleted δ18O (<23.8 ‰ pdb) in ostracod valves, and sediments with greater Ca content indicating a greatly expanded glacial drainage system with sediment sources closer to Hudson Bay where carbonate bedrock and tills occur. 3) A transition period followed beginning with a return to rhythmic sedimentation as before which graded into laminated sediment while ostracod δ18O composition continued to decline (from -21 to -23.5 ‰ pdb) indicating increased meltwater input. 4) During the final ~200 years, ostracod concentration and diversity increased, and isotopic composition became more enriched, as the site environment evolved into a shallow isolated embayment, subjected to dry climate indicated by pollen from grassland vegetation around the lake.
