T138 Glacial Lake Agassiz—Its History and Influence on North America and on Global Systems: In Honor of James T. Teller

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room L100FG

EVOLUTION OF LATE GLACIAL LAKE AGASSIZ AS SHOWN BY ITS OFFSHORE SEDIMENTARY RECORD BENEATH NORTHERN LAKE WINNIPEG, MANITOBA, CANADA

Paper number: T138-5
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

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 C14 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.