USING OSTRACODES TO DESCRIBE THE TERMINAL PHASE AND DRAINAGE OF GLACIAL LAKE AGASSIZ-OJIBWAY

Glacial Lake Agassiz-Ojibway was a massive pro-glacial lake that emptied into the North Atlantic Ocean when a final ice barrier of the Laurentide Ice Sheet (LIS) broke. The prevailing theory is that the catastrophic release of this water had drastic effects on North Atlantic circulation, triggering an abrupt global climatic cooling event known as the 8200 cal yr BP event. This study utilizes a sediment core from Gardiner Lake in northern Ontario, within the Lake Agassiz-Ojibway basin, to detail the nature and timing of Lake Agassiz-Ojibway drainage. Ostracode concentrations (exclusively Candona subtriangulata, a benthic, cold water species) constrain sedimentation rates in terminal Lake Agassiz-Ojibway sediments. Ostracode valves are absent in thick glaciolacustrine varves, and have a low abundance within thinner, more ice distal varves (~1 valve/gram wet sediment). Valves are more abundant in a unit above the varves that has been described as a drainage breccia, but the higher valve density in this unit requires a lower sedimentation rate than the varves below. We attribute this unit to ice distal sedimentation, with ice rafted debris, during rapid retreat of the LIS margin into what is now Hudson Bay. The drainage event is an abrupt contact and transition to reworked Lake Agassiz-Ojibway sediments. Valves are rare in the post-drainage, reworked sediments. Pending oxygen isotopic data will provide information about the amount of glacial meltwater in Lake Agassiz-Ojibway relative to meteoric water, and perhaps assess the rate of ice sheet decay.