The subglacial origin of the lake Agassiz-Ojibway final outburst flood

Deglaciation of North America resulted in the development of the ice-dammed lake Agassiz-Ojibway along the southern margin of the Laurentide Ice Sheet(1-5) and its catastrophic northward drainage similar to 8.47 kyr ago(6). This sudden outburst of fresh water may have weakened the Atlantic ocean overturning circulation and triggered the cold event that occurred 8.2 kyr ago(6,7). Geological evidence of this flood has been documented in a red sedimentary bed in cores collected in Hudson Strait(6,8) and by submarine features in Hudson Bay(9). However, there have been few constraints on the manner in which the lake drained: for example, by flow over the ice sheet or beneath it, in one or several pulses(10) and where the flood routes were located(5,11). Here we present seafloor images obtained using multibeam sonar, which reveal that the outburst flood displaced icebergs to produce arcuate (arc-shaped) scours on the seafloor with a dominant east-northeast-west-southwest orientation. The flood also produced sandwaves in areas unaffected by the arcuate scours, indicating they were protected from iceberg scouring by overlying ice during the event. We suggest that these sandwaves, along with submarine channels inferred from the data, indicate that Laurentide ice was lifted buoyantly, enabling the flood to traverse southern Hudson Bay under the ice sheet.