Reconstruction of the Last Outburst Flood of Glacial Lake Agassiz-Ojibway in Hudson Bay and Hudson Strait

Hudson Bay and Hudson Strait were the sites of a rapid deglaciation that culminated in the catastrophic drainage of proglacial Lake Agassiz-Ojibway into the North Atlantic at ~8.47 cal kyr BP. It has previously been suggested that this sudden outburst of freshwater may have weakened the thermohaline circulation and triggered the 8200 cal BP cold event recorded in Greenland ice cores. Evidence for the outburst flood included geomorphic features observed on the seafloor of southern Hudson Bay and the identification of a centimeter to decimeter- thick hematite-rich red layer present in Hudson Strait sediment cores. However, unequivocal evidence is still lacking in order to define the way the lake drained (i.e., either by a breach through the ice-dam, a supraglacial spillover or a subglacial flood), whether it drained by one or more pulses and the location of the northward flood routes toward Hudson Bay. In this paper, we present new seafloor images and sediment cores collected onboard the CCGS Amundsen in Hudson Bay and Hudson Strait in 2004 and 2005 that shed light on the dynamics of the final drainage of the ice-dammed lakes. We found that this sudden outburst flood combined with subsequent currents displaced icebergs back-and-forth in a former calving bay to produce preferentially oriented arc-shaped scours on the seafloor. In addition, fields of giant sandwaves were identified in southern Hudson Bay in areas unaffected by arcuate iceberg scours, suggesting that they were protected from iceberg scouring by overlying glacier ice during the lake drainage event. The subglacial origin of the widely distributed sandwaves and the occurrence of many submarine channels lead us to propose that the drainage of Lake Agassiz-Ojibway took place by a buoyant lifting of the rapidly thinning LIS along many subglacial routes that spread over the entire southern Hudson Bay region. We also reveal that the red bed contains two layers deposited by hyperpycnal flows (hyperpycnites). These two red layers can be correlated to a red bed previously identified as a regional isochron in Hudson Strait and associated with the final drainage of Lake Agassiz-Ojibway around 8500 cal BP. Regardless of the exact timing of the catastrophic drainage, these hyperpycnites suggest that they were deposited following two pulses, which is in agreement with one of the scenarios proposed in previous studies for lake final drainage.