Lake Agassiz outburst age and routing by Labrador Current and the 8.2 cal ka cold event

Abstract

Catastrophic drainage of glacial Lake Agassiz, impounded by the Laurentide Ice Sheet (LIS), into Hudson Bay and Strait is widely thought to have caused the 8.2 cal ka cold event, though the path of its drainage in the North Atlantic is poorly known. Analysis and compilation of 10 sediment cores from 8 sites distributed on the eastern Canadian continental shelf and upper slope south of Hudson Strait revealed distinct beds with elevated detrital carbonate (DC) concentrations, known by their distinct composition to originate from erosion of carbonate bedrock and carbonate-rich glacial sediments in the Hudson Bay-Hudson Strait region. At all but one site, radiocarbon ages, bracketing and within the thickest and most widespread of these beds, are coeval with the age constraints for the Agassiz floods in Hudson Strait and southeastern Hudson Bay. The DC distribution and concentration of this bed are consistent with southward transport in the Labrador Current. Transfer function analysis of dinoflagellate assemblage data from cores on the central Labrador Shelf, in Hudson Bay, and on the Northeast Newfoundland Shelf reveal that early Holocene seasonal ice cover was 5–6 months longer than at present, requiring the application of a 200-year reduction to the radiocarbon age constraints on the Lake Agassiz outburst floods. The age of the floods is reduced to about 8.33 cal ka (7.5 14C ka) from the previous determination of 8.47 cal ka (7.7 14C ka), with a similar reduction in their error ranges to 8.04–8.49 cal ka from 8.16–8.74 cal ka. Based on present current routing, the Agassiz floodwaters and icebergs are inferred to have travelled south in the Labrador Current, and to have possibly become incorporated into the North Atlantic Current, and transported to the Nordic seas. There the freshwater additions would have lessened deepwater production with a concomitant reduction in northward heat transport in the upper limb of the Atlantic meridional overturning circulation, thereby contributing to the 8.2 cal ka cold event. Variation in the calcite and dolomite content of the Agassiz DC bed, two salinity reductions during Agassiz bed deposition, and low-level shorelines of lakes Agassiz and Ojibway suggest that more than one outburst was delivered during the final Agassiz drainage. The 8.2 cal ka cold event registered in Greenland was coeval with a second or later flood, which possibly accompanied the final collapse of the LIS in Hudson Bay.