ENIGMAS SURROUNDING THE FINAL DRAINAGE OF GLACIAL LAKE AGASSIZ AND THE 8.2 CAL KA COLD EVENT

The final drainage of Glacial Lake Agassiz closely preceded the cold period 8200 cal yrs ago. These events’ timing, paired with model-derived assumptions regarding the sensitivity of the North Atlantic overturning circulation to freshwater inputs, led to the hypothesis that altered ocean circulation, and reduced northward heat transport caused the observed cooling. Despite wide acceptance, there remain flaws and unanswered questions regarding this scenario for the 8.2 cal ka cold event. The idea that Labrador Sea Water (LSW) formation ceased due to freshening during the event is challenged by reconstructions suggesting that LSW did not begin forming until about 7 cal ka (Hillaire-Marcel et al., 2001. Nature 410:1073-1077). Therefore, an oceanic explanation for the event requires impacts beyond the Labrador Sea. Although various climate records show changes during the event, it is almost unnoticeable in most North Atlantic marine records, and there is little evidence of freshening in the Labrador Sea. This is puzzling given the geological data implying an immense freshwater flux (1 – 5 Sv) from Lake Agassiz via Hudson Strait to the Labrador Sea (Clarke et al., 2004, Quat. Sci. Rev. 23:389-407). This influx of 100,000 cubic km of freshwater might be expected to cause a distinct drop in the oxygen isotopic compositions of planktonic foraminifera shells, but no clear isotopic excursion is observed. The brief duration of the freshwater pulse (< 1 yr) may be among various reasons for the lack of an observed signal. The fact that clearly identifiable marine O-18 depletions accompany Heinrich events, even though the freshwater fluxs were much lower, signifies a fundamental difference between the two types of freshwater delivery to the ocean. Whereas the lake outburst probably tracked the coast as do modern river plumes, some of the H-event icebergs crossed the Atlantic intact, and thereby supplied freshwater to distant deepwater formation sites. This may be why the lower H-event freshwater flux influenced the overturning circulation more than the Agassiz outburst. In testing alternative, non-oceanic explanations for the 8.2 ka cold event, modelers should consider the sudden change in land surface properties over central Canada (cf. Hostetler et al., 2000. Nature 405:334-337; Dean et al., 2002. Quat. Sci. Rev. 21:1763-1775).