May 8, 2007: An Ice Age megaflood and the 8200 BP cold event: Goldilocks or fool's gold?

Garry Clarke, University of British Columbia


The most conspicuous climate event of the past 10,000 years occurred while the Northern Hemisphere was exiting from the last Ice Age and is commonly known as the “8200 BP cold event”. The timing of this event appears to coincide with a geologically remarkable flood - the final draining of glacier-dammed Lake Agassiz, situated along the southern margin of the North American ice sheet. This was the largest freshwater flood of the past 100,000 years and the volume of released water was more than ten times that of Lake Superior , Earth's largest contemporary freshwater lake. Model-based estimates place the peak discharge at around 5 Sv and the flood duration at less than a year. The coincidence of a gigantic freshwater flood and an abrupt climate event has emboldened some to dub this the “Goldilocks event”, because everything seems to be just right. But were they causally linked? Earlier abrupt climate change events seem to have been associated with ocean circulation changes in response to freshening of the North Atlantic , either by redirection of deglacial meltwater or by melting of iceberg armadas launched from the North American ice sheet. The switching mechanism for which there is the strongest evidence is that associated with changing the operation of the North Atlantic meridional overturning circulation (MOC). Thus, from this standpoint, the Agassiz megaflood presents a near-ideal case for testing this idea because the volume, site, and rate of delivery of the freshwater are well constrained. Several recent modeling studies, aimed at simulating the 8.2 kyr event, support the idea that the flood triggered a change in the MOC but there is scant evidence in the marine sedimentary record to support this claim. In this study we combine hydraulic modeling of the flood forcing, with coupled ocean-atmosphere modeling of the climate response to explore approaches to reconciling the model predictions with the paleoenvironmental evidence. The story that emerges owes more to Alice than to Goldilocks.