THE SEA-LEVEL SIGNATURE OF THE CATASTROPHIC FINAL DRAINAGE OF THE LAKE AGASSIZ SYSTEM

The proglacial Lake Agassiz system that occupied large parts of North America during the retreat of the Laurentide Ice Sheet, catastrophically drained into different directions at several occasions during its ~5000 year lifetime. The final drainage of Lake Agassiz is now believed to be the largest megaflood of the past 100 kyr, triggering the “8.2 ka event”, a prolific, century-scale climate swing characterized by abrupt cooling in the North Atlantic region and beyond. The catastrophic meltwater release led to sudden freshening of surface ocean waters, reduction of thermohaline circulation, and diminished heat transport to high latitudes. So far, no attempts have been made to measure the sea-level rise associated with the 8.2 ka event. The amount of near-instantaneous sea-level rise would enable an estimate of the total freshwater flux that caused the 8.2 ka event, a critical boundary condition in coupled ocean-atmosphere models that attempt to elucidate the climatic response to such forcing.

We have collected new sea-level data in the western Mississippi Delta where we identified two sites with sedimentary successions ~14 m below present mean sea level that predate and postdate the 8.2 ka event, respectively. At the stratigraphically deeper site, the Pleistocene basement is onlapped by 2 cm of peat which in turn is sharply overlain by a lagoonal deposit with brackish-water bivalves. In contrast, the shallower site contains 60 cm of basal peat. These data suggest that the deeper site was rapidly drowned due to sea-level rise that exceeded the rate of peat accumulation, while at the shallower site slower rates of sea-level rise prevailed. The intervening transgressive event provides a smoking gun for abrupt sea-level rise. Radiocarbon dating confirms that the two sites bracket the 8.2 ka event. The vertical separation between the two dated levels is 1.19 m, constraining the maximum amount of sea-level rise that can be attributed to the 8.2 ka event. This meltwater signal reflects liquid water released directly from the Lake Agassiz system, plus meltwater from the rapidly collapsing Hudson Ice Dome. Although our findings leave open the possibility that the actual amount of sea-level rise may have been considerably less, they do rule out some estimates of meltwater volumes, as high as 5∙10^14 m3 (equivalent to ~1.4 m of sea-level rise), that have been used in the recent literature.