THE RAPID RISE IN SEA LEVEL AND COASTAL FLOODING RELATED TO THE FINAL DRAINAGE OF LAKE AGASSIZ, WITH SPECULATIONS ABOUT A TSUNAMI

The final drainage of glacial Lake Agassiz at ~8.4 cal (7.6 ¹⁴C) ka occurred in less than a year. This outburst of 5.2 Sv from the world’s largest lake (840,000 km²) occurred when the Laurentide Ice Sheet was breached, allowing ~150,000 km³ of stored water from the southern side of the LIS to pass through Hudson Bay and Hudson Strait into the Labrador Sea and North Atlantic Ocean. This outburst resulted in a rise in global sea level of about 0.4 m. Previous outbursts from Lake Agassiz, which only partially drained the lake, typically released <10,000 km³ of water, and each of those resulted in only a few centimeters of sea level rise. Sediment records along the flood path of the 8.4 cal ka drainage, and records elsewhere in the world, suggest that the final drainage may have occurred in two stages of 3.6 and 1.2 Sv, separated by a few decades. The abrupt rise in sea level resulting from the final drainage probably would have been recorded in shallow-water marine environments such as lagoons and marshes around the world, and would have included abrupt changes in biota and grain size related to changes in water depth, wave energy, and chemistry. Teller et al. (2005, Science of Tsunami Hazards) speculated that this 5 Sv outburst of water may have generated a tsunami in the Labrador Sea, perhaps with an initial height of 10-16 m, and may have been recorded in places around its periphery. The close coincidence in time of the drainage of Lake Agassiz with the Storegga landslide off the coast of Norway, dated at about 8.1-8.2 cal ka, and the subsequent tsunami recorded along the coast of Norway, Scotland, and the Shetland and Faeroe Islands is interesting.