An outstanding climate anomaly 8200 years before the present (B.P.) in the North Atlantic is commonly postulated to be the result of weakened overturning circulation triggered by a freshwater outburst. New stable isotopic and sedimentological records from a northwest Atlantic sediment core reveal that the most prominent Holocene anomaly in bottom water chemistry and flow speed in the deep limb of the Atlantic overturning circulation begins at '8.38 thousand years B.P., coeval with the catastrophic drainage of Lake Agassiz.