DID LAKE AGASSIZ CAUSE THE YOUNGER DRYAS COLD EVENT?

Since the mid 1970’s, Lake Agassiz and its larger drainage basin have been hypothesized to have caused a reduction in Atlantic meridional overturning circulation (AMOC) strength during the last deglaciation when its runoff was routed eastward through the St. Lawrence River to the northern North Atlantic upon southern Laurentide Ice Sheet retreat out of the Great Lakes. A reduction in freshwater discharge down the Mississippi River to the Gulf of Mexico at the onset of the Younger Dryas cold event ~12.9 ka suggested that such a routing event likely caused this decrease in AMOC strength. If the drop in the level of Lake Agassiz occurred in 1 yr, the routing of runoff away from the Mississippi River could have been accompanied by an ~0.3 Sverdrup (1 Sv=1x10⁶ m³ s^-1) flood. Others have concluded, however, that a freshwater anomaly was lacking in the Gulf of St. Lawrence during the Younger Dryas, that the eastern outlets of Lake Agassiz which showed evidence for a flood were still ice covered at the start of the Younger Dryas, and that the fall in Agassiz lake level may have occurred after the start of the Younger Dryas. These observations have led to the suggestion that Lake Agassiz runoff was routed to the Arctic Ocean at the start of the Younger Dryas, or that an enclosed basin existed in Western Canada during much of the Younger Dryas and thus Lake Agassiz runoff did not cause the Younger Dryas. Note that coupled climate models consistently show that short-lived floods of the magnitude suggested from Lake Agassiz during the Younger Dryas do not significantly impact AMOC. The lack of evidence for a flood is therefore not proof that Lake Agassiz runoff did not cause the Younger Dryas. Here I will review the terrestrial and marine geologic evidence that constrain the routing of Lake Agassiz runoff during the Younger Dryas. I will show that 1) the drop in Lake Agassiz level and abandonment of the southern outlet were concurrent with the opening of the eastern outlets of Lake Agassiz at the start of the Younger Dryas. 2) The greatest freshwater proxy-anomaly in the North Atlantic during the Younger Dryas was off of the Gulf of St. Lawrence, consistent with eastward routing of Lake Agassiz runoff. 3) Northward routing of Lake Agassiz runoff more likely occurred near the end of the Younger Dryas at ~12.0 ka rather than at its beginning.