MELTWATER AND PRECIPITATION RUNOFF TO THE NORTH ATLANTIC, ARCTIC, AND GULF OF MEXICO FROM THE LAURENTIDE ICE SHEET AND ADJACENT REGIONS DURING THE YOUNGER DRYAS

Runoff from North America may have played a significant role in ocean circulation and climate change during the last deglaciation. Because the driving force behind such changes may have been related to salinity of the north flowing Atlantic Ocean conveyor circulation, it is critical to know the volume, timing, and location of fresh water entering the North Atlantic from the melting Laurentide Ice Sheet. During the Younger Dryas cold episode, 11,000-10,000 years B. P., there was a two-fold increase in the volume of meltwater plus precipitation runoff, to more than 1700 km(3) yr(-1), flowing through the St. Lawrence valley to the North Atlantic, mainly because retreating ice allowed the glacial Lake Agassiz basin to drain eastward into the Great Lakes at this time. There was a corresponding decline in discharge from Lake Agassiz through the Mississippi River to the Gulf of Mexico. Runoff to the Arctic Ocean also increased at about the beginning of the Younger Dryas, from 740 to 900 km(3) yr(-1), because of the capture of what is now the head-water region of the Mackenzie River watershed. This, in combination with rising sea level and warming climate, may have increased the amount of pack ice reaching the North Atlantic through the Norwegian Sea from the Arctic Ocean. At 10,000 years B. P., eastward overflow from the western interior of North America was blocked by advancing ice, again forcing overflow to the Gulf of Mexico and, possibly, to the northwest into the Arctic Ocean. Although total runoff to the oceans from all regions draining from the Laurentide Ice Sheet did not vary substantially between 12,000 and 9000 years B. P., if discharge to the Gulf of Mexico is excluded, fresh water reaching the North Atlantic averaged 4000 km(3) yr(-1) during the Younger Dryas, in contrast to 2870 km(3) yr(-1) just before this cold episode and 3440 km(3) yr(-1) just after it.