The amphi-atlantic distribution of the younger dryas paleoclimatic oscillation

Abstract

Interest in significant climatic fluctuations affecting large geographic areas but having a time range that is too brief to adduce the Milankovitch radiation cycles has focused on the Alleröd/Younger Dryas event, which is a well-established phenomenon of western Europe manifested by glacial and vegetational reconstructions. It is also shown by the foraminiferal evidence for shifts in the polar front in the North Atlantic. Recent pollen and lithostratigraphic investigations in the Maritime Provinces of eastern Canada and in the till plains of west-central Ohio on opposite sides of the late-glacial spruce forest indicate that the event also left a record in eastern North America. Experiments with an atmospheric general circulation model, with the temperature of North Atlantic surface waters depressed to full-glacial levels, lowered summer temperatures in ‘upwind’ periglacial eastern North America by 2°C — just as far inland as they did downwind in western Europe, where the paleoecological record of cooling is clear. A leading hypothesis for the cause for the Younger Dryas cooling appeals to the influx of tabular icebergs released from arctic ice shelves around the wasting Scandinavian and Siberian ice sheets during the Alleröd warming. The resulting cooling of the North Atlantic waters during the Younger Dryas could be facilitated or precipitated by the abrupt influx of glacial meltwater derived from almost the entire southern margin of the Laurentide ice sheet and emerging from the Gulf of St Lawrence, as brought about by a major shift in drainage direction from the Mississippi River to the St Lawrence River about 10.8 ka BP. A supplemental influx of fresh water may have come from the contemporaneous drainage of the Baltic Ice Lake. The diversion of Laurentide meltwater to the Atlantic was caused by retreat of an ice lobe in northwestern Ontario north of Minnesota, allowing the huge Glacial Lake Agassiz in Saskatchewan and Manitoba to drain into the Great Lakes and more than double the inflow of meltwaters in the North Atlantic, thereby diverting the Gulf Stream and permitting the influx of icebergs from the north to cool the ocean surface. Minor readvance of the ice lobe about 10 ka BP temporarily diverted the Lake Agassiz drainage to the south. With the final retreat of the ice, the drainage again went to the east, but by this time the ice sheets were much diminished and the regional climate had warmed enough so that the effects of the meltwater flux on the Gulf Stream were muted, and the ice sheets bordering the North Atlantic had retreated onto the land and no longer delivered enough icebergs to cool the surface waters. The Younger Dryas interval thus was terminated, and the warming manifested in the Alleröd was renewed.