Climatic effects of glacial Lake Agassiz in the midwestern United States during the last deglaciation

Abstract

Stable isotope and pollen analyses of a sediment core from Deep Lake, Minnesota, provide new information on the climatic effects of glacial Lake Agassiz in Minnesota and insights into the cause of the prominent Picea recurrence during the Younger Dryas in the southern Great Lakes region. Bulk-carbonate δ18O exhibited large fluctuations between 12.0 and 9.1 ka (dates are in calendar years throughout this paper unless indicated otherwise), probably reflecting the effects of glacial Lake Agassiz superimposed on climatic warming related to large-scale climatic controls. In particular, a 3‰ decrease in δ18O 11.2–10.2 ka interrupted the δ18O enrichment of 1‰ from 12.0 to 11.2 ka and 3.5‰ from 10.2 to 9.1 ka. This δ18O decrease coincided with the expansion of Lake Agassiz. We interpret this decrease as a result of decreased summer temperature and increased precipitation derived from the cold and isotopically light meltwaters of Lake Agassiz. During this δ18O decline, Pinus pollen continued to increase at the expense of Picea pollen at Deep Lake, as at other Minnesota sites, providing evidence that climatic cooling induced by Lake Agassiz did not cause a reversal to a Picea -dominated vegetation. The absence of such a vegetational response implies that the prominent Picea recurrence during the Younger Dryas in the southern Great Lakes region was not caused solely by climatic cooling due to increased flux of meltwater from Lake Agassiz into the Great Lakes. Instead the Picea recurrence might have been driven primarily by the westward penetration of the Younger Dryas cooling.