CLIMATIC EFFECTS OF GLACIAL LAKE AGASSIZ IN THE MIDWESTERN UNITED-STATES DURING THE LAST DEGLACIATION

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 delta(18)O exhibited large fluctuations betwee n 12.0 and 9.1 ka (dates are in calendar years throughout this paper u nless indicated otherwise), probably reflecting the effects of glacial Lake Agassiz superimposed on climatic warming related to large-scale climatic controls, In particular, a 3 parts per thousand decrease in d elta(18)O 11.2-10.2 ka interrupted the delta(18)O enrichment of 1 part s per thousand from 12.0 to 11,2 ka and 3.5 parts per thousand from 10 .2 to 9.1 ka This delta(18)O decrease coincided with the expansion of Lake Agassiz, We interpret this decrease as a result of decreased summ er temperature and increased precipitation derived from the cold and i sotopically light meltwaters of Lake Agassiz, During this delta(18)O d ecline, Pinus pollen continued to increase at the expense of Picea pol len 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 respo nse implies that the prominent Picea recurrence during the Younger Dry as in the southern Great Lakes region was not caused solely by climati c cooling due to increased flux of meltwater from Lake Agassiz into th e Great Lakes. Instead the Picea recurrence might have been driven pri marily by the westward penetration of the Younger Dryas cooling.