CHRONOLOGICAL STATUS OF GLACIAL LAKE AGASSIZ

Over the past decade there has been a considerable increase in the temporal understanding of glacial Lake Agassiz. With the new chronology data, especially the age on the Big Stone Moraine, an age can now be assigned to the beginning of the lake. Interestingly, the initiation of the lake coincides with a trend towards heavier 18O isotopes in the Gulf of Mexico records before the southern outlet is abandoned. One interpretation is for evaporative enrichment of the lake water and increased runoff from the lake's drainage basin as it was progressively deglaciated. New OSL ages within a few hundred years of each other from the Herman, Norcross and Upham beaches (Lepper et al., 2007-Geology) record rapid incision of the southern outlet during the Lockhart Phase. An interpretation of these closely-spaced ages would require catastrophic recession of the ice sheet even without the embedded readvances historically proposed for the Red River lobe if indeed the Herman beach does trace as far northwards as Riding Mountain, Manitoba. Alternatively, the age gap (as afforded by the error bars) between the beaches is greater, and/or the correlations of beaches northward is over simplified. The Moorhead Phase lowstand is well dated with in situ terrestrial macrofossils, but the chronology for the lowering after the beginning of the Younger Dryas cold period is based upon only one date. Ice-margin and strandline chronology within the basin remains poorly constrained, making it difficult to refine paleo discharge values. The Campbell and Ojata Beaches are the best dated strandlines and the beginning of the Emerson Phase is pushed forward to 9400 rather than 9900 14C yr BP. Our understanding of the lake continues to increase, but clearly, we are far from developing precise paleogeographic reconstructions of the lake until additional ice margins and beaches are dated.