2 EPISODES OF MELTWATER INFLUX FROM GLACIAL LAKE AGASSIZ INTO THE LAKE-MICHIGAN BASIN AND THEIR CLIMATIC CONTRASTS

Two episodes of meltwater influx from glacial Lake Agassiz are recorde d as prominent sedimentologic, isotopic, magnetic, and faunal signatur es in southern Lake Michigan profundal sediments. As a tributary to th e main path of eastward Lake Agassiz flow, southern Lake Michigan reco rded only the largest, catastrophic discharges. The distinctive Wilmet te Bed, a massive gray mud that interrupts laminated red glaciolacustr ine clays, marks the first episode, which occurred near the beginning of the Younger Dryas cooling event. The associated discharge may have played a role in the inception or severity of the Younger Dryas event. An oxygen isotope excursion in biogenic carbonate and changes in ostr acode assemblages mark the second episode, which appears to have had a t least two pulses, dated by accelerator mass spectrometer C-14 ages o n biogenic carbonate at about 8.9 and 8.6 ka. The second episode occur red during the early Holocene peak in global meltwater discharge and a pparently had little widespread climatic or oceanographic effect. The contrast between the effects associated with these two episodes of mel twater discharge emphasizes the complexity of the ice sheet-ocean-clim ate system.