LATE-GLACIAL PALEOHYDROLOGY OF LAKE AGASSIZ AND THE UPPER GREAT LAKES: INSIGHTS FROM LAKE SUPERIOR’S OXYGEN ISOTOPE RECORD
We suggest the record from Lake Superior reflects summer pulses of glacial meltwater by hyperpycnal flows rather than the average isotopic composition of the greater water body. Average Lake Superior water was probably closer to 18 SMOW (perhaps heavier), rather than 26 SMOW, which would have been required to produce the lightest values recorded by the ostracodes. The much heavier values recorded in Huron may reflect the influx of Lake Agassiz water sourced from regional precipitation, which ponded over Lake Superior's thermocline during the summer months. Age models from cores in Lake Huron and Michigan are re-evaluated to show a strong correlation between a thick varve sequence and light isotopic values in Lake Superior, and the negative Late Stanley isotopic anomalies in Huron and Michigan. This massive 15 excursion in Huron and Michigan records extreme fluxes of water from Lake Superior between 9,400 and 9,000 cal ybp [8.5-8.1 14C ka]. The initial pulses may have been Lake Agassiz catastrophic overflow events (previously labelled A2' by Colman et al., 1994. Geology, 22: 547-550), but later pulses were more likely sourced from anomalous glacial meltwater discharge during glacial surging and moraine formation.