LATE-GLACIAL PALEOHYDROLOGY OF LAKE AGASSIZ AND THE UPPER GREAT LAKES: INSIGHTS FROM LAKE SUPERIOR’S OXYGEN ISOTOPE RECORD

Late glacial ostracode and bivalve records from the Great Lakes and Lake Agassiz are characterized by extreme oxygen isotopic variations, ranging from values that reflect a source that is primarily glacial in origin (~–26 ‰ PDB) to much heavier values characteristic of a regional meteoric source (~–5 ‰ PDB). Benthic ostracodes (Candona subtriangulata) from Caribou Basin in Lake Superior, have values of –15 ‰ PDB at around 10,500 cal ybp [9.4 ¹⁴C ka], gradually decrease to –23 % PDB by around 9,000 cal ybp [8.1 ¹⁴C ka], then sharply increase to –18 ‰ PDB: in a manner similar to a co-eval record from the northern Lake Winnipeg (Agassiz) basin (Rodrigues and Lewis, 2000. Geological Survey of Canada Open File 3470: 791-794). Paradoxically, coeval records from Lake Huron are much heavier (–10 to –15 ‰ PDB), which increase upsection until around 9,000 cal ybp, then decrease dramatically to –20 ‰ PDB and remain low for several hundred years (the Late Stanley lowstand) (Moore et al., 2000. Paleoceanography 15(1): 4-18).

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.