PALYNOLOGICAL EVIDENCE OF CLIMATE- DRIVEN HYDROLOGIC ISOLATION OF LAKE SUPERIOR FROM THE REST OF THE GREAT LAKES; IMPLICATIONS FOR FUTURE CLIMATE SCENARIOS

For more than two millennia following the recession of the Laurentide ice sheet and diversion of glacial meltwater inputs from the Great Lakes basin, katabatic winds continued to influence climate in the proglacial region and the dry Arctic and Pacific airstreams predominated over the Superior basin. Lake Superior was dependent on only precipitation within the immediate watershed for hydrologic input which may have been reduced by as much as 15% from modern values, based on pollen transfer function reconstructions from profiles from small lakes all around the basin. Most of the Lake Superior basin underwent a prolonged period of aridity that is represented in the palynological record by the early Holocene pine zone, with relatively high abundances of xeric herb pollen (e.g., Artemisia cheno-am, grasses) and boreal parkland taxa (Populus, Salix, Picea). Until ca. 7000 cal BP, fossil-pollen based reconstructions of climate record significantly colder winter temperature contributing to greater seasonality and a commensurate reduction in precipitation. A simple hydrologic model suggests that these conditions, combined with isostatic depression as great as 390m northeast of the basin and 128m at Sault St. Marie, isolated the low-level Lake Superior from the other Great Lakes. Current discharge from Lake Superior via the St. Mary’s River is 2,140 m³/s, comprising >3/4 of hydrologic input into Lakes Michigan and Huron. Cessation of this input rapidly led to their hydrologic closure by 7900 cal BP, producing the Chippewa, Stanley, and Hough lowstands in the Michigan, Huron and Georgian Bay basins. This lowstand affected water quality, producing noticeable increases in salinity in late Lake Hough, and elevated a land bridge across the main basin of Lake Huron that allowed caribou and humans to migrate between Alpena MI and Amberley ON, but isolating sub-basins of Lake Stanley. Sensitivity of Lake Superior to reduced precipitation and increased evaporation resulting from increased seasonality only 8000 years ago is an important consideration regarding future trends in climate. Even with such a vast volume of water stored above the Sault St. Marie outlet, a negative water budget would initiate a cascading effect in Great Lakes downstream of Lake Superior.