A LOW-LEVEL PHASE IN GREAT LAKES HOLOCENE HISTORY, NEWLY-REVEALED BY STUDY OF LAKE SEDIMENTS AND GEOMORPHOLOGY

Evidence of water levels below the present surfaces of the Laurentian Great Lakes was explained historically by overflow through northern outlets which were still differentially depressed by the retreating Laurentide Ice Sheet. This interpretation was challenged following a review of radiocarbon-dated, upwarped Late Wisconsinan and Holocene shorelines which led to reconstruction and comparison of the original elevations of former lakes and outlets. The comparison revealed, in the Huron basin, for example, that water levels had declined into hydrologic closure (>20 m below the basin overflow outlet) between about 7900 and 7500 radiocarbon (~8770 and 8290 cal) years BP.

This hypothesis of closed lowstands has been validated by newly-acquired and re-interpreted older evidence of low water levels in sedimentary records. In the Huron and Georgian Bay basins these data, which also apply to the Michigan basin, include seismo- and litho-stratigraphic evidence of a widespread erosion surface attributed to wave erosion during a period of low lake level, microfossil indications of brackish conditions likely related to enhanced evaporation, and in situ tree stumps up to 43 m below present lake level. Seismostratigraphic evidence of mud-buried beaches implying former water levels 40 to 105 m below present in western Lake Superior has been reported. An investigation of sediments in local basins of the former outlet valley of the upper Great Lakes basins revealed sedimentary hiatuses associated with detrital organics that are explained by cessation of outlet river flow and reduced water levels. A submerged beach represents the equivalent lowstand in the Erie basin, and published paleo-environmental findings have shown that shallow-water conditions occurred in Lake Ontario at the same time.

The closed lowstands at 7900 radiocarbon years BP occurred when the supply of meltwater was cut off from the Great Lakes basins, and the resulting lake water balances became negative as evaporative losses exceeded precipitation in the drier-than-present early Holocene climate. This unique hydrological event provides an opportunity to assess the sensitivity of the Great Lakes system under climatic conditions different than present, a useful parameter for modeling and projecting future lake levels under global warming.