EARLY TO MIDDLE HOLOCENE HYDROLOGIC LAKE CLOSURE: DRY CLIMATE IMPACT ON THE GREAT LAKES OF NORTH AMERICA

Lines of C-14-dated evidence based on >20 stratigraphic sites registering regression or transgression, >1000 km of seismic profiles revealing lowstand sediment unconformities, biostratigraphic and paleolimnological information, and 8 submerged tree stumps in growth position define the lake-level history in Huron and Georgian Bay basins between 11 and 6 ka BP. Altitudes of both the lake-level indicators and possible lake overflow outlets were reconstructed to their original elevations by application of a new regional model of exponential glacial rebound fitted to observed upwarping of paleo-lake shorelines in each basin. A plot of the reconstructed elevations vs age shows that 2-3 lake lowstands between highstands were metres to tens of metres below possible outlets at 7.9-7.5 ka, 9.3-9.1 ka, and possibly before 9.8 ka, and thus were closed basins. These newly recognized closed lake phases could only result from former dry climates in which evaporation outpaced inflows from precipitation and runoff. We infer a dry climate persisted throughout the early Holocene, 9.8-7.5 ka; the resulting closed lakes were interrupted by highstands caused by episodic inflows from upstream glacial Lake Agassiz. A low-level beach and shoreface beneath eastern Lake Erie also exist without an overflowing outlet about the time of the Huron-Georgian Bay lowstands, as the age of overlying sediment indicates the shoreface erosion occurred prior to 7.6 ka. Thus the former dry climate was spatially extensive and probably affected the entire Great Lakes drainage system.

As the termination of the youngest lowstand is coincident with the demise of the ice sheet in Hudson Bay and the drainage of associated large glacial lakes Agassiz and Ojibway to the sea, an association of the Laurentide ice sheet and the dry climate in the Great Lakes basin is implicated. We infer the Great Lakes drainage basin was subject to evaporative stress throughout the deglacial period by outbreaks of dry air off the Laurentide ice sheet, in combination with dominant dry airflows from the west, indicated by recent findings of Pacific-type ENSO signals in Lake Huron rhythmites after 10 ka, and by middle Holocene desiccation of southern Lake Winnipeg, west of Superior basin, associated with eastward migration of dry grassland vegetation.