Extreme Changes in Great Lakes Paleo-Levels in the Early Holocene

Results

The past low lake conditions were a response to the combined effects of an early Holocene dry climate and an abrupt shortage in water supply when upstream glacial meltwater drainage from Lake Agassiz switched from the Great Lakes (Superior) basin to the Ottawa River valley.

Evidence supporting this hypothesis includes the following:

• Tree stumps have been found in growth position under water up to 43 m below Georgian Bay. These trees would have required lower past water levels.
• Buried organic-rich sand beds from the French River area, an outlet at the time, reveal two separate occurrences when lake outflow stopped and closed-lake conditions commenced.
• Microfossils indicate brackish conditions in Georgian Bay, consistent with a regime of high evaporation.
• Submerged beaches and deep wave-eroded zones establish low water levels in Michigan, Huron, Georgian Bay, and Erie basins. This also occurred in Lake Simcoe, a tributary lake (below).
• Shallow-water fossils and peat deposition beneath deep-water sediments establish low lake levels in the Ontario basin consistent with low water levels in Hamilton Harbour (Delorme, 1996; Duthie et al., 1996).
• A low-level beach was confirmed in the Superior basin (Wattrus 2007, with U. Minnesota-Duluth).

The state of the climate during times of low levels in the Great Lakes is indicated by data derived from the sedimentary record in small lakes within the Great Lakes basin. For example, pollen from Preston Lake sediments reflect the past vegetation and show that the climate during the time of closed-lake conditions was characterized by reduced precipitation and temperature.

Experimental hydrologic modeling of lake closure in terms of precipitation decrease and temperature increase (consistent with expectations of future climate) by the Great Lakes Environmental Research Laboratory shows that the climate during the time of low lake levels in the Great Lakes was substantially drier than today.

Experimental hydrological modeling shows that, relative to the present climate, large decreases in precipitation coupled with increases in mean air temperature would be required to drive each of the present Great Lakes below their outlets. The conditions required in order for the lakes to become closed are indicated along the blue lines for each lake. These values indicate that the past closed-lake climate was substantially drier than the present climate (from Croley and Lewis, 2006). Larger image

Plot of reconstructed water level (blue) and outlet (gray) elevations in the Huron basin. Once the Great Lakes entered the present non-glacial hydrological regime (1), the water level descended to a closed-lake (2) about 20 m below the basin outlet level. This was due to the dry early Holocene climate in which water losses by evaporation from the lake surface exceeded water gains by precipitation in the drainage basin (from Lewis et al., 2008). Larger image

Underwater photograph of a tree stump in growth position in Georgian Bay. Such stumps have been discovered down to 43 m water depth and require past lake surfaces to have been below their root levels for growth (Blasco et al., 1997). Larger image

Logs of cores from a basin within the ancient outlet channel (French River valley) of the upper Great Lakes show buried, organic-rich, sand beds (inset photos), separated by lake deposits. The sand beds represent intervals of closed conditions when receding upper Great Lake water levels fell below the outlet threshold, isolating the basin and reducing its water level (diagram courtesy of Dr. G.R. Brooks, Geological Survey of Canada). Larger image

Wave-eroded zones are seen in the configuration of sediment layers in the lakebed of Lake Simcoe. Low lake levels in Lake Simcoe led to an increase in wave erosion in the basin. This is indicated by the discontinuous bedding planes (shown in the diagram as dark lines within the green unit) at the top of the white arrows. Bedding planes are normally continuous in an accumulating body of sediment (seen in the red, purple, and blue units), indicating limited wave erosion due to higher lake levels (from Todd et al., 2008). Larger image

Pollen diagram from Preston Lake, a small basin between lakes Ontario and Simcoe, with estimates of past annual precipitation and mean monthly air temperatures. During the time when the Great Lakes were very low (long horizontal red rectangle), precipitation was about 200 mm/year less than today. The presence of aquatic peat at that time indicates that Preston Lake nearly dried up in the dry climate (diagram courtesy of J.H. McAndrews, University of Toronto.) Larger image.