Chronology of the Great Lakes lobes: implications for the dynamics of the Laurentide ice sheet

The Great Lakes Lobes of Laurentide Ice Sheet lie closer to the equator than the north pole. These lobes then reflect the dynamics of a temperate sector of the largest relic ice sheet. This location also affords abundant organic materials for radiocarbon dating that has been underway for several decades and continues. The resulting several hundred individual ages constraint the ice sheet in space and time. Review of this historic and new ages from these lobes, filtered with reproducibility and quality criteria when combined with consideration of stratigraphic context has yield a new chronology. Our preliminary results suggest the following: 1) nearly equal 2/3 of the region displays a similar history of ice sheet growth and decay, the remaining 1/3 has one time slice with a common history; 2) the ice sheet expanded into the larger region three times ( nearly equal 27 ka; 23.5 ka; 21.5 ka); 3) the ice sheet retreated from the larger region after nearly equal 21 ka rapidly at first, but in a step wise manner forming moraines from 20.0 to nearly equal 16.5 ka, 4) the smaller area was not reoccupied at 21.5 ka. One implication is the Last Glacial Maximum was maintained for between 5 and 10 ka with three fluctuations. Two working hypothesis emerge 1) the smaller area was dynamically linked with the larger area for only one expansion. If so, reconstructions of the flow line geometries require reevaluation. 2) the main source of ice for the Great Lake Lobes was from the NE, not the N as most current modeling experiments suggest. If so, the existing models of mass delivery could be updated. Both imply that a simple climate forcing for the ice sheet is inadequate.