THE SOUTHERN MARGIN OF THE LAURENTIDE ICE SHEET DURING TERMINATION 1

The Laurentide Ice Sheet (LIS) reached well into the middle latitudes during the LGM. The advance and retreat of the south-central lobes of the LIS were perhaps the most responsive to temperature. Here we present a refined chronology for these lobes to assess factors that influenced the last termination. By about 29.0 ka, the Lake Michigan lobe (LML) had extended as far south as 41.5° N latitude. After retreating north of Milwaukee, WI, the LML readvanced to its terminal position at 23.0 ka, about the same time as other lobes in southern Ohio. Both regions experienced minor (10’s km) oscillations occurred from 22.0 to 17.5 ka with a major retreat after 16.9 ka. From 16.5 to 13 ka the retreat rates increased, and by 13 ka the ice sheet margin had retreated ~500 km. This collapse of ice sheet cannot be fully explained by the transition from terrestrial to lacustrine margins and associated calving. The LML had shortest retreat distance was in the deepest water; the Des Moines lobe had the longest retreat distance, and was land based. Overall this pattern of glacial advance and retreat is not in phase with generally accepted climate proxies, such as the NGRIP ice core, nor is it parallel to the insolation signal at this time and latitude. New isotope records from New Mexico speleothems have a closer correspondence with the southern lobes record suggesting position of the jet stream may explain the ice margin changes between 28 and 17.5 ka. After ~16.9 ka the collapse begins at the same time as increased global CO₂ levels suggesting the forcing(s) that influenced the collapse of the LIS originated in the southern hemisphere.