Deglaciation Patterns of the Southern Lobes of the Laurentide Ice Sheet from the Last Glacial Maximum to the Late Glacial

Patterns of meltwater delivery to the ocean are dictated by both rates of meltwater production and routing of that water. The Laurentide Ice Sheet (LIS) was the largest producer of meltwater, hence we here report an undated chronology for its southern margin from the Last Glacial Maximum to the Late Glacial. Evidence indicates the ice sheet margin was south of the Great Lakes by 28.0 ka yr and reached its southernmost extent at 23.0 ka yr not only south of the Great Lakes, but also further west in the Des Moines lobe. Minor (10’s km) oscillations superposed on ~ 100 km retreat occurred from 22 to 17.5 ka yr. From 16.5 to 13 ka yr the retreat rates increased such that by 13 ka yr the ice sheet margin had retreated ~ 500 km. It may be that there is some lag between retreat rates of the three lobes as the Lake Erie and Lake Michigan lobes transitioned from land to lake proximal margins at this time. However the lobe with the smallest retreat, Lake Michigan was in the deepest water so something besides calving drove this retreat. The pattern of glacial advance and retreat is not in phase with what would be predicted from generally accepted climate proxies, such as NGRIP, for the northern hemisphere. However new isotope records from New Mexico suggest variable hydrological patterns during this time interval. For example between 28 and 17.5 ka yr for there is some correlation between that hydrology signal and changes in the ice margin. This may imply accumulation patterns along the ice sheet margin were important. After ~ 17.5 ka yr the large retreat parallels rising global CO2 levels. This suggests that warming from greenhouse gases and insolation were sufficient to drive a major retreat of the Laurentide Ice Sheet.