The Late Glacial Advance of the James Lobe, South Dakota, Suggests Climate-Driven Laurentide Ice Sheet Behavior

The relationship between climate and independent glacier masses is now understood, but what is not understood is how ice sheets respond during times of rapid climate change. At its maximum extent the southern Laurentide Ice Sheet (LIS) was sourced from two domes that terminated in multiple lobes across central North America. The extent and timing of the eastern lobes, which were sourced from the Labrador Dome are relatively well constrained. Although the extent of the lobes sourced from the western Keewatin Dome is better understood, there is little chronologic data on them. Twenty-six radiocarbon ages recovered from within the drift of the James Lobe from South Dakota are used to reconstruct the timing of late-glacial fluctuations of the James Lobe. Lithologic logs from 21 South Dakota counties were analyzed and provide stratigraphic context for the radiocarbon ages. Analysis of the stratigraphy reveals two distinct glacial till units with a distinct, widespread layer of silt between them. The silt is interpreted here as evidence for interstadial conditions between two separate advances of the James Lobe. Radiocarbon ages of organics from this silt layer and from within the uppermost oxidized till indicate that interstadial conditions persisted from ~15.8 to 13.7 ka, followed by an advance of the James Lobe of at least 230 km to its maximum position at the Missouri River. Comparison to other locations in Wisconsin, northern lower Michigan, and western New York reveals a similar period of interstadial conditions followed by ice margin advance. We correlate this advance across ~1000 km and suggest that the simplest explanation is reduced summer ablation caused by widespread climatic cooling.