An assessment of the minimum timing of ice free conditions of the western Laurentide Ice Sheet

This study presents an investigation of the minimum timing of retreat of the western Laurentide Ice Sheet, which covered parts of Canada and the northern United States during the Wisconsin glaciation. The retreat of the ice sheet is poorly constrained due to low spatial resolution of chronological data that indicate ice free conditions. A limitation of radiocarbon and luminescence dating methods is that it is only possible to determine the time when it is certain that ice is absent from a region, which may differ substantially from the true date of ice margin retreat. The minimum timing of ice free conditions is determined using a model of ice sheet retreat, inferred from landforms that formed during the late stages of glaciation, and extrapolating the calibrated probability distribution of chronological data in the opposite direction. The analysis excludes radiocarbon samples from materials that have been shown to give artificially old ages, such as bulk sediments. The results of the analysis identify 55 dates that have the most impact on the model. The retreat of the western Laurentide Ice Sheet commenced by at least 15 000 cal yr BP, while the minimum timing of ice free conditions between the Laurentide Ice Sheet and the Cordillera is after 11 000 cal yr BP. A lack of data in the Keewatin sector prevents the determination of a precise timing of ice free conditions in the last vestiges of the Laurentide Ice Sheet. The model provides a guide to where additional samples could improve chronological control on ice margin location.