LUMINESCENCE CHRONOLOGY CONSTRAINS GLACIAL HISTORY OF THE GREAT LAKES/ST. LAWRENCE RIVER SECTOR OF THE LAURENTIDE ICE SHEET DURING MIS3

The Great Lakes/St.Lawrence River (GL-SLR) drainage network offers unique paleogeographical constraints for deciphering the evolution of the Laurentide Ice Sheet (LIS) during the last glaciation. Indeed, the paleogeography requires that from the moment the LIS had crossed the SLR in the Quebec City area, a significant fraction of the surface drainage in NE North America is diverted towards the upper reaches of the S.LR drainage basin. Physical stratigraphy exposed in the St. Lawrence Lowlands and the Appalachians of Southern Quebec as well as along the shores of Lake Ontario and Lake Erie suggests a complex history of ice margin fluctuation with consequent periodical damming of meltwater and short but intense pulse of fluvioglacial sedimentation. Fossil organic and carbonate (mostly shells) material is found to be older or at the limit of the radiocarbon method. Fortunately, a large number of state-of-the-art Infrared Stimulated Luminescence (IRSL) dates have yielded depositional ages for these waterlaid lithostratigraphic units. The luminescence ages measured on K-feldspar are based on classical fading-corrected methods as well as on a new post-IRSL approach known as post-isothermal IRSL (pIt-IRSL) in which an age is obtained from presumably stable and well-bleached traps. Thence, a more robust age control for the events leading to the Last Glacial Maximum (LGM; MIS2) is now available. The transition from the geographically restrained and probably short-lived glacigenic paleoenvironments of MIS4 to the early MIS3 ice free landscape is poorly exposed and probably obscured by erosion. However, several Mid-Wisconsinan sections expose aggrading distal sandy and silty outwash in glacio-isostatically subsiding basinal areas. The largely similar IRSL ages of ca 35 to 50 ka obtained for stratigraphically equivalent sediments collected in different sections of the GL-SLR might indicate rapid ice build-up and expansion in the mid part of MIS 3 or it may reflect a lack of age resolution in the IRSL chronology. Nonetheless, the chronological and paleogeographical constraints emerging from these studies shall contribute to better assess the age and continental ice volume during MIS3 in the NE sector of the LIS.