Burial Ages for Middle Pleistocene Glacial Deposits of the Laurentide Ice Sheet

Subglacial erosion and sediment dynamics influence the size, stability, and climatic sensitivity of large ice sheets as well as the landscape they leave behind. In the regions surrounding the former Laurentide Ice Sheet (LIS) thick deposits of glacial sediment contain a rich record of a) the extent and dynamics of the ice sheet during many glacial cycles, and b) the rate and timing of Pleistocene glacial erosion of the Canadian Shield. This record is poorly accessible at present due to difficulties in determining the age of glacial deposits older than the limit of radiocarbon dating. In order to better interpret this record of long-term ice sheet erosion, we have attempted to determine the age of a series of glacial and interglacial sediments in Minnesota, USA, using a technique of "burial dating" based on the different rates of decay of the in-situ-produced cosmogenic isotopes 10Be and 26Al in sediments first exposed to cosmic rays at the earth's surface and then buried for an extended time. We measured 10Be and 26Al on eight samples of fluvial or glaciofluvial sediment interbedded between tills previously thought to be middle Pleistocene in age. 26Al/10Be ratios were lower in stratigraphically lower units, and ratios were indistinguishable in units previously correlated based on their position and lithology. Thus, this may be a viable method for determining the age of these othewise hard-to-date sediments and may result in significant progress toward correlating terrestrial and marine records of Pleistocene glaciation. Results for sandy sediments within one set of tills are consistent with 10,000-14,000 years of exposure followed by 0.8 +/- 0.1 Myr burial; those within another group of tills indicate similar exposure times followed by 1.1 +/- 0.1 Myr burial. Subject to several assumptions, these results give the age of overlying till units. If correct, they corroborate other evidence suggesting that the LIS achieved its maximum known extent prior to the large oxygen-isotope excursions of the late Pleistocene.