AL-26/BE-10 BURIAL AGES FOR EARLY AND MIDDLE PLEISTOCENE GLACIAL DEPOSITS OF THE LAURENTIDE ICE SHEET

The thick sequences of glacial sediment near the margins of the former Laurentide Ice Sheet directly record the extent and dynamics of this ice sheet during many glacial cycles. They are also hard to interpret or relate to other Pleistocene climate records because of the difficulty in determining the age of glacial deposits older than the limit of radiocarbon dating. In order to better interpret these deposits, we have attempted to determine the age of a series of glacial and interglacial sediments in Minnesota and South Dakota, USA, using a technique of "burial dating" based on the different rates of decay of the in-situ-produced cosmogenic nuclides Be-10 and Al-26 in sediments initially exposed to cosmic rays at the earth's surface and then buried for an extended time. We measured Be-10 and Al-26 on 27 samples of fluvial or glaciofluvial sediment interbedded with pre-Wisconsinan tills. Al-26/Be-10 ratios are lower in stratigraphically lower units and indistinguishable between units previously correlated based on their position and lithology. Simple burial ages calculated from these ratios range from 0.8 Myr to 3.4 Myr and suggest that the two major unconformity-bounded packages of tills in the field area record glaciations at approximately 0.8 +/- 0.1 Myr and 1.1 +/- 0.1 Myr. These ages are stratigraphically consistent with the age of the Lava Creek Ash, the only independent chronostratigraphic marker near the field area. However, analogous modern and Wisconsinan glacial sediments have anomalously low Al-26/Be-10 ratios (equivalent to 0.2 - 0.4 Myr burial), suggesting that we may need to account for incomplete resetting of recycled glacial sediments in calculating numerical ages from our data. Overall, this may be a viable method for determining the age of hard-to-date older glacial sediments and may result in significant progress toward correlating terrestrial and marine records of Pleistocene glaciation. If correct, these initial results would corroborate other evidence suggesting that the LIS achieved its maximum known extent prior to the large oxygen-isotope excursions of the late Pleistocene.