BE-10/AL-26 ISOCHRON AGES FOR EARLY AND MIDDLE PLEISTOCENE ADVANCES OF THE LAURENTIDE ICE SHEET INTO MISSOURI

The glacial sediments of central Missouri consist of five widely recognized tills, all of which predate the 'Illinoian' (Stage 6) glaciation, that are underlain and separated by paleosols. We have used a method based on the cosmic-ray-produced radionuclides beryllium-10 and aluminum-26 to date these tills. As a paleosol forms, it is exposed to the cosmic-ray flux, and accumulates Be-10 and Al-26. When the paleosol is buried by an overlying till that is thick enough to effectively shield it from cosmic rays, nuclide production stops, and the accumulated inventories of Be-10 and Al-26 decay. Al-26 decays more rapidly than Be-10, so the Al-26/Be-10 ratio decreases with increasing burial time. This basic idea is complicated by i) the unknown amount of inherited Be-10 and Al-26 in the tills that form the parent material for the paleosols, and ii) mixing during soil formation and subglacial deformation, which disturb the depth-nuclide concentration relationship in the paleosol. These complications can be overcome by measuring Be-10 and Al-26 concentrations at several depths in the soil; these measurements should lie on a straight line whose slope reflects the burial age of the soil. Be-10 and Al-26 measurements from five paleosols indicate that the Laurentide Ice Sheet advanced into Missouri near 2.6 Ma, 1.2 Ma, and 0.8 Ma, and twice between 0.5 Ma and 0.25 Ma.