BE-10/AL-26 BURIAL AGES FOR THE MISSOURI TILL SEQUENCE

The glacial sediments of central Missouri consist of five widely recognized tills that are underlain and separated by well-developed paleosols. As a paleosol forms at the surface, it is exposed to the cosmic-ray flux, and accumulates the cosmic-ray-produced radionuclides beryllium-10 and aluminum-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 steadily decreases over time. Thus, the Al-26 and Be-10 concentrations in paleosols buried by tills can be used to determine the time the overlying till was emplaced. We are applying this technique to date the central Missouri till sequence. In previous work, we showed that the lowest till, the Atlanta till, was 2.4 million years old, and appears to record the first major glaciation of North America. In the present work, we are measuring Be-10 and Al-26 in paleosols underlying the other four tills in the sequence, in an effort to establish the chronology of succeding glaciations as well. We will describe the dating method, discuss some complications caused by vertical mixing and other soil-forming processes, report the ages of these tills, and discuss their relationship to other records of the timing of major Laurentide Ice Sheet advances.