NUMERICAL AGES FOR PLIO-PLEISTOCENE TILL-PALEOSOL SEQUENCES BY COSMOGENIC-NUCLIDE DATING

Although the northern continents are covered with thick sequences of glacial sediment that record the advance and retreat of continental ice sheets during the last several million years, most of what we know about these ice sheets is indirect information extracted from marine sediment records. This is because it is difficult to determine the age of tills that are too old for radiocarbon or luminescence dating techniques, which makes it equally difficult to correlate most pre-late-Pleistocene till sections with each other or with other paleoclimate records. Here we show that the cosmic-ray-produced radionuclides Al-26 and Be-10, which are produced in quartz exposed at the surface and then decay when this quartz is buried by the emplacement of younger sediment, can be used to date pre-late-Pleistocene till-paleosol sections. We have applied this method to tills in Missouri, Nebraska, Minnesota, and South Dakota. Tills in Missouri, where long periods of surface exposure and soil formation result in well-developed paleosols and high nuclide concentrations, can be dated very accurately. These tills record some of the earliest advances of the Laurentide Ice Sheet (LIS) and show that the initial development of the LIS was in fact coeval with Northern Hemisphere cooling inferred from marine records near 2.4 Ma. The thick section of tills in southwestern Minnesota and adjacent South Dakota is harder to date because periods of surface exposure were shorter, soils are poorly developed, and nuclide concentrations are lower. The oldest till we have yet identified in this region was deposited 1.5-2 Ma, and an intermediate-age till was deposited 1-1.2 Ma. Overall, the technique is most successful where well-developed paleosols are buried by thick tills. Thus, our eventual goal is to locate such sites throughout the glaciated area of central North America, and thus develop a better picture of the early history of the LIS.