AN ISOCHRON METHOD FOR COSMOGENIC-NUCLIDE DATING OF BURIED SOILS AND SEDIMENTS

We describe an improved method for dating buried paleosols using measurements of the cosmic-ray-produced radionuclides (10)Be and (26)Al in quartz grains, and apply it to a sequence of intercalated tills and paleosols in central Missouri, USA, that record Plio-Pleistocene advances of the Laurentide Ice Sheet. A buried paleosol implies a period of surface exposure and nuclide accumulation, followed by burial and a halt to nuclide production. If the paleosol is formed in a sedimentary unit such as till, this unit may also have been emplaced with unknown (26)Al and (10)Be concentrations inherited from past surface exposure. If the inherited nuclide concentrations are the same at all depths in the soil-as is true for well-mixed sediments such as till-then the (26)Al and (10)Be concentrations at different depths in the paleosol will show a linear relationship. The slope of this line depends on the duration of burial of the paleosol, but not on the inherited nuclide concentrations or on the sample depths. Thus, one can date strata overlying buried paleosols by measuring (26)Al and (10)Be at multiple depths in the paleosol and calculating the burial age of the paleosol from the resulting isochron. We focus on applying this approach to till-paleosol sequences, but 2 the basic idea of forming an (26)Al-(10)Be burial isochron with a set of samples that share the same burial age, but differ in other aspects of their exposure history, applies to other stratigraphic settings as well. The method yields ages for four tills in Missouri that are stratigraphically consistent, agree with paleomagnetic age constraints, and show that ice advanced into Missouri near 1.25 Ma, near 0.8 Ma, and twice between ca. 0.4 and 0.2 Ma.