Dating soils and alluvium with cosmogenic 21Ne depth profiles: case studies from the Pajarito Plateau, New Mexico, USA

Abstract

21Ne depth profiles measured from amalgamated samples of sand-sized quartz from the Quaternary Bandelier Tuff were used to estimate inheritance-corrected exposure ages in stream terraces and an alluvial fan on the Pajarito Plateau. Stable alluvium and associated soils display a downward decreasing (Type I) nuclide profile pattern. Bioturbated colluvium with uniform (Type III) nuclide concentrations has buried these alluvial surfaces. Another possible profile type with downward increasing nuclide concentrations (Type II) is described but was not found on the Pajarito Plateau. We present mathematical models for each of the profile patterns relating nuclide concentration to inheritance, time-averaged bulk density, and either exposure time or burial rate. Curve fits of Type I profiles are used to estimate the age of alluvial landforms. A burial correction technique for deposits buried by younger colluvium is also presented. Inheritance- and burial-corrected 21Ne surface exposure ages of 144±17 ka and 63±8 ka for two Pajarito Plateau stream terraces agree within error with independent soil profile development index dates of 160 ka and 68–78 ka. A soil overlying the 50–60 ka El Cajete pumice yields an inheritance-corrected 21Ne exposure age of 44±11 ka that also agrees closely with a 14C age on soil charcoal of 41 ka. These results demonstrate that reliable cosmogenic 21Ne exposure ages can be obtained for fine-grained alluvial surfaces with the depth profile technique.