USING LUMINESCENCE DATING TO DECIPHER TIMING AND ORIGIN OF EOLIAN AV SOIL HORIZONS, SOUTHWEST US

The age and origin of eolian-derived vesicular A (Av) soil horizons developed beneath desert pavements has been widely debated. Here, we apply optically stimulated luminescence (OSL) dating to determine the age distribution of wind blown sediment in these soil horizons formed on Holocene deposits. Our technique provides a range of ages for grains that have accumulated in the soil over thousands of years, providing a more detailed picture of when dust deposition led to Av soil horizon development. This research focuses on Holocene soils developed primarily on alluvial fans and reveals that 1) Av dust began to accumulate on fan surfaces soon after alluvial fan deposition ceased, 2) Av horizons are constructed by numerous pulses of dust over time, and 3) the outer margins of Av soil peds are younger than ped interiors. Timing and source of dust pulses is likely a function of pluvial lake desiccation, alluvial fan activity, and intermittent eolian sand transport. One model of Av horizon development includes incorporation of sand and silt into Av horizons along vertical ped faces, with finer silt and clay moved towards ped centers along horizontal, platy fabric. Particle size analyses reveal that clay percent increases in ped interiors over time. Our OSL ages support this model of soil development by demonstrating that ped interiors are consistently older than ped exteriors. Soil forming processes in Av horizons translocate grains downward and into underlying soil horizons, resulting in an accumulation of dust in subsoil horizons that is thoroughly mixed.