WHEN DID VESICULAR A (AV) HORIZONS FORM IN THE DESERT SW U.S.: ELUCIDATING BETWEEN SOIL PROCESSES AND LUMINESCENCE AGES

Vesicular A (Av) horizons, and associated overlying desert pavement, are ubiquitous features across desert environments. Extensive research has demonstrated that Av horizons develop from the incorporation of dust during soil development; however, two conflicting models have emerged regarding the age of Av horizon formation. Numerous published luminescence (OSL) ages suggest that Av horizons are Holocene, with reported ages commonly ≤5 ka. In addition, other studies have suggested Av horizons and desert pavements are Holocene in age because Late Pleistocene environmental conditions largely destroyed desert pavements and Av horizons prior to the Holocene, especially for surfaces above 400 m elevation. In contrast, time-related trends in the morphology of Av horizons suggest that Av horizons and pavements must have existed prior to the Holocene.

Geochronology and soil morphology from two soil chronosequences formed on alluvial fans in the Mojave Desert (soils ~0.5 ka to ~100 ka, ~900 m asl) and in the Sonoran Desert (soils ~0.5 ka to ~250 ka; ~200 m asl) indicate that Av horizons existed prior to the Holocene, that the strength of Av development coincides with increasing age of the fan surface. In both chronosequences, Av horizon properties of eolian derived silt and clay, development of soil structure, and horizon thickness, all systematically increase with surface age on soils with no evidence of past erosion or substantial soil mixing. Soil morphology and depth profile relations further support that soil profiles are intact with no evidence of erosion or mixing. OSL dates of Av horizons are considerably younger than soil profiles dated with cosmogenic nuclides and OSL. Some examples include: Av: 5 ka/soil: 8-12 ka; Av: 1-3 ka/soil: 16-21 ka; Av: 2-6 ka/soil: 50-60 ka; Av: 1 ka/soil: 250 ka. Mixing of the Av and episodic addition of Holocene dust cannot account for age inconsistencies. Recent research using OSL for thermochronology indicates that the closure of electron traps occurs between 35^o to 50^o C. Measured hourly temperatures in Av horizons (Sonoran Desert) commonly exceed 50^o C May through September. We suggest that younger OSL ages for Av horizons may be due to high soil temperatures.