MEASURING THE RELATIVE CONTRIBUTION OF AEOLIAN SALTATION TO DESERT SILT PRODUCTION

Evaluating processes of silt production in desert environments is necessary when using loess deposits as a paleoclimate indicator. Silt in the earth’s major loess deposits has been linked to both glacial and desert processes. The relative production of loess-sized particles from desert processes is unknown, though it might be significant. In this study, we assess if silt in warm deserts originated within the desert through intergranular collisions, or if upwind processes control the amount of silt exiting deserts to form loess and dust deposits, e.g., glacial grinding and fluvial transport operating beyond the desert margins. To determine the origin of the silt, iron oxide grain coatings are utilized to identify if the silt is the product of recent collisions of sand grains. The process of rubification (i.e., reddening), which coats sand and silt grains with iron oxides, is estimated to take much longer than the time required to produce silt grains through aeolian abrasion. We are testing the hypothesis that examination of silt grains individually with an optical microscope, or in populations with a visible light spectrometer, can be used to measure creation of fresh fracture faces. A fresh fracture face indicates a relatively recent fracture through collision. An abundance of newly fractured silt grains would shift the spectral reflectance curve.

Samples were collected from eight sites in the Algodones dunes of southern California. Silt was separated from the dune sand and inspected for iron oxide coating coverage and fresh fracture faces. A visible light spectrometer was used to look for changes in population color and an optical microscope was used to check individual grains for fresh fracture faces. Preliminary results indicate that reflectance values in the visible range vary with sample location, but only slightly. This might indicate non-uniformity of iron oxide coatings of silt grains throughout the dune field caused by fracturing. Comprehensive results for sample surface reflectance will be presented along with optical microscope observations of silt grain coatings. Quantifying the relative abundance of both populations of silt grains is needed to predict if silt expelled by a desert is controlled by environmental conditions in the desert or by processes outside the desert.