Biolevitation of pebbles on desert surfaces
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Certain desert surfaces called desert pavements are characterized by a nearly stone-free layer of fine granular material, mostly silt (diameter of hundredths of mm) and sand (tenths of mm) capped by a monolayer of pebble-sized (few cm) stones. The fine granular material is deposited as dust from the air, so the stones must be levitated to accommodate emplacement of this material. In so-called “normal grading” of stratigraphic deposits of geologic material, as in many stream deposits, finer grained material tends to settle on top of coarser grains which have faster settling velocities. Anecdotal observations in the field suggested that the observed “reverse grading” of pavement surfaces is a consequence of the activity of small animals that provide through their activities sufficient input of energy to the surface to cause large particles to “float” on the accumulating fine sediment. To test this idea a laboratory experiment with a test organism, the large desert beetle Eleodes, was carried out to observe possible sorting process. Essentially random foraging motions of the beetles in an environment in which fine sediment was periodically added from above led to sorting of stones that were moveable by the organisms, burial of immovable stones, as well as burial and diffusion of marked grains that were comparable in size to the fine sediment. These results help explain the existence of a geologic deposit with an unusual layering of grain sizes.
KeywordsDesert pavement Sedimentation Bioturbation Size sorting
I would like to thank B. T. Werner, T. A. Tombrello, and D. J. Furbish for their help.
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