Paper No. 12-5
Presentation Time: 10:55 AM
STONGLY DUST-INFLUENCED SOILS AND LANDSCAPE DYNAMICS IN VEGETATED ARIDLANDS OF THE SOUTHWESTERN U.S
Dust plays an important role in the development of aridland soils, given abundant dust influx and the limited magnitude of chemical weathering. Research in the Cima volcanic field in the Mojave Desert showed that dust-dominated soil development associated with the formation of desert pavements is different from that proscribed by the “canonical” A/B/C model of V. Dokuchaev, the profile model most familiar to many Earth scientists. The “upward growth” model of desert pavement formation profoundly contrasts with the “deflation” model and also is associated with a fundamentally different mode of soil profile development: accretionary soil development. Accretionary soil development is also an important mode of soil profile development in semi-arid regions where desert pavements cannot form. Increased plant density favors dust entrapment rates necessary to promote it and provides a surface stabilizing function. The hillslopes of aridlands are excellent dust traps, and in favorable circumstances, sustained dust entrapment promotes thick, vegetated and smooth soil-mantled hillslopes. Toposequence studies in the southwestern U.S. indicate that dust-entrapping colluvium is probably necessary to maintain these soils on hillslopes over long periods of time. Vegetation also promotes entrapment of dust, but vegetation on hillslopes is highly sensitive to episodic drought and/or fires that temporarily eliminate or drastically reduce vegetation cover. Episodically increased erosion potential prevents sustained accretionary soil development. Certain types of bedrock in aridland hillslopes weather rapidly and favor the development of soil-mantled, transport-limited hillslopes, but thin, weakly developed soils are prone to rapid erosion, especially when subject to a combination of extreme drought followed by large increases in rainfall. The results of a half century of “CLORPT”-based soil geomorphological research in aridlands, complemented by studies of the behavior of associated biotic communities, suggest that maintaining two necessary conditions fundamental in the application of soil production functions in studies of hillslopes (transport solely by diffusive processes and steady state soil thickness) for thousands to tens of thousands of years is highly unlikely in many aridland settings.