Long-term accumulation of atmospheric dust in rocky deserts

The spatial pattern of long-term (hundreds to thousands of years) accumulation of dust in rocky deserts was investigated in the northern Negev Desert of Israel. The concentration of dust in the desert subsoil was measured at 41 locations in a 53 ha test area for which detailed information exists on contemporary dust deposition and dust erosion rates. Results show that the spatial pattern of the long-term accumulation is strongly affected by topography. Highest accumulation occurs in the valleys, especially those having a large catchment area, and on flat surfaces in a plateau position. Less, but still important, accumulation is observed on concave windward slopes and also, but still less, on concave lee slopes. Moderate accumulation occurs where the curvature of windward slopes changes from concave to convex, and also on slopes parallel to the wind. Little accumulation is observed on the convex windward slopes. Finally, the least accumulation occurs on convex lee slopes. The dust accumulation pattern in the test area reflects the effects exerted by aeolian as well as hydraulic processes. The pattern is clearly hydraulic in the valleys, especially on the valley floors (which are characterised by thick water-supplied deposits), and also, but not always, on the lower valley slopes (where colluvia may occur). Outside the valleys and the lower slope sections the dust pattern is merely aeolian.