Æolian dust deposition rates, particle-sizes and contributions to soils along a transect in semi-arid New South Wales, Australia

Abstract

The entrainment, transport and deposition of æolian dust are important processes affecting soil development at the margins of deserts. To assess accurately the impact of deposited æolian dust on soil development, it is important not only to measure total dust deposition rates, but to distinguish additions of remotely and regionally sourced dust from locally derived material as well (which should not be viewed as new soil material as it is derived from æolian re-mobilization of an existing soil). Because of the well-established relationship between dust particle-size and distance travelled from source, the particle-size distribution of deposited dusts can be used to identify the distance to probable source regions, in addition to identifying matching topsoil particle populations. Three dust traps were located along a transect of semi-arid south-eastern Australia, to measure æolian dust deposition rates between late 2000 and late 2001. The particle-size distributions of selected dust deposits were measured and, with the aid of meteorological data, probable dust source regions determined. Particle-size distributions of the topsoils at each dust trap location were also measured to determine if any dust and soil particle populations matched. Although the sampling period was relatively short, and there was relatively little dust storm activity in this part of Australia during this time, a clear pattern of diminishing total dust deposition was measured downwind along the south-east dust path. Dust deposition rates were also moderate to low by global standards. Source regions of deposited dust were interpreted as being: (i) a combination of south-western and local sources; (ii) a combination of north-western and local sources; and (iii) a mixed source where dust was rained-out. The relative importance of these sources was correlated strongly with seasonal weather conditions, although the proportion of local dust in each deposit was greater at the arid (western) end of the transect. The effects of deposited dust on soil profile development are greatest at the western and central transect locations. At the western location, a fine-grained particle population distinctive of the south-western and north-western regional dusts and a coarse silt-sized particle population characteristic of local dusts, are also present in the topsoil, altering the texture of this Arenosol derived from dune sand. At the central location, where a Calcisol has formed in lacustrine lunette sediment, local dust of the same size as the dominant silt population of the topsoil continues to be deposited, while minor topsoil populations of very fine-grained silt match regional and long-distance dusts deposited at the site. Deposited dust appears to have been a less important soil development factor at the more humid eastern site because of the much smaller rates of dust deposition there. Nevertheless, a minor population of very fine silt particles in the Vertisol topsoil matches long-distance dusts deposited at the site, suggesting a small input of dust to this alluvial soil. The particle-size methodology used here has applications in other studies of dust contributions to peri-desert soils. In particular, particle-size distribution 'smoothness' may provide an indication of whether soils have simple or complex origins.