Æolian dust contributions to soil of the Namoi Valley, northern NSW, Australia
AEolian processes have been regarded as having little relevance to pedogenesis in the subhumid regions of eastern Australia, due perhaps to a perceived paucity of clear evidence of dust deposition and to difficulties of measuring an aeolian dust component within soils. Contrary to earlier opinions, an aeolian soil component may occur in eastern Australia, as appreciable amounts of dust are deposited annually. We report dust deposition rates and some dust characteristics at Gunnedah, and measured granulometric, mineralogical and geochemical attributes of four soil profiles found in different physiographic areas of the Namoi Valley. Estimates of dust deposition rates indicate that a deposit at least 20 cm thick could have formed over the last 13,000 years. The soil profile which most clearly indicates the presence of dust overlies basalt at Bald Hill. Here, the topsoil contains a conspicuous population of particles with a modal diameter of 40-50 mum and has been enriched in Si by the addition of quartz, which cannot have originated from the basalt or from colluvial or alluvial processes. A clayey floodplain soil profile (Boonawa) also contained a large dust-particle population and also illite and moderate concentrations of K of possible aeolian origin. Two sandy profiles at Round Swamp displayed the same conspicuous dust-particle populations, and one exhibited an interstratified illite-smectite component, suggesting dust additions. Local aeolian processes were also indicated by the well-sorted, coarse sand-sized particles of the Round Swamp East profile, located on a low lunette adjacent to the swamp. These fine-grained, quartz-rich, aeolian dust deposits differ considerably from the benchmark Australian aeolian dust mantles, which form the clay-aggregated, calcareous parna soils of southern New South Wales.