Using OSL to assess hypotheses related to the impacts of land use change with the early nineteenth century arrival Of europeans in southeastern Australia: An exploratory case study from Grabben Gullen Creek, New South Wales

A common explanation for intense soil erosion and gullying in SE Australia is the introduction by Europeans of new land use practices following their arrival in Australia in the late 18th century. Eucalyptus woodlands were cleared to introduce farming, and valley bottoms, characterized by chains of ponds with organic-rich swampy meadow (SM) soils, were subsequently buried by thick deposits of 'post-settlement alluvium' (PSA) generated by erosion in the catchment. In this study, optically stimulated luminescence (OSL) is used to evaluate the source(s) of the PSA in Grabben Gullen Creek (GGC), Australia. We use a portable OSLreader to measure total photon counts on bulk polymineral and polygrain-size samples from nine profiles along the Creek. We use these luminescence signals as geotracers of sediment source(s) and transport pathways. We obtained higher luminescence signals in the PSA than in the SM sediments, suggesting different sources and fluvial transport conditions for these two widespread sedimentary units. Portable OSL reader data from soils in the GGC catchment that are potential sources for the SM sediments and PSA show that the high luminescence signals recorded in the PSA are similar to those fromsubsoil samples in granite soils, suggesting that the PSAwas derived by gullying of granite subsoils. In the SM sediments, luminescence signals decrease upwards from the base of the profile, as expected in well-reset fluvial deposits, but with one or more changes in gradient in the profile of photon counts with depth, most likely indicating changes in sediment deposition rates. To calculate deposition rates in the SM sediments, several samples were dated using OSL. The OSL ages produced low scatter in the equivalent doses, confirming the well-reset nature of the grains composing the SM and indicating a process of sediment transport in dilute flows, as is interpreted from the portable OSL signals.