Climate controls on coupled processes of chemical weathering, bioturbation, and sediment transport across hillslopes

Most hillslope studies examining the interplay between climate and earth surface processes tend to be biased towards eroding parts of landscapes. This limitation makes it difficult to assess how entire upland landscapes, which are mosaics of eroding and depositional areas, evolve physio-chemically as a function of climate. Here we combine new soil geochemical data and published ¹⁰Be-derived soil production rates to estimate variations in chemical weathering across two eroding-to-depositional hillslopes spanning a climate gradient in southeastern Australia. At the warmer and wetter Nunnock River (NR) site, rates of total soil (–3 to –14 g m^-2 yr^-1; negative sign indicates mass loss) and saprolite (–18 to –32 g m^-2 yr^-1) chemical weathering are uniform across the hillslope transect. Alternatively, the drier hillslope at Frog's Hollow (FH) is characterized by contrasting weathering patterns in eroding soils (–30 to –53 g m^-2 yr^-1) vs. depositional soils (+91 g m^-2 yr^-1; positive sign indicates mass addition). This difference partly reflects mineral grain size sorting as a result of upslope bioturbation coupled with water-driven soil erosion, as well as greater vegetative productivity in moister depositional soils. Both of these processes are magnified in the drier climate. The data reveal the importance of linking the erosion–deposition continuum in hillslope weathering studies in order to fully capture the coupled roles of biota and erosion in driving the physical and chemical evolution of hillslopes. Our findings also highlight the potential limitations of applying current weathering models to landscapes where particle-sorting erosion processes are active.