Estimating millennial-scale rates of dust incorporation into eroding hillslope regolith using cosmogenic nuclides and immobile weathering tracers

Dust fluxes are of wide interest because of the effects of dust on climate, oceanic primary productivity, terrestrial biogeochemical cycles, and regolith composition. Estimating long-term dust deposition rates, however, can be difficult, especially in steep, eroding terrain. Here we present a geochemical mass balance method for estimating long-term average rates of dust incorporation into regolith on steadily eroding hillslopes. This method requires measurements of the local regolith production rate and the concentrations of two immobile elements in the regolith, its parent rock, and dust. Dust incorporation rates inferred with this method are averaged over the long timescale of regolith residence on the hillslope (typically 10³-10⁵ years), and thus may serve as long-term averages against which modern-day dust fluxes may be compared. We apply this model to 17 field sites in the South Fork of the Salmon River in the Idaho Batholith, where rock and regolith compositions imply that mafic-rich material has been added to the otherwise granitic regolith. We suggest that the most likely source of this mafic material is dust sourced from the same glacial outburst flood sediments that generated the Palouse loess on the Columbia Plateau, and we use the published composition of these sediments to infer dust incorporation rates of 3-13 t km⁻² yr⁻¹ at these sites, comparable to modern-day dust fluxes elsewhere in the western United States.