Could atmospheric dust deposition be an important contributor to Earths riverine silicate weathering flux?

Chemical weathering of the materials on Earths surface plays a central role in the regulation of biogeochemical cycles over geologic time. This weathering is usually attributed to chemical transformation of rock, but chemical transformation of dust may also play an important role in supplying dissolved loads to global rivers. Unlike rock, fine-grained mineral dust is transported over long distances atmospherically and precipitates onto the Earth surface. Dusts high reactive surface area could lead to much higher weathering rates than those of bedrock in some regions. However, dusts similar composition to upper continental crust makes distinguishing rock from dust weathering contributions difficult. Here, we test the hypothesis that weathering of dust could possibly account for a significant proportion of dissolved loads in global rivers. To do this, we first compare measurements of dissolved load for ~40 global rivers to modeled dust precipitation estimates from global general circulation model for modern and Last Glacial Maximum time periods. Second, we set up a steady-state weathering model to calculate dust and rock weathering from silicate minerals in the critical zone considering their reaction kinetics and material residence time. Model parameters are optimized based on minimization of error between predicted and observed Si and silicate-derived Ca + Mg (SCM) dissolved fluxes for 14 rivers where sufficient information is available for modeling. Our model predicts that 31% of modern Si flux and 47% of SCM flux could be dust-derived, which is an appreciable fraction of the weathering-derived solutes observed within these catchments. Our model also predicts that contributions from dust weathering during the Last Glacial Maximum vary significantly by locality. While our results do not rule out the long-held view that rock weathering in soil dominates riverine solute fluxes and thus continental weathering fluxes, they also do not rule out dust accounting for a significant fraction of global dissolved loads. Our results also highlight the need to better understand the dynamics of dust deposition and subsequent weathering when assessing global weathering impacts on biogeochemical cycling over geologic time.