DUST INPUT TO REGOLITH AND CHEMICAL EROSION OF CARBONATE HILLSLOPES: A MASS BALANCE APPROACH

Carbonate minerals are susceptible to dissolution, so any signal from enhanced chemical erosion controlled by climate might be magnified in landscapes underlain by carbonate rocks. If chemical erosion reduces the magnitude of sediment flux, an important factor in shaping landscapes, it could be one mechanism by which climate modifies landscapes. In contrast, arid regions often accumulate significant amounts of dust over millennia, and so dust accumulation could overprint sediment loss due to chemical erosion.

Through a mass balance framework, we quantify the fluxes of dissolved material and dust for two carbonate hilltop sites in central Arizona (site MT1; mean annual precipitation = 450 mm/yr) and south-east Spain (NQ; mean annual precipitation = 550 mm/yr). In this conceptual model, chemical weathering of bedrock produces fine sediment (<2 mm), insoluble oxides and residual material, whereas mechanical weathering produces lithic clasts (> 2mm). Concentrations of cosmogenic 36-Chlorine in hilltop bedrock samples and amalgamated clasts constrain their respective mass fluxes. The clasts are assumed to share the erosion rate of the measured bedrock sample, and their 36-Chlorine concentrations are used to quantify the maximum additional exposure that they could have accumulated since eroding. The fine sediment flux, which is a combination of the insoluble products of bedrock dissolution and dust, is estimated by the concentration of meteoric 10-Beryllium in hillslope sediments <2mm. The proportion of dissolved material flux to the flux of insoluble fraction is set by the bedrock carbonate mineral composition.

Bedrock fluxes (F_br) at NQ were 29.9±7.0 g m^-2 yr^-1 and 20.0±4.0 g m^-2 yr^-1 at MT1. Fine sediment fluxes were 12±1% of F_br at NQ and 33±2% of F_br at MT1, part of which was attributed to dust flux, which for NQ was 8±2% of F_br and for MT1 was 28±6%. Coarse sediment flux was 8±2% and 12±3% of F_br at NQ and MT1, respectively. Dissolved flux at NQ (88% of F_br; 26±7 g m^-2 yr^-1) is higher than at MT1 (83%; 16±4 g m^-2 yr^-1). These estimates of dissolved fluxes indicate that even in arid regions like central Arizona, chemical erosion could account for more than 50% of mass flux from landscapes.