The potential influence of dust flux and chemical weathering on hillslope morphology: Convex soil-mantled carbonate hillslopes in the Eastern Mediterranean

Abstract

Convex, soil-covered hillslopes are ubiquitous in various tectonic and climatic settings and their morphologic characteristics can be significantly altered by dust flux and chemical weathering. Carbonate hillslopes across the Eastern Mediterranean display a classic convex morphology despite high rates of dust deposition, which are apparently similar in magnitude to denudation rates and despite a highly soluble bedrock lithology. These high rates of dust flux and bedrock dissolution result in soil composition which is strikingly different from the underlying bedrock.In this study we apply a modified regolith mass balance equation which includes dust input and chemical depletion and utilize a combination of field work and geochemical methods (immobile and major elements, XRD) to estimate the fraction of dust-derived materials in the soil and the fraction of the soil that was lost via dissolution. Soil and bedrock were sampled at 7 Eastern Mediterranean study sites, across a prominent north-to-south gradient in both precipitation (250 to 900 mm yr−1) and dust flux (150 to 40 g m−2 yr−1) and compared with dust composition. Our results indicate that the dust fraction in the soil correlates with modern measurements of dust deposition rates and decrease from nearly 100% of the <2 mm soil material in the southern sites, to <10% in the northern sites. Chemical weathering increases with mean annual rainfall, from 0 to 20% in the arid south to 60–95% in the wetter sub-humid north. In addition, the soil also contains 30–50% rock fragments (>2 mm) indicating that mechanical weathering of the underlying bedrock contributes to regolith formation and plays a role in shaping carbonate hillslopes. Using a commonly used soil transport model, we show that the observed dust fraction and chemical weathering can potentially account for a 50% change in hillslope curvature. Furthermore, the predicted steady state hilltop curvature, based on our results, fits well the observed curvature based on high-resolution LiDAR-derived topographic data.Our results highlight the potential importance of dust and chemical weathering in soil production formulations and hillslope evolution models and we propose that these contributors to soil mass balance could also be relevant for silicate terrain. In addition, we also demonstrate that the source material of Mediterranean soils varies with environmental factors such as rainfall, dust deposition rates and bedrock mechanical properties.