Spatiotemporal denudation rates of the Swabian Alb escarpment (southwestern Germany) dominated by anthropogenic impact, lithology, and base-level lowering

Abstract

Surface denudation rates, a composite of physical erosion and chemical weathering, are governed by the tectonic, lithologic, climatic, and biotic conditions of a landscape and anthropogenic disturbances. Quantifying rates and disentangling their causes is challenging but important for understanding and predicting landscape evolution over space and time. In this study, we focus on a low-relief and mixed-lithology mountain range (Swabian Alb, southwestern Germany), whose 200-400 m high escarpment and foreland drain to the Neckar River to the north and whose plateau drains into the Danube River to the southeast. These two drainage systems are subjected to similar uplift rates and climate and biotic conditions but contain different lithologies, base levels, and topography. We calculate decadal-Timescale chemical weathering and physical erosion rates based on 43 locations with suspended and dissolved river load measurements and compare them to published longer-Term rates (e.g., denudation, incision, uplift). Chemical weathering rates (based on the dissolved river load and discharge and corrected for atmospheric and anthropogenic input) range from 0.003-0.070 mmyr-1, while physical erosion rates (calculated from suspended river load and discharge) range from 0.001-0.072 mmyr-1. The catchment-wide denudation rates range from 0.005-0.119 mmyr-1, resulting in ratios of chemical weathering over total denudation rate (W/D) between 0.40 and 0.99. These high values indicate that chemical weathering is generally the dominant denudation process in this cool to temperate humid setting dominated by sedimentary rocks. Both physical erosion and chemical weathering rates are higher in tributaries draining towards the north/Neckar River than in rivers draining towards the southeast/Danube River, resulting in southeastern escarpment retreat rates of 1.0-8.1 mmyr-1. The anthropogenic effects on denudation rates were evaluated using the Human Footprint and Connectivity Status indices (HFI and CSI, respectively) and the area of artificial constructions for each catchment. After a simplified correction for either index, the natural (non-Anthropogenic) denudation rates are estimated to be lower than the values reported above, although it is unclear how to accurately correct rates with either index. Regardless of how the correction of anthropogenic impact is applied, we find denudation rates are consistently higher for the Neckar Swabian Alb tributaries. Comparison of W/D values from the Swabian Alb to other study areas in different tectonic, lithologic, and climatic settings with W/D values ranging from 0.11.0 suggests the W/D values in the Swabian Alb (>0.4) result from high and lithology-dependent chemical weathering rates. The high W/D ratio likely results from late Cenozoic base-level lowering of the Neckar River that resulted in south-To-southeast-directed escarpment retreat across southwestern Germany. Differences in chemical weathering and physical erosion rates across the escarpment divide may arise from either the contrast in topographic relief or the exposure of lithologies in the Neckar catchment that are more susceptible to chemical weathering and physical erosion.