Spatiotemporal denudation rates of the Swabian Alb escarpment (Southwest Germany) dominated by base-level lowering and lithology

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. Disentangling and quantifying these rates 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 escarpment, Southwest Germany), whose 200 to 400 m high escarpment front and foreland drain into 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/biota but incorporate different lithologies and have different base-levels and topography. We calculate decadal-scale chemical weathering and physical erosion rates based on 30 locations with suspended and dissolved river load measurements and compare them to published longer-term rates to evaluate how these differences influence landscape evolution.

Chemical weathering rates (based on the dissolved river load and corrected for anthropogenic input) range from 0.009 to 0.082 mm/yr, while physical erosion rates (calculated from suspended river load and discharge) range from 0.001 to 0.072 mm/yr. The catchment-wide denudation rates range from 0.005 to 0.137 mm/yr, resulting in chemical depletion fractions between 0.48 and almost 0.99. These high values indicate that chemical weathering is generally the dominant erosion process in this cool to temperate, humid mountain range dominated by chemical 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 southeast escarpment retreat rates of 1.2 to 9.3 mm/yr.

Results indicate that the evolution of the Swabian Alb and its escarpment is dominated by base-level lowering and lithology. Decadal-scale denudation rates based on river load may provide insights into the evolution of the escarpment over million-year timescales. The chemical depletion fractions CDFs of the Swabian Alb are compared to other study areas in different tectonic, lithologic, and climatic settings with CDFs ranging from 0.1 to 1.0. We interpret the high CDF values of >0.5 in the Swabian Alb to result from high chemical weathering rates of the recently exposed lithologies, continuously brought to the surface as a product of late-Cenozoic base-level lowering and consequent south to southeast-directed escarpment retreat across southern Germany. Differences in chemical weathering and physical erosion rates across the escarpment divide may arise from either the contrast in topographic relief, or exposure of bedrock units that are more susceptible to chemical weathering and physical erosion.