Quantifying erosion over timescales of one million years: A photogrammetric approach on the amount of Rhenish erosion in southwestern Germany

The Lein valley in southwestern Germany possesses well-preserved Pliocene to mid Pleistocene land surfaces featuring a gentle relief and sediments accumulated by former tributaries of the Danube. This ancient Danubian land surface was captured and incised by mid Pleistocene to Holocene tributaries of the River Rhine. In a photogrammetric approach we calculated the volume of material extracted by Rhenish erosion providing a first quantification of the effects of stream piracy on timescales of about 1 Ma. Using stereoscopic surface modelling software a DEM was generated with a resolution of 5 m. From borehole data, literature, geological maps, and own field observations we determined the morphometric parameters of the ancient Danubian Ur-Lein valley. The gradient was imported as a 3D-breakline into the model where it controls the reinterpolation of surrounding data points. The result is a high-resolution DEM of the valley of the Ur-Lein. Subtraction of the DEM of the actual landscape from the DEM of the ancient Ur-Lein valley yields a model representing the rock volume eroded by the Rhenish Lein which totals 1.39 km3 and converts into a rate of erosion between 63 and 74 mm/ka over a period of 700 to 600 ka, respectively, in accordance with figures obtained elsewhere in Central Europe through cosmogenic nuclides. It reflects the dominance of frequent fluctuations in climate and is considered to be mainly a product of strong changes in temperature and related processes during the transitional times between mid to late Pleistocene warm and cold states. A filtering procedure applied to cold and transitional state erosion rates of the Middle and Late Pleistocene yielded peak values between 66 and 77 mm/ka, up to three times higher than the modern rate or the rate of warm-state episodes. An assessment of the contribution of Rhenish stream piracy on long-term mid Pleistocene denudation under changing climate conditions resulted in a maximum 4.9-fold acceleration.