Significant increase in relief of the European Alps during mid-Pleistocene glaciations

Some of Earth’s greatest relief occurs where glacial processes act on mountain topography1,2. This dramatic landscape is thought to be an imprint of Pleistocene glaciations3,4. However, whether the net effect of glacial erosion on mountains is to increase5–7 or decrease8–10 relief remains disputed. It has been suggested that in the European Alps, the onset of widespread glaciation since the mid-Pleistocene climate transition11 led to the growth of large, long-lived and strongly erosive alpine glaciers12,13 that profoundly influenced topography14. Here we use 4He/3He thermochronometry15 and thermal-kinematic models to show that the Rhône Valley in Switzerland deepened by about 1–1.5 km over the past one million years. Our results indicate that while the valley was incised and back-cut, high-altitude areas were preserved from erosion. We find an approximately two-fold increase in both local topographic relief and valley concavity, which occurred around the time of the mid-Pleistocene transition. Our results support the proposed link12–14 between the onset of efficient glacial erosion in the European Alps and the transition to longer, colder glacial periods at the middle of the Pleistocene epoch. Understanding how the topographic relief of alpine landscapes develops and what controls this evolution requires quantifying the relative efficiency of fluvial, glacial and hillslope processes. Recent studies have suggested that glacial erosion efficiently shapes mountain-belt topography through glacial valley carving and cirque retreat1,5–7, leading to significant relief increase at the valley scale (∼5–10 km) and isostatic mountain-peak uplift in response to late Cenozoic climate cooling4,6. However, others have proposed that, because glacial erosion is most efficient around the Equilibrium Line Altitude (ELA), it should act as a ‘glacial buzzsaw’ to limit mountain height and therefore reduce topographic relief8–10 at the drainage-basin scale. Therefore, understanding how mountain topography develops in response to climatic forcing (that is, glacial/interglacial oscillations) requires detailed quantitative information on the spatial and temporal patterns of erosion over ∼106-year timescales. In this study, we combine apatite (U–Th–Sm)/He dating16 with 4He/3He thermochronometry15 to constrain the timing and rate of glacial incision of a major Alpine valley and thus evaluate relief development at the valley scale. The (U–Th–Sm)/He system in apatite (AHe) is sensitive to cooling below ∼80 C and records bedrock exhumation through the uppermost 2–4 km beneath the Earth’s surface16. Apatite 4He/3He thermochronometry constrains the spatial distribution of 4He within individual apatite crystals and therefore the cooling history of specific samples below the AHe closure temperature15 (see Methods and Supplementary Information). The resulting cooling histories may be compared at different