10Be-derived assessment of accelerated erosion in a glacially conditioned inner gorge, Entlebuch, Central Alps of Switzerland

Inner gorges often result from the propagation of erosional waves related to glacial/interglacial climate shifts. However, only few studies have quantified the modern erosional response to this glacial conditioning. Here, we report in situ 10Be data from the 64?km2 Entlen catchment (Swiss Alps). This basin hosts a 7?km long central inner gorge with two tributaries that are >100?m-deeply incised into thick glacial till and bedrock. The 10Be concentrations measured at the downstream end of the gorge yield a catchment-wide erosion rate of 0.42?+/-?0.04?mm?yr-1, while erosion rates are consistently lower upstream of the inner gorge, ranging from 0.14?+/-?0.01?mm?yr-1 to 0.23?+/-?0.02?mm?yr-1. However, 10Be-based sediment budget calculations yield rates of similar to 1.3?mm?yr-1 for the inner gorge of the trunk stream. Likewise, in the two incised tributary reaches, erosion rates are similar to 2.0?mm?yr-1 and similar to 1.9?mm?yr-1. Moreover, at the erosional front of the gorge, we measured bedrock incision rates ranging from similar to 2.5?mm?yr-1 to similar to 3.8?mm?yr-1. These rates, however, are too low to infer a post-glacial age (1520?ka) for the gorge initiation. This would require erosion rates that are between 2 and 6 times higher than present-day estimates. However, the downcutting into unconsolidated glacial till favored high erosion rates through knickzone propagation immediately after the retreat of the LGM glaciers, and subsequent hillslope relaxation led to a progressive decrease in erosion rates. This hypothesis of a two- to sixfold decrease in erosion rates does not conflict with the 10Be-based erosion rate budgets, because the modern erosional time scale recorded by 10Be cover the past 23?ka only. These results point to the acceleration of Holocene erosion in response to the glacial overprint of the landscape. Copyright (c) 2012 John Wiley & Sons, Ltd.