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

Abstract

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 ~1.3 mm yr‐1 for the inner gorge of the trunk stream. Likewise, in the two incised tributary reaches, erosion rates are ~2.0 mm yr‐1 and ~1.9 mm yr‐1. Moreover, at the erosional front of the gorge, we measured bedrock incision rates ranging from ~2.5 mm yr‐1 to ~3.8 mm yr‐1. These rates, however, are too low to infer a post‐glacial age (15–20 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 2–3 ka only. These results point to the acceleration of Holocene erosion in response to the glacial overprint of the landscape. Copyright © 2012 John Wiley & Sons, Ltd.

Deciphering the driving forces of erosion rates on millennial to million‐year timescales in glacially impacted landscapes: An example from the Western Alps

“…Recently, several studies quantified transient gorge incision into glacially formed hanging valleys [e.g., Norton et al ., ; Valla et al ., ; Van den Berg et al ., ], demonstrating that incision rates of these features can exceed catchment‐wide erosion rates by at least an order of magnitude. Depending on the ratio between the area of the gorge/catchment and erosion rate of the gorge/catchment, catchment‐wide erosion rates can be strongly raised, e.g., in the Entlen catchment in central Switzerland by a factor of ~2 [ Van den Berg et al ., ]. We speculate that small tributary catchments have a larger fraction of rapidly eroding area as a result of migrating knickpoints (knickzones) developed in incised hanging valleys (convex river segments, e.g., Figure ), compared to larger upstream trunk valleys.…”

“…Smaller catchments from the Pelvoux‐Ecrins massif also follow this trend, but above a threshold of ~50 km 2 , erosion rates seem to be independent of basin area. Recently, several studies quantified transient gorge incision into glacially formed hanging valleys [e.g., Norton et al ., ; Valla et al ., ; Van den Berg et al ., ], demonstrating that incision rates of these features can exceed catchment‐wide erosion rates by at least an order of magnitude. Depending on the ratio between the area of the gorge/catchment and erosion rate of the gorge/catchment, catchment‐wide erosion rates can be strongly raised, e.g., in the Entlen catchment in central Switzerland by a factor of ~2 [ Van den Berg et al ., ].…”

“…Vermeesch , ]: Randomly select a number of counted induced tracks ( N ij ) from the measurement database. Select a bedrock fission‐track age ( t j ) from the interpolated surface age map, letting the probability of sampling a specific pixel depends on its erosion rate relative to the mean catchment erosion rate. We tested six different temporally constant erosion models: (A) spatially uniform erosion, as expected for a steady state landscape with spatially uniform tectonic and climatic forcing [e.g., Whipple , ]; (B) erosion decreasing or (C) increasing with elevation, as a result of, e.g., changes in (local) base‐level or frost‐cracking processes, respectively [e.g., Delunel et al ., ; van den Berg et al ., ]; (D) erosion increasing with slope, as observed in landscapes characterized by hillslopes below the threshold gradient [e.g., Montgomery and Brandon , ]; (E) erosion increasing with Myr‐scale erosion rates from interpolated AFT ages, as expected for steady state landscapes with spatially variable, but temporally constant tectonic and climatic forcing [e.g., Willett and Brandon , ], and (F) as (E), but increasing relative erosion rates of areas with a Myr‐scale erosion rate <0.4 km/Myr by a factor of two. There is no direct theoretical justification for the latter threshold approach; it is rather based on the observed relation between millennial and Myr‐scale erosion rates (see section 5).…”

“…Locally, glacial valley incision rates exceeded preglacial rates by up to 1 order of magnitude [ Muttoni et al ., ; Haeuselmann et al ., ; Valla et al ., ], leaving behind partly overdeepened trough valleys and associated hanging tributary valleys formed during repeated glaciations. Less is known about the glacial/interglacial variability of erosion rates, but erosion rates are assumed to have decreased since peak erosion rates of 1.1 to 2.9 m/kyr estimated from the postglacial sediment flux of the Alps [e.g., Hinderer , ; Van den Berg et al ., ]. Wittmann et al .…”

Improved discrimination of subglacial and periglacial erosion using 10Be concentration measurements in subglacial and supraglacial sediment load of the Bossons glacier (Mont Blanc massif, France)

“…Heimsath and McGlynn, 2008;Ward and Anderson, 2011), to evaluate the erosion/exposure history of eroded surfaces in the vicinity of a glacier (e.g. Goehring et al, 2012;Berg et al, 2012) or to characterize sediment transfer processes (e.g. .…”