Abstract
Abstract. Denudation of steep rockwalls is driven by rock fall processes of various sizes and magnitudes. Rockwalls are sensitive to temperature changes mainly because thermo-cryogenic processes weaken bedrock through fracturing, which can precondition the occurrence of rock fall. However, it is still unclear how the fracturing of rock together with cryogenic processes impacts the denudation processes operating on steep rockwalls. In this study, we link data on long-term rockwall denudation rates at the Eiger (Central Swiss Alps) with the local bedrock fabric and the reconstructed temperature conditions at these sites, which depend on the insolation pattern. We then estimate the probability of bedrock for failure through the employment of a theoretical frost cracking model. The results show that the denudation rates are low in the upper part of the NW rockwall, but they are high both in the lower part of the NW rockwall and on the SE face, despite similar bedrock fabric conditions. The frost cracking model predicts a large difference in cracking intensity from ice segregation where the inferred efficiency is low in the upper part of the NW rockwall but relatively large on the lower section of the NW wall and on the SE rock face of the Eiger. We explain this pattern by the differences in insolation and temperature conditions at these sites. Throughout the last millennium, temperatures in bedrock have been very similar to the present. These data thus suggest the occurrence of large contrasts in microclimate between the NW and SE walls of the Eiger, conditioned by differences in insolation. We use these contrasts to explain the relatively low denudation rates in the upper part of the NW rockwall and the rapid denudation in the SW face and in the lower part of the NW rock face where frost cracking is more efficient.
Highlights
Steep bare bedrock faces are a common feature of Alpine landscapes
Denudation rates can be estimated from in situ cosmogenic nuclide depth profile modeling (e.g, Hidy et al, 2010; Braucher et al, 2009). This method has been applied for estimating the local denudation rates in steep Alpine rockwalls, and it has been very useful for extracting information on the mechanisms of denudation, as shown in a recent study where denudation rates were measured with concentrations of cosmogenic 36Cl in bedrock depth profiles at the Eiger (Mair et al, 2019)
We complemented the Mair et al (2019) denudation rate dataset by measuring concentrations of cosmogenic 10Be for five samples in a bedrock depth profile at site EW-01 (Fig. 1), and we modeled the local denudation rate over millennia
Summary
Steep bare bedrock faces are a common feature of Alpine landscapes They are situated at various elevations but are especially prominent in high-altitude environments. Previous research has resulted in the generally accepted notion that among the various mechanisms leading to rockwall denudation, rock fall and rockslide processes are the most important agents (e.g., Krautblatter et al, 2012; Moore et al, 2009), mainly because all loosened material is eventually removed by gravitational processes In this context, laboratory experiments disclosed a close relationship between rock fracturing and temperature variations (e.g., Draebing and Krautblatter, 2019; Murton et al, 2016).
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