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Glacial Erosion by the Trift Glacier (Switzerland): Deciphering the Development of Riegels, Rock Basins and Gorges

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Glacial Erosion by the Trift Glacier (Switzerland): Deciphering the Development of Riegels, Rock Basins and Gorges Research Collection Journal Article Glacial erosion by the Trift glacier (Switzerland): Deciphering the development of riegels, rock basins and gorges Author(s): Steinemann, Olivia; Ivy-Ochs, Susan; Hippe, Kristina; Christl, Marcus; Haghipour, Negar; Synal, Hans- Arno Publication Date: 2021-02-15 Permanent Link: https://doi.org/10.3929/ethz-b-000458333 Originally published in: Geomorphology 375, http://doi.org/10.1016/j.geomorph.2020.107533 Rights / License: Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Geomorphology 375 (2021) 107533 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Glacial erosion by the Trift glacier (Switzerland): Deciphering the development of riegels, rock basins and gorges Olivia Steinemann a,⁎, Susan Ivy-Ochs a, Kristina Hippe a, Marcus Christl a, Negar Haghipour b, Hans-Arno Synal a a Laboratory of Ion Beam Physics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zürich, Switzerland b Institute of Geology, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland article info abstract Article history: A long-lasting question in glacial geology is how and how fast glaciers were able to shape the distinctive land- Received 10 June 2020 scapes of the Alps. This study contributes to the understanding on the formation of overdeepened basins, espe- Received in revised form 18 November 2020 cially the processes and the amount of time involved. We examine the remarkably high (150 m) cross-valley Accepted 23 November 2020 bedrock riegel and the associated overdeepening located in front of the Trift glacier in the central Swiss Alps. A Available online 26 November 2020 combined approach of field survey with measurements of two cosmogenic nuclides, 10Be and in-situ 14C, and a Keywords: numerical model was used to determine the spatial glacial erosion patterns on the bedrock riegel. Ten samples fl Cosmogenic 10Be were taken along two transects; one perpendicular to the glacier ow direction, from outside of the Little Ice In-situ 14C Age (LIA) extent down to the centre of the riegel, and the other following the former ice-flow direction across Glacial erosion rates the riegel. Analysis of measured nuclide concentrations shows that the sample outside of the LIA was constantly Swiss Alps exposed since the retreat of the Egesen stadial Trift glacier (~11.5 ka). The samples inside the LIA extent indicate a Overdeepening distinct trend of increasing glacial erosion rates from 0 mm/a near the LIA ice margin to high erosion all across the Inner gorge top of the riegel. The resulting minimum glacial erosion rates from samples on the riegel are 0.5–1.1 mm/a (10Be) and 0.6–>1.8 mm/a (in-situ 14C) which correspond to minimum erosion depths of 1.6–>3 m (10Be) and 1–>5 m (14C). The extremely low nuclide concentrations measured at the riegel highlight the substantial erosion (predominantly abrasion) of the bedrock surface during late Holocene glacier coverage. Field observations suggest that the formation of the overdeepening and, as a consequence, the riegel is due to a combination of valley shape, bedrock structures, glacier confluence and hydrology. We further hypothesise that the gorge is a key factor responsible for this impressive overdeepening, by lowering the threshold for the subglacial meltwater, effectively decoupling the height of the riegel from the depth of the overdeepening. © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction main trunk valleys, in tributary valleys and in cirques, overdeepend rock basins form because glaciers have the ability to erode their beds Glaciers and rivers shape valleys in completely different ways. This is below the fluvial profile (Penck, 1905). This leads to the distinctive gla- evident not only in the cross-profile, U- vs. V-shape, but in the longitu- cial longitudinal valley profiles which were already described by McGee dinal profile. While rivers tend towards a graded concave up profile, gla- in 1894 as: “… irregularly terraced – i.e., made up of a series of rude ciers uniquely and characteristically erode ‘unevenly’ into bedrock steps in variable form and dimension, - and some of the terraces are leaving cross-valley bedrock bars or riegels and rock basins in their so deeply excavated as to form rock-basins occupied by lakelets…” wake (Sugden and John, 1976; Evans, 2008, 2013). Glacially shaped (McGee, 1894). The rock basins, which are often tens to hundreds of overdeepenings are closed depressions found in the Alpine forelands meters deep, are usually followed downstream by a steep upward (Anselmetti et al., 2010; Brückl et al., 2010; Dehnert et al., 2012; Dürst (adverse) slope, which can transform into a cross-valley riegel if intense Stucki and Schlunegger, 2013; Buechi et al., 2018; Burschil et al., erosion has also occurred on the lee side of the bedrock ridge. Adverse 2019) where abundant drill core data (up to hundreds of meters slopes are often 10–30° steep, but even steeper adverse slopes have deep) and/or reflection seismic data have enabled an increased under- been reported (Röthlisberger, 1968; Hooke, 1991; Alley et al., 1997; standing. But overdeepenings are also frequent landforms of high Al- Creyts et al., 2013; Haeberli et al., 2016). In some cases, a narrow inci- pine areas (Pfiffner et al., 1997; Frey et al., 2010; Preusser et al., 2010; sion is eroded in the bedrock step or riegel, that can deepen with time Reitner et al., 2010; Linsbauer et al., 2012; Haeberli et al., 2016). In the to form a gorge across the riegel, also called an inner gorge. The origin and evolution of inner gorges is enigmatic (Montgomery and Korup, ⁎ Corresponding author. 2011; Dürst Stucki et al., 2012). Although several authors have attrib- E-mail address: [email protected] (O. Steinemann). uted their formation to predominantly fluvial processes during https://doi.org/10.1016/j.geomorph.2020.107533 0169-555X/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). O. Steinemann, S. Ivy-Ochs, K. Hippe et al. Geomorphology 375 (2021) 107533 interglacial periods (Montgomery and Korup, 2011; Ziegler et al., 2013; abraded bedrock, of how much rock was removed by the glacier (Briner Leith et al., 2014) pressurised, sediment-charged, subglacial meltwater and Swanson, 1998; Fabel and Harbor, 1999; Goehring et al., 2011; unequivocally has the ability to cut deep gorges into bedrock (Creyts Wirsig et al., 2016, 2017; Young et al., 2016; Steinemann et al., 2020). et al., 2013; Jansen et al., 2014; Beaud et al., 2016; Werder, 2016; Cosmogenic nuclides are produced in the upper few meters of bedrock Blomdin and Harbor, 2017). surfaces as long as they are exposed to cosmic rays, whereas during gla- In the last decade, investigation of overdeepenings has gained new cial coverage the bedrock is completely shielded and no nuclides are attention, as improved understanding of their formation (Cook and produced (Dunai, 2010). Measurement of cosmogenic nuclides in bed- Swift, 2012) and distribution is sought (Patton et al., 2015); especially rock surfaces is commonly used to date the time of glacier retreat, in the context of investigation of suitable sites for deep geological repos- thus, the time of exposure after deglaciation. However, if a glacier did itories for nuclear waste (Fischer and Haeberli, 2012). The occurrence of not remove enough rock (at least 2–3 m) to re-zero the nuclide concen- glacially scoured rock basins is often linked to areas of glacier conflu- tration built up during previous exposures, those inherited nuclides re- ence (Anderson et al., 2006; MacGregor et al., 2009) and past equilib- sult in ‘too old’ exposure ages (Fabel et al., 2004). Fortuitously, this rium line altitude (ELA) positions (Hooke, 1991; Brocklehurst and excess of cosmogenic nuclides can be used to determine erosion depths Whipple, 2004), underlining the importance of ice thickness and ice- in the bedrock below a glacier, and if the time periods of glacial coverage flow velocity (Hooke, 1991; Alley et al., 1997). But overdeepenings are are known or can be inferred from independent data, glacial erosion also found at (past) terminus positions, including cirques, where forma- rates can be derived (Briner and Swanson, 1998). tion is attributed to upward flow towards the ablating ice surface (Alley The focus of this study is the polished bedrock inside the footprint of et al., 2003; Cook and Swift, 2012; Evans, 2013; Haeberli et al., 2016). the Little Ice Age (LIA) extent of the Trift glacier (Fig. 1), especially the Several authors have emphasized the control of tectonic structures conspicuous cross-valley bedrock riegel. The methodological approach or weak lithologies, promoting intensified plucking, on the location taken is a combination of geomorphological and geological observa- of overdeepenings (Sugden and John, 1976; Augustinus, 1995; tions, cosmogenic nuclide analysis and a numerical model to determine Dühnforth et al., 2010; Hooyer et al., 2012; Becker et al., 2014). Despite glacial erosion depths and rates (Wirsig et al., 2017; Steinemann et al., extensive research carried out in the last years, questions about the 2020). Measuring two nuclides, in this study 10Be and in-situ 14C, can temporal framework remain unanswered. Crucially, constraints on provide additional information about the duration of glacier coverage, how much a glacier can reasonably erode during a glacial period or dur- taking advantage of their different half-lives and depth profiles (Miller ing a single glaciation are rare (Preusser et al., 2010). et al., 2006; Hippe, 2017).
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