Last-glacial-cycle glacier erosion potential in the Alps

Earth Surf. Dynam., 9, 923–935, 2021 https://doi.org/10.5194/esurf-9-923-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Last-glacial-cycle glacier erosion potential in the Alps Julien Seguinot1 and Ian Delaney2 1 Independent scholar, Anafi, Greece 2 Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland Correspondence: Julien Seguinot () Received: 15 February 2021 – Discussion started: 25 February 2021 Revised: 14 June 2021 – Accepted: 7 July 2021 – Published: 3 August 2021 Abstract. The glacial landscape of the Alps has fascinated generations of explorers, artists, mountaineers, and scientists with its diversity, including erosional features of all scales from high-mountain cirques to steep glacial valleys and large overdeepened basins. Using previous glacier modelling results and empirical inferences of bedrock erosion under modern glaciers, we compute a distribution of potential glacier erosion in the Alps over the last glacial cycle from 120 000 years ago to the present. Despite large uncertainties pertaining to the climate history of the Alps and unconstrained glacier erosion processes, the resulting modelled patterns of glacier erosion include persistent features. The cumulative imprint of the last glacial cycle shows a very strong localization of erosion potential with local maxima at the mouths of major Alpine valleys and some other upstream sections where glaciers are modelled to have flowed with the highest velocity. The potential erosion rates vary significantly through the glacial cycle but show paradoxically little relation to the total glacier volume. Phases of glacier advance and maximum extension see a localization of rapid potential erosion rates at low elevation, while glacier erosion at higher elevation is modelled to date from phases of less extensive glaciation. The modelled erosion rates peak during deglaciation phases, when frontal retreat results in steeper glacier surface slopes, implying that climatic conditions that result in rapid glacier erosion might be quite transient and specific. Our results depict the Alpine glacier erosion landscape as a time-transgressive patchwork, with different parts of the range corresponding to different glaciation stages and time periods. 1 Introduction The glacial erosion landscape of the Alps has fascinated generations of explorers, artists, mountaineers, and scientists for centuries. Its cultural impact is indeed so far-reaching, that in English, a non-Alpine language, the adjective “alpine” with non-capital “a” is now casually used to describe an Alpine-like, glacially modified mountain landscape outside the Alps, while the proper noun “Alps” has been applied to nickname Alpine-like, glacier-eroded mountain ranges in Norway (Lyngsalpene), New Zealand (Southern Alps), Japan ( , Nihon Arupusu), and elsewhere. Some mountain ranges are predominantly characterized by cirque glaciation (e.g. Uinta Mountains), glacial valleys (e.g. Putorana Plateau), or large-scale overdeepenings (e.g. Patagonia). But other regions, including the Alps (e.g. Penck, 1905), present a higher variety of glacier erosional landforms, whose impli- cations for glacial history are yet to be understood. In parallel to such landscape diversity, the cosmogenic nuclide memory of bedrock erosion provides a more quantitative but equally varied picture of glacier erosion effectiveness both within and between glaciated regions (Jansen et al., 2019; Steinemann et al., 2020, 2021). Glacially eroded topography has sometimes been used as a proxy for mapping palaeoglacier extent (e.g. Margold et al., 2011; Fu et al., 2012). Cold-based glaciers, however, have been observed to preserve landforms as fragile as sand beaches (Kleman, 1994), leading to the reinterpretation of complex glacial landscapes within a binary conceptual framework of cold and temperate basal thermal regimes (Kleman et al., 2008, 2010; Fabel et al., 2012; Fu et al., 2013). Nevertheless, it remains elusive whether glacial topography is a proxy for temperate ice-cover duration and why some glaciated regions show preserved periglacial block- Published by Copernicus Publications on behalf of the European Geosciences Union. 924 J. Seguinot and I. Delaney: Last-glacial-cycle glacier erosion potential in the Alps fields topped by erratic boulders and others, including the Alps, do not (Wirsig et al., 2016; Seguinot et al., 2018). On a larger scale, glacier erosion also likely governs, at least in part, the height of mountains (Egholm et al., 2009; Thomson et al., 2010) through the “glacier buzzsaw”. In turn, evidence suggests that increased mountain erosion rates coincided with global cooling and Pleistocene glaciations from roughly 2.6 million years before present (Herman and Champagnac, 2016). However, oceanic isotopic proxies for global weathering rates remained constant over this time period (Willenbring and Von Blanckenburg, 2010). Examination of contemporary erosion rates across different climates suggests that increased temperatures within glaciated regions lead to higher erosion rates (Koppes and Montgomery, 2009; Koppes et al., 2015; Fernandez et al., 2016). However, understanding the link between climate and glacier erosion in the sedimentary record remains difficult due to timescale biases (Ganti et al., 2016) and the non-linear relationship between atmospheric temperature and erosion (e.g. Anderson et al., 2012; Mariotti et al., 2021). Numerical glacier modelling may provide insights into such complexity. Observing and quantifying long-term glacier erosion and sedimentation processes has been a challenge, and two general methods have been adopted to quantify erosion (Alley et al., 2019). Physically based models describe the quarrying process and abrasion of bedrock by debris-laden ice (e.g. Alley et al., 1997; Iverson, 2012; Beaud et al., 2014). Describing the processes physically provides a basis for understanding erosion processes (Hallet, 1979; Ugelvig et al., 2018) and yields insight into the formation of glacial landforms, such as tunnel valleys and eskers (Beaud et al., 2018; Hewitt and Creyts, 2019). Yet, these models prove difficult to implement in many cases due to the large number of poorly constrained parameters and processes, i.e. water pressure fluctuations and subglacial debris concentration (e.g. Hallet, 1979; Seguinot, 2008; Ugelvig et al., 2018). Instead, empirical relationships between glacier sliding and erosion can represent erosional quantities well and can recreate important glacial landforms (e.g. Harbor et al., 1988; MacGregor et al., 2000). For instance, Humphrey and Raymond (1994) correlated temporal variations in ice velocity and suspended sediment load during a surge of Variegated Glacier to establish a linear erosion law. Koppes et al. (2015) quantified sediment yields in 15 Patagonian and Antarctic Peninsula fjords, concluding there was a predominant control (...truncated)