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Postglacial Erosion of Bedrock Surfaces and Deglaciation Timing: New Insights from the Mont Blanc Massif (Western Alps) Benjamin Lehmann1*, Frédéric Herman1, Pierre G

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Postglacial Erosion of Bedrock Surfaces and Deglaciation Timing: New Insights from the Mont Blanc Massif (Western Alps) Benjamin Lehmann1*, Frédéric Herman1, Pierre G https://doi.org/10.1130/G46585.1 Manuscript received 31 May 2019 Revised manuscript received 30 September 2019 Manuscript accepted 13 October 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 22 November 2019 Postglacial erosion of bedrock surfaces and deglaciation timing: New insights from the Mont Blanc massif (western Alps) Benjamin Lehmann1*, Frédéric Herman1, Pierre G. Valla2,3, Georgina E. King1, Rabiul H. Biswas1, Susan Ivy-Ochs4, Olivia Steinemann4 and Marcus Christl4 1 Institute of Earth Surface Dynamics, University of Lausanne, CH-1015 Lausanne, Switzerland 2 Institut des Sciences de la Terre (ISTerre), University of Grenoble Alpes, University of Savoie Mont Blanc, Centre National de Recherche Scientifique (CNRS)–Institut de Recherche pour le Développement (IRD)–Institut Français des Sciences et Technologies des Transports, de l’Aménagement et des Réseaux (IFSTTAR), 38000 Grenoble, France 3 Institute of Geological Sciences and Oeschger Center for Climate Research, University of Bern, 3012 Bern, Switzerland 4 Laboratory of Ion Beam Physics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland ABSTRACT ture variation and snow cover and their effects Since the Last Glacial Maximum, ∼20 k.y. ago, Alpine glaciers have retreated and thinned. on the evolution of bedrock surfaces over diur- This transition exposed bare bedrock surfaces that could then be eroded by a combination nal to decadal time scales (e.g., Łoziński, 1909; of debuttressing or local frost cracking and weathering. Quantification of the respective Matsuoka and Murton, 2008). However, bridging contributions of these processes is necessary to understand the links between long-term the temporal gap between these erosion estimates climate and erosion in mountains. Here, we quantified the erosion histories of postglacial remains challenging, in part because of the sto- exposed bedrock in glacial valleys. Combining optically stimulated luminescence and ter- chastic nature of geomorphic processes (Koppes restrial cosmogenic nuclide (TCN) surface exposure dating, we estimated the erosion rate and Montgomery, 2009; Ganti et al., 2016). of bedrock surfaces at time scales from 101 to 104 yr. Bedrock surfaces sampled from the To address these issues, we adopted a new flanks of the Mer de Glace (Mont Blanc massif, European Alps) revealed erosion rates that method that combined optically stimulated lumi- vary from 3.5 ± 1.2 × 10−3 mm/yr to 4.3 ± 0.6 mm/yr over ∼500 m of elevation, with a nega- nescence (OSL) and TCN surface exposure dat- tive correlation between erosion rate and elevation. The observed spatial variation in erosion ing (Sohbati et al., 2018; Lehmann et al., 2019). rates, and their high values, reflect morphometric (elevation and surface slope) and climatic In this study, we applied this methodology to (temperature and snow cover) controls. Furthermore, the derived erosion rates can be used investigate how erosion rates have evolved over to correct the timing of deglaciation based on TCN data, potentially suggesting very rapid time scales of 101–104 yr on bedrock surfaces of ice thinning during the Gschnitz stadial. the Mer de Glace (Mont Blanc massif, European Alps) and how morphometric and climatic fac- INTRODUCTION of periglacial processes during interglacial pe- tors control their evolution. Then, we addressed To understand the long-term evolution of riods (Burbank et al., 1996; Ballantyne, 2002; how the variability in erosion rates can be used Alpine landscapes, the respective contributions Scherler, 2015). Yet, the rate at which bare-bed- to correct TCN exposure ages, leading to very of surface erosion, sediment production, and rock surfaces weather and erode during intergla- different possible scenarios of ice thinning dur- sediment transport must be quantified. During cials remains poorly quantified (e.g., Colman, ing the Gschnitz stadial (a period of regional- the Quaternary period, the alternation between 1981; Zimmerman et al., 1994; André, 2002; ly extensive glacier advance in the European glacial and interglacial periods has modulated Nicholson, 2008; Kirkbride and Bell, 2010). Alps, temporally between the breakdown of the the efficiency of glacial, fluvial, and hillslope The erosion of hillslopes in periglacial envi- Last Glacial Maximum piedmont lobes and the processes (Koppes and Montgomery, 2009). In ronments is governed by a combination of land- beginning of the Bølling warm interval). that context, changes in bedrock morphology sliding, rock shattering, and weathering (e.g., and corresponding sediment delivery have been Anderson and Anderson, 2010). During the last STUDY SITE related to glacier extent, and glacial erosion is decades, the development of terrestrial cosmogen- We collected samples along two elevation often thought to be the most efficient erosional ic nuclide (TCN) methods, mainly using in situ– profiles at the Mer de Glace Fig. 1( ). Six bed- and sediment transport mechanism in mountain produced 10Be in quartz crystals, has improved rock surfaces were sampled below the Mont environments (e.g., Hallet et al., 1996; Brozović our ability to quantify bedrock surface erosion Blanc Tête de Trélaporte (MBTP sample sites, et al., 1997; Montgomery, 2002; Mitchell and over time scales from 104 to 106 yr, assuming west side of the glacier, from 2545 to 2094 m Montgomery, 2006; Egholm et al., 2009; Herman that erosion occurs steadily through time (Balco above sea level [masl]; Fig. 1), and three bed- et al., 2013; Herman and Champagnac, 2016). et al., 2008; von Blanckenburg and Willenbring, rock surfaces were sampled below the Ai- Recent studies have also revealed the importance 2014; Hippe, 2017). Over modern time scales, guille du Moine (MBAM sample sites, east geomorphologists working on frost cracking have side of the glacier, ranging from 2447 to 2259 *E-mail: [email protected] also highlighted the feedbacks between tempera- masl; Fig. 1). All surfaces were from the same CITATION: Lehmann, B., et al., 2020, Postglacial erosion of bedrock surfaces and deglaciation timing: New insights from the Mont Blanc massif (western Alps): Geology, v. 48, p. 139–144, https://doi.org/10.1130/G46585.1. Geological Society of America | GEOLOGY | Volume 48 | Number 2 | www.gsapubs.org 139 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/2/139/4926750/139.pdf by guest on 13 February 2020 A C 6.93°E 6.95°E phenocrystalline granitic lithology of the Mont Switzerland N 2000 Blanc massif, and selected sampling sites can all be classified as glacially eroded bedrock 1 B 45.92° surfaces (see the GSA Data Repository ). The 2500 surfaces are rough and exhibit a weathered Trélaporte France texture without glacial striations. All studied bedrock surfaces were located between the ice- Italy 3000 surface elevations of the Little Ice Age (LIA) and the Last Glacial Maximum (LGM; Cout- B N terand and Buoncristiani, 2006; Vincent et al., Chamonix 2014) and were therefore likely deglaciated 45.91° C sometime between ca. 20–18 ka (Wirsig et al., 2016) and A.D. 1850. Moine 3000 METHODS 10 2500 We measured both Be concentrations (e.g., Gosse and Phillips, 2001; Ivy-Ochs and Briner, 2014) and OSL profiles (Sohbati et al., Mont-Blanc 4810 0 4 km 01500 000 m NN45.90°N 2012) on exposed granitic rock samples (see D the Data Repository). The 10Be concentrations 2800 Trélaporte profile Moine profile provided us with constraints on the time since trimline the rocks were exposed to cosmic rays and 2600 trimline MBTP1 with a temporal framework for the possible MBTP2 MBAM1 erosion histories since exposure. OSL profiles 2400 MBAM2 constrained the erosion history since the rock MBTP11 MBAM3 exposure to light following ice decay (Lehm- MBTP5 ann et al., 2019). Note that it is the differ- 2200 MBTP9 LIA 10 Elevation [masl] MBTP6 ence in sensitivity between Be and OSL that LIA 2000 makes it possible to quantify surface erosion rate histories over short (<102 yr) and long SSW NNESW NE (>104 yr) time scales (Sohbati et al., 2018; 1800 0 500 1000 1500 2000 2500 Lehmann et al., 2019). Horizontal distance [m] The evolution in time of the OSL bleaching front into a rock surface depends on exposure age, Figure 1. Study sites sampled along Mer de Glace glacier (Mont Blanc massif, European surface erosion, electron trapping and detrapping Alps) at (A) regional, (B) massif, and (C) local scale. Blue area shows extent of Mer de Glace determined from aerial images in 2004 CE (Rabatel et al., 2016). Red lines depict two vertical profiles, Trélaporte and Moine, along which bedrock surfaces were sampled (each colored 1GSA Data Repository item 2020042, supplemen- dot represents a specific sample; round and triangle dots are from the Trélaporte and Moine tal details on sample preparation, measurement, and profiles, respectively). (D) Topographic cross sections of Trélaporte and Moine vertical profiles analysis, as well as a sample list with their character- along the Mer de Glace and corresponding sampled surfaces (MB—Mont Blanc, TP—Trélaporte, istics and measured 10Be concentrations, calibration AM—Aiguille du Moine). Gray lines represent elevation of Last Glacial Maximum (LGM) trim- details, and luminescence signal, is available online lines (Coutterand and Buoncristiani, 2006); blue lines represent Little Ice Age (LIA) elevation at http://www.geosociety.org/datarepository/2020/, or (Vincent et al., 2014); masl—meters above sea level. on request from [email protected]. ABC Figure 2. Schematic representation of four different erosion scenarios through time (A,B) and their resulting luminescence signal (C), 10 where t0 is uncorrected Be exposure age, tS is onset time of erosion (yr), tC is corrected exposure age, and ε is erosion rate (mm/yr). Note that luminescence plots in C are not model outputs but drawings, with the aim of conceptualizing how the experiments were designed. 140 www.gsapubs.org | Volume 48 | Number 2 | GEOLOGY | Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/2/139/4926750/139.pdf by guest on 13 February 2020 (bleaching) rates, and athermal loss (Lehmann tC = the corrected exposure age.
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