The Bossons glacier protects Europe's summit from erosion C. Godon a,n , J.L. Mugnier a,n , R. Fallourd b , J.L. Paquette c , A. Pohl d , J.F. Buoncristiani d a ISTerre, UMR 5275, CNRS, Université de Savoie, UJF, Campus Scientifique, 73376 Le Bourget du Lac cedex, France b LISTIC, Université de Savoie, Polytech' Annecy-Chambéry, BP 80439, 74944 Annecy le Vieux cedex, France c Laboratoire Magmas & Volcans, Université Blaise Pascal, CNRS et IRD, 5 rue Kessler, 63038 Clermont-Ferrand, France d Biogéosciences, Université de Bourgogne, 6 Boulevard Gabriel, 21000 Dijon, France article info Article history: Received 28 September 2012 Received in revised form 29 April 2013 Accepted 9 May 2013 Editor: J. Lynch-Stieglitz Available online 28 June 2013 Keywords: glacial erosion glacial transport detrital zircon geochronology denudation rate abstract The contrasting efficiency of erosion beneath cold glacier ice, beneath temperate glacier ice, and on ice- free mountain slopes is one of the key parameters in the development of relief during glacial periods. Detrital geochronology has been applied to the subglacial streams of the north face of the Mont-Blanc massif in order to estimate the efficiency of erosional processes there. Lithologically this area is composed of granite intruded at 303 Ma within an older polymetamorphic complex. We use macroscopic features (on 10,000 clasts) and U–Pb dating of zircon ( 500 grains) to establish the provenance of the sediment transported by the glacier and its subglacial streams. The lithology of sediment collected from the surface and the base of the glacier is compared with the distribution of bedrock sources. The analysis of this distribution takes into account the glacier's surface flow lines, the surface areas beneath temperate and cold ice above and below the Equilibrium Line Altitude (ELA), and the extent of the watersheds of the three subglacial meltwater stream outlets located at altitudes of 2300 m, 1760 m and 1450 m. Comparison of the proportions of granite and metamorphics in these samples indicates that (1) glacial transport does not mix the clasts derived from subglacial erosion with the clasts derived from supraglacial deposition, except in the lower part of the ice tongue where supraglacial streams and moulins transfer the supraglacial load to the base of the glacier; (2) the glacial erosion rate beneath the tongue is lower than the erosion rate in adjacent non-glaciated areas; and (3) glacial erosion beneath cold ice is at least 16 times less efficient than erosion beneath temperate ice. The low rates of subglacial erosion on the north face of the Mont-Blanc massif mean that its glaciers are protecting “the roof of Europe” from erosion. A long-term effect of this might be a rise in the maximum altitude of the Alps. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Glaciers play a major role in the formation of surface relief (Hallet et al., 1996; Hay, 1998). The erosional work accomplished by glaciers is widely recognized; they carve broad and deep valleys as attested by recent studies of overdeepenings in the Alps (e.g. Preusser et al., 2010; Herman et al., 2011). While some studies have shown that an erosion-induced isostatic rebound contributes to the uplift of mountain ranges (e.g. Molnar and England, 1990; Cederbom et al., 2004; Schlunegger et al., 2011; Norton and Schlunegger, 2011) others have suggested that glacial erosion limits elevations in mountain belts worldwide (e.g. Brozovic et al., 1997; Egholm et al., 2009). Korup and Montgomery (2009) have suggested that glacial advances retard regressive fluvial incision while Thomson et al. (2010) and Braun (2010) have reported that glacier caps protect mountain tops. Understanding the large-scale relief control involves determining the temporal (Valla et al., 2011) and spatial (Herman et al., 2011) variations in rates of erosion within a glaciated domain and more specifically determining the efficiency of subglacial erosion compared to erosion rates on the ice-free margins of the glacier (Alley et al., 1997). To address these questions, a method based on the study of the detrital flux of a present-day active glacier system is developed and applied to the Bossons glacier (north face of Mont-Blanc, France; Fig. 1). If the mean efficiency of erosional processes within a watershed is classically determined from the detrital flux at its outlet (e.g. Holeman (1968) and Milliman and Meade (1983) for fluvial watersheds, and Hallet et al. (1996) for glacial environ- ments), making a detailed detrital study of a glaciated watershed requires an understanding of the glacial transport pattern. In particular, such a study must investigate how far glacial transport leads to mixing of clasts produced by frost cracking on the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/epsl Earth and Planetary Science Letters 0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.05.018 n Corresponding authors. Tel.: +33 616 604 150. E-mail addresses: cecile.godon@etu.univ-savoie.fr, cecile.godon@hotmail.fr (C. Godon). Earth and Planetary Science Letters 375 (2013) 135–147