Godon, Cécile; Guillon, Hervé; Buoncristiani, Jean-François; Mugnier, Jean-Louis (2013): Grain size distributions of the Bossons glacier (France) [dataset publication series]. PANGAEA, https://doi.org/10.1594/PANGAEA.835991,

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Supplement to: Godon, Cécile (2013): L'érosion dans les environnements glaciaires : exemple du Glacier des Bossons (Massif du Mont-Blanc, Haute-Savoie, France) = Erosion in glacial environments : example of the Glacier des Bossons (Massif du Mont-Blanc, Haute-Savoie, France). Université de Grenoble, v1, 212 pp, https://theses.hal.science/tel-00909603

The study presented in this PhD memory aim at better define and quantify the present timeerosion processes in glacial and proglacial domain. The Glacier des Bossons, situated in theMont-Blanc massif (Haute-Savoie, France), is a good example of a natural and nonanthropizedsystem which allows us to study this topic. This glacier lies on two mainlithologies (the Mont-Blanc granite and the metamorphic bedrock) and this peculiarity is usedto determine the origin of the glacial sediments. The sediments were sampled at the glaciersurface and at the glacier sole and also in the subglacial streams in order to understand themechanisms of mechanical erosion and particle transportation in glacial domain. The study ofthe granulometric distribution and the origin of the sediments were performed by a lithologicanalysis at macro-scale (naked-eye) and a geochemical analysis at micro-scale (U-Pb datingof zircons). These analyses allowed specifying the characteristics of glacial erosion andtransport. (1) the supraglacial sediments derived from the erosion of the rocky valley sides aremainly coarse and the glacial transport does not mix these clasts with those derived from thesub-glacial erosion, except in the lower tongue; (2) the sub-glacial erosion rates areinhomogeneous, erosion under the temperate glacier (0.4-0.8 mm/yr) is at least sixteen timesmore efficient than the erosion under the cold glacier (0.025-0.05 mm/yr); (3) the sub-glacialsediments contain a silty and sandy fraction, resulting from processes of abrasion andcrushing, which is evacuated by sub-glacial streams. The high-resolution temporal acquisitionof hydro-sedimentary data during the 2010 melt season, between the May 5th and theSeptember 17th, allowed defining the seasonal behavior of the hydrologic and sedimentaryfluxes. The sediment exportation occurs mainly during the melt season therefore, quantify thesediment fluxes in the Bossons stream and measure regularly the topographic evolution of thefluvio-glacial system allows to perform a sedimentary balance of the erosion of glacial andnon-glacial domains. During the year 2010, about 3000 tons of sediments were eroded with430 tons settled on the fluvio-glacial system. By analyzing the evolution of suspendedparticulate matter concentrations in the Bossons stream upstream and downstream the fluvioglacialsystem, the part of glacial erosion and non-glacial denudation in the sedimentarybalance could be proportioned. The erosion during the stormy events of the uncoveredmoraines, confining the fluvio-glacial system of the Bossons stream, furnishes at least 59% ofthe sediments exported by the Bossons stream and glacial erosion (41% of the flux) istherefore less efficient comparatively. The long-term evolution of the glacial systems inperiod of global warming would show a sustained erosion of proglacial environments(mountain sides and moraines) recently exposed and therefore an increasing of the detritalfluxes. The Glacier des Bossons protects the summit of the Mont-Blanc, the differentialerosion between zones under the ice and non-glacial could lead to an increase of thedifference of altitude between valleys and summits.

This dataset presents sediment load grain size distributions of Bossons stream, one of the subglacial streams draining Bossons glacier meltwater.

Bossons glacier is rapidly retreating and its proglacial area is deglaciated for ~ 30 years. Within this area, Bossons stream flows through an alluvial plain with day-to-day morphological evolution. It is an intriguing location to study periglacial and proglacial erosion processes which requires to characterize transported sediments. Since coarse particles are difficult to monitor in such alpine stream environment, this dataset focuses on fine grained particles in suspension load, saltation load at 5 cm from bed and bedload.

Grain size analysis of suspended particles requires few grams of material. Ten L of water were sampled at the same location in Bossons stream on four summer days (July 2nd (n=5), July 16th (n=2), August 2nd (n=1) and August 3th 2010 (n=1)) and one flood event (June 10th 2010 (n=1)).

In 2009, a sediment trap constituted by a sole 1.5L-bottle was used to acquire 68 samples. The trap was pickep up every morning (9h00) and evening (20h00) between July 2nd and September 13th 2009. During 2010, the sediment trap was constituted by two 1L-bottles kept at 5 cm from bed by a metal rod (following Delft bottle principle). The trap was picked up every evering during spring and two times a day during summer totaling 238 samples.

Sediment trap for bedload was constituted by a 235x240 mm landing net with 3 mm mesh. Behind this net, a non-woven geotextile with a 0.1 mm mesh was set. Coarse particles were trapped in the first net whereas fine particles were sampled in the geotextile. This sampling system was installed on a wear and was able to monitor about one tenth of the stream width.Sampling was carried out in different hydrological conditions during 22 days from June to August 2011 totaling 92 samples.

After decantation, drying and weighting, Grain Size Distribution (GSD) of each samples has been determined by laser micro-granulometry. Some sample were analyzed twice to ensure data consistency. Only 12 bedload samples have been analyzed.

Suspended particles GSD is bimodal when sand can be transported, unimodal otherwise. The dominant mode is 18 µm. Saltating particles GSD is unimodal and dominant mode is always greater than 500 µm. Bedload particles GSD is slightly bimodal : a clear and dominant mode at 528±84 µm and a less prominent one between 5 and 8 mm. Hence, on normal hydrological condition, suspended load, saltating load and bedload correspond respectively to silts, medium to coarse sands and medium to coarse sands together with granular gravels.