Reconstructing recent environmental changes from proglacial lake sediments in the Western Alps (Lake Blanc Huez, 2543 m a.s.l., Grandes Rousses Massif, France)

Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 586 – 600 www.elsevier.com/locate/palaeo

Reconstructing recent environmental changes from proglacial lake sediments in the Western Alps (Lake Blanc Huez, 2543 m a.s.l., Grandes Rousses Massif, France) Emmanuel Chapron a,⁎, Xavier Faïn b , Olivier Magand b , Laurent Charlet c , Maxime Debret b , Marie Antoinette Mélières b a

Geological Institute ETH, Universitätstrasse 16, CHN E23, CH-8092 Zürich, Switzerland Laboratoire de Glaciologie et Géophysique de l’Environnement, UMR 5183, Domaine Universitaire, BP 96, F-38402 Saint Martin d’Hères, France Laboratoire de Géophysique Interne et Tectonophysique, UMR 5559 CNRS, Observatory for Earth and Planetary Sciences (OSUG), Joseph Fourier University, BP 53, F-38041 Saint Martin d’Hères, France b

c

Received 11 October 2006; received in revised form 19 April 2007; accepted 28 May 2007

Abstract The evolution of high-altitude glaciers and human activities in the Grandes Rousses massif is documented by high-resolution seismic reflection profiling and multiproxy analysis of short sediment cores in proglacial Lake Blanc Huez. These lacustrine data are compared with historical chronicles, geomorphological features and glaciological studies in this region of the western Alps and they allow the documentation of recent environmental changes. The specific geometry of high-amplitude reflections in the uppermost seismic unit, the lithology of short cores and the available limnological data in the lake suggest that clastic particles eroded by the glaciers and transported in suspension by glacial melt waters in early summer essentially develop homopycnal flood events in the lake. A conceptual model linking fluctuations of glacier equilibrium line altitudes in the catchment area with sedimentary facies retrieved in the lake basin is proposed. This approach allows reconstructing continuous glacier fluctuations since AD1820–1850 and suggests several phases of glacier fluctuations during the Little Ice Age (LIA). These reconstructions are based on changes in lacustrine sediment laminations, density, magnetic susceptibility, reflectance spectra, organic matter and Arsenic content. The age-depth model of short sediment cores is provided by 210Pb, 137 Cs and 241Am radionuclide dating. This chronology is further supported by identifying in lacustrine sediments the impact of (i) the nearby M 5.3 Corrençon earthquake in AD 1962, (ii) the development of the ski resort at high-altitude close to the lake and (iii) the last advance of glaciers during the LIA in AD1820–1850 and the following phase of glacier retreat observed in the alpine region at the end of the LIA in AD 1880. Frequent sandy layers enriched in organic matter and associated with fluctuations in the Arsenic concentrations may result from hydraulic remobilisation of Middle Age mine tailings at the lake shore by snow melt or heavy rain fall events during the LIA. © 2007 Elsevier B.V. All rights reserved. Keywords: French Alps; Proglacial lakes; Little Ice Age; Seismic stratigraphy; Glacier fluctuations; High-altitude mining activities

1. Introduction

⁎ Corresponding author. Tel.: +41 44 632 68 49; fax: +41 44 632 10 75. E-mail address: [email protected] (E. Chapron). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.05.015

The interaction of climate and human impact on Holocene environmental changes in the Alpine region and on clastic sediment delivery to lake basins is frequently

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discussed (Wessels, 1998; Noël et al., 2001; Dearing et al., 2001; Chapron et al., 2002; Dearing and Jones, 2003; Arnaud et al., 2005). On the one hand, millennial-scale Holocene climate fluctuations have been documented by lake-level fluctuations, archeological and palynological records from many small lakes in the Jura Mountains and several larger peri-alpine lakes in the NW French Alps and in the Swiss Plateau (see Magny et al., 2003; Magny, 2004; Holzhauser et al., 2005 for a review). Within radiocarbon dating uncertainties, cold and wet periods favouring highlake levels match well-documented Swiss and Austrian glacier advances and tree-line decent in the Central and Southern Alps. Holocene glacier fluctuations in the French Western Alps still need to be reconstructed in more detail. However, these Holocene cold and wet periods match a continuous reconstruction of enhanced Rhone River

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flooding activity in the peri-alpine Lake Le Bourget (Fig. 1A) draining the Mont Blanc glaciers (Arnaud et al., 2005; Chapron et al., 2005). Since the end of the Late Bronze Age (i.e. 2800 yrs cal BP), a drastic increase in clastic sedimentation and some periods with enhanced flood deposits in this proglacial lake may result from increasing land use within this large alpine drainage basin. Hence, one way to disentangle the impact of climate and land use in this alpine region is to investigate high-altitude catchments dynamics, from a source-to-sink perspective, and to further document sedimentary processes associated with glacier advances and enhanced glaciofluvial regimes. Over the past decades, Matthews and Karlén (1992), Leeman and Niessen (1994), Leonard (1997), Ariztegui et al. (1997), Nesje et al. (2000) and Dhal et al. (2003) have demonstrated in various mountain ranges of the world that

Fig. 1. Location of Lake Blanc Huez and Alpe d'Huez ski resort in the Western Alps. Lake Blanc Huez is located at c. 50 km from Lake Le Bourget (LDB) in the alpine foreland and from the epicentres of Corrençon (AD 1962) and Monteynard (AD 1963) earthquakes discussed in the text (A). Lake Blanc Huez was submitted to Middle Age mining activities concentrated in the SW part of the Grandes Rousses Massif and especially in the Brandes village (B). In the massif two glaciers are instrumented (St. Sorlin and Sarennes) and several belts of moraines were previously documented as discussed in the text. Lake Blanc Huez is a narrow basin 37 m deep (C) and its catchment area (dotted line) include Rousses and Herpie glaciers, their Little Ice Age (LIA) moraines and an earlier stage of glacier fluctuation in the Holocene. This catchment was also significantly submitted to human activities, such as the change of the main tributary's position in 2003 (white dashed line) for the displacement (white arrow) of a ski lift (black line). (1) frontal moraines; (2) glaciers; (3) temporary torrents; (4) torrents; (5) shallow lakes; (6) alluvial plain; (7) gondola; (8) ski lift and (9) tunnel built to connect the Sarennes glacier with the ski resort. Isobaths in Lake Blanc Huez are 10 m.

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Fig. 2. Illustrations of the main sedimentary environments in the catchment area of the lake and of the main sedimentary processes occurring in early summer in the lake. Dirty snow avalanche deposits near the Middle Age mine tailings at the Eastern shore of the lake (A). Detail of dirty snow avalanche deposits on icebergs (B). Development of a homopycnal flows in the lake waters during a snow melt flood (C). General view of the upper part of Lake Blanc Huez catchment area illustrating the turbid waters of a new ice contact lake formed at the front of the Rousses glacier in summer 2003 (D).

proglacial lakes can provide continuous high-resolution sedimentary records of glacial activity in their catchment area. Glacier size itself is controlled by climatic parameters such as mean air summer temperature and winter precipitation. Glacial abrasion of the bedrock is maximal for a temperate glacier at the equilibrium line altitude (ELA) of the glacier (cf. Dhal et al., 2003) and produces fine rock and mineral fragments. During summers, a relatively large amount of silt- and clay-sized particles originating from glacial abrasion are transported in suspension by glacial melt waters and are deposited into proglacial lakes. Because erosion rate increases with glacier size and thickness, variations through time in the accumulated amount of generally more magnetic and denser silt-and clay-sized mineral fraction in proglacial lake sediments provides a reliable high-resolution record of glacier activity and thus, of climate changes. However, little is known about the geometries of proglacial lacustrine

basin fills and the dominating sub-aqueous sedimentary processes occurring at high-altitude. In the present study we document for the first time the basin-fill geometries of a high-altitude proglacial lake in the western Alps and describe the main sedimentary processes occurring in the upper meter of this lacustrine infill. Based on a high-resolution multiproxy study of the sediments, radionuclide dating and arsenic profiles, we then establish an age-depth model and discuss the fingerprints of major recent environmental changes in this lake basin. 2. Setting Today the Grandes Rousses massif (45° 7′N; 6°6′E) is characterised by plateau and cirque glaciers developed between 2500 and 3400 m altitudes and the glacier ELA is around 2900 m in the northern part of the massif (Vincent,

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Fig. 3. Location of seismic profiles and sediment cores in Lake Blanc Huez (left) and illustration of a transversal seismic section in the main basin with its interpretation (right). The upper 3 m of the basin fill are characterized by high-amplitude reflections with a pounded geometry, but below, only few low-amplitude horizontal reflections are visible and covering an acoustically transparent unit. The acoustic substratum is suggesting that the bedrock is U shaped. MWD: mass wasting deposits. The location of the longitudinal seismic section shown is Fig. 4 is also indicated.

2002). Two glaciers located on the southern (Sarennes) and northern (St Sorlin) side of the Grandes Rousses massif (Fig. 1B) have been instrumented since 1949 and 1957, respectively, and are generally considered as representative for the NW Alpine region (Torinesi et al., 2002; Vincent, 2002). Since the 20th century variations in temperate glacier ELAs in the Alps are essentially controlled by mean summer temperatures (Vincent, 2002), while during the Little Ice Age (LIA, between the 15th to the 19th centuries) they were previously driven by winter precipitation regimes (Vincent et al., 2005). Five generations of former moraine belts were mapped in the massif by Flusin et al. (1909) and Monjuvent and Chardon (1989) between 1700 m and 2600 m (Fig. 1B). The two oldest ones were recognized near the ski resort of Alpe d’Huez at ca. 1700 m and 1900 m, respectively. They were related to the Older and Younger Dryas pollen zones, respectively, based on pollen analyses and two mid Holocene 14C ages in condensed sections of post-glacial lacustrine and bogs sediments (Chardon, 1991). The higher moraine belt is free of any vegetation cover and is located on average at less than 100 m below the present-day glacier snout positions in the massif. This belt was related to the last advance of glaciers during the LIA in AD 1820–1850, based on historical records (Flusin et al., 1909; Edourard, 1978; Monjuvent and Chardon, 1989; Edouard, 1994). Glacial lakes occurring above 2600 m were at least partly covered and eroded by glaciers advances during the LIA. However, proglacial lakes occurring along the south western and northern edges of the massif at ca. 2000 m and ca. 2500 m were formed after the Lateglacial–Holocene transition

(Chardon, 1991; Edouard, 1994) and have the potential to contain a continuous record of Holocene environmental changes in this massif. Lake Blanc Huez (880 m long, 350 m wide and 37 m deep) is a narrow and over-deepened basin of glacial origin orientated along a NS axis (Fig. 1C). The lake is located at an altitude of 2543 m above a Holocene moraine (alt. 2340 m) and below the LIA moraine (alt. 2680 m). Glacial erosion of the bed rock (consisting of gneiss, schist and Triassic sedimentary rocks) in this area has been enhanced by the development of intense NS and NW–SE faulting during the formation of the Grandes Rousses massif. This specific tectonic setting is reflected in the bathymetry of the lake, and is associated with ground water flows feeding several springs downstream from the catchment area of the lake in fractured rocks. The southern end of the lake is dammed by a glacial rock bar. To the North, three sub-basins with increasing depth and size can be distinguished. The main sub-basin directly receives the melt waters of the Rousses and Herpie glaciers. In the northern part of the lake a well-developed gilbert type delta is associated with a small alluvial plain that essentially drains the Rousses glacier (Fig. 1C). This steep cirque glacier exposed to the WNW had a surface of c. 0.4 km2 in 1970 and its front oscillated around an altitude of 2750 m over the last decades. Since the summer 2003 significant ice melting favoured the formation of a small ice-contact lake at the front of the glacier (Fig. 2). The catchment area of Lake Blanc Huez is rather small (c. 3.2 km2) but steep and snow-covered more than 6 months per year. On

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Fig. 4. Location and illustration of a longitudinal seismic section in the lake with a schematic representation of dominating sedimentary processes discussed in the text. The location of Fig. 3 is also indicated. The acoustic substratum is developing three sub-basins characterized by high-amplitude reflections draping the basin fill morphologies and locally developing onlaps in the subsurface.

average, the lake is largely frozen from mid-November to mid-June and the outlet is dry from January to March when the level of the lake is several meters lower than the altitude of the bedrock sill. Each year tributary floods resulting from snow melt events occurs from mid-May to mid-July. Snow avalanches also frequently reach Lake Blanc Huez and can typically transport coarse particles eroded from the catchment to the lake ice as illustrated in Fig. 2. On the south-western and western sides of the Grandes Rousses massif, archeological investigations highlighted an important silver and lead mining industry concentrated in the village of Brandes at alt. 1830 m (Fig. 1B) during the 12th to 14th century, and on the eastern shore of Lake Blanc Huez (Fig. 1C) from AD 1236 to AD 1330. These high altitude mining industries extracted Barite from veins that locally outcrop in the massif are well-documented in Bailly-Maitre and Bruno-Dupraz (1994). The rocks were weakened by wood fires in both open-pit mining or deep gallery extractions and such industries required significant transport of wood from lower altitudes to the mines. More recently, the development of the Alpe d’Huez ski resort induced important changes in the south-western side of the massif and in the catchment area of Lake Blanc Huez. While the first ski lifts were installed in the 1930’s, they were significantly developed between 1950 and 1970. In 1964 and 1965 a tunnel was built under the Pic Blanc and a ski track connecting the Sarennes glacier with the

Rousses glacier and Lake Blanc Huez was opened (Fig. 1C). In the mid 1980’s important geomorphic changes were induced at the edges of the catchment area of Lake Blanc Huez by the construction of small roads connecting the ski resort (2000 m) with the arrivals of a ski lift at 2500 m, the gondola station at 2600 m and the Dôme des Rousses at 2810 m. In 2000 a new road connected the Dôme des Rousses with the tunnel near the Rousses glacier at 2900 m. Finally, in June 2003 the displacements of the ski lift in the alluvial plain upstream from Lake Blanc Huez required a significant shift of the course of the main tributary of the lake toward the East (c.f. white arrow and dotted line in Fig. 1C). The western Alps are also affected by large active tectonic features such as basement thrusts and strike-slip faults (Thouvenot et al., 2003) and our study area has been historically subjected to several moderate magnitude earthquakes (Nomade et al., 2005). The Corrençon (local Richter magnitude ML 5.3) and the Monteynard (ML 4.9) earthquakes in 1962 and 1963, respectively, where the two strongest events over the last 350 years and their epicentres were located less than 50 km away from Lake Blanc Huez (Fig. 1A). 3. Materials and methods In September 2003, the sedimentary infill of Lake Blanc Huez was imaged with the ETH Zurich 3.5 kHz pinger

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Fig. 5. Lithology and multiproxy characterisation of core LB03-1, illustrating the signature of sedimentary facies discussed in the text. The down core changes in first derivative values of sediment reflectance spectra measured by spectrophotometry is plotted on a 3D diagram where the X axis represent the wavelengths, Y is the depth in core and Z the derivative value for the corresponding wavelength (in nm) expressed by a code of colour. The higher the first derivative the more the colour tends towards the red; the lower its value the more it tends towards blue. Laminated sediments (1); finely laminated sediments (2); dark lamina (3); sandy layer (4); angular drop stones (5); organic debris (6).

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system mounted on an inflatable cataraft that was pushed by an inflatable boat. Conventional GPS navigation allowed the acquisition of a dense grid of seismic reflection profiles with a mean line spacing of 100 m (Fig. 3). In total, 6.5 km of single channel high-resolution reflection profiles were digitally recorded. The system imaged an up to 10 ms two-way travel time (ms TWT) thick sedimentary, except near the main delta, where no acoustic penetration occurred (Fig. 4). Data processing was carried out at the Limnogeology laboratory of ETH and included band pass filtering and automatic gain control. Following this sub-bottom profiling survey, two short cores up to 0.8 m in length were retrieved from two locations using a gravity corer in the deepest part of Lake Blanc Huez (36 m water depth, core LB03-1) and along the eastern slope of the main basin (22 m water depth, core LB03-2) (Fig. 3). In the laboratory, the cores were scanned at 5 mm intervals by a multi-sensor core logger (MSCL; Geotek Ltd.) for gamma density and pwave velocity. Afterwards, the cores were opened, described and photographed with a 400 d.p.i resolution digital camera. Sedimentological investigations on core LB03-1 (Fig. 5) involved detailed measurements of sediment grain size, magnetic susceptibility (MS), spectrophotometry, total organic carbon (TOC) and inorganic carbon (IC). Laser grain size measurements were realized every centimeter using a Malvern Mastersizer 2000. Organic Carbon content of the sediments was determined every 2 cm by the subtraction of IC from TOC measurements using a Coulometer 5011 UIC, as detailed by Gilli et al. (2003). High-resolution measurements of MS and spectrophotometry were performed every 3 mm with a Bartington point sensor MS2E1 and a Minolta 2600D spectrophotometer, respectively. Sediment colour analyses by spectrophotometry provided reflectance intensity measurements for visible wavelengths between 400 and 700 nm. Calibration was performed using a white calibration tile referenced to an international BaSO4 standard. In this study, the first derivative method was used and applied to the visible spectrum as detailed by Debret et al. (2006) in order to investigate down core variations in sediment composition (Fig. 5). The bulk mineralogy of the sediments was also punctually established in each sedimentary facies by X-ray diffraction measurements with a SIEMENS D 501 diffractometer. Finally, arsenic (As) concentrations in core LB03-1 were determined every 2 cm by ICPAES (inductively coupled plasma-atomic emission spectrometry) on a Perkin Elmer Optima 3300. The recent chronology of core LB03-1 was determined by radiometric measurements every 5 mm on the first

20 cm of one half of the core. Samples were dried at 60 °C for 3 days, and weighted to estimate dry bulk density and cumulative massic depth (Fig. 6). Excess 210Pb is determined by subtracting the activity of its parent 226Ra (deduced from measured 214Pb activities in each sediment samples) from the total 210Pb activity. This calculation is based on the assumption that the intermediate daughter 222 Rn is in equilibrium with 226Ra. Samples were sealed before Gamma counting to prevent 222Rn escape and aged for 3 weeks to allow 222Rn to equilibrate with 226Ra. Samples were then measured in a low background germanium well detector-P type (Canberra Company) and all the radiometric measurements were performed in low-level background laboratory. These precautions are particularly necessary for the isotopes of interest here (210Pb, 214Pb, 137Cs and 241Am). Details of the method are given in Pinglot and Pourchet (1995) and Arnaud et al. (2006). Accuracies for our measurements are respectively about 5% for 214Pb, 20% for 137Cs and 210Pb and 50% for 241 Am. 4. Results 4.1. Seismic data The seismic signal easily penetrated the relatively thin and locally regularly stratified lacustrine sediments of Lake Blanc Huez but became scattered and absorbed at the acoustic basement. The topography of the acoustic basement on NS longitudinal profiles is characterized by an undulating morphology dividing the lake into three subbasins (Fig. 4). Conversely, along WE transverse profile it is steep and asymmetric and it locally produces side echo (e.g. Fig. 3). The youngest and uppermost seismic unit is made of laterally continuous high-amplitude reflections characterized by a draping pattern that thins toward the South and develops onlap configurations at the contact with the acoustic substratum (Figs. 3 and 4). This upper seismic unit is also characterized by several small lensshaped bodies with low-amplitude chaotic internal reflections occurring close to the lake floor in the middle and northern sub-basins. Larger lens-shaped bodies are as well recognized in the sub-surface of the northern sub-basin in front of the tributary prodelta (Fig. 4). 4.2. Core data Core LB03-1 is composed of laminated sandy silts frequently containing small (b 1 cm) angular drop stones and sandy layers (Fig. 5). P-wave velocities oscillate around 1.5 km/s down the core in these carbonate-free sediments made of quartz, feldspars (albite and

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Fig. 6. Natural and artificial radionuclide profiles of core LB03-1: total (B); 137Cs and 241Am specific activities (C).

microline), mica (muscovite) and kaolinite. The core is also characterized by several sedimentary facies (F1 to F4) defined by correlated fluctuation of magnetic susceptibility, gamma densities and first derivative reflectance spectra, but anti-correlated fluctuations of TOC. Facies F1 (between 0 and 1.2 cm) is a tan to light-brown homogenous layer composed of clayey silts and characterized by low MS and gamma density values, by a reflectance peak centred at 525 nm and low TOC content (1%). Facies F2 (from 1.2–8 cm) is characterized by sandy silts developing an alternation of black, light-brown or slightly organge-tint and light-grey millimetric to plurimillimetric laminas. Sediments with high MS and density values contain two reflectance peaks in the upper part of the facies culminating at 445 and 525 nm. Below 4 cm, sediments become more greyish with higher organic content (up to 1.8% of TOC) and lower SM and densities values. Facies F3 (from 8–28 cm to 52–57 cm) consist of light grey finely laminated sandy silts characterized by

210

Pb and

214

Pb specific activities (A); Excess

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210

Pb specific activities

high MS and density values (around 1.8 g/cm2), low TOC and two strong reflectance peaks centred at 445 and at 525 nm. Only one thin sandy layer, bearing some organic debris at 18 cm depth, contains more than 1% of TOC. Few angular drop stones occur in this facies. Facies F4 (from 28–52 cm to 57–83 cm) corresponds to dark-grey finely laminated silts containing more sand layers and angular drop stones. These sediments have correlated oscillating MS and density values and contain more than 1% of TOC. In addition, a fluctuating sediment composition in this facies is associated with the occurrence or disappearance of reflectance peaks centred at 445 and 525 nm that are well expressed in facies F3. In core LB03-2 only facies F1 and the upper part of F2 are recognized and can be correlated to those in LB03-1 (Fig. 7). Most of core LB03-2 (from 4–69 cm) consists of greyish to light-brown remoulded sediments. Locally fine laminations are preserved but tilted, while some parts of the core (from 4–21 cm) are made of patchy facies or deformed laminas.

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Fig. 7. Synthetic age-depth model in the upper meter of Lake Blanc Huez sedimentary infill based on the correlation of sedimentary facies in cores LB03-1 and LB03-2. The summer 2003 event has been retrieved in both cores and corresponds to a major hyperpycnal flood deposit resulting both from significant snow and ice melting and human impact on the alluvial plain upstream from the lake.

4.3.

210

Pb and

137

Cs records of core LB03-1

The total 210Pb profile (Fig. 6A) shows a regular decrease in specific activity with depth to a constant value (80 +/− 10 Bq/kg) where total 210Pb and 214Pb activities are similar. Radioactive equilibrium is reached at core depth 10.5 cm. However, relative low values of total 210Pb activity can be noticed close to the surface in facies F1 (from 0 to 0.76 g/cm2) and should be interpreted carefully as discussed later. Below facies F1 and down to 3.2 g/cm2 (i.e. 10.5 cm in core LB03-1), an exponential decrease in excess 210Pb is shown by logarithmic representation (Fig. 6B) and allows the use of excess 210Pb for dating the sediments with the Constant Flux Constant Supply (C.F.C.S.) model (Goldberg, 1963). With this model we obtained a mean mass sedimentation rate of 15.6 mg/ cm2/yr− 1 corresponding to a mean sedimentation rate of c. 0.55 mm/yr. The 137Cs and 241Am atmospheric fall-out radionuclide concentrations are highlighted by a sharp peak between 0.9 and 1.1 g/cm2 (Fig. 6C) corresponding to sediments retrieved between 3 and 3.7 cm in core LB03-1. The 137Cs specific activity is detectable from 1.6 g/cm2 (i.e. 6.5 cm depth) to the top of the core: the concentrations first increases rapidly from 6.5 cm to 3–3.7 cm core depth and

reach a 439+/− 13 Bq/kg maximum specific activity. Caesium concentrations then form a plateau ranging from 359+/− 8 Bq/kg to 372 +/− 11 Bq/kg between 3 and 1.2 cm depth, and finally abruptly drop in the upper 1.2 cm of the core. The 241Am profile depicts a single peak rising sharply from a no-detection level to a 7.5+/− 2 Bq/ kg level, between 0.9 and 1.1 g/cm2 depth. 4.4. Arsenic concentration in sediments Arsenic concentration profiles in the LB03-1 sediments (Fig. 5) show contrasting values from less than 40 ppm in facies F1 to up to 120 ppm in facies F2 (at 4 cm depth in the orange-tint layer). Deeper in the sediment core, concentrations oscillate between 40–60 ppm in facies F3 and 50– 100 ppm in facies F4. 5. Interpretation and discussion 5.1. Sedimentary processes in a proglacial lake The pounded geometry of the upper seismic unit across transversal profiles in each sub-basin and its draping pattern thinning toward the south on longitudinal sections suggest that sedimentation in this proglacial

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lake is controlled by the formation of homopycnal flows during tributary floods (c.f. Brodzikowski and Van Loon, 1991; Ashley, 1995). These sediment plumes, rather than following a specific level within the lake, expand across the lake, while river and lake waters are gradually mixed and the suspended sediment load moves by advections throughout the water column (Bates, 1954; Ashley et al., 1985). As a result, proglacial and ice-contact lake waters during homopycnal flows are characterized by a high-suspension sediment load and have, a brownish colour in Lake Blanc Huez (Fig. 2C) and in the ice-contact lake formed recently in front of the Rousses glacier (Fig. 2D). In general, snow melt floods occur in May or June when the lake-ice starts to break up. At this time of the year the lake waters are still cold and homogenous (5 °C in upper 25 m of the water column) and favour the formation of homopycnal flows. Six measurements of lake water temperatures profiles in the centre of the lake from end of June to mid September 2005 have shown that a thermocline at 10 m only formed at the end of August when flooding typically ceases and very little suspended sediment load is provided to the lake (Delicourt, 2006). In the north of the lake, limited acoustic penetration is observed on the steep slopes of the prodelta. It slightly increases in the delta bottomsets where the continuous high-amplitude reflections are recurrently deformed by mass wasting deposits (MWD) occurring as chaotic lens-shaped bodies (Fig. 4). The upper most out standing reflection is occurring at c. 50 cm below the lake floor in the deepest part of the basin (Fig. 3) and can be correlated with abrupt gamma density fluctuations between 45 and 58 cm depth in core LB03-1 (Fig. 5). These density fluctuations are both associated with the intercalation of frequent dropstones and sandy layers in facies F4 and abrupt changes in the dominating sediment composition of facies F4 and F3 reflected by spectrophotometry and MS measurements. The first derivative reflectance peak at 445 nm, the one centred at 525 nm and those occurring in the 605–695 nm band, are indicative of specific sediment components such as iron oxide minerals (555, 565, 575 nm), the oxyhydroxide goethite (445 and 525 nm) and organic components of the sediment (605 to 695 nm) according to former studies in lacustrine and marine sediments (Deaton and Balsam, 1991; Balsam and Deaton, 1996; Balsam and Beeson, 2003; Nomade, 2005; Debret et al., 2006). Higher MS values in facies F3 are thus interpreted as induced by these iron and goethite oxides, while lower MS values in facies F4 would be a consequence of lower concentrations of oxides but higher amounts of organic matter (Fig. 5). This interpretation is in agreement with the TOC measurements

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and is also suggested by fluctuations in the reflectance band of organic components from 605 to 695 nm. Based on Bates (1954) definition of homopycnal flows, stronger flows would transport sands toward the deep basin, while sands would settle near the delta during less powerful flows. Fluctuating sand content in core LB03-1 is thus interpreted as resulting essentially from changes in the intensity of homopycnal floods through time. Several sandy layers in facies F3 and F4 are however associated with higher TOC values and/or higher As content (e.g. at 18 cm, 41 cm and 61 cm core depths; Fig. 5). Since our coring site is located less than 300 m from a Middle Age mining industry at the lake shore (cf. Fig. 1A) where mining activities required frequent fires and drainage of galleries by re-routing small torrents along the steep surrounding slopes (Bailly-Maitre and Bruno-Dupraz, 1994), the sandy layers rich in organic matter or in As in LB03-1 may be due to human activities associated with the mine. These sandy layers could result either from reworking of debris accumulated in the galleries during the mining activities or heavy rain fall or snow melt events occurring over time since the end of the mining activities. The occurrence of angular rock debris in the sediment retrieved in LB03-1 are interpreted as dropstones from dirty snow-avalanches eroding the valley sides and deposited on the frozen lake as illustrated in Fig. 2A and B. Similar deposits where described in a Norwegian proglacial lake and used to reconstruct Holocene snowavalanche activity (Seierstad et al., 2002). Once the lakeice starts to break and melt, the icebergs rich in avalanche debris in Lake Blanc Huez are transported by winds into different parts of the lake over several days or weeks. As a result, snow avalanche debris can form drop stones at any location in the lake infill when the icebergs finally melt (Fig. 4). This transport by icebergs thus prevents the use of dropstones at one single coring-site to reconstruct snowavalanche activity back in time. 5.2. Age-depth model Cores LB03-1 and LB03-2 were retrieved form the lake in September 2003 some weeks after the re-routing of the course of the main tributary in the alluvial plain (Fig. 1C). During this work, snow melt water supply together with the remobilisation of a large amount of sediments trapped in the alluvial plain developed a large flood event in the lake and its waters kept a striking light-brown colour during several days (F. Clomart. oral com.). Lake sediments in the first 1.2 cm (facies F1) are further characterized by a light colour, a homogenous texture and finer grain size. Facies F1 is thus interpreted as an exceptional flood deposit triggered in summer 2003 by the

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combination of snow melt and release of large volumes of old and fine grained sediments trapped in the alluvial plain by human activities. Following Arnaud et al. (2002), Nomade et al. (2005) and Arnaud et al. (2006) we excluded the 210Pb and 137Cs values associated with this flood deposit and applied a CFCS model starting in 2003 to excess 210Pb specific activity profile between 1.2 and 10.5 cm depth in core LB03-1. The application of a mean sedimentation rate of 0.55 mm/yr allows dating the base of facies F2 at 8 cm to AD 1882+/− 6 years (Fig. 7). This age-depth model is strongly and independently supported by the calculation of a 16.5 +/− 1.2 mg/cm2/yr (i.e. 0.53 mm/yr) mean sedimentation rate based on the depth of both 241Am and 137Cs peaks corresponding to the culmination in AD 1963 of atmospheric nuclear test fall-out in the northern hemisphere (Appelby et al., 1991). Sedimentation rate deduced from natural radionuclide activities allow dating the striking orange-tint layer occurring between 3.7 and 4.3 cm in core LB03-1 to the AD 1955+/− 7 and AD 1943 +/− 9 time period. The origin of this layer, characterized by its colour and a high content in Arsenic (Fig. 5), is not completely understood. The high As content may result either from the use of As-rich explosives during the development of the ski resort at high elevation sites and in the catchment area of Lake Blanc Huez inducing both direct As fall out in the lake or transitory As accumulations in the poorly developed soils of the watershed (Tomkins et al., 2001; Hooijschuur et al., 2002). Or it may result from the release of As in anoxic groundwater in the watershed during the transformation of predominantly Fe (III) oxyhydroxide coatings on sand particles to Fe (II) or mixed Fe (II/III) solid phases, as indicated by a reflectance peak at 520 nm and high MS values (Horneman et al., 1998). In both hypotheses, this striking layer occurring just before the culmination of atmospheric nuclear test in AD 1963 is interpreted as resulting from the impact of the development of the ski resort at higher elevations during the fifties and early sixties around the lake watershed. The orange-tint horizon is also clearly visible in core LB03-2 between 3.7 and 4 cm depth (Fig. 7) and suggests rather similar recent sedimentation rates at both coring sites. In LB03-2 this layer is thinner and draping the top of a slide deposit made of tilted stratified blocks of sediments and locally containing remoulded mud clasts. Based on stratigraphic correlations shown in Fig. 7, this slide was triggered before AD 1963. In addition this slide seems to be synchronous with two recent MWDs in the middle subbasin visible in seismic profiles (Fig. 4) suggesting that an external triggering factor has destabilised the sediments which were draping several slopes of the lake (cf.

Schnellmann et al., 2002; Chapron et al., 2004). Since no exceptional lake-level changes affected the lake in the early 1960’s, we interpret this sediment slide as resulting from earthquake ground accelerations during the 25/04/ 1962 Corrençon earthquake (ML 5.3, maximum MSK intensity 7.5) which is the strongest event of the last 350 years in the western Alps (Thouvenot et al., 2003; Nomade et al., 2005). Middle Age silver and lead mining activities in the galleries next to Lake Blanc Huez released several volatile metallic components, such as Arsenic (Bailly-Maitre and Bruno-Dupraz, 1994). Fluctuations in Arsenic content in the facies F3 and F4 of core LB03-1 may thus be used as a proxy of the impact of mining activities on the environment via atmospheric and/or hydraulic metallic contaminations. Arsenic content in the sediments can however only be used as a chronostratigraphic marker if low background levels can be clearly recognized in the host sediments and used as a pre-anthropogenic times base level (cf. Shotyk and Krachler, 2004). Unfortunately, this does not seem to be the case in core LB03-1 where most of fluctuating As concentrations in facies F3 and F4 do not clearly show a low background level at the base of the core. Since higher As concentrations are mainly associated with fluctuations in the sand fraction and in TOC (Fig. 5) we interpret them as essentially resulting from hydraulic metallic contaminations during or after the mining activities. These interpretations are further supported by the recent study of Lin (2006) showing that levels of higher As concentrations highlighted in the present study are also rich in other metals, possibly related to the exploitation of Barite at the lake shore (i.e. Lead (Pb) and Antimony (Sn)). Based on available data we speculate that the succession of facies F3 and F4 in core LB03-1 represent most of the Little Ice Age period, but not the onset of mining activities at Lake Blanc Huez in AD 1236. Future studies involving similar multiproxy analyses on a longer core should clearly establish the geochemical background of Lake Blanc Huez sediments and allow using metallic contaminations as valuable chronostratigraphic markers. 5.3. High-altitude glacier fluctuations Glacier fluctuations during the LIA are documented in the NWAlps by glacier snout fluctuations over the last four centuries in the Mont-Blanc area (Mougin, 1912; Reynaud and Vincent, 2002; Vincent et al., 2004). Significant regional trends in Swiss and French alpine glacier length fluctuations are also given in Vincent et al. (2004) and Holzhauser et al. (2005) and can be summarized as follow: two main phases of glacier advance during the 17th and 19th centuries, large fluctuations during the 18th century

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and a long phase of glacier retreat locally initiated after AD1830 but widely evidenced after AD1860 and accelerating between AD1860 and 1880. This important phase of glacier retreat in the Alps at the end of the LIA matches a major change in the lithology of Lake Blanc Huez sediments from facies F3 to F2 dated to AD 1882 +/− 6 yrs (Fig. 7). The transition from facies F3 (i.e. finely laminated sediments (rythmites) bearing higher density and magnetic susceptibility values), to the more organic rich and less laminated F2 facies, suggests a significant reduction of glacier erosion in the catchment according to similar lithological changes in other proglacial case studies (Leeman and Niessen, 1994; Ariztegui et al., 1997; Nesje et al., 2000). Following the approach of Dhal et al. (2003), less erosion in the catchment would result from the rapid rise of the Rousses and Herpie glaciers ELA. The timing of this transition from facies F3 to F2 in Lake Blanc Huez is also in agreement with glaciological studies in the Alps and in the Grandes Rousses massif highlighting a major retreat of glaciers (Vincent et al., 2004). Based on the reconstruction of the cumulative net mass balance for Sarennes glacier since AD1880, a decrease of c. 15 m water equivalent was estimated by Torinesi et al. (2002) between AD1885 and AD1906. The sharp change in Lake Blanc Huez sedimentation from F3 to F2 facies is thus interpreted as resulting from a significant retreat of the Rousses and Herpie glaciers at the end of the LIA. Similarly, fluctuations in sediment reflectance spectra, densities, TOC and MS from finely laminated facies F3 to F4 may be due to Rousses and Herpie glaciers ELA fluctuations during the LIA. On the one hand, the presence of more minerogenic material reflected by spectrophotometry and by higher density and MS values in F3 facies, suggests enhanced glacier erosion resulting from lower glacier ELAs in the catchment. This interpretation is supported by the correlation of facies F3 from 8 to 28 cm in core LB03-1 with the well-documented last advance of glaciers in the massif (Flusin et al., 1909; Edouard, 1994) during the 19th century from AD 1820–1850 to ca. 1880 (Fig. 7). On the other hand, less minerogenic and more organic sediments are present in facies F4 and indicate less glacier erosion and thus an elevation of glacier ELAs in the drainage basin. As a working hypothesis, we speculate that the accumulation in core LB03-1 of F4 facies from 28 to 52 cm could reflect significant glacier fluctuations documented in the alpine region during the 18th century, while the formation of F3 facies from 52 to 57 cm may result from one of the well-documented glacier advances described in the Alps during the 17th century in relation with the Maunder solar minima. Facies F4 from 57 cm till the base of core LB03-1 would then possibly reflect glacier fluctuations from the beginning of the LIA.

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The sensitivity of Lake Blanc Huez sedimentation to small glacier fluctuations during the 20th century is documented by a clear increase in sediment density and magnetic susceptibility but a decrease in TOC values between 3.5 and 2 cm in F2 facies of core LB03-1 (Fig. 5). According to our age-depth model these data provide the first evidences for an increase in glacier activity during the late sixties-early nineties time period and can be correlated with the well-documented glacier advances in the alpine region culminating during the 1980’s (Six et al., 2001; Reynaud and Vincent, 2002; Vincent et al., 2004). 5.4. A conceptual model for Lake Blanc Huez sedimentary record of natural environmental changes Based on former case studies in proglacial lakes and on the Lake Blanc Huez basin fill multiproxy analysis presented above, the following conceptual model can be proposed to reconstruct recent high-altitude environmental changes in this part of the Alps. The lowering of the glacier equilibrium line altitude in the catchment area of the lake during periods of climatic changes (either affecting temperatures or precipitations) will increase glacier erosion of the bedrock and the amount of clastic particles transported in suspension by glacial melt waters in early summer. Coalescing glacial melt waters with high sediment suspension loads essentially focus towards the main tributary delta of the lake. At this time of the year, the lake waters, partly covered by icebergs, are still cold and homogenous, witch favour the formation of homopycnal flows across the lake during tributary floods. These sediment plumes develop the progradation of a gilbert type delta, but also result in sediment focusing in the deepest sub-basins and the formation of a lacustrine drape along more gentle slopes. Additionally, sediment accumulation in each sub-basin generally thins towards the lake outlet. Laminated proglacial lacustrine sediments are slightly enriched in organic matter (sedimentary facies F2) during periods when glacial activity is reduced in the catchment. Increasing glacial activity, until a certain threshold is reached in the catchments is, on the contrary, favouring the accumulation of less organic but finely laminated lacustrine sediments (sedimentary facies F3 and F4). In both periods, short term lowering of glacier equilibrium line altitudes in the catchments are reflected in the sedimentary facies F2, F3 and F4 by enhanced accumulations of iron oxide minerals and oxyhydroxide goethite but lower concentrations of organic matter. These changes in sediment composition are reflected by dominating reflectance spectra centred at 445 and 525 nm and by higher values of sediment magnetic susceptibility and gamma density.

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The accumulation of a specific sedimentary facies in Lake Blanc Huez can be related to some periods of welldocumented glaciers snout positions by historical chronicles but also to recent glaciological studies. The last deposition of facies F3 most probably resulted from the last glaciers advance during the LIA in AD 1820– 1850 and the deposition of a moraine belt down to the altitude of approximately 2680 m. This glacier advance was due to at least, a 25% higher mean winter precipitations compared to the 20th century according to Vincent et al. (2005). The accumulation of facies F2 is associated to the 20th century period that follows a major glacier retreat at the end of the LIA essentially resulting from a winter precipitation decrease and not a temperature increase (Vincent et al., 2005). Glacier fluctuations since the 20th are thus essentially controlled by mean summer temperatures and today glacier ELA is around 2900 m in the massif (Vincent, 2002). A modest lowering of the glacier ELAs measured in the massif during the 1980’s (Vincent et al., 2004) is detected in Lake Blanc Huez by MS, density and spectrophotometry records in the upper part of facies F2. Superimposed on the general patterns and trends discussed above, which are essentially controlled by climatic conditions, sediment slumping along the slopes of the lake can be triggered by the active seismo-teconic setting of the area. The occurrence of layers rich in angular drop stones in the lacustrine sediments can as well be related to the impact of frequently observed dirty snow avalanches on the lake-ice in early summer. 6. Conclusions The pounded geometry of the uppermost seismic unit across transversal profiles in each sub- basin and its draping pattern thinning towards the lake outlet on longitudinal profiles suggest that clastic sedimentation is principally controlled by the formation of homopycnal flows during tributary floods in this proglacial lake. This interpretation is in agreement with present-day lacustrine sedimentary processes observed during snow melt events in early summer and with available measurements of lake water temperature profiles. A conceptual model linking fluctuations of glacier equilibrium line altitudes in the catchment area with sedimentary facies retrieved in this lake basin is also proposed. This model allows reconstructing continuous glacier fluctuations since AD 1820–1850 and to document several periods with contrasting glacier activities in the catchment during the LIA. The occurrence of layers rich in angular drop stones in Lac Blanc Huez sediments are related to the impact of

dirty snow avalanches on the lake-ice in early summer but their spatial occurrence is related to the influence of winds on the transport of melting icebergs in the lake. Former high-altitude mining activities during the Middle Age period at the lake shore seems to be responsible for the deposition of sandy layers enriched in organic matter and the associated fluctuations of the arsenic concentration. Enhanced metallic concentrations in sediments may essentially result from hydraulic metallic remobilisation from Middle Age mine tailings by snow melt or heavy rain fall events during the LIA. The highest concentration in arsenic in our lake sediments is however documented during the 20th century in finer sediments deposited just before the AD 1963 peaks in 137Cs and 241Am. The origin of this contamination in arsenic is still not fully understood but is contemporaneous to the development of the ski resort at higher elevations near the lake. This study shows that (i) sub-bottom profiling in highaltitude proglacial lakes is required to select the most suitable coring sites for high-resolution reconstructions of environmental changes, (ii) proglacial lacustrine sediment gamma density, magnetic susceptibility and first derivative reflectance spectra are good and simple proxies to reconstruct glacier activity in the catchment area and (iii) metallic concentrations in Lake Blanc Huez sediments are good indicators of human activities in its drainage basin. Future studies with similar environmental proxies measured on a long piston core at site LB03-1 will allow further documentation on the timing of Holocene glacier fluctuations in the catchment area, the detailed composition of the basin fill and the onset of human impact at these high-altitudes. Acknowledgments The authors would like to greatly acknowledge Flavio Anselmetti (ETH Zurich) for his support during the realisation of the TICO 2003 seismic survey, Jérome Nomade (LGCA Grenoble) for the spectrophotometry data processing, Sylvain Grangeon (LGIT, Grenoble) for the measurements of TOC and all the staff from the ski resort of Alpe d’Huez for the logistical support. Thanks to Marc Desmet and Christian Beck (LGCA, Chambéry) whom kindly provided access to their laboratory for spectrophotometry and magnetic susceptibility analyses. We would also like to thank Urs Gerber (ETH Zürich) for the core photographs and Edward A. Button (ETH Zürich) for his contribution to English editing. Two anonymous reviewers are also acknowledged for the improvement of the manuscript. This project was funded by the French Mountain Institute.

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