Elena Serra
email: elena.serra@geo.unibe.ch

Pierre G Valla

Natacha Gribenski

Fabio J Guedes Magrani

Julien Carcaillet

Reynald Delaloye

Bernard Grobéty

Braillard Serra

Gribenski, N Guedes Magrani

Grobéty, B. & Luc Braillard

Fabio J Guedes Magrani

Geomorphic response to the Lateglacial-Holocene transition in high Alpine regions (Sanetsch Pass, Swiss Alps)

Geomorphic response to the Lateglacial-Holocene transition in high Alpine regions (Sanetsch Pass, Swiss Alps

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 3

High-elevation Alpine regions are active environments, where landforms are constantly reshaped by the interplay between a variety of geomorphic processes. Throughout the Quaternary, the nature and dominance of these processes have been largely determined by climatic oscillations between glacial and interglacial conditions. During glacial periods, glacier erosion, sediment transport and deposition leave behind characteristic landforms and deposits such as Ushaped valleys, glacial cirques, polished bedrock, moraines and erratic boulders (e.g. [START_REF] Benn | Glaciers and Glaciation[END_REF]. During or soon after glacier retreat at the glacial/interglacial transition, glacial processes are gradually replaced by paraglacial and periglacial processes, which transiently transform the ice-free landscape [START_REF] Church | Paraglacial sedimentation: A consideration of fluvial processes conditioned by glaciation[END_REF][START_REF] Ballantyne | Paraglacial geomorphology[END_REF][START_REF] French | The Periglacial Environment[END_REF][START_REF] Mercier | Paraglacial geomorphology: Processes and paraglacial context[END_REF]. Paraglacial processes involve various geomorphic agents (hillslope, fluvial, aeolian) resulting in a wide range of depositional and erosional landforms (e.g. rockfall deposits, debris cones, alluvial fans, gully incisions and aeolian drapes; [START_REF] Ballantyne | Paraglacial geomorphology[END_REF][START_REF] Cossart | Slope instability in relation to glacial debuttressing in alpine areas (Upper Durance catchment, southeastern France): Evidence from field data and 10 Be cosmic ray exposure ages[END_REF][START_REF] Borgatti | Landslides as a geomorphological proxy for climate change: A record from the Dolomites (northern Italy)[END_REF][START_REF] Cossart | An overview of the consequences of paraglacial landsliding on deglaciated mountain slopes: typology, timing and contribution to cascading fluxes[END_REF][START_REF] Geilhausen | Sediment discharge from the proglacial zone of a retreating Alpine glacier[END_REF][START_REF] Gild | Discovery of a landscape-wide drape of Lateglacial aeolian silt in the western Northern Calcareous Alps (Austria): First results and implications[END_REF].

Periglacial processes are also present and give origin to permafrost-related landforms like rock glaciers and patterned ground [START_REF] French | The Periglacial Environment[END_REF][START_REF] Colombo | Geomorphology of the Hohsand basin (Western Italian Alps)[END_REF].

The European Alps have been repeatedly glaciated during the Late Pleistocene, with glacier expansion culminating at the Last Glacial Maximum (LGM, 26.5-19.0 ka ago; [START_REF] Clark | The Last Glacial Maximum[END_REF]Wirsig et al. 2016a). Post-LGM warming and rapid ice decay were followed by episodes of glacier stillstands and readvances during the Lateglacial period (Lateglacial stadials, 19.0-11.6 ka ago;[START_REF] Ivy-Ochs | Cosmogenic nuclides and the dating of Lateglacial and Early Holocene glacier variations: The Alpine perspective[END_REF][START_REF] Hippe | Chronology of Lateglacial ice flow reorganization and deglaciation in the Gotthard Pass area, Central Swiss Alps, based on cosmogenic 10 Be and in situ 14 C[END_REF][START_REF] Ivy-Ochs | Glacier variations in the European Alps at the end of the last glaciation[END_REF], due to temporary climate deteriorations [START_REF] Schmidt | North Atlantic climate impact on early Lateglacial climate oscillations in the south-eastern Alps inferred from a multiproxy lake sediment record[END_REF]. Following the Younger Dryas (YD, 12.7-11.6 ka ago; [START_REF] Ivy-Ochs | Cosmogenic nuclides and the dating of Lateglacial and Early Holocene glacier variations: The Alpine perspective[END_REF]) and the Early Holocene Preboreal Oscillation cold event (11.4-11.3 ka ago;[START_REF] Rasmussen | Early Holocene climate oscillations recorded in three Greenland ice cores[END_REF][START_REF] Schimmelpfennig | Holocene glacier culminations in the Western Alps and their hemispheric relevance[END_REF], Alpine glaciers eventually retreated

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 4

and remained behind ice limit of the Little Ice Age (LIA, 1300-1860 CE;[START_REF] Hanspeter | Glacier and lake-level variations in west-central Europe over the last 3500 years[END_REF][START_REF] Ivy-Ochs | Latest Pleistocene and Holocene glacier variations in the European Alps[END_REF][START_REF] Steinemann | Quantifying glacial erosion on a limestone bed and the relevance for landscape development in the Alps[END_REF], in response to Early and Middle Holocene abrupt warming [START_REF] Schimmelpfennig | Holocene glacier culminations in the Western Alps and their hemispheric relevance[END_REF].

Palaeoenvironmental conditions during the Lateglacial and at the Holocene transition have been investigated through several palaeoclimate proxies [START_REF] Heiri | Palaeoclimate records 60-8 ka in the Austrian and Swiss Alps and their forelands[END_REF]. However, the geomorphic response to this major environmental transition has been only punctually examined, focusing on specific geomorphic processes like rockfall activity (e.g. [START_REF] Zerathe | Mid-Holocene cluster of large-scale landslides revealed in the Southwestern Alps by 36 Cl dating. Insight on an Alpine-scale landslide activity[END_REF][START_REF] Ivy-Ochs | Geomorphology and age of the Marocche di Dro rock avalanches (Trentino, Italy)[END_REF] or fluvial incision (e.g. [START_REF] Korup | Bedrock landsliding, river incision, and transience of geomorphic hillslope-channel coupling: Evidence from inner gorges in the Swiss Alps[END_REF][START_REF] Valla | Fluvial incision into bedrock: Insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys (Western Alps, France)[END_REF][START_REF] Rolland | Inner gorges incision history: A proxy for deglaciation? Insights from Cosmic Ray Exposure dating Accepted Manuscript[END_REF]. Likewise, the timing of paraglacial activity, as well as the question of sediment cascade and landscape connectivity, are still poorly understood (Mercier & Etienne 2008 and references therein).

Our study aimed to contribute to the reconstruction of post-LGM palaeoenvironmental conditions and geomorphic connectivity within high-altitude Alpine regions. We focused on the Sanetsch Pass area (2252 m a.s.l., western Swiss Alps; Fig. 1) where multiple geomorphic processes have left an extensive record of landforms and sediment deposits during the Lateglacial and the Holocene (Fig. 2). Three landforms representative of the high Alpine landscape were investigated: a silty deposit covered by patterned ground, an alluvial fan and a hummocky moraine covered by rockfall deposits (Figs 1, 2, 3). These three deposits were selected with the goals of (i) reconstructing their geomorphological history, (ii) investigating a potential connectivity between the different records and (iii) assessing the suitability of glacial, paraglacial and periglacial landforms as proxies for palaeoenvironmental reconstruction in high Alpine regions. To that aim, we performed a detailed geomorphological and sedimentological

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 5

field investigation and collected samples for multi-methodological analyses on the different landforms.

Study area and sampling locations

The study area is located next to the Sanetsch Pass (2252 m a.s.l., western Swiss Alps; Fig. 1).

The local bedrock is composed of Cretaceous and Paleogene sedimentary rocks belonging to the Helvetic Diablerets and Wildhorn nappes [START_REF] Badoux | Map sheet 1286 St-Léonard with explanatory note 35[END_REF][START_REF] Masson | Helvetic Alps of Western Switzerland[END_REF]. Three geological units are of particular interest for our study: calcareous shales containing ca. 20% of fine detrital quartz (Palfris Formation; [START_REF] Föllmi | Unlocking paleoenvironmental information from Early Cretaceous shelf sediments in the Helvetic Alps: stratigraphy is the key![END_REF]) form the Arpille Ridge, siliceous limestones (Helvetic Kieselkalk Formation; [START_REF] Föllmi | Unlocking paleoenvironmental information from Early Cretaceous shelf sediments in the Helvetic Alps: stratigraphy is the key![END_REF]) form the Arpelistock cliff, and quartzose sandstones (Fruttli Member of the Klimsenhorn Formation, Menkveld-Gfeller 1997) from a 5-m thick layer at the top of the calcareous Les Montons cliff (Fig. 2).

The Sanetsch Pass is currently ice free, with the nearest glacier front (Tsanfleuron Glacier) 3 km to the southwest (Fig. 1). During the LGM, the Rhône glacier reached an altitude of about 2000 m a.s.l. in the main valley above Sion (Fig. 1; [START_REF] Bini | Switzerland during the Last Glacial Maximum[END_REF]). The Sanetsch Pass area was partly covered by ice with an estimated average thickness of 200 m and some mountain peaks were protruding above the ice surface as nunataks [START_REF] Bini | Switzerland during the Last Glacial Maximum[END_REF]).

The Tsanfleuron glacier was flowing both northwards and southwards from the pass, as testified by lateral morainic ridges (Fig. 2; [START_REF] Badoux | Map sheet 1286 St-Léonard with explanatory note 35[END_REF]. Following the deglaciation of the Swiss foreland c. 21 ka ago [START_REF] Ivy-Ochs | Timing of deglaciation on the northern Alpine foreland (Switzerland)[END_REF]), the Rhône Valley became ice free c. 17 ka ago [START_REF] Hantke | Eiszeitalter -Die jüngste Erdgeschichte der Alpen und ihrer Nachbargebiete[END_REF][START_REF] Hinderer | Late Quaternary denudation of the Alps, valley and lake fillings and modern river loads[END_REF]. The onset of sedimentation in the Pond Emines (at 2288 m a.s.l., 1 km southwest from the Sanetsch Pass; Fig. 2) at c. 12 ka ago [START_REF] Berthel | Impact of Holocene climate changes on alpine and treeline vegetation at Sanetsch Pass, Bernese Alps, Switzerland[END_REF], and the apparent post-12 ka exposure ages [START_REF] Steinemann | Quantifying glacial erosion on a limestone bed and the relevance for landscape development in the Alps[END_REF] 3D).

Methods

Geomorphological and sedimentological field investigation

In the present study, we performed a new geomorphological mapping over an area of 16 km 2 around the Sanetsch pass (Fig. 2), based on the sheet St-Léonard (1286) [START_REF] Badoux | Map sheet 1286 St-Léonard with explanatory note 35[END_REF] of the Geological Atlas of Switzerland (1:25000). To this aim, we combined the geomorphological map of Quaternary superficial deposits [START_REF] Badoux | Map sheet 1286 St-Léonard with explanatory note 35[END_REF] with new field investigations and remote sensing mapping based on the swissALTI3D DEM and orthophotos (swisstopo).

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 7

Detailed stratigraphic outcrops were logged for the ARP and CRE deposits. Two stratigraphic profiles ( 40 cm-deep) were dug out in the ARP high-elevation silty deposit, below the polygonal patterned ground (Fig. 3B). The CRE log was recorded from a natural outcrop exposed in a gully incising the alluvial fan (4.5 m deep; Fig. 3C). Sedimentary units were identified based on their grain-size range, sorting, clast shape, compaction, sedimentary structures, colour and lithology. Reaction to HCl on the field and nature of contacts between individual units were also recorded.

Grain-size, micromorphology and optical petrography

Eight bulk sediment samples were collected for grain-size analyses (G1-G6 from CRE site, G7

and G8 from ARP site; Figs 5 and4, respectively), in order to understand the genesis of the ARP and CRE deposits and a potential connectivity between them. Conventional grain-size distribution measurements were carried out by sieving the air-dried sand and granule/pebble fractions [START_REF] Rivière | Méthodes granulométriques, techniques et interprétation[END_REF][START_REF] Hadjouis | Granulométrie des fractions sableuses[END_REF]. Analyses of the silt and clay fractions were performed using a laser Malvern MS20 diffraction system (Department of Environmental Sciences, University of Basel, Switzerland). Sorting indexes (So) were calculated using millimetres units (So = (D 75 /D 25 ) 0.5 , where D 25 and D 75 are respectively the first and the third quartile of the grainsize distribution), providing different sediment sorting categories: very well sorted (So < 2.5), well sorted (So between 2.5 and 3.5), normally sorted (So between 3.5 and 4.5) and poorly sorted (So > 4.5).

Three CRE undisturbed sediment samples were collected in Kubiena boxes (8x11 cm) for micromorphological analyses (M1-M3; Fig. 5), with the aim to identify depositional and postdepositional features in the alluvial fan sediments. The samples were first air-dried, impregnated

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 8

with an acetone-diluted epoxy resin and then cut with a diamond saw (Department of Environmental Sciences, University of Basel, Switzerland). Two covered thin sections (4.5x4.5 cm) per sample were prepared (Th. Beckmann, Braunschweig, Germany) and examined optically on a Leitz DM-RXP microscope, both in plane-polarized and crossed-polarized light. Bedrock thin sections were additionally prepared (Department of Geosciences, University of Fribourg, Switzerland) and analysed for four bedrock samples representative of the catchment lithology (two from the siliceous limestone, L1 and L2, and two from the calcareous shale, S1 and S2;

Table S1), in order to obtain further information on the silicate fraction of the bedrock (grain shape and size) and to discuss a potential connectivity between the local bedrock and the CRE sediments.

X-Ray diffraction and geochemistry X-Ray Diffraction (XRD) analyses were performed on a total of 22 samples, including 18 bulk sediment samples from CRE and ARP sites (CRE01-CRE16, Fig. 5; ARP01 and ARP03, Fig. 4) and 4 bedrock samples (L1, L2, S1, S2), to gain information on their mineral composition and assess sediment provenance and potential connectivity of the ARP and CRE deposits. Bulk sediments and bedrock samples were ground with a mechanical crusher, and sample powders were analysed through a Rigaku Ultima IV diffractometer system using Cu-Kα radiation (Department of Geosciences, University of Fribourg, Switzerland). Mineral identification was performed using the Rigaku's PDXL-2 software and the ICDD Powder Diffraction File 2017 database (International Centre for Diffraction Data). Mineral quantification was made by Rietveld refinement [START_REF] Rietveld | A profile refinement method for nuclear and magnetic structures[END_REF], with weighted residuals of the whole pattern (Rwp) and the

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 9

goodness of fit (GOF) considered as Rietveld fit criteria [START_REF] Toby | R factors in Rietveld analysis: How good is good enough?[END_REF]). In the present refinements,

Rwp and GOF values range from 4.84% to 6.74% and from 1.73 to 2.90, respectively.

Concentrations of total carbon (TC) and total inorganic carbon (TIC) were measured on the same ARP and CRE sediment samples, using a Bruker G4 Icarus elemental analyser (Institute of Geology, University of Bern, Switzerland). The percentage of total organic carbon (TOC) was determined from the difference between TC and TIC. These analyses and calculations were performed to examine the potential presence of palaeosols in the deposits.

Geochronology

Portable optically stimulated luminescence. Luminescence signal intensities were measured for the CRE (cre01-cre16; Fig. 5) and ARP (arp01-arp05; Fig. 4) sites, with the aim of investigating potential stratigraphic variation in the luminescence response, possibly reflecting differences in sediment provenance and depositional environment. Indeed, the luminescence signal of bulk sediment is mainly dependent, among other factors, on the pre-deposition light exposure of sediments (i.e. signal resetting or " bleaching"; [START_REF] King | Understanding processes of sediment bleaching in glacial settings using a portable OSL reader[END_REF] S2A), which includes an IRSL stimulation at 50 °C (100 s) prior to the blue OSL stimulation (at 125 °C for 100 s; [START_REF] Lowick | Luminescence dating of Middle Pleistocene proglacial deposits from northern Switzerland: methodological aspects and stratigraphical conclusions[END_REF]. IRSL D e measurements were conducted on polymineral (non-separated) fractions for all the ARP samples using the post-IR IRSL protocol of [START_REF] Buylaert | Testing the potential of an elevated temperature IRSL signal from K-feldspar[END_REF] (this protocol targets feldspar luminescence signal only;

Table S2B). After a preheat treatment (250 °C for 60 s), luminescence measurement cycles of this modified single-aliquot regenerative (SAR) dose protocol [START_REF] Murray | Dating quartz using an improved single-aliquot regenerative-dose (SAR) protocol[END_REF] consist in a first IRSL stimulation at 50 °C (100 s) followed by a second IRSL stimulation at 225 °C (100 s). A 40s IRSL stimulation at 290 °C was added at the end of each SAR cycle in order to limit signal carry-over throughout successive cycles [START_REF] Wallinga | A test case for anomalous fading correction in IRSL dating[END_REF]. No post-IR IRSL measurements were performed on CRE samples due to the absence of feldspar IRSL signal.

Fading rates (g-value; Aitken 1985) were measured on aliquots used for D e determination, following [START_REF] Auclair | Measurement of anomalous fading for feldspar IRSL using SAR[END_REF]. Final fading corrected D e values were calculated following the fading correction procedure of [START_REF] Huntley | Ubiquity of anomalous fading in K-feldspars and the measurement and correction for it in optical dating[END_REF], using the Luminescence R package [START_REF] Kreutzer | Introducing an R package for luminescence dating analysis[END_REF]. Signals used for data analysis were integrated over the first 1. Final ages were calculated using the Central Age Model (CAM; Galbraith & Roberts 2012). 10 Be surface exposure dating. Three sandstone boulders rockfall-derived lying on the hummocky moraine, one on a flat area, one on a hummock ridge and one in an intra-hummock hollow (boulders 2-m wide and 1-m high, SAN site; Fig. 3C), were sampled for 10 Be surface-exposure dating, in order to obtain a minimum age constraint for the hummocky moraine formation.

Samples were collected with hammer and chisel from flat surfaces on top of the boulders, with evidence for minimal erosion [START_REF] Gosse | Terrestrial in situ cosmogenic nuclides: theory and application[END_REF]. Sample crushing and sieving were performed to isolate the 250-400 µm grainsize fraction. Quartz was isolated using magnetic separation and repeated leaching in a H 2 SiF 6 -HCl mixture. Conventional chemical treatment adapted from [START_REF] Brown | Examination of surface exposure ages of Antarctic moraines using in situ produced 10 Be and 26 Al[END_REF] and [START_REF] Merchel | An update on radiochemical separation techniques for the determination of long-lived radionuclides via accelerator mass spectrometry[END_REF] was followed to complete 10 Be extraction (GTC platform, ISTerre, University Grenoble Alpes, France). 10 Be/ 9 Be ratios were measured at ASTER French National AMS facility (CEREGE, Aix-en-Provence, France; Arnold

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 13 et al. 2010) against the in-house Be standard [START_REF] Braucher | Preparation of ASTER in-house 10 Be/ 9 Be standard solutions[END_REF], whose assumed isotope ratio is 1.191x10 11 . Correction for a full process blank ratio of 10 Be/ 9 Be = 5.4±0.6x10 -15 was applied.

Surface-exposure ages were computed with the online CREp program [START_REF] Martin | The CREp program and the ICE-D production rate calibration database: A fully parameterizable and updated online tool to compute cosmic-ray exposure ages[END_REF]; https://crep.otelo.univ-lorraine.fr/#/init). Calculations were performed using a 10 Be production rate by neutron spallation at sea level and high latitude (SLHL) of 4.16±0.10 at g -1 a -1 [START_REF] Claude | The Chironico landslide (Valle Leventina, southern Swiss Alps): age and evolution[END_REF]) scaled at the sample sites using the LSDn scaling scheme [START_REF] Lifton | Scaling in situ cosmogenic nuclide production rates using analytical approximations to atmospheric cosmic-ray fluxes[END_REF]. The ERA-40 reanalysis data set [START_REF] Uppala | The ERA-40 re-analysis[END_REF]) and the Lifton-VDM2016 geomagnetic database [START_REF] Lifton | Implications of two Holocene time-dependent geomagnetic models for cosmogenic nuclide production rate scaling[END_REF] were used to correct atmospheric pressure and geomagnetic field fluctuations, respectively. Corrections for topographic shielding based on field measurements were also applied [START_REF] Dunne | Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces[END_REF]. For assessing the effect of potential postdepositional surface covering (e.g. snow) or surface degradation processes, two end-member scenarios were considered: one without any post-depositional correction and one including snow-cover and surface-erosion corrections. For surface-erosion correction, an erosion rate of 1 mm ka -1 was applied, following the argument of Wirsig et al. (2016b). The snow-cover correction factor was computed for all boulders following eq. (3.76) in [START_REF] Gosse | Terrestrial in situ cosmogenic nuclides: theory and application[END_REF], considering 6 months per year persistence of a 50-cm thick snow cover (Wirsig et al. 2016b), a snow density of 0.3 g cm -3 and a spallation attenuation length in snow of 109 g cm -2 [START_REF] Delunel | Snow shielding factors for cosmogenic nuclide dating inferred from long-term neutron detector monitoring[END_REF].

Results

Geomorphological and sedimentological field investigation

We present an updated map of superficial deposits in Fig. 2, showing the Quaternary deposits occurring in the study area and the location of the three study sites (ARP, CRE and SAN).

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 14

Sedimentlogical observations for ARP and CRE deposits are summarised in the stratigraphic logs showed in Figs 4 and 5, respectively (detailed descriptions are given in Tables S3 andS4). The stratigraphy of the ARP deposit (Fig. 4, Table S3) consists of two sediment units lying on top of non-weathered siliceous-limestone bedrock. The 10-cm thick top unit (unit 1) is clast-supported, with siliceous-limestone fragments in a silty matrix. The 30-cm thick basal unit (unit 2) consists of a silty layer with a well-developed platy structure, containing few siliceouslimestone fragments.

The stratigraphy of the CRE alluvial fan (Fig. 5, Table S4) displays the current soil horizon (unit 1) and the recent (i.e. Holocene) coarse alluvial fan deposit (unit 2) in the first 1.5 m of depth. Underneath, a succession of three categories of sediment units can be observed: homogenous clayey silt (units 6, 7, 9, 10, 17, called "fine layers" hereafter), silty clay or loam containing weathered granules and pebbles (units 3, 4, 5, 8, 11, 12, 13, 15, 19, called "heterogeneous layers" hereafter), and gravel layers clast-supported with imbricated elements (units 14, 16, 18).

Grain-size and micromorphology

Grain-size distributions (Fig. 6) show strong similarities between the ARP silty unit (unit 2, samples G7 and G8), and the CRE fine (units 6, 10, 17, samples G2, G3 and G6) and heterogeneous (units 5, 12, 15, samples G1, G4 and G5) layers. Frequency curves are multimodal and characterised by one primary peak around 35 µm (coarse silt), a secondary peak around 4-8 µm (fine/very fine silt), and a minor peak around 150 µm (fine sand). An additional minor peak around 10 4 µm (pebble) is also visible for samples G4 (CRE site, heterogeneous layer) and G8 (ARP site), which reflects the presence of some isolated clasts as observed in the

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 15

stratigraphy (Figs 4,5). Median grainsize values (D 50 ) range between 11 and 16 µm (fine silt).

Calculated So vary between 2.7 and 3.4, reflecting good sorting (with the exception of G4 and G8 which have a So of 5.5 and 4.5 respectively, but reduced to 3.3 and 2.9 if only the clayey, silty and sandy fractions are considered). A low proportion (<2%) of small lithic fragments (<2 mm; not visible in Fig. 7A but similar to the one shown in Fig. 7C) containing quartz grains (20 to 300 𝜇m size) is also recognized. In thin section M2 (fine unit 7; Fig. 7B) and M3 (heterogeneous unit 13), welldeveloped horizontal laminae (1 to 5-mm thick) with normal grading are observed. In addition, the lower part of thin section M2 (heterogeneous unit 8; not visible in Fig. 7B) contains a higher proportion (5-10%) of weathered (partly-decarbonated) lithic fragments (2-5 mm) in a clayeysilty matrix. Thin section M3 (heterogeneous units 12 and 13; only unit 13 shown in Fig. 7C)

shows 40% of weathered (partly-to completely-decarbonated) lithic fragments (3-10 mm) containing quartz grains (20 to 150 µm size), which are embedded in a clayey-silty carbonated matrix. These fragments present different weathering stages, but no weathering cortex on their edges. Lastly, bedrock thin sections display quartz grains of 10 to 150 µm size (the main population ranging between 30 and 50 µm), representing 5-15% of the siliceous limestone and 15-30% of the calcareous shales (Fig. 7D).

X-Ray diffraction and geochemistry

Accepted shown in Fig. 9 (samples arp01-arp05) and Fig. 10 (samples cre01-cre16), respectively (luminescence counts are reported in Table S5). For ARP site (Fig. 9), both the quartz OSL and feldspar IRSL intensity profiles could be measured, with more than one order of magnitude difference between OSL (10 3 -10 4 counts) and IRSL (10 1 -10 2 counts) signals. While the IRSL counts increase consistently with depth (Fig. 9B), the OSL intensity profile is more complex: sample arp03 displays large uncertainty due to the significant difference between the two

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 17

replicate measurements, and sample arp05 deviates significantly from the increasing-signal trend with depth (Fig. 9A).

For CRE site, the general low-feldspar content in the analysed sediments, as highlighted by XRD-analyses (Fig. 8), prevented the measurement of IRSL signal along the section, and only an OSL profile could be obtained (Fig. 10). The measured profile does not follow a stratigraphic coherence (i.e. luminescence signal intensity increasing with depth), but alternates between two signal-intensity groups characterised by low (20-25x10 3 counts) and high (30-50x10 3 counts) OSL counts. These two groups correspond respectively to fine (low signal intensities) and heterogeneous (high signal intensities) layers (Fig. 10).

Conventional OSL dating. Sample specific information and results, including D e values, relevant dose-rate data and final CAM ages [START_REF] Galbraith | Statistical aspects of equivalent dose and error calculation and display in OSL dating: An overview and some recommendations[END_REF], are shown in Tables 1 and2, and Figs 9 and 10. Both OSL and IRSL ages were obtained for two ARP samples (ARP01 and ARP02; Fig. 9), while only an IRSL age was measured for sample ARP03 (due to the lack of material for quartz purification). ARP final IRSL and OSL ages range between 2.5 and 7 ka, with high variability between samples and signals despite sample collection at relatively similar depths within unit 2 ( 25-cm deep; Fig. 4). For both ARP01 and ARP02, the IRSL and OSL ages appear to differ without any systematics (i.e. younger IRSL age for ARP01, the opposite for ARP02).

Quartz OSL data for CRE site (Fig. performed on CRE samples due to the absence of feldspar IRSL signal. 10 Be surface exposure dating. Sample specific information, as well as measured 10 Be concentrations and calculated exposure ages (with and without erosion and snow corrections), are summarized in Table 3 and Fig. 11. A high consistency of the 10 Be exposure ages obtained for the three SAN sandstone boulders is evident (9.3±0.4, 9.4±0.4 and 9.8±0.4 ka, without snow and erosion corrections). The three boulder ages agree within uncertainty and yield a weightedaverage age estimates of 9.5±0.3 ka (no snow and erosion correction) and 10.3±0.3 ka (with snow and erosion correction).

Discussion

Landform and sediment (post-)depositional histories High-elevation platform (ARP). Field stratigraphic observations (Table S3) and the different results obtained from the multi-method approach suggest (i) that the silty sediments (unit 2) from the ARP high-elevation platform correspond to an aeolian deposit covered by in situ produced cryoclasts (i.e. bedrock limestone clasts derived from freeze and thaw cycles), and (ii) that the whole deposit has been reworked by recent (possibly still ongoing) cryoturbation.

Excluding the coarser grain-size fraction (>2 mm, most probably deriving from isolated cryoclasts) from G7 and G8 grain-size analyses (Fig. 6), the good sorting indexes (2.5 and 3. [START_REF] Guélat | Les Loess de la Vallée du Rhône[END_REF]and pers. comm. 2016). We interpret the different grain-size characteristics between ARP and Rhône Valley loess as the result of a mixed provenance with both local and allochtonous input for ARP deposit.

Several arguments allow us to exclude the hypothesis that the ARP silty sediments derive from in situ weathering of the underlying siliceous limestone (L1-L2; Fig. 8): (i) the sharp (with no weathering) contact between the base of the ARP aeolian deposit and the underlying bedrock, (ii) the complete absence of carbonates, and (iii) the presence of albite in ARP samples (ARP01 and ARP03; Fig. 8). The albite content (4-7%) supports the hypothesis of an allochtonous component within the ARP aeolian deposit, possibly related to the Rhône Valley loess at lower elevations, which is dominated by quartz, micas, chlorite and feldspar mineralogy [START_REF] Spaltenstein | Pédogenèse sur calcaire dur dans les Hautes Alpes calcaires[END_REF]M. Guélat pers. comm. 2016). Such a loessic input was indeed highlighted in a nearby soil profile located 1.7 km south of the Sanetsch Pass [START_REF] Spaltenstein | Caractérisation de deux sols alpins minces sur calcaire dur dans le lapiaz du Sanetsch[END_REF]. However, XRD analyses alone cannot discriminate the dominant source area(s) between Alpine or more distal origin (e.g. [START_REF] Martignier | Assessment of a locally-sourced loess system in Europe: The Swiss Jura Mountains[END_REF] for the ARP aeolian material. Further geochemical analyses would be necessary (Újvári et al. 2015) to quantitatively discuss the origin of such aeolian sediments in the context of atmospheric palaeo-circulations at the scale of the European Alps (e.g. [START_REF] Muhs | Loess record[END_REF][START_REF] Rousseau | European loess records[END_REF].

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 20

Conventional luminescence dating on ARP silty sediments provides ages ranging between 2 and 7 ka, with an observed discrepancy between OSL and IRSL ages and no clear systematics (Fig. 9, Tables 1,2). This mismatch is further highlighted by portable luminescence measurements, with different patterns in OSL and IRSL intensity profiles (i.e. higher OSL variability compared to depth-increasing IRSL signal; Fig. 9). Different luminescence signal resetting rates under light exposure (i.e. bleaching) have been reported in the literature [START_REF] Murray | Identifying well-bleached quartz using the different bleaching rates of quartz and feldspar luminescence signals[END_REF], with quartz OSL signal bleaching more rapidly than feldspar IRSL. Our observations can be explained by (i) partial bleaching of the ARP silt before deposition, which would lead to overestimation of the true depositional age, and/or (ii) differential post-depositional bleaching through cryoturbation, which would cause underestimation of the true depositional age. Based on the sedimentology, we favour the post-depositional bleaching hypothesis: there is high probability that the ARP silt was completely bleached prior to deposition, since aeolian sediment transport is commonly associated with efficient sunlight exposure [START_REF] Li | Luminescence sensitivity change due to bleaching of sediments[END_REF]). After deposition, active cryoturbation processes resulted in both sediment reworking and formation of the overlying patterned ground (unit 1; Fig. 4). Frost and thaw processes have continually mobilized sediment patches from various depths [START_REF] Bertran | Experimental soft-sediment deformation caused by fluidization and intrusive ice melt in sand[END_REF], causing light exposure of reworked sediments and the (partial) resetting of their luminescence signal. Similarly, stratigraphic inconsistency and large variability in luminescence ages have been observed for periglacial sediment deposits, and have been attributed to post-depositional reworking due to active cryoturbation processes [START_REF] Bateman | Luminescence dating of periglacial sediments and structures[END_REF][START_REF] Andrieux | The chronology of Late Pleistocene thermal contraction cracking derived from sand wedge OSL dating in central and southern France[END_REF] Alluvial fan (CRE site). Insights into the depositional history of the CRE alluvial fan and its connection to the surrounding landforms and deposits also emerge from our multi-method results. XRD and micromorphological analyses confirm field stratigraphic observations (Table S4), revealing a local bedrock origin for the clasts contained in the heterogeneous and gravel layers of the fan (Figs 5,7,8). This is suggested by the moderate calcite content (5-39%; Fig. 8)

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 22

in most of the heterogeneous CRE units, which likely relates to the presence of bedrock-derived weathered granules and pebbles (55-66% calcite content in both siliceous limestone and calcareous shales; Fig. 8). The absence of calcite in some heterogeneous layers can be explained by clast weathering and complete decarbonation as observed in the field (very little/no reaction to HCl; Table S4). Thin-section analyses further highlight mineralogical and grain-size similarities between weathered clasts and bedrock (Figs 7C, D), with abundant quartz grains of similar sizes (10-150 µm) in both bedrock and clasts. The absence of a developed weathering cortex around the clasts suggests that weathering did not occur in situ (i.e. after clast deposition in the alluvial fan), but is probably inherited from a previous alteration phase (e.g. within morainic and/or slope parental deposits). Further support to this hypothesis is given by the low TOC along the fan stratigraphy, which implies absence of paleo-soils and therefore little in situ weathering by pedogenesis.

Field stratigraphic observations, grain-size and XRD analyses also show similarities between the fine/heterogeneous CRE layers and the high-elevation ARP deposit. Grainsize distributions closely overlap for CRE and ARP samples (G1 to G6, and G7 and G8, respectively; Fig. 6), with multiple modes (4-8, 35 and 150 µm), good sorting (2.7-3.3) and silty-fraction dominance (D 50 between 10.9 and 16.5 µm). Mineralogical analyses further support this observation, with the absence of calcite (except for CRE15, which probably integrates clasts from overlying/underlying gravel units) and the dominance of quartz (40-54%) and micas (23-43%). Stratigraphy and micromorphological analyses also show (i) fining-up sedimentary sequences within some CRE units (i.e. from weathered calcareous granules and pebbles at the base to silty loam at the top; Fig. 5, Table S4), (ii) a moderately-developed sub-angular blocky microstructure (unit 6; Fig. 7A) and (iii) well-developed 1 to 5-mm thick horizontal laminae

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 23

(units 7 and 13; Fig. 7B). All together, these observations point towards an alluvial reworking of aeolian sediments to form the fine units and the matrix of the heterogeneous units of the CRE alluvial fan. Deposition of reworked aeolian sediments could have occurred in a lake, or possibly in a pond within the alluvial fan, as suggested by the fining-up sedimentary sequences (units 3, 4 and 5) and the horizontal laminae (units 7 and 13). Isolated pebbles and cobbles (fine units 7, 9

and 10) can be interpreted as possible dropstones, and would support the lake hypothesis. Based on the presence of slickensides and mottling (unit 6; Fig. 7A) as well as Fe-Mn precipitation, we hypothesize ephemeral lake/pond conditions with wetting/drying cycles and low-sedimentation phases, during which the original depositional microstructure (Fig. 7B) was destroyed.

Luminescence measurements (both portable and conventional OSL), show a general alternating pattern between two populations of low-counts/young-ages and high-counts/old-age along the fan stratigraphy (Fig. 10). We propose that these OSL trends reflect differences in quartz provenance as well as transport processes. For the heterogeneous layers, we expect the high OSL counts and old OSL age to result from the mixing and signal-averaging between grains from weathered clasts carrying an inherited signal (i. The rockfall event investigated here was probably followed by a second event, characterized by smaller extent (Fig. 11). Age constraint for this subsequent event is however not available, but may be related to Holocene landslide/rockslide clusters observed elsewhere in the European Alps [START_REF] Zerathe | Mid-Holocene cluster of large-scale landslides revealed in the Southwestern Alps by 36 Cl dating. Insight on an Alpine-scale landslide activity[END_REF][START_REF] Ivy-Ochs | Geomorphology and age of the Marocche di Dro rock avalanches (Trentino, Italy)[END_REF].

Lateglacial to Holocene palaeoenvironmental reconstruction

By combining our field observations and multi-method results, we propose a palaeoenvironmental reconstruction of the Sanetsch Pass area during the Lateglacial-Holocene transition, with the aim to highlight the interplay between different sedimentary and geomorphic processes (Fig. 12).

Throughout the LGM and Lateglacial periods, the area north of Sanetsch Pass was icecovered, while some peaks were protruding above the ice surface (nunataks, Fig. 12A; [START_REF] Bini | Switzerland during the Last Glacial Maximum[END_REF], similar to the overall European Alps. These conditions may have favoured deposition of

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 26

aeolian drapes on nunatak platforms already at that time [START_REF] Gild | Discovery of a landscape-wide drape of Lateglacial aeolian silt in the western Northern Calcareous Alps (Austria): First results and implications[END_REF], with an allochtonous contribution of the wind-blown material as confirmed by XRD analyses (ARP site), although local vs. more distal sources cannot be discriminated by our multi-method approach and would require further investigations. Lateral morainic ridges and glacial deposits in the Sanetsch plain (Fig. 2) do not have tight chronological constraints but they are likely related to ice retreat stadials during the Lateglacial [START_REF] Ivy-Ochs | Cosmogenic nuclides and the dating of Lateglacial and Early Holocene glacier variations: The Alpine perspective[END_REF]. Several indications point towards an onset of glacier retreat for the Sanetsch Pass and our investigated area at the end of the YD (Fig. 12B), in close agreement with other palaeo-glacial and lacustrine chronologies in the Swiss Alps [START_REF] Schwander | Oxygen isotopes of lake marl at Gerzensee and Leysin (Switzerland), covering the Younger Dryas and two minor oscillations, and their correlation to the GRIP ice core[END_REF][START_REF] Heiri | Palaeoclimate records 60-8 ka in the Austrian and Swiss Alps and their forelands[END_REF][START_REF] Schimmelpfennig | A chronology of Holocene and Little Ice Age glacier culminations of the Steingletscher, Central Alps, Switzerland, based on high-sensitivity beryllium-10 moraine dating[END_REF]. The Pond Emines (Fig.

2 and/or regional winds [START_REF] Bullard | Dust generation on a proglacial floodplain, West Greenland[END_REF][START_REF] Martignier | Assessment of a locally-sourced loess system in Europe: The Swiss Jura Mountains[END_REF]. Post-YD paraglacial relaxation favoured hillslope dynamics and sediment entrainment [START_REF] Sanders | Post-last glacial alluvial fan and talus slope associations (Northern Calcareous Alps, Austria): A proxy for Late Pleistocene to Holocene climate change[END_REF], with catastrophic rockfall events such as the rockslope failure of Les Montons cliff (Figs 3, 11;[START_REF] Ivy-Ochs | Geomorphology and age of the Marocche di Dro rock avalanches (Trentino, Italy)[END_REF]. The alluvial fan (CRE site) was deposited at the downstream end of the fluvioglacial plain, probably within an ephemeral ice-dammed lake contemporary to the hummocky moraine formation. Exposed catchment-slope deposits (i.e. aeolian, glacial and talus)

were remobilized and transported downhill into the plain, where they were redeposited to form a stratigraphic succession of coarse and fine layers within the CRE alluvial fan [START_REF] Beaudoin | Holocene palaeoenvironmental record preserved in a paraglacial alluvial fan, Sunwapta Pass, Jasper National[END_REF]. We propose that the aeolian deposit identified on the high-elevation plateau (ARP site) is a remnant of a broader aeolian drape. The ARP aeolian sediments have been preserved due to the low connectivity between the platform and the downstream hillslopes ( 3C) at low elevations, and (ii) active periglacial processes at high elevations, with aeoliansediment cryoturbation (ARP site) and rock glaciers within scree deposits (Fig. 2).

Conclusions

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 28

Based on a combined approach of geomorphological and sedimentological field investigations S3. The deposit lies on top of non-weathered siliceouslimestone bedrock. S4. S6. S7. Table 1. Sample luminescence details and corresponding quartz OSL dating results. For analytical details about the measurement protocol, see Table S2A. CAM = Central Age Model; OD = Over Dispersion [START_REF] Galbraith | Statistical aspects of equivalent dose and error calculation and display in OSL dating: An overview and some recommendations[END_REF]. Table 3. Details on cosmogenic 10 Be samples, concentrations and exposure ages. Samples thickness and density are 2.5 cm and 2.65 g cm -3 , respectively. the LGM ice extent [START_REF] Ehlers | Quaternary Glaciations-Extent and Chronology: part I: Europe[END_REF]. S3. The deposit lies on top of non-weathered siliceouslimestone bedrock. S4. Table S4. Stratigraphic description and field interpretation of the sedimentological units forming the upper 4.5 m of the CRE alluvial fan. The thickness of the units is given in Fig. 5. *Reaction to HCl on the field: no (-), little (-/+), strong (+), very strong (++) reaction. S5. Luminescence signal intensities measured for CRE and ARP sites using the SUERC portable OSL reader [START_REF] Sanderson | Using simple portable OSL measurements and laboratory characterisation to help understand complex and heterogeneous sediment sequences for luminescence dating[END_REF], and following the measurement sequence of [START_REF] Muñoz-Salinas | Using OSL to assess hypotheses Accepted Manuscript[END_REF]. IRSL counts are the total photon counts obtained after the first 30 s of IRSL stimulation. OSL counts are the total photon counts obtained after the 60 s of OSL stimulation. The counts and respective errors were obtained by averaging between two replicate measurements of each bulk sediment sample. Dim IRSL counts were measured along the CRE section, due to the low-feldspar content in the analysed sediments. For this reason, only the OSL signal intensity profile is represented in Fig. 10.

List of Tables

List of Tables

Sample

  of glacially-polished bedrock outside Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 6 the Tsanfleuron LIA moraine ( 1 km west of the Sanetsch Pass; Fig. 2) imply ice-free conditions at the Sanetsch Pass during or soon after the YD. Paraglacial and periglacial processes following the deglaciation have resulted in a rich record of landforms reflecting the transient landscape evolution (Fig. 2). Three representative landforms of the Sanetsch Pass landscape are investigated in the present study (Figs 2, 3). The first landform consists of a thin sediment blanket draping a bedrock platform located on the southern slope of the Arpelistock peak, in contact with glacial deposit from a small local glacier (ARP site, 2660-2715 m a.s.l., 0.23 km 2 in area; Fig. 3A). The surface of the ARP deposit exhibits locally a polygonal patterned ground (Fig. 3B; Gobat & Guenat 2019). The second landform is an alluvial fan located in the plain north of the Sanetsch Pass (CRE site, 2090-2130 m a.s.l., 0.14 km 2 in area; Fig. 3A, C). The third landform is a hummocky moraine located north of the plain (SAN site, 2040-2100 m a.s.l., 0.13 km 2 in area; Figs. 3A, C, D). The distinct hummocky morphology, with hills up to a few meters high, is partially covered by rockfall deposits composed of sandstone and limestone boulders detached from the nearby overhanging cliff (Les Montons peak; Figs 2,

Fig. 7

 7 Fig. 7 highlights the main micromorphological features observed in thin sections from the

  ) became ice-free during the same time period (c. 12 ka;[START_REF] Berthel | Impact of Holocene climate changes on alpine and treeline vegetation at Sanetsch Pass, Bernese Alps, Switzerland[END_REF], when the Tsanfleuron glacier finally retreated close to the most external LIA morainic ridges ( 1 km west of the Sanetsch Pass; Fig.2), similar to other palaeo-glaciological reconstructions in the Alps[START_REF] Schimmelpfennig | Holocene glacier culminations in the Western Alps and their hemispheric relevance[END_REF][START_REF] Kronig | Holocene evolution of the Triftje-and the Oberseegletscher (Swiss Alps) constrained with 10Be exposure and radiocarbon dating[END_REF]. A disconnected ice body, thick and sediment rich, persisted at the downstream end of the Sanetsch plain until c. 10 ka and formed a hummocky morphology overridden by rockfall deposits.Glacier retreat in the Sanetsch Pass area was associated with rapid (re-)establishment of paraglacial and periglacial geomorphic processes, promoting sediment connectivity and transport, resulting in major landscape changes. All these processes illustrate and synthesize post-glacial alpine dynamics[START_REF] Church | Paraglacial sedimentation: A consideration of fluvial processes conditioned by glaciation[END_REF][START_REF] Mercier | Paraglacial geomorphology: Processes and paraglacial context[END_REF] Fig. 12B), as summarized below. Firstly, ice retreat promoted slope processes, as well as aeolian remobilization of glacially-derived erosion products. Fine sediments in glacial outwash plains, within the Sanetsch Pass area or other nearby valleys (e.g. the Rhône Valley), were probably entrained and redeposited on mountain flanks and low-relief areas by local katabatic winds Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 27

  Figs 2, 3), but this potentially more-extensive deposit has been eroded elsewhere in the Sanetsch Pass area, with the reworked fine sediments being eventually incorporated in the alluvial fan downslope. Based on our OSL chronology, we hypothesize that short-lived sedimentation in the CRE alluvial fan stopped or slowed down already during/soon after the YD, resulting from a drastic change in base-level and sedimentation dynamics due to the melting of the ice-dam. Since then, landscape dynamics around the Sanetsch Pass seem to have been dominated by (i) hillslope denudation and localized stream incision within bedrock and glacial/alluvial sediments (Figs 2,

Figure 1 .

 1 Figure 1. Location of the study area (swissALTI3D DEM from swisstopo, authorization

Figure 2 .

 2 Figure 2. Detailed map of the Quaternary deposits occurring in the study area (swissALTI3D

Figure 3 .Figure 4 .

 34 Figure 3. Field photographs. A. General view of the three study sites (see Fig. 2 for locations)

Figure 5 .

 5 Figure 5. Stratigraphy of the CRE site (left: photograph, right: log). Sedimentological units and

Figure 6 .

 6 Figure6. Grain-size distributions of ARP (G7 and G8, in red) and CRE (G1-G6, in blue)

Figure 7 .

 7 Figure 7. Main micromorphological features observed in CRE fine and heterogeneous units (A-

Figure 8 .

 8 Figure 8. XRD bulk mineralogical compositions of CRE (CRE01-CRE16), ARP (ARP01 and

Figure 9 .

 9 Figure 9. Portable and conventional luminescence measurements for the ARP site. A. OSL

Figure 10 .

 10 Figure 10. Portable and conventional luminescence measurements for the CRE site. OSL signal

Figure 11 .

 11 Figure 11. 10 Be surface exposure ages from rock-fall sandstone boulders lying on the hummocky

Figure 12 .

 12 Figure 12. Schematic palaeoenvironmental reconstruction of the study area. A. Glacier extent

Figure 1 .

 1 Figure 1. Location of the study area (DEM from swisstopo, authorization 5701367467/000010).

Figure 2 .

 2 Figure 2. Detailed map of the Quaternary superficial deposits occurring in the study area

Figure 3 .

 3 Figure 3. Field photographs. a) General view of the three study sites (see Figure 2 for locations)

Figure 4 .

 4 Figure 4. Stratigraphy of the ARP site (left: photograph, right: log). Sedimentological units and

Figure 5 .

 5 Figure 5. Stratigraphy of the CRE site (left: photograph, right: log). Sedimentological units and

Figure 6 .

 6 Figure 6. Grain-size distributions of ARP (G7 and G8, in red) and CRE (G1-G6, in blue)

Figure 7 .

 7 Figure 7. Main micromorphological features observed in CRE fine and heterogeneous units (a-c)

Figure 8 .

 8 Figure 8. XRD bulk mineralogical compositions of CRE (CRE01-CRE16), ARP (ARP01 and

Figure 9 .

 9 Figure 9. Portable and conventional luminescence measurements for the ARP site. a) OSL

Figure 10 .

 10 Figure 10. Portable and conventional luminescence measurements for the CRE site. OSL signal

Figure 11 .

 11 Figure 11. 10 Be surface exposure ages from rock-fall sandstone boulders lying on the hummocky

Figure 12 .

 12 Figure 12. Schematic paleo-environmental reconstruction of the study area. a) Glacier extent

  (mean diameter 5-10 cm, max. 30 cm) of slightly weathered shale (ca. 70%) in a greyish brown loamy matrix (ca. 30%). Clast-supported with imbricated elements.++Coarse alluvial fan deposit. 3 Three sub-units each composed of slightly weathered granules and pebbles (shale and limestone) at the base and yellowish to grayish brown silty loam at the top. clayey silt with black and red mottles (Fe-Mn hydromorphism) especially at the base. Massive structure. -Aeolian sediments (in situ deposited or reworked by fluvial transportand granules (ca 60%) in a yellowish brown loamy matrix (40%). Clast-supported sediment with weathered imbricated elements. ++ Table

  Manuscript published at Boreas (doi.org/10.1111/bor.12480) 16XRD results show four distinct average mineralogical compositions (Fig.8). Bedrock samples

	(samples L1-L2 and S1-S2) present similar composition, largely dominated by carbonates (55-
	66% calcite and up to 5% dolomite). Silicates are secondary components, mainly represented by
	quartz (23-33%) and micas (3-16%), with some chlorite appearing only in the calcareous shales
	(S1-S2, up to 4%). ARP sediment samples (ARP01 and ARP03) are carbonate-free and contain
	67-70% of quartz and 19% of micas as main components (with up to 8% of chlorite and 4-7% of
	albite). For CRE samples, two main groups can be distinguished from the mineralogical
	compositions. Nine samples (CRE02, 03, and 05 to 11) are carbonate-free and contain mainly
	quartz (40-54%), micas (36-52%) and chlorite (1-14%). The seven other samples (CRE01, 04,

and 12 to 16) are characterized by variable calcite content (5-39%), in addition to quartz

(32- 43%)

, micas (23-43%) and chlorite (6-11%) components. CRE04 also contains a small percentage of albite (11%), as found in the ARP samples. Geochemistry analyses result in low TOC percentages for all the CRE (0.3-1.2%) and ARP (0.8 and 1.2%) samples.

Geochronology

Portable OSL measurements. ARP and CRE depth-profiles for luminescence signal-intensity are

  Accepted Manuscript published atBoreas (doi.org/10.1111/bor.12480) 21 minimum age estimate is in agreement with low-elevation loess from the Rhône Valley attributed to the late YD/Early Holocene, based on luminescence dating (Guélat 2013; Parriaux et al.To summarize, the high-elevation ARP deposit reflects Lateglacial to Holocene aeolian dynamics in the Sanetsch area, in line with previous aeolian sediment reconstructions in the Swiss Alps and foreland[START_REF] Pochon | Apport allochtone dans les sols jurassiens (Jura vaudois et Jura neuchâtelois)[END_REF][START_REF] Guélat | Les Loess de la Vallée du Rhône[END_REF][START_REF] Martignier | Assessment of a locally-sourced loess system in Europe: The Swiss Jura Mountains[END_REF]. The ARP primary aeolian deposit presents a mixed autochthonous (bedrock and moraine-deposit deflation) and

	2017).
	. Our young OSL (2.3-4.1
	ka) and IRSL (2.5-2.8 ka) ages are therefore probably the result of recent/ongoing post-

depositional bleaching through periglacial sediment reworking, while the IRSL age of sample ARP02 (7.1±0.7 ka) is interpreted as minimum age for the high-elevation aeolian deposit. This allochtonous (nearby alluvial plains such as the Rhône Valley) origin. Luminescence dating suggests that deposition occurred at least c. 7 ka ago, however this only remains a minimum age estimate due to recent/ongoing active cryoturbation and sediment reworking at the ARP site. The ARP aeolian deposit may give also insights into the presence of high-elevation ice-free areas (nunataks) in the Alps already during the LGM or the early Lateglacial

[START_REF] Bini | Switzerland during the Last Glacial Maximum[END_REF][START_REF] Martignier | Assessment of a locally-sourced loess system in Europe: The Swiss Jura Mountains[END_REF][START_REF] Gild | Discovery of a landscape-wide drape of Lateglacial aeolian silt in the western Northern Calcareous Alps (Austria): First results and implications[END_REF]

, where aeolian sediment could accumulate, in proximity of glacially-covered areas. Furthermore, luminescence analyses (both portable and conventional dating) have highlighted the occurrence of (ongoing) cryoturbation processes at the ARP site, providing potential palaeoclimatic proxy for the Holocene but requiring further investigation (e.g.

[START_REF] Vandenberghe | Cryoturbation Structures[END_REF][START_REF] Rousseau | European loess records[END_REF]

.

Table 2 .

 2 Sample luminescence details and corresponding polymineral IRSL dating results. See

Table 1 (

 1 ARP samples) for radionuclides concentration and water content. For analytical details about the measurement protocol, see TableS2B. CAM = Central Age Model; OD = Over Dispersion[START_REF] Galbraith | Statistical aspects of equivalent dose and error calculation and display in OSL dating: An overview and some recommendations[END_REF].

	Accepted Manuscript published at Boreas
	(doi.org/10.1111/bor.12480)
	44

Table 1 .

 1 Sample luminescence details and corresponding quartz OSL dating results. For analytical details about the measurement protocol, see Supplementary TableS2a.*CAM: Central Age Model, **OD: Over Dispersion[START_REF] Galbraith | Statistical aspects of equivalent dose and error calculation and display in OSL dating: An overview and some recommendations[END_REF].

	Sample	Depth below surface (m)	Radionuclide concentration U (ppm) Th (ppm) K (%)	Water content (%)	Total dose rate (Gy ka -1 )	CAM* D e (Gy)	Aliquots number	OD** (%)	Age (ka)
	ARP01	0.15	3.4±0.2 10.5±0.5 1.6±0.1	18	3.53±0.13	14.51±0.45	11	8.6	4.1±0.2
	ARP02	0.15	3.4±0.2 10.5±0.5 1.6±0.1	18	3.53±0.13	8.14±0.36	12	14.7	2.3±0.1
	CRE01	2.35	3.2±0.2 12.0±0.6 1.8±0.1	28	3.36±0.12	33.45±1.51	12	15.1	10.0±0.6
	CRE02	2.65	3.4±0.3 11.7±0.6 1.8±0.1	27	3.40±0.13	41.90±1.78	12	14.0	12.3±0.7
	CRE03	3.17	2.9±0.2 10.4±0.5 1.6±0.1	33	2.89±0.10	78.44±1.71	12	4.5	27.2±1.1
	CRE04	3.73	3.1±0.2 10.3±0.5 1.8±0.1	31	3.08±0.11	28.18±1.38	7	8.9	9.1±0.6

Table 2 .

 2 Sample luminescence details and corresponding polymineral IRSL dating results. The reader is referred to Table 1 (ARP samples) for radionuclides concentration. For analytical details about the measurement protocol, see Supplementary TableS2b.

	*CAM: Central Age Model, **OD: Over Dispersion (Galbraith & Roberts 2012).
	Sample	Depth below surface (m)	Total dose rate (Gy ka -1 )	CAM* Uncorrected D e (Gy)	Aliquots number	OD** (%)	g 2days (%/decade)	CAM* Corrected D e (Gy)	Age (ka)
	ARP01	0.15	4.13±0.15	10.57±0.35	11	0	1.26±1.03	11.77±1.21	2.8±0.3
	ARP02	0.15	4.13±0.15	26.01±1.02	10	9.82	1.39±0.88	29.50±2.88	7.1±0.7
	ARP03	0.15	4.13±0.15	9.02±0.47	12	14.47	1.43±0.98	10.19±1.08	2.5±0.3

Table 3 .

 3 Details on cosmogenic10 Be samples, concentrations and exposure ages. Samples thickness and density are 2.5 cm and 2.65 g cm -3 , respectively. * 10 Be production rate of 4.16±0.10 at.g -1 yr -1[START_REF] Claude | The Chironico landslide (Valle Leventina, southern Swiss Alps): age and evolution[END_REF] and LSDn scaling scheme[START_REF] Lifton | Scaling in situ cosmogenic nuclide production rates using analytical approximations to atmospheric cosmic-ray fluxes[END_REF]. **50 cm snow cover for 6 months per year(Wirsig et al. 2016a), snow density of 0.3 g cm -3 and spallation attenuation length in snow of 109 g cm -2[START_REF] Delunel | Snow shielding factors for cosmogenic nuclide dating inferred from long-term neutron detector monitoring[END_REF]. Erosion rate of 1 mm ka -1(Wirsig et al. 2016b; Wirsig et al. 2016a).

	Sample	Location WGS 84 (°N/°E)	Elevation (m a.s.l.)	Topographic shielding*	10 Be/ 9 Be blank corrected (10 -13 at g -1 )	10 Be/ 9 Be uncertainty (%)	10 Be concentration (10 5 at g -1 )	10 Be exposure age (ka)** no incl. snow/erosion snow/erosion***
	SAN18_01	46.35111/ 7.28751	2055	0.92	1.46	3.33	1.86±0.06	9.3±0.4	10.1±0.4
	SAN18_02	46.35117/ 7.28900	2056	0.96	1.55	3.38	1.96±0.07	9.4±0.4	10.1±0.4
	SAN18_03	46.35137/	2054	0.94	1.76	3.39	2.01±0.07	9.8±0.4	10.6±0.4
		7.28874

*Topographic shielding correction according to

[START_REF] Dunne | Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces[END_REF]

. **

Table S1 .

 S1 Samples locations, corresponding geomorphic units and conducted analyses.

Table S2 .

 S2 Luminescence protocols. A. Post-IR OSL protocol, after[START_REF] Murray | Dating quartz using an improved single-aliquot regenerative-dose (SAR) protocol[END_REF], applied on quartz separates from CRE and ARP sample (except for sample ARP03, which did not provide enough material for quartz purification). B. Post-IR IRSL protocol, after[START_REF] Buylaert | Testing the potential of an elevated temperature IRSL signal from K-feldspar[END_REF], applied on polymineral ARP samples. No post-IR IRSL measurements were performed on CRE samples due to the absence of feldspar IRSL signal.

	Table S2A
	Step	Treatment		8
	1	Natural/regenerative dose		9
	2	Preheat	200°C, 60 s
	3	IRSL	10 50°C, 100 s
	4	OSL	125°C, 100 s
	5	Test dose	500 s	11
	6	Preheat	200°C, 60 s
	7	IRSL	12 50°C, 100 s
	8	OSL	125°C, 100 s
	9	Return to step 1		13
	Table S2B
	Step	Treatment
	1	Natural/regenerative dose
	2	Preheat	250°C, 60 s
	3	IRSL	50°C, 100 s
	4	IRSL	225°C, 100 s
	5	Test dose	500 s
	6	Preheat	250°C, 60 s
	7	IRSL	50°C, 100 s
	8	IRSL	225°C, 100 s
	9	IRSL	290°C, 40 s
	10	Return to step 1

Table S3 .

 S3 Stratigraphic description and field interpretation of the sedimentological units forming the ARP high-elevation platform deposit. The thickness of the units is given in Fig.4. *Reaction to HCl on the field: no (-), little (-/+), strong (+), very strong (++) reaction.

	Unit	Description	HCl*	Field interpretation
		Angular platy fragments of grey siliceous limestones (60%) in a
	1	greyish brown silty matrix (40%). Clast-supported, no preferred orientation of elements. Well-developed forms of sorted polygonal	-	In situ cryoclasts bedrock derived, reworked by frost and thaw processes.
		patterned grounds on the surface.
		Light yellowish brown silt containing few limestone angular
	2	fragments. Well-developed platy structure.	++	Aeolian deposit.

Table S6 .

 S6 Grain-size distributions of CRE (G1-G6; S6A) and ARP (G7 and G8; S6B) samples. Individual cumulative and frequency percentages of the different grain sizes are reported. Details about grain-size distribution measurements are given in the main text.

Table S6A .

 S6A Grain-size distributions of CRE samples.

	2000	99.90		0.40	99.50		2.60	98.20		0.40	92.60		0.60	99.10		0.60	97.30		0.60
	5000	100.00		0.10	99.60		0.10	99.10		0.90	96.30		3.70	99.80		0.70	98.50		1.20
		100.00		0.00	100.00		0.40	100.00		0.90	100.00		3.70	100.00		0.20	100.00		1.50
		100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00
		100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00
	Sample	100.00	G1	0.00	100.00	G2	0.00	100.00	G3	0.00	100.00	G4	0.00	100.00	G5	0.00	100.00	G6	0.00
		100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00	100.00		0.00
	Grain size (µm)	Cumulative (%) 100.00	Frequency (%) 0.00	Cumulative (%) 100.00	Frequency (%) 0.00	Cumulative (%) 100.00	Frequency (%) 0.00	Cumulative (%) 100.00	Frequency (%) 0.00	Cumulative (%) 100.00	Frequency (%) 0.00	Cumulative (%) 100.00	Frequency (%) 0.00
	0.11	0.00 100.00		0.00 0.00	0.00 100.00		0.00 0.00	0.00 100.00		0.00 0.00	0.00 100.00		0.00 0.00	0.00 100.00		0.00 0.00	0.00 100.00		0.00 0.00
	0.58 100000	0.91 100.00		0.91 0.00	0.95 100.00		0.95 0.00	1.04 100.00		1.04 0.00	1.30 100.00		1.30 0.00	0.96 100.00		0.96 0.00	0.85 100.00		0.85 0.00
	1.06 200000	3.09 100.00		2.18 0.00	3.36 100.00		2.40 0.00	3.74 100.00		2.70 0.00	4.16 100.00		2.86 0.00	3.29 100.00		2.33 0.00	2.80 100.00		1.95 0.00
	1.95 300000	7.99 100.00		4.90 0.00	9.07 100.00		5.72 0.00	10.02 100.00		6.28 0.00	10.24 100.00		6.08 0.00	8.89 100.00		5.60 0.00	7.38 100.00		4.57 0.00
	4.19 400000	19.93 100.00		11.94 0.00	23.76 100.00		14.69 0.00	25.21 100.00		15.19 0.00	23.87 100.00		13.63 0.00	23.35 100.00		14.46 0.00	19.66 100.00		12.28 0.00
	7.72 500000	32.97 100.00		13.04 0.00	39.65 100.00		15.89 0.00	41.29 100.00		16.07 0.00	34.12 100.00		10.25 0.00	39.51 100.00		16.16 0.00	34.56 100.00		14.90 0.00
	14.22	46.51		13.53	54.23		14.58	56.60		15.31	48.11		14.00	55.80		16.29	50.87		16.30
	35.56	68.44		21.94	71.81		17.58	75.66		19.06	69.41		21.30	77.41		21.61	73.50		22.63
	63	78.70		10.26	77.80		5.99	82.10		6.44	77.90		8.49	85.00		7.59	80.80		7.30
	125	89.50		10.80	87.90		10.10	90.00		7.90	83.70		5.80	89.90		4.90	86.60		5.80
	250	95.70		6.20	92.90		5.00	95.00		5.00	89.10		5.40	94.50		4.60	92.40		5.80
	500	97.70		2.00	94.70		1.80	96.40		1.40	90.10		1.00	96.30		1.80	94.10		1.70
	1000	99.50		1.80	96.90		2.20	97.80		1.40	92.00		1.90	98.50		2.20	96.70		2.60

Table S6B .

 S6B Grain-size distributions of ARP samples.

	Sample	G7		G8
	Grain size	Cumulative	Frequency	Cumulative	Frequency
	(µm)	(%)	(%)	(%)	(%)
	0.11	0.00	0.00	0.00	0.00
	0.58	0.91	0.91	1.11	1.11
	1.06	3.62	2.72	4.53	3.42
	1.95	9.50	5.88	11.39	6.86
	4.19	22.65	13.15	24.35	12.96
	7.72	36.76	14.11	36.46	12.11
	14.22	51.55	14.79	47.88	11.42
	35.56	73.43	21.88	63.97	16.09
	63	82.50	9.07	71.80	7.83
	125	90.90	8.40	78.20	6.40
	250	95.20	4.30	83.80	5.60
	500	96.20	1.00	87.50	3.70
	1000	97.40	1.20	88.40	0.90
	2000	97.80	0.40	88.60	0.20
	5000	98.40	0.60	93.00	4.40
	10000	100.00	1.60	100.00	7.00
	20000	100.00	0.00	100.00	0.00
	30000	100.00	0.00	100.00	0.00
	40000	100.00	0.00	100.00	0.00
	50000	100.00	0.00	100.00	0.00
	60000	100.00	0.00	100.00	0.00
	80000	100.00	0.00	100.00	0.00
	100000	100.00	0.00	100.00	0.00
	200000	100.00	0.00	100.00	0.00
	300000	100.00	0.00	100.00	0.00

Supporting information

Table S1. Samples locations, corresponding geomorphic units and conducted analyses. Table S2. Luminescence protocols. A. Post-IR OSL protocol, after [START_REF] Murray | Dating quartz using an improved single-aliquot regenerative-dose (SAR) protocol[END_REF], applied on quartz separates from CRE and ARP sample (except for sample ARP03, which did not provide enough material for quartz purification). B. Post-IR IRSL protocol, after [START_REF] Buylaert | Testing the potential of an elevated temperature IRSL signal from K-feldspar[END_REF], applied on polymineral ARP samples. No post-IR IRSL measurements were performed on CRE samples due to the absence of feldspar IRSL signal. Table S3. Stratigraphic description and field interpretation of the sedimentological units forming the ARP high-elevation platform deposit. The thickness of the units is given in Fig. 4. *Reaction to HCl on the field: no (-), little (-/+), strong (+), very strong (++) reaction. Table S4. Stratigraphic description and field interpretation of the sedimentological units forming the upper 4.5 m of the CRE alluvial fan. The thickness of the units is given in Fig. 5. *Reaction to HCl on the field: no (-), little (-/+), strong (+), very strong (++) reaction. Table S5. Luminescence signal intensities measured for CRE and ARP sites using the SUERC portable OSL reader [START_REF] Sanderson | Using simple portable OSL measurements and laboratory characterisation to help understand complex and heterogeneous sediment sequences for luminescence dating[END_REF], and following the measurement sequence of [START_REF] Muñoz-Salinas | Using OSL to assess hypotheses Accepted Manuscript[END_REF]. IRSL counts are the total photon counts obtained after the first 30 s

Accepted Manuscript published at Boreas (doi.org/10.1111/bor.12480) 45

of IRSL stimulation. OSL counts are the total photon counts obtained after the 60 s of OSL stimulation. The counts and respective errors were obtained by averaging between two replicate measurements of each bulk sediment sample. Dim IRSL counts were measured along the CRE section, due to the low-feldspar content in the analysed sediments. For this reason, only the OSL signal intensity profile is represented in Fig. 10.

Table S6. Grain-size distributions of CRE (G1-G6; S6A) and ARP (G7 and G8; S6B) samples.

Individual cumulative and frequency percentages of the different grain sizes are reported. Details about grain-size distribution measurements are given in the main text.

Table S6A. Grain-size distributions of CRE samples.

Table S6B. Grain-size distributions of ARP samples.


Natacha Gribenski

Pierre G Valla

Frank Preusser

Thibault Roattino

Christian Crouzet

Jean-François Buoncristiani

Out-of-phase Late Pleistocene glacial maxima in the Western Alps reflect past changes in North Atlantic atmospheric circulation

Paleoglacier reconstructions in the northern and southern forelands of the European Alps indicate a synchronous Late Pleistocene glacial maximum during Marine Isotope Stage (MIS) 2, in phase with global ice volume records. However, strong controversy remains in the western foreland, where scarce and indirect dating as well as modelling studies suggest out-of-phase glacial maxima with the rest of the Alps. New luminescence dating brings the first direct Late Pleistocene glacial chronology for the western Alpine foreland and reveals two major glacier advances of similar maximum extent, at ca. 75-60 and ca. 40-30 ka, coinciding with MIS 4 and late MIS 3. We propose that asynchrony in glacial maxima between the western and the northern/southern Alpine forelands results from a progressive spatial reorganization of the

INTRODUCTION

Ice-core and marine paleoclimate archives show two main periods of global temperature minima during the Late Pleistocene (LP; the last ca. 130 kyr; NGRIP, 2004), coinciding with Marine Isotope Stage (MIS) 4 and 2 (71-57 and 29-14 ka; [START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic 18 O records[END_REF]. Marine records indicate the main peak in global ice volume, also largely reported from terrestrial glacier archives, during the Last Glacial Maximum (LGM; ca. 27-19 ka; Clark et al., 2009). However, local glacial maxima well before the LGM are increasingly recorded in polar and mountain regions [START_REF] Hughes | Timing of glaciation during the last glacial cycle: evaluating the concept of a global 'Last Glacial Maximum[END_REF][START_REF] Batchelor | The configuration of Northern Hemisphere ice sheets through the Quaternary[END_REF]. The emerging picture of variable ice expansion in both time and space likely reflects the regional effect of dynamic atmospheric circulations [START_REF] Löfverström | Evolution of the large-scale atmospheric circulation in response to changing ice sheets over the last glacial cycle: Climate of the Past[END_REF]. Accurate glacial reconstructions across the globe are thus crucial to constrain past atmospheric circulation and paleo-precipitation patterns (e.g. [START_REF] Kuhlemann | Regional synthesis of Mediterranean atmospheric circulation during the Last Glacial Maximum[END_REF].

In the European Alps, major LP glaciations are recorded in prominent morainic lobes spreading over several tens of kilometers in the forelands (Ehlers et al., 2011; Fig. 1). In the northern and southern forelands, synchronous LP glacial maxima at ca. 26-23 ka have been documented [START_REF] Monegato | The Alpine LGM in the boreal ice-sheets game[END_REF]. On the western side, the most extensive LP glaciation occurred in the Lyon area (Fig. 1a), where an ice lobe draining the Arve (western Mont Blanc massif) and the Isere catchments developed ~50 km westward into the piedmont [START_REF] Coutterand | Le lobe glaciaire lyonnais au maximum würmien : glacier du Rhône ou/et glaciers savoyard?[END_REF]. In contrast to other Alpine forelands, scarce and indirect radiocarbon data in this area suggest older LP ice-maxima than the global LGM [START_REF] Mandier | Chronostratigraphie des accumulations würmiennes dans la moyenne vallée du Rhône[END_REF]. Such asynchronous paleoglacial activity is supported by numerical simulations suggesting transgressive glacier dynamics across the Alps, with the observed maximum ice extent in the Western Alps only reached when considering paleo-precipitation partitioning or older (i.e. MIS 4) glacier advance [START_REF] Becker | Last Glacial Maximum precipitation pattern in the Alps inferred from glacier modelling[END_REF][START_REF] Seguinot | Modelling last glacial cycle ice dynamics in the Alps: The Cryosphere[END_REF].

To further examine the hypothesis of asynchronous paleoglacial dynamics across the Alps, a robust chronology in the Western Alps is now critical. Radiocarbon and cosmogenic nuclide exposure dating are challenging in this area due to the scarcity of organic material incorporated within glacial deposits and in-place moraine boulders, respectively. Here, we present the first direct chronology of LP ice-maximum extent in the Lyon area, using optically stimulated luminescence (OSL) dating of glaciofluvial sediments. These results not only enlighten the debate around the paleoglacial history of the Western Alps, but also have major implications in terms of past atmospheric circulation over the North Atlantic and Western Europe.

SETTING AND METHODS

Two main moraine complexes, separated by ~25 km, define the Middle (≥MIS 6; External Moraine Complex, EMC) and Late (Internal Moraine Complex, IMC) Pleistocene ice-maximum extents in the Lyon area [START_REF] Buoncristiani | Quaternary glaciations in the French Alps and Jura[END_REF]Fig. 1a). While the EMC presents discrete, highly-degraded and discontinuous moraine crests, the IMC is characterized by a series of well-defined frontal moraine ridges (Fig. 1a).

Glaciofluvial valleys (i.e. inactive today) expand over several tens of kilometers westward from the IMC (Fig. 1a), containing large sediment infills organized in step terraces. In the western and southern sectors, the IMC lays on a bedrock topographic step rising abruptly >200 m above the present-day lowland (Fig. 1b). This topographic configuration allows to unambiguously relate glaciofluvial sediment deposition and terraces formation to major ice-extent periods (i.e. IMC ridges), when ice and meltwater could overtop this escarpment. Previous geomorphic and sedimentological studies identified at least three main terrace levels which, from high (T1/T2) to low (T4), directly connect to external (M2) to internal (M4) moraine ridges from the IMC [START_REF] Mandier | Le relief de la moyenne vallée du Rhône au Tertiaire et au Quaternaire: essai de synthèse paléogéographique[END_REF][START_REF] Roattino | Geometry of glaciofluvial deposits and dynamics of the Lyonnais lobe ice front during the last glacial period[END_REF]Fig 1a), and thus were formed by aggradation during periods of glacial advances [START_REF] Marren | Magnitude and frequency in proglacial rivers: a geomorphological and sedimentological perspective[END_REF]. Typical coarse braided-river facies exposed along the terraces do not permit to argue for major discontinuity in terrace construction. Discrete outermost IMC ridges (M1) are visible only in the NW sector, suggesting an additional glacial stage hardly preserved through subsequent glacial periods (overridden moraine patterns; Fig. DR1 in the GSA Data repository), and with possible remnants in the east and south sectors of the IMC indistinguishable from the M2 deposits. In the absence of direct numerical dating, it remains challenging to quantitatively relate the observed glacial stages to distinct glaciations or short-term glacier oscillations.

We targeted the three main terrace levels (T1/T2 to T4; Fig. 1a), with six samples collected from well-sorted sand lenses exposed in quarry sections (Table 1, Appendix DR1). Samples were taken as close as possible to the terrace surfaces to capture the last depositional period related to a glacial stage. Two additional samples were collected from a glaciofluvial unit underlying till from an M1 ridge in the NW sector (PIZ site; Fig. 1a,Table 1), providing maximum age constraints of the LP outermost ice extent. [START_REF] Galbraith | Statistical aspects of equivalent dose and error calculation and display in OSL dating: An overview and some recommendations: Quaternary Geochronology[END_REF]. Feldspar burial doses were corrected for anomalous fading (20-30% increase, Table DR3) and final ages were calculated using sample-specific dose rate (2-3 Gy ka -1 range, Tables DR4,5), derived from element concentrations measured by laboratory gamma spectrometry.

RESULTS

SG quartz OSL data could only be acquired for three samples (EYP1, OYT1, SPC5; Table 1), for which ~2% of measured grains emit a suitable luminescence signal. This is consistent with low luminescence sensitivity typically observed in quartz from crystalline bedrock and/or with limited transport [START_REF] Sakawuchi | Thermal history versus sedimentary history: OSL sensitivity of quartz grains extracted from rocks and sediments: Quaternary Geochronology[END_REF]. All eight samples reveal good feldspar luminescence characteristics (20-40% suitable grains).

SG quartz D e distributions exhibit normal-like to moderately-skewed shapes, while SG feldspar D e distributions are widely spread, asymmetric and multimodal, with a significant fraction of saturated grain for post-IRIR 225 (Figs. 2b,DR6,7). In addition, feldspar CAM D e are significantly larger (>50%) than for quartz. Both complex SG D e distributions and CAM D e differences between feldspar and quartz are strong indicators for limited pre-depositional exposure to light, typical for proximal glaciofluvial sediments [START_REF] Duller | Single grain optical dating of glacigenic deposits: Quaternary Geochronology[END_REF]. Partial bleaching, leading to age overestimate, was diagnosed for most of the samples and especially for feldspar signals. We thus applied statistical techniques (i.e. FMM model) to extract D e from the best-bleached grain subpopulations, giving more accurate age determination. For final age calculation, quartz OSL and feldspar IR 50 were selected, based on their best bleaching potential.

Quartz OSL and feldspar IR 50 ages obtained from the highest terrace in the central part of the study area (T1/T2: SPC5; Fig. 1a) and from the intermediate (T3: ART2, OYT1, EYP1) to lower (T4: ART1) terrace levels overlap between ca. 42 and 39 ka (Fig. 3b; Table 1). Where both available, quartz OSL and feldspar IR 50 ages agree within uncertainties, bringing further confidence in the statistical treatment applied to complex feldspar IR 50 D e distributions. The southern highest terrace level (PEN1) resulted in a distinctly older IR 50 age of 63.0±11.4 ka, coinciding with two IR 50 ages of 60.8±12.3 and 76.4±15.1 ka obtained for the glaciofluvial unit underlying the north-western M1 ridge (PIZ1 and PIZ2; Table 1).

DISCUSSION

Luminescence dating indicates two periods of glaciofluvial aggradation beyond the IMC: between ca. 60-75 ka and ca. 30-40 ka, coinciding with MIS 4 and late MIS 3 respectively. This result strongly suggests that western Alpine glaciers reached the IMC at least twice during the LP, with similar maximum ice-extent configuration. The main terrace-moraine sequence (T2/T3/T4 -M2/M3/M4) observed in the area would hence result from glacier margin oscillations during a single glaciation period (i.e. late MIS 3), whereas scarce and outer remnant deposits (T1 -M1) were preserved from an earlier MIS 4 glacial maxima.

Our new MIS 4 chronology further corroborates earlier studies based on scarce chronological data in the Swiss Alpine foreland (Gaar et al., 2019 and references therein), and in the nearby Pyrenees [START_REF] Delmas | Paleogeography and 10Be exposure-age chronology of Middle and Late Pleistocene glacier systems in the northern Pyrenees: implications for reconstructing regional paleoclimates: Palaeogeography[END_REF], as well as results from numerical simulations (Fig. 3a; [START_REF] Seguinot | Modelling last glacial cycle ice dynamics in the Alps: The Cryosphere[END_REF], all of which proposed significant glaciations in the Western European mountain ranges at that time. The late MIS 3 glaciation clearly predates the last glacial maxima recorded in the northern and southern Alpine forelands during MIS 2 [START_REF] Monegato | The Alpine LGM in the boreal ice-sheets game[END_REF]. However, an extended MIS 3 ice advance has been suggested in the southwestern Alpine foreland (e.g. [START_REF] Ivy-Ochs | New geomorphological and chronological constraints for glacial deposits in the Rivoli Avigliana end-moraine system and the lower Susa Valley[END_REF], while valleys and forelands in the northern and central Alps were reported to be ice-free before 30 ka [START_REF] Preusser | Quaternary glaciation history of northern Switzerland[END_REF]Barret et al., 2017). The absence of MIS 2 ages in our dataset, in particular for the lowest (T4) terrace level (ART 1, 33.8±7.3 ka), suggests that no glacier advance reached the IMC after late MIS 3. This is further supported by radiocarbon ages (Fig. 1a; [START_REF] Mandier | Chronostratigraphie des accumulations würmiennes dans la moyenne vallée du Rhône[END_REF] of ca. 27 to >35 ka from a paleosol above till (i.e. minimum ages) in the NW sector of the IMC (Balan) and of 28-30 ka from glacio-lacustrine deposits 10-20 km upstream of the IMC (Moras and Malville), which suggest shorter glacier extent in the area during MIS 2. It is unlikely that the time lag (ca. 10 kyr) observed for the last major advance between the Lyon and northern/southern central Alpine forelands is caused by methodological uncertainties, as the applied dating approaches have shown age consistency on such timescales in similar glacial settings (e.g. [START_REF] Smedley | Luminescence dating of glacial advances at Lago Buenos Aires (~46°S)[END_REF][START_REF] Gribenski | Re-evaluation of MIS 3 glaciation using cosmogenic radionuclide and single grain luminescence ages, Kanas Valley[END_REF].

In addition to multiple and similar LP glacial maxima in the western Alpine foreland predating the global LGM, our data indicates spatially and temporarily variable paleo-glacier extent patterns across the European Alps, which likely reflect differential precipitation distribution combined with periods of global cooling. In contrast to the modern dominance of north-west atmospheric circulation, it has been proposed that south-dominated moisture advection prevailed across the Alps during the LGM [START_REF] Florineth | Alpine evidence for atmospheric circulation patterns in Europe during the Last Glacial Maximum[END_REF][START_REF] Luetscher | North Atlantic storm track changes during the Last Glacial Maximum recorded by Alpine speleothems[END_REF][START_REF] Monegato | The Alpine LGM in the boreal ice-sheets game[END_REF]. The southward latitudinal shift of the North Atlantic storm track, as far as 40°N, was forced by the development of large ice masses and sea ice in the North Hemisphere (NH; Fig. 3c), as indicated by atmospheric general circulation models (e.g. [START_REF] Hofer | Simulated winter circulation types in the North Atlantic and European region for preindustrial and glacial conditions[END_REF].

The extensive MIS 4 glaciation in the western Alpine foreland coincides with a major cooling period observed at the global and regional scales (NGRIP, 2004;[START_REF] Helmens | The Last Interglaciale Glacial cycle (MIS 5-2) re-examined based on long proxy records from central and northern Europe[END_REF][START_REF] Moseley | NALPS19: sub-orbital-scale climate variability recorded in northern Alpine speleothems during the last glacial period: Climate of the Past[END_REF]. Further elaborating on the above circulation model, we propose that the moderate expansion of the NH ice sheets during MIS 4, as indicated by deep-marine sediment records [START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic 18 O records[END_REF], could have similarly initiated a more discrete storm-track migration southward [START_REF] Löfverström | Evolution of the large-scale atmospheric circulation in response to changing ice sheets over the last glacial cycle: Climate of the Past[END_REF]. Moisture was thus dominantly brought from the west and promoted high precipitation in the west-facing massifs of the Western Alps, enabling favorable conditions (i.e. wet and cold) for large piedmont glaciers. While cold MIS 4 conditions are expected across the entire Alps, limited precipitation in the Central and Eastern Alps may have resulted in more restricted ice extents in the main northern/southern central forelands, with glacial deposits likely eroded during the greater MIS 2 extent.

During MIS 3, global ice records indicate a decrease of temperatures already at 40 ka before reaching minima at 25-20 ka (NGRIP, 2004). Few existing long-term regional records suggest similar trends in SW Europe, even with a potentially earlier cooling onset at 45 ka [START_REF] Moreno | A compilation of Western European terrestrial records 60-8 ka BP: towards an understanding of latitudinal climatic gradients[END_REF][START_REF] Moseley | NALPS19: sub-orbital-scale climate variability recorded in northern Alpine speleothems during the last glacial period: Climate of the Past[END_REF]. After significant shrinkage during early MIS 3, NH ice sheets started to expand again from ca. 40 ka [START_REF] Batchelor | The configuration of Northern Hemisphere ice sheets through the Quaternary[END_REF]. Similar to our proposed MIS 4 scenario, dominant western moisture advection would have been maintained throughout mid/late MIS 3 with a progressive southward shift of the polar storm track, again favoring glacier extent in the Western Alps. This configuration changed with the ongoing growth of NH ice sheets towards the LGM, which further pushed the polar storm track southward and lead shortly after 30 ka to the main southerly moisture advection over the European Alps [START_REF] Luetscher | North Atlantic storm track changes during the Last Glacial Maximum recorded by Alpine speleothems[END_REF].

Interestingly, the lacustrine record at Les Echets (Fig. 1a) also points towards a major change in lake productivity at ca. 28-30 ka, with a climatic transition from highly-oscillating to relatively stable, dry and cold conditions [START_REF] Veres | Climate-driven changes in lake conditions during late MIS 3 and MIS 2: a high-resolution geochemical record from Les Echets[END_REF]. We propose that this change in atmospheric circulation resulted in significant moisture decrease over the Western Alps, while enabling maximum glacier extent in the southern and northern Alpine forelands during MIS 2.

CONCLUSIONS

Our luminescence chronology constitutes the first direct evidence of two major Late Pleistocene glaciations reaching similar maximum extents in the western Alpine foreland, during MIS 4 (76-60 ka) and late MIS 3 (42-29 ka). These multiple and early glacial maxima revealed in the western foreland contrast with MIS 2 maximum ice extent recorded in the northern and southern sides of the European Alps, in phase with the global LGM. Such spatial variability in glacier fluctuations has been observed elsewhere around the globe and likely translates swaying atmospheric circulation and precipitation partitioning during the last glacial cycle. We propose that such configuration may be related to the gradual migration of the polar storm track, forced by the progressive development of NH ice sheets. This study further highlights the importance of regional paleo-glacier reconstructions and numerical chronologies for quantifying past changes in atmospheric circulation patterns.

Figure 1: (a) Glacial geomorphology of the study area [START_REF] Mandier | Le relief de la moyenne vallée du Rhône au Tertiaire et au Quaternaire: essai de synthèse paléogéographique[END_REF][START_REF] Roattino | Geometry of glaciofluvial deposits and dynamics of the Lyonnais lobe ice front during the last glacial period[END_REF].

Inset: geomorphic limits of the LP ice maximum in the Alps (Ehlers et al., 2011) with main

LGM ice flows (light blue arrows; [START_REF] Wirsig | Dating the onset of LGM ice surface lowering in the High Alps[END_REF], and location of the study area (black box) with main ice-drainage catchments (dark blue limit, AR: Arve, IS: Isère). (b) Longitudinal topographic profile (LP1) showing bedrock escarpement underlying the IMC ridges and downstream glaciofluvial sediments (undifferentiated), when the glacier reached its LP maximum extent. (c) Topographic cross-profile (ART site; CP1) showing the main three terrace levels. The bedrock/sediment contact is extrapolated from borehole data [START_REF] Roattino | Geometry of glaciofluvial deposits and dynamics of the Lyonnais lobe ice front during the last glacial period[END_REF].

  Luminescence signals and equivalent doses (D e ) were measured from quartz (green OSL) and feldspar (infra-red stimulation at 50°C and 225°C, IR 50 and post-IRIR 225 ) single grains (SG, 200-250 µm fraction). Methodological details are given in the GSA Data repository (DRII). Total burial dose since sample deposition was determined using the Central Age Model (CAM D e ) or Finite Mixture statistical model (FMM D e ) in case of partial bleaching diagnosis

Figure 2 :

 2 Figure 2: Quartz normal-like and feldspar (no fading correction) complex single-grain burial age

Figure 3 :

 3 Figure 3: (a) Late Pleistocene Northern Alpine speleothems δ 18 O (purple; Moseley et al., 2020) and global marine sediments δ 18 O (grey curve with MIS in shaded bars; Lisiecki and Raymo,

Figure 1 Figure 2

 12 Figure 1

TableTable 1 .

 1 Sample locations and luminescence ages

	Sample	Lat./ Long.	Elevation	Depth	Geomorphic	Quartz	Feldspar
	ID	(°N/°E)	(m a.s.l.)	(m below	unit	(OSL) age	(IR 50 ) age
				surface)		(ka)	(ka)
	EYP1	45.496/5.009	262	5	T3	32.9±2.3	29.4±2.8
	OYT1	45.575/5.030	265	3.2	T3	36.4±3.8	32.4±4.3
	SPC5	45.666/5.015	250	5	T1/T2	38.6±4.5	41.6±7.9
	ART1	45.537/5.182	430	3	T4	-	33.8±7.3
	ART2	45.536/5.182	420	12	T3 or T4	-	40.6±4.2
	PEN1	45.363/5.186	313	12	T1/T2	-	63.0±11.4
	PIZ1	45.876/5.088	250	13	underlying M1	-	60.8±12.3
	PIZ2	45.876/5.088	256	2	underlying M1	-	76.4±15.1

GSA Data Repository item

201Xxxx, [Details of samples' geomorphological and sedimentological context, and luminescence dating], is available online at www.geosociety.org/pubs/ft20XX.htm, or on request from editing@geosociety.org.

ACKNOWLEDGMENTS

This work was supported by the SNSF (PP00P2_170559), INSU and USMB (AAP Montagne).

We thank Gachet TP, Eiffage, Cemex Granulat Rhône Méditerranée, Nord Isère Matériaux and Vicat Granulat for providing access to gravel pits. We thank the four anonymous reviewers for their valuable suggestions.