Olivier Moine

Pierre Antoine

Sylvie Coutard

Gilles Guérin

Christine Hatté

Clément Paris

Ségolène Saulnier-Copard

Intra-interstadial environmental changes in Last Glacial loess revealed by molluscan assemblages from the Upper Palaeolithic site of Amiens-Renancourt 1 (Somme, France)

Keywords: Terrestrial molluscs, Loess, Upper Weichselian, Interstadial, Upper Palaeolithic

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Introduction

In continental Europe, only a few (sub)continuous speleothems and sedimentological, pollen and diatom records from lacustrine and peat deposits document the environmental impact of the millennial-timescale milder oscillations, i.e. interstadials, of the last glacial period (Weichselian) north of the Mediterranean domain [START_REF] Ampel | Modest summer temperature variability during DO cycles in western Europe[END_REF][START_REF] Luetscher | North Atlantic storm track changes during the Last Glacial Maximum recorded by Alpine speleothems[END_REF]Fankheuser et al., 2016;[START_REF] Sirocko | The ELSA-Vegetation-Stack: Reconstruction of Landscape Evolution Zones (LEZ) from laminated Eifel maar sediments of the last 60,000 years[END_REF][START_REF] Duprat-Oualid | Vegetation response to abrupt climate changes in Western Europe from 45 to 14.7k cal a BP: the Bergsee lacustrine record (Black Forest, Germany)[END_REF].

Conversely, loess deposits cover vast areas in European plains [START_REF] Lehmkuhl | Loess landscapes of Europe -Mapping, geomorphology, and zonal differentiation[END_REF] and offer numerous sequences showing alternations of loess units and pedogenetic horizons, i.e. tundra gleys or arctic/boreal brown soils [START_REF] Antoine | High-resolution record of the last Interglacialglacial cycle in the Nussloch loess-palaeosol sequences, Upper Rhine Area, Germany[END_REF][START_REF] Antoine | High-resolution record of the environmental response to climatic variations during the Last Interglacial-Glacial cycle in Central Europe: the loess-palaeosol sequence of Dolní Věstonice (Czech Republic)[END_REF][START_REF] Antoine | Les séquences loessiques pléistocène supérieur d'Havrincourt (Pas-de-Calais, France) : stratigraphie, paléoenvironnements, géochronologie et occupations paléolithiques[END_REF][START_REF] Jary | Late Pleistocene loess-palaeosol sequences in Poland and western Ukraine[END_REF][START_REF] Meszner | Late Pleistocene landscape dynamics in Saxony, Germany: Paleoenvironmental reconstruction using loess-paleosol sequences[END_REF][START_REF] Haesaerts | The Late Pleistocene loess-palaeosol sequence of Middle Belgium[END_REF][START_REF] Zens | OSL chronologies of paleoenvironmental dynamics recorded by loess-paleosol sequences from Europe: Case studies from the Rhine-Meuse area and the Neckar Basin[END_REF], which respectively deposited during stadials and formed during interstadials [START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF]. This scheme is particularly well-expressed between 35 and 20 ka owing to high sedimentation rates, i.e. about 1 m/ka, and supported by radiocarbon chronologies derived from classical (charcoal, bones, terrestrial mollusc shells) or new (earthworm calcite granules, n-alkanes) dating materials [START_REF] Haesaerts | Climatic signature and radiocarbon chronology of Middle and Late Pleniglacial loess from Eurasia: comparison with the marine and Greenland records[END_REF][START_REF] Moine | The impact of Last Glacial climate variability in west-European loess revealed by radiocarbon dating of fossil earthworm granules[END_REF][START_REF] Újvári | Coupled European and Greenland last glacial dust activity driven by North Atlantic climate[END_REF][START_REF] Zech | Comparative 14C and OSL dating of loess-paleosol sequences to evaluate post-depositional contamination of n-alkane biomarkers[END_REF].

Contrary to pollen, terrestrial mollusc shells are present in most European loesses [START_REF] Ložek | Molluscs in loess, their paleoecological significance and role in geochronology -Principles and methods[END_REF]. Their 5-40 % calcium carbonate (CaCO3) content [START_REF] Bosq | Geochemical signature of sources, recycling and weathering in the Last Glacial loess from the Rhône Valley (southeast France) and comparison with other European regions[END_REF] favoured both shell construction and preservation, but also the preservation of bone remains in Human occupation layers as well. As assemblages of these organisms are highly sensitive to their medium, they are well-suited to reconstruct millennial timescale palaeoenvironmental changes (e.g. [START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF][START_REF] Moine | Enregistrements malacologiques à haute résolution dans les loess et les gleys de toundra du Pléniglaciaire weichselien supérieur : premiers exemples du nord de la France[END_REF][START_REF] Sümegi | High-resolution proxy record of the environmental response to climatic variations during transition MIS3/MIS2 and MIS2 in Central Europe: The loess-paleosol sequence of Katymár brickyard (Hungary)[END_REF]. They thus provide a useful complement to the chronostratigraphical frame for Middle and Upper Palaeolithic sites of northern France [START_REF] Antoine | Upper Pleistocene loess-palaeosol records from Northern France in the European context: Environmental background and dating of the Middle Palaeolithic[END_REF].

Continuous molluscan records compiled for the 35-20 ka period revealed a similar environmental response to stadial-interstadial cycles across western Europe. Increases in both the total abundance of molluscs and the proportions of hygrophilous and palustral species respectively indicate warmer and more humid conditions during the formation and degradation of tundra gley horizons [START_REF] Remy | Würmzeitliche Molluskenfaunen aus Lößserien des Rheingaues und der nördlichen Rheinhessens[END_REF][START_REF] Kerney | A Middle Weichselian deposit at Halling, Kent[END_REF][START_REF] Schiermeyer | Würmzeitliche Lößmollusken aus der Eifel[END_REF][START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF][START_REF] Moine | Enregistrements malacologiques à haute résolution dans les loess et les gleys de toundra du Pléniglaciaire weichselien supérieur : premiers exemples du nord de la France[END_REF]. The disappearance of the most vegetation-requiring taxa systematically leads to a richness minimum at the top of tundra gleys interpreted as a harmful effect of ground ice on vegetation [START_REF] Moine | Weichselian Upper Pleniglacial environmental variability in north-western Europe reconstructed from terrestrial mollusc faunas and its relationship with the presence/absence of human settlements[END_REF]. In northern France, due to the poor mollusc diversity, this effect is less marked and a few species may even briefly appear at the basis of tundra gleys, indicating a short phase of vegetation improvement following harsh stadial conditions.

The Amiens-Renancourt 1 site offered us a new opportunity to document environmental changes associated with stadial-interstadial cycles with a time resolution rarely reached until now but which is necessary to characterise features specific to each cycle and event. This site also yielded the youngest Gravettian occupation of northern France preceding the abandonment of the region during the Last Glacial Maximum (LGM) [START_REF] Antoine | Upper Pleistocene loess-palaeosol records from Northern France in the European context: Environmental background and dating of the Middle Palaeolithic[END_REF].

Its study will thus also allow for the precise positioning of the archaeological layer in its climatic and environmental context. This is thus a fundamental test to demonstrate that such an approach may improve correlations between archaeological layers and regional or global climate and environmental records. Such correlations basically rely on the relative -by typology -or absolute -by radiocarbon or thermoluminescence -dating of the archaeological material, especially when the stratigraphy has been reworked, is unclear or complex and environmental proxies are absent or inconclusive. However, combining high resolution palaeoenvironmental information from bioindicators for open air sites located in clearly stratified loess sequences may allow much more precise correlations. Reaching the expected (multi-)centennial-timescale will definitely deepen the comprehension of the relationships between climate and human settlements in the European Plain during the last glacial period.

Site settings

The archaeological site of Amiens-Renancourt 1 is located in northern France (Fig. 1) at the edge of the western suburb of Amiens in the Renancourt district (49°54'5.21"N; 2°15'50.58"E). The excavation is located on the left-hand side of the Somme River valley, several meters above the level of its present alluvial plain, at the foot of the north exposed slope of a slight promontory separating the Grâce dry valley from the Selle River valley (Fig. 2).

The most complete loess succession is 6.75-m-thick and overlies a chalky flint gravel bed reached at a depth of 8.3 m using a manual auger (Fig. 3). No Quaternary sediments are found below this gravel bed. It overlies the bedrock constituted of Upper Cretaceous chalk outcropping westward at short distance and constituting the promontories that separate the different valleys [START_REF] Kuntz | Carte géologique de la France à 1/50 000[END_REF].

The first archaeological discoveries in the surroundings date back to the beginning of the 20 th century [START_REF] Commont | Les hommes contemporains du Renne dans la Vallée de la Somme[END_REF] owing to the numerous brickearth quarries in operation at that time.

A concentration of Palaeolithic flint artefacts and large mammal bone remains was excavated on this occasion in the former Devalois quarry, but then lost. Fortunately, a new concentration, named Amiens-Renancourt 1, was discovered in 2011. Excavated every year since, the archaeological layer has already yielded many horse bone fragments, thousands of lithic remains attributed to the Recent-Final Gravettian technocomplex as well as ornamental items and several Venus figurines in chalk [START_REF] Paris | Premières observations sur le gisement gravettien à statuettes féminines d'Amiens-Renancourt 1 (Somme)[END_REF][START_REF] Paris | Les gisements gravettiens d'Amiens-Renancourt 1 et 2 (Somme, France) : premières données palethnologiques[END_REF].

Based on these archaeological discoveries, Amiens-Renancourt 1 is thus one of the best preserved and most important Gravettian sites for northern France [START_REF] Paris | La période du Gravettien dans la zone loessique du Nord de la France[END_REF], and is even unique regarding the presence of Venus figurines. It is clearly positioned in the stratigraphy, almost undisturbed and with a very limited vertical dispersion. The high-resolution palaeoenvironmental study of this site, along with an adequate chronology, will thus contribute to documenting the impact of the millennial-timescale climate variability on continental loess environments and to exploring their relationship with this Human settlement.

Material and methods

Sampling strategies

The studied profile was positioned on a side of the excavated area so that it passes through the archaeological layer, which can be optimally associated with palaeoenvironmental proxies.

After a careful cleaning of the profile, a precise drawing of the sequence was made at the scale 1:20 annotated with pedostratigraphic features and the position of all the samples (Fig. 3). Following the continuous column sampling (CCS) established by [START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF], 48 samples were collected for grain size analyses by cutting successive 5-cm-thick and 200-g-large slices of sediment, as well as 44 5-cm-thick and 10-litre-large samples for molluscan analyses taken side by side in parallel (Fig. 3) for precise comparisons. Samples (H: -60/-65, H: -65/-70, H: -70/-75) located at the base of the upper profile close to and within the archaeological layer were taken afterwards during the excavation of the square crossed by the profile. As their volume was respectively 30, 20.25 and 16.80 litres of sediments, their molluscan abundances were normalised to 10 litres to be comparable with those of the other samples.

In addition, 6 samples were taken for optically stimulated luminescence (OSL) datings using copper cylinders (L = 14 cm, Ø = 3.5 cm) hammered horizontally into the sediment. After extraction they were sealed at both ends to prevent exposure to light. To measure dosimetric parameters, about 2 kg of the sediment were taken for each sample in a radius of 30 cm around each cylinder. Ten radiocarbon (hereafter 14 C) datings were also performed on samples of earthworm calcite granules (hereafter ECG) extracted from molluscan samples following the methodology of [START_REF] Moine | The impact of Last Glacial climate variability in west-European loess revealed by radiocarbon dating of fossil earthworm granules[END_REF] to complete the four obtained on bone, burnt bone and charcoal fragments from the archaeological layer [START_REF] Paris | Du Gravettien final dans le Nord de la France ? Nouvelles données à Amiens-Renancourt[END_REF][START_REF] Paris | Premières observations sur le gisement gravettien à statuettes féminines d'Amiens-Renancourt 1 (Somme)[END_REF]. All dating samples were distributed throughout the record to constrain the age of the archaeological layer and of the main pedostratigraphic horizons of the studied part of the sequence.

Analytical protocols

Granulometry

Grain size analyses were performed on 2 to 5 g homogenised samples using a Beckman Coulter LS1330XR laser particle size analyser. The samples were first dispersed by sodium hexametaphosphate (5 ‰) during a period of one night in a rotating agitator, and then sieved at 160 μm to remove the coarse fraction including non-sedimentary Fe-Mn concretions, calcified roots and shell fragments that may bias the analysis. The coarse fraction was dried and weighed. For the decarbonated analyses, carbonates were removed using HCl (4 %) until the effervescence subsided. Then 1 to 2 ml of HCl (20 %) was added to check that the reaction was complete. The samples were rinsed by centrifugation before dispersion. Finer fractions were measured on the analyser at least three times to ensure good reproducibility.

The limits of the grain size classes (clay: ≤ 6 μm; coarse silts: 20-61 μm; fine sands: 61-160 μm) were adjusted using the comparison between classical sieve-pipette and laser grain size analyses for a set of test samples from the loess sequence of Havrincourt about 60 km to the north-east [START_REF] Antoine | Les séquences loessiques pléistocène supérieur d'Havrincourt (Pas-de-Calais, France) : stratigraphie, paléoenvironnements, géochronologie et occupations paléolithiques[END_REF]. The grain size index (hereafter GSI) defined as the ratio between coarse silts and fine silts plus clay [START_REF] Rousseau | Abrupt millennial climatic changes from Nussloch (Germany) Upper Weichselian eolian records during the Last Glaciation[END_REF], which is sensitive to minor changes in the loess deposition dynamics, was calculated for the characterization of millennial-timescale climate changes, using class size limits defined for the laser particle size analyser [START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF].

Luminescence dating

Luminescence analyses were performed on the fine (4-11 µm) quartz fraction. Preparation techniques followed usual procedures [START_REF] Lang | Procedures used for optically and infrared stimulated luminescence dating of sediments in Heidelberg[END_REF] and aliquots were prepared by settling of a 1 mg/ml suspension on 10 mm diameter stainless steel discs. The final quartz deposit was about 1 mg/cm 2 . All luminescence measurements were performed using a Risø TL/OSL-DA-15 system equipped with blue (470 nm) light-emitting diodes and an IR (830 nm) laser diode [START_REF] Bøtter-Jensen | Enhancements in luminescence measurement techniques[END_REF]. The luminescence signals were detected through a 7.5 mm thick Hoya U-340 filter. Irradiations were performed with the 90 Sr/ 90 Y source of the TL-OSL reader (dose-rate on 01/01/2015 was 0.132 Gy/s).

The determination of equivalent doses was achieved using a single-aliquot regenerative-dose (SAR) procedure [START_REF] Murray | Luminescence dating of quartz using an improved single aliquot regenerative-dose protocol[END_REF]. We used a measurement protocol identical to the SAR datings of quartz from Havrincourt loess [START_REF] Guérin | Chronology of the Upper Pleistocene loess sequence of Havrincourt (France) and associated Palaeolithic occupations: A Bayesian approach from pedostratigraphy, OSL, radiocarbon, TL and ESR/U-series data[END_REF] as all samples have an OSL behaviour close to those of this site. Thus, the parameters of the SAR protocol were fixed individually for each sample so as to satisfy the preheat plateau test performed for preheat temperatures ranging from 180 to 320°C with 20°C steps. For all samples it was checked that the recycling ratio was unity within a few percent as well as the dose recovery test [START_REF] Murray | The single aliquot regenerative dose protocol: potential for improvements in reliability[END_REF]. The alpha efficiency factor was set to 0.0420 ± 0.0020, the mean value measured on quartz from Havrincourt loess.

Annual dose rates were calculated using the data in table 1 and the last revised conversion factors [START_REF] Guérin | Dose-rate conversion factors: update[END_REF]. The contribution of cosmic rays was calculated using the depth from the surface and a sediment density of 2 [START_REF] Prescott | Cosmic ray contribution to dose rates for luminescence and ESR dating: large depths and long-term time variations[END_REF]. U, Th and K concentrations were measured on the sediment samples taken around the OSL cylinders. Prior to gamma spectroscopy, the sediment was dried for 3 days at 105°C. Th and K activities were calculated using the specific activities of 234 Th, 228 Ra and 40 K, the latter only used for K. The water saturation content was measured for each sample and the incidence of moisture was calculated by assuming a mean water content of the sediment since its deposition between 60 and 100 % of the saturated value [START_REF] Aitken | Thermoluminescence Dating[END_REF].

Radiocarbon dating

For the archaeological layer, four samples (two bones, one burnt bone and one charcoal) were carefully selected for radiocarbon dating. Earthworm calcite granules were also selected following the protocol of [START_REF] Moine | The impact of Last Glacial climate variability in west-European loess revealed by radiocarbon dating of fossil earthworm granules[END_REF]. For each molluscan sample, the 50 selected granules (0.8 < Ø < 1 mm) provided between 50 and 70 mg of CaCO3. Radiocarbon measurements were performed in Poznan, Saclay and Lyon radiocarbon laboratories (Tab. 2).

From the basal auger sediment sample, 3 granules were extracted for each of the three aliquots, providing between 1.1 and 1.51 mg of CaCO3. Given the small volume of the sediment sample, the minimum size limit of selected granules was lowered to 0.5 mm. They were treated at LSCE, Gif-sur-Yvette (GifA/ECHo prefixes as chemistry ID and measurement ID respectively) according to the same chemical protocol. CO2 evolved from carbonate acid attack was collected and flame-sealed in Pyrex tubes. Physical measurement was handled on the CO2 gas itself (no graphitization step) by ECHoMICADAS [START_REF] Synal | MICADAS: A new compact radiocarbon AMS system[END_REF][START_REF] Tisnérat-Laborde | ECHoMICADAS: A new compact AMS system to measuring 14 C for Environment[END_REF] through the gas source [START_REF] Ruff | A gas ion source for radiocarbon measurement at 200kV[END_REF]. This procedure makes it possible to measure samples as small as some dozens of micrograms of carbon.

Malacology

Sediment samples taken for the molluscan analysis were wet sieved on a 425 μm mesh and then dried. All useful shells and fragments were sorted, identified, counted and stored. The number of individuals per taxa was estimated according to [START_REF] Ložek | Quartärmollusken der Tschechoslowakei[END_REF]. Taxonomic attributions were based on Welter-Schultes (2012), the results of [START_REF] Nekola | Species assignment in Pupilla (Gastropoda: Pulmonata: Pupillidae): integration of DNA-sequence data and conchology[END_REF] for Pupilla species and the latest additions to the MolluscaBase (https://molluscabase.org/), and the ecological classification of taxa followed the groups established by [START_REF] Ložek | Quartärmollusken der Tschechoslowakei[END_REF] and [START_REF] Puisségur | Mollusques continentaux quaternaires de Bourgogne[END_REF].

Results

Stratigraphy and sedimentology

According to the field description (Fig. 3), five sub-sequences can be distinguished:

-Sub-sequence I (unit 13 to 12) is made of loessic material partly reworked by hillwash processes and accumulated at the foot of the slope directly on the chalk bedrock, testifying to the occurrence of a major erosion event directly preceding the deposition of the first sandy loess of unit 13. The decreasing content in chalk granules and gelifracted flint fragments from the base to the top suggests that the contribution of the bedrock, eroded on the overlooking plateau, progressively decreased with the increasing loess cover deposition in the area.

Unit 12 features are characteristic of a tundra gley horizon; -Sub-sequence II (unit 11 to 6) is made of homogeneous loess units alternating with a weakly developed arctic brown soil (unit 10), a tundra gley (unit 8) and two micro-gleys. Its lower boundary is marked by the onset of both high GSI values and of an increasing trend in clay, suggesting an enhanced contribution of aeolian deposition processes (Fig. 4). In the absence of a synchronous GSI decrease, the decrease in fine sand percentage in the middle of unit 11 associated with the disappearance of sandy layers probably indicates the end of hillwash processes. A few meters away from the profile, unit 10 turns into a tundra gley horizon.

Locally, its brownish facies may thus result from a better drainage favouring oxidation processes owing to a steeper slope. This recalls the BS5 units described at Ringen as an "intermediated tundra gley/boreal soil" [START_REF] Zens | OSL chronologies of paleoenvironmental dynamics recorded by loess-paleosol sequences from Europe: Case studies from the Rhine-Meuse area and the Neckar Basin[END_REF]. The archaeological layer is in the median part of the tundra gley (unit 8). Each of these two horizons is characterised by fine sand and GSI decreases with minima occurring 5-10 cm below the surface of unit 10 and 10-15 cm above that of unit 8. Slight decreases in the fine sand percentage are associated with the micro-gley located in the middle of unit 7 and in the thin upper tundra gley horizon (unit 6), whereas GSI values respectively show a plateau and an increase; -Sub-sequence III (unit 5 and 4) is made of laminated sandy loess with cryo-desiccation micro-crack levels corresponding to small (Ø ~20-30 cm) polygons. Its sharp lower boundary and the associated GSI increase (Fig. 4) highlight a rapid change toward a stronger loess deposition dynamics; -Sub-sequence IV (unit 3 and 2) is made of homogenous loess. It began with an erosive humid and cold event marked by a humic tundra gley in which one ice-wedge cast was observed. The loess deposition dynamics reverts to similar conditions to those prevailing during sequence II, albeit much more stable; -Sub-sequence V (unit 1 and 0) is made of two horizons belonging to the Lateglacial/Holocene soil complex strongly eroded by anthropogenic processes (agriculture).

Chronology

Distributed from the top of unit 12 to the top of unit 7, ECG 14 C ages range from 40926-45484 to 26036-26929 cal BP without inversions (Tab. 2; Fig. 4). Scattered between 40926-45484 and 30667-31646 cal BP, the three ages provided by the three small aliquots of the lowermost sample indicate that ECG of different ages were gathered along the slope by reworking processes. The younger one provides a minimum age estimate of the reworking phase. Such a mix explains the gap of about 6 ka that separates both ages from unit 11. The two lowermost ages of column B and the two oldest ones of the auger sample were therefore rejected. The 14 C age obtained on earthworm calcite granules in the archaeological layer is in very good agreement with those obtained on burnt bone and charcoal. This demonstrates the reliability of this material for 14 C datings in loess contexts from north-western Europe. The other two ages obtained on bone appear significantly younger (no recovery between their 2σ age ranges and that of the ECG 14 C of the archaeological layer). Such shifts between ages obtained from different materials of a unique layer often occur in archaeological contexts.

They may result from the quality of the samples or from the dated carbon fraction, which depends on the nature or the preservation state of the samples. The two 14 C ages (Lyon-632 and Lyon-633) obtained on an isolated bone found in a test pit in 1997 about 100 m SE of the excavation site [START_REF] Fagnart | Le Paléolithique supérieur ancien dans le Nord de la France[END_REF] demonstrate the occurrence of erosional events and hillwash processes during the deposition of the thick laminated loess of sequence III. The topographically higher position of this bone also suggests an occupation of the area by prehistoric humans that largely exceeded the limits of the excavation. According to the uppermost 14 C, i.e. 26036-26929 cal BP, sequence II should end around 26.5-26 ka.

Statistically different, the 14 C ages obtained in unit 10 and 8 confirm the occurrence of two independent pedogenic phases. Distributed between units 13 and 7, OSL ages range from 31.0 ± 1.6 to 27.2 ± 1.5 ka (Tab. 1).

Both 14 C and OSL age ranges are thus in good agreement. The uppermost one, i.e. REN1 14-03 (27.6 ± 1.4 ka), is slightly younger than the closest 14 C age of sample H: -15:-10 (26036-26929 cal BP), though this difference is not statistically significant. Nor is the apparent shift of the youngest OSL age of the archaeological layer, i.e. REN1 14-04 (27.2 ± 1.5 ka), which takes the error bars into account. The discrepancy is greater for the OSL age located in unit 13. Sample REN1 15-01 appears significantly younger than sample REN1 14-02 located above (Tab. 1). However, its water content (23.3 ± 2.3 %), which is significantly lower than that of all the other samples, is certainly incorrect and was therefore assumed to result from the destructuring of the sample between the field and the laboratory, leading to a decreased capacity to absorb water. Recalculated with the water content value of a physically close sample taken on another profile of the site (Fig. 3, Tab. 1), REN1 15-01* yields an age of 30.5 ± 2.0 ka, close to that of sample REN1 14-02 and to the 14 C of sample A: 0-20 (30667-31646 cal BP). It is assumed here that the strong reworking undergone by unit 13 may have reduced its porosity and water absorption capacity, leading to younger ages. Furthermore, the lower positions of samples REN1 15-01* and A: 0-20 suggest a rapid deposition of reworked units 13 and 12, possibly during Greenland interstadial (GI) 5.1.

Consequently, the Amiens-Renancourt 1 loess sequence covers the period between about 31 and 22 ka. Its four main pedogenic horizons are synchronous with GI-5.1, GI-4, GI-3 and GI-2, the third one including a unique archaeological layer solidly dated around 27.5-28 ka (Fig. 4).

Malacology

The number of individuals per assemblage (hereafter abundance) varies between 8 and 3855, and the number of taxa (hereafter richness) between 3 and 6 (Tab. 3; Fig. 5). Extreme values of both parameters are similar to those from other northern France molluscan records [START_REF] Moine | West-European malacofauna from loess deposits of the Weichselian Upper Pleniglacial: compilation and preliminary analysis of the database[END_REF][START_REF] Moine | Enregistrements malacologiques à haute résolution dans les loess et les gleys de toundra du Pléniglaciaire weichselien supérieur : premiers exemples du nord de la France[END_REF]. Moreover, owing to the occurrences of Arianta arbustorum, Vallonia costata, Punctum pygmaeum and Clausilia rugosa parvula, the composition of the molluscan fauna is in agreement with the position of the site at the boundary between the western and central malacological domains [START_REF] Moine | Weichselian Upper Pleniglacial environmental variability in north-western Europe reconstructed from terrestrial mollusc faunas and its relationship with the presence/absence of human settlements[END_REF]. The poor diversity and the ecological preferences of the few molluscan taxa (Tab. 4) almost always reflect a microenvironmental mosaic dominated by damp areas in which palustral and dry loci of variable extension occur. Moreover, the limited changes in the composition of the molluscan fauna and in taxa relative abundances (Fig. 5) indicate that the vegetation cover was most of the time discontinuous and poorly diversified, with only short phases of densification possibly accompanied by isolated patches of shrubs. However, despite the relative stability of the composition of the molluscan fauna, the thorough observation of variations in absolute and relative taxa abundances revealed the existence of four types of phases (Tab. 5) succeeding in a pattern of seven cycles (C1-7) (Fig. 5).

Phase I

All the samples attributed to this type of phase are located in homogeneous (units 11, 9 and 7) or laminated (unit 5) loess units. Their very low abundance rarely exceeds 100 individuals (Tab. 5, Fig. 5). As most taxa of the site are able to survive freezing temperatures down to -12 to -15°C [START_REF] Ansart | Is cold hardiness size-constrained? A comparative approach in land snails[END_REF], cold temperatures during the activity period most probably limited the development of the molluscan population during these phases. Indeed, rearing experiments at less than 15°C attempted during the cited study proved to be ineffective.

Phase II

This type of phase is characterised by a simultaneous increase in all common taxa of the site, with a first weak ("start") sub-phase up to a few hundred individuals then a stronger ("acceleration") sub-phase up to several thousands. Moreover, as the hygrophilous species Succinella oblonga reacts more promptly and slightly more intensively than the others, it always predominates in phase II. The "start" sub-phase is common to all cycles from C2 to C6. It occurs at the top of loess units underlying the main pedogenic horizons in the full cycles C3 and C4, and in the micro-gleys and thin tundra gley of the incomplete cycles C2, C5 and C6. Although dominated by the palustral species P. alpicola, the sample of the uppermost micro-gley was included in phase II of cycle C6 given its 40 % of S. oblonga and low abundance. The "acceleration" sub-phase occurs only in the full cycles C3 and C4 and is located at the basis of the main pedogenic horizons (units 10 and 8).

In the full cycle C3, the "acceleration" sub-phase is also characterised by the appearance of the mesophilous species Trochulus hispidus and the presence of Clausilia rugosa parvula and Punctum pygmaeum, two vegetation-demanding species, which all benefit from the moderate moisture increase. In cycle C4, only Trochulus hispidus is present. During the "start" subphase of cycle C5 Clausilia rugosa parvula briefly occurs with half-forested species Arianta arbustorum. The richness thus increases from phase I to phase II. Consequently, both the increases in abundance and the progressive predominance of S. oblonga indicate a two-step increase in both temperature and moisture throughout phase II. A damp micro-environmental mosaic develops in which wet loci extend and the vegetation cover densifies including a few shrub patches during the "acceleration" sub-phase or from the "start" sub-phase.

In the uppermost sample of phase II in cycle C4, the taxa relative abundance remains the same as in the underlying one also included in the "acceleration" sub-phase, despite a sudden drop in species abundances. Contemporaneous to the dense archaeological layer, this abundance drop rather results from anthropic effects (trampling, etc.) than from a marked temperature decrease in such an improved climatic context.

Phase III

This phase occurs in the full cycles C3 and C4, and also possibly in cycle C1. In cycle C3, a "humid" sub-phase precedes a "dry" one. In the "humid" sub-phase, the total abundance is maximum and richness decreases mostly due to the disappearance of the vegetationdemanding taxa that appeared in phase II. P. alpicola prevails as it increases slightly more than slugs and P. muscorum. S. oblonga begins to decrease, slightly in C3 but much more in C4, i.e. in the sample overlying the archaeological layer. However, it remains unclear here whether this strong decrease results from environmental conditions or from the incapacity of S. oblonga to recolonize this anthropized area as all other dominant taxa return to almost the same level of abundance as before the Human occupation. During the "dry" sub-phase, the total abundance remains maximum, as P. muscorum, which predominates, and slug increases compensate the decreases in S. oblonga and P. alpicola. T. hispidus increases significantly but none of the vegetation-demanding taxa reappear.

Phase III occurs only in the arctic brown soil (unit 10) and in the tundra gley (unit 8). It is characterised by milder temperatures in C3 than in C4 and by a damp micro-environmental mosaic. During the "humid" phase, the extension of wet loci was maximum and more prominent in C4 than in C3. This is coherent with the typical reduced facies of the tundra gley (unit 8) inherited from the waterlogging of the active layer which was less well drained than during the formation of the arctic brown soil (unit 10). During the "dry" sub-phase, most of the damp areas turn to dry ones, which become prominent, whereas wet loci only slightly regress. In the absence of aquatic taxa and of a marked micro-topography, the succession of a seasonal thaw followed by a drier phase during the activity period appears more probable than the persistence of localized water ponds or releases in a drier landscape to explain the persistence of wet loci. During both sub-phases, the vegetation remains discontinuous and poorly diversified, owing to the harmful effects of waterlogging and ice lenses on plant roots, as in other tundra gley horizons [START_REF] Moine | Weichselian Upper Pleniglacial environmental variability in north-western Europe reconstructed from terrestrial mollusc faunas and its relationship with the presence/absence of human settlements[END_REF].

Phase IV

This phase only follows phase III in the full cycles C1, C3 and C4. As all taxa decrease, the total abundance drops strongly but remains above 500 individuals in most of the samples. As slugs decrease less sharply, their proportions peak and the taxon sometimes prevails as in cycle C3. Phase IV of cycles C1 and C3 takes place in the upper part of geliflucted tundra gley (unit 12) and arctic brown soil (unit 10). In cycle C1, the micro-environmental mosaic resembles that of phase III in cycle C3 despite lower temperatures. The co-dominance of P. alpicola and P. muscorum indicate the development of a micro-environmental mosaic with both wet and dry areas. The low proportion of the hygrophilous species S. oblonga suggests that water is restrained in delimited loci or in time, e.g. during the seasonal thaw, which fits with the geliflucted facies of unit 12. Though scarce, the occurrence of Pisidium sp. suggests at least temporary local ponds. As for the brief occurrence of V. costata, it indicates an increase in the herbaceous component of the vegetation cover in dry areas. In cycle C3, the micro-environmental mosaic appears well-balanced between dry, damp and wet areas.

However, the vegetation remains sparse despite a moderately moist and stable soil resulting from a higher clay content which favoured the development of slugs.

Phase IV of cycle C4 takes place at the base of the homogeneous loess (unit 7) directly overlying the tundra gley (unit 8). The molluscan fauna reflects a cooling context and a damp micro-environmental mosaic in which wet loci tend to disappear. Only a slight increase in the herbaceous component is indicated by a brief occurrence of V. costata in the basal sample. This environmental trend is contrary to that observed in phase II. The presence of Pisidium sp.

suggests the persistence of temporary ponds in the surroundings. Despite relatively high temperatures, the moderate moisture increase, the absence of water-driven sedimentological features and the resumption of loess deposition underline the limited seasonal water release but its possibly longer persistence during the activity period of the previous phase III.

Discussion

Stadial-interstadial cycles

The 5-cm sampling resolution of the Amiens-Renancourt 1 molluscan record provides a new opportunity to confirm the conceptual model linking palaeoenvironmental changes associated with loess-tundra gley alternations with Weichselian Upper Pleniglacial stadial-interstadial cycles [START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF] based on the combination of high resolution stratigraphy, grain size [START_REF] Antoine | High-resolution record of the last Interglacialglacial cycle in the Nussloch loess-palaeosol sequences, Upper Rhine Area, Germany[END_REF][START_REF] Rousseau | Abrupt millennial climatic changes from Nussloch (Germany) Upper Weichselian eolian records during the Last Glaciation[END_REF] and molluscan [START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF] analyses at Nussloch (Rhine Valley, Germany). The correlation of large abundance increases, which occur in tundra gley horizons and imply milder temperatures, with interstadial phases [START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF] was recently confirmed by radiocarbon datings [START_REF] Moine | The impact of Last Glacial climate variability in west-European loess revealed by radiocarbon dating of fossil earthworm granules[END_REF]. Conversely, all low abundance samples from loess units were linked with stadial phases. Furthermore, by combining the intra-interstadial environmental succession reconstructed from the molluscan record and its chronological frame, a correlation is proposed between molluscan phases and major variations of several high-resolution global and regional climate records. NorthGRIP (NGRIP) δ 18 O and calcium ion concentrations ([Ca 2+ ]), hereafter dust concentration, in ice core records mostly reflect local temperature and atmospheric dust loading respectively (Rasmussen et al., 2004), whereas the Sieben Hengste (7H) speleothem δ 18 O record reflects the balance between moisture reaching the site directly from the North Atlantic and by crossing the Alps from the south [START_REF] Luetscher | North Atlantic storm track changes during the Last Glacial Maximum recorded by Alpine speleothems[END_REF]. Indeed, at the depth of the site (215 m below the ground surface), the temperature was assumed to be constant and δ 18 O variations to result from precipitation isotope values, i.e. provenance. [START_REF] Luetscher | North Atlantic storm track changes during the Last Glacial Maximum recorded by Alpine speleothems[END_REF] also pointed out that since the North Atlantic is the main moisture source in the northern Alps, the total moisture amount increases with its proportions.

Phase I

According to the cold temperatures implied by its low molluscan abundances, phase I is linked with stadials (Fig. 7). The molluscan assemblages of Amiens-Renancourt 1 reflect poor and discontinuous steppe/tundra vegetation. This is in agreement with the rare pollen data in loess sequences. North-eastward, Weichselian Upper Pleniglacial pollen samples from Kesselt (eastern Belgium) indicate a grass-dominated steppe [START_REF] Bastin | Premiers résultats de l'analyse pollinique des loess en Belgique[END_REF]. Southward, Weichselian Middle Pleniglacial pollen samples, from Villiers-Adam (northern France), Upper Pleniglacial being sterile, also indicate a grass dominated vegetation with rare woody taxa resembling modern Nordic tundra [START_REF] Limondin-Lozouet | Biocénoses pléistocènes des séquences loessiques de Villiers-Adam (Val d'Oise, France) : études malacologiques et palynologiques[END_REF]. At Amiens-Renancourt 1, owing to the low local richness, no cyclical variations of molluscan richness occur during stadials as in Nussloch, except brief occurrences of V. costata in phases I of C4 and C5 (Fig. 6). The ecological preferences of additional taxa present in Nussloch reflect a much more diversified vegetation with the presence of shrubs. The Bergsee Lake (Black Forest, Germany) pollen record shows a "steppe dominated landscape with some elements of boreal forest" during stadials, and GS-3 and GS-5.1 are distinguished by their maximum of xerophytic steppe element, i.e. Artemisia [START_REF] Duprat-Oualid | Vegetation response to abrupt climate changes in Western Europe from 45 to 14.7k cal a BP: the Bergsee lacustrine record (Black Forest, Germany)[END_REF]. Interestingly, the driest phases I at Amiens-Renancourt 1, i.e. in C2 and C7, are respectively correlated with these two stadials contemporaneous to HE 3 and 2 respectively, whereas at Nussloch molluscan richness maxima suggest a densification of the vegetation cover and increases in shrubs.

Phase II

Phase II, and especially its two sub-phases (start and acceleration), had not yet been precisely depicted. At Curgies (northern France), "MZ2" located at the basis of a tundra gley presents the characteristics of phase II, including a short occurrence of a vegetation-requiring species typical of cold and wet mountain environments, i.e. Columella columella (see [START_REF] Moine | Enregistrements malacologiques à haute résolution dans les loess et les gleys de toundra du Pléniglaciaire weichselien supérieur : premiers exemples du nord de la France[END_REF]. At Nussloch, only a "MP-3" phase with higher frequencies in hygrophilous Succinella oblonga was recognized (Fig. 6) and occurred within all tundra gley horizons and microgleys. Increases in the proportions of S. oblonga also preceded the high abundance "MP-0" phases at the basis of both G3 and G4 tundra gleys. It must be kept in mind here that hydromorphic processes leading to gley formation are linked to the functioning of a permafrost active layer and develop mainly downward in previously deposited sediments [START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF]. This can explain the apparent discrepancy between the two phases II taking place at the base of tundra gleys at Nussloch and in underlying loess at Amiens-Renancourt 1. Hence, as the large molluscan abundance increase occurs only during the "acceleration" sub-phase, "start" sub-phases are correlated with small excursions of 7H δ 18 O towards less depleted values contemporaneous to limited NGRIP dust concentration decreases that precede GI-4 and GI-3 but also occur within GS-3 based on 14 C dating. As for "acceleration" sub-phases, they are linked with a subsequent increased shift in both 7H δ 18 O and NGRIP dust concentration preceding GI-4 and GI-3. Active layer annual freeze-thaw cycles probably began during the "acceleration" sub-phase given its high abundances and trend toward palustral conditions. The 7H δ 18 O record also indicates a larger and earlier moisture increase before GI-4 than before GI-3. In the absence of local effects, this may explain the greater thickness of both the arctic brown soil (unit 10) at Amiens-Renancourt 1 and G3 tundra gley at Nussloch compared to the above (unit 8) and G4 tundra gleys respectively (Fig. 6). At Amiens-Renancourt 1, the untypical arctic brown soil (unit 10) also implies reduced waterlogging periods and soil moisture content, and thus a limited harmful impact of diurnal freeze-thaw cycles on plant roots [START_REF] Walker | History and pattern of disturbance in Alaskan arctic terrestrial ecosystems: a hierarchical approach to analysing landscape change[END_REF].

In addition, the richer molluscan fauna occurring in GI-4 phase II in both sites also suggest a greater lengthening of the growing season allowing for a more complex seasonal succession of vegetation resulting in an apparently more diversified micro-environmental mosaic. The occurrences of Clausilia rugosa parvula and Punctum pygmaeum in phase II of cycle C3 and C5 (only C. r. parvula for the latter) are also remarkable as original in northern France and as no other elements suggest their possible allochthonous or reworked origin in this sequence.

Contrary to Arianta arbustorum and Vallonia costata, which are present eastward and southward in loess from the Paris Basin, C. r. parvula and P. pygmaeum are only known in the Rhine Valley during the Weichselian Upper Pleniglacial [START_REF] Moine | Weichselian Upper Pleniglacial environmental variability in north-western Europe reconstructed from terrestrial mollusc faunas and its relationship with the presence/absence of human settlements[END_REF]. Their discovery raises the question of whether they persist in cryptic refuge areas in the region or come from abroad and by which means of transport. Surprisingly, P. pygmaeum also occurs at Nussloch (Rhine Valley, Germany) only at the top of the loess underlying G3 tundra gley during the studied time interval. Otherwise, such scarce occurrences indicate cryptic populations that may have been missed elsewhere in the sequence whereas these rarer taxa were still living in the surroundings. The lack of precise knowledge on Last Glacial loess molluscan fauna still prevents any conclusion as to whether occurrences of C. r. parvula and P. pygmaeum at Amiens-Renancourt 1 result from brief passive colonization phases from the Rhine Valley by an efficient means of transport, or from active expansion phases from local cryptic refuges.

Whatever the case, the development of shrub patches at Amiens-Renancourt 1 may have benefited from its position sheltered from (north-)western winds by the higher Ferme de Grâce plateau on the opposite slope of the Grâce valley (Fig. 2).

Phase III

In all Weichselian Upper Pleniglacial loess sequences where they have been detected, large abundances characteristic of phase III (e.g. "MP-0" phases at Nussloch) only occur in thick cryoturbated tundra gley horizons or arctic brown soils [START_REF] Remy | Würmzeitliche Molluskenfaunen aus Lößserien des Rheingaues und der nördlichen Rheinhessens[END_REF][START_REF] Schiermeyer | Würmzeitliche Lößmollusken aus der Eifel[END_REF][START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF][START_REF] Moine | Enregistrements malacologiques à haute résolution dans les loess et les gleys de toundra du Pléniglaciaire weichselien supérieur : premiers exemples du nord de la France[END_REF], and never in thin micro-gleys (Fig. 6). In most cases associated with maximum proportions in hygrophilous and/or palustral taxa, they formed during the functioning of the active layer, which requires milder temperatures [START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF][START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF]. This thus supports the correlation of the different phases III with interstadial phases marked by maximum δ 18 O values and minimum dust concentration in NGRIP records (Fig. 7). Accompanying changes in the structure of the Pupilla populations support a lengthening of the activity period of snails over the year implying milder temperatures in spring, and to a lesser extent in autumn, rather than in summer [START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF][START_REF] Moine | Weichselian Upper Pleniglacial environmental variability in north-western Europe reconstructed from terrestrial mollusc faunas and its relationship with the presence/absence of human settlements[END_REF]. This is in agreement with palaeoenvironmental reconstructions from Les Echets (central-eastern France) diatom assemblages [START_REF] Ampel | Modest summer temperature variability during DO cycles in western Europe[END_REF] and modelling results from [START_REF] Flückiger | Modeled seasonality of glacial abrupt climate events[END_REF]. As already underlined above, interstadial environmental conditions did not favour the diversification of the vegetation, but at least benefited the development of Betula populations as recorded in the Bergsee Lake [START_REF] Duprat-Oualid | Vegetation response to abrupt climate changes in Western Europe from 45 to 14.7k cal a BP: the Bergsee lacustrine record (Black Forest, Germany)[END_REF] but also in a tundra gley from the loess sequence of Kesselt [START_REF] Bastin | Premiers résultats de l'analyse pollinique des loess en Belgique[END_REF], most of the other tree taxa being allochthonous, e.g. Pinus [START_REF] Duprat-Oualid | Vegetation response to abrupt climate changes in Western Europe from 45 to 14.7k cal a BP: the Bergsee lacustrine record (Black Forest, Germany)[END_REF].

Interestingly, the 7H δ 18 O signal shifts back to its stadial level in the middle of GI-4 and GI-3.

Slight decreases in NGRIP δ 18 O values occur synchronously. The shift back to stadial values follows immediately in GI-3 but occurs after a period of stability in GI-4. Both interstadials have thus their own course and different palaeoenvironmental responses. During GI-4, the decrease in North Atlantic moisture supply whereas temperatures are still high probably explains the occurrence of an additional dry sub-phase after the humid one within phase III in the arctic brown soil (unit 10). During this dry sub-phase, the dust concentration in the NGRIP ice core remains low, which fits with minimum grain size values in Amiens-Renancourt 1. Owing to the strong cryoturbation of the top of thick tundra gley horizons that generally leads to their mix with the basis of the overlying loess unit (Fig. 6), this environmental phase has to our knowledge never been described up to now.

Finally, the correlation of phase III of cycle C1 must be considered with caution owing to the strongly cryoturbated facies of the sediment of the lowest tundra horizon (unit 12). A possible reworking of older material cannot be excluded though the composition changes in the molluscan fauna at the basis of the record appear realistic and the shells are well-preserved despite being sensitive to transport owing to their thinness.

Phase IV

As for phase III, pending phase IV is only linked with thick tundra gleys. Its large molluscan abundance decrease to minimum values reflects a strong temperature shift towards stadial levels, which obviously leads to correlating it to interstadial-stadial conditions characterised by a large decrease in the NGRIP δ 18 O record. During phase IV of cycle C3, the molluscan abundance decrease is significantly sharper than during phase IV of cycle C4, which fits the quicker δ 18 O decrease in GI-4 than in GI-3. Regarding the aeolian dynamics, the loess GSI, as well as the fine sand percentage (Fig. 4), at the top of the arctic brown soil (unit 10) increases in parallel to the dust concentration at the top of GI-4 (Fig. 7).

Conversely, the link is not that clear in phase IV of cycle C4 which shows an almost flat loess GSI signal, and only a slight increase in the fine sand percentage, at the top of the upper tundra gley (unit 8), whereas the dust concentration increases at the same pace at the end of GI-3 as at the end of GI-4 (Fig. 7). Other striking features are the relatively high molluscan abundance, intermediate between interstadial and stadial values, and the high proportions in hygrophilous taxa throughout this phase IV, suggesting the persistence of a moisture supply, whereas 7H δ 18 O values are the same for both phases IV. Up to now, these discrepancies remain unexplained, but interestingly the same evolution characterises this transition at the top of the upper tundra gley horizon at Amiens-Renancourt 1 and at the top of G4 tundra gley at Nussloch (Fig. 6), suggesting in both sites a similar forcing that affected neither NGRIP nor 7H records.

Early LGM interstadial-like oscillations

Owing to the reliable and precise radiocarbon chronology, more tenuous stratigraphical horizons than loess units and arctic brown soil/tundra gleys can be correlated between Amiens-Renancourt 1 and Nussloch sequences. Indeed, careful field observations revealed the presence at Amiens-Renancourt 1 of micro-gleys (Fig. 3, unit 6 and thin diffuse horizons with oxidation stains within units 7 and 11) presenting the same features as those already described in several profiles from Nussloch, i.e. P3 (Fig. 4). There, they are characterised by GSI minima [START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF] and both increases in the percentage of hygrophilous taxa and decreases in molluscan richness [START_REF] Moine | The impact of Dansgaard-Oeschger cycles on the loessic environment and malacofauna of Nussloch (Germany) during the Upper Weichselian[END_REF]. At Amiens-Renancourt 1, this concerns molluscan cycles C5 and C6 (Fig. 5). They are marked by a slowdown in the increasing trends of both GSI and the fine sand fraction (Fig. 4), increases in the percentage of hygrophilous and to a lesser extent of palustral taxa, a decrease in molluscan richness, slight and only in C5 as it is already minimum at the top of unit 7, and slight abundance increases (Fig. 5). These features are similar to those characterising the "start" sub-phase of stadial-interstadial transitions at the beginning of full cycles associated to the arctic brown (unit 10) soil and the tundra gley (unit 8). Further east, the loess section of Krems-Wachtberg (Austria) also exhibits supernumerary micro-gleys characterised by slight decreases in GSI and middle plus coarse sand values and similar, though weaker, trends in colorimetric parameters to those of tundra gleys [START_REF] Sprafke | Paleoenvironments from robust loess stratigraphy using high-resolution color and grain-size data of the last glacial Krems-Wachtberg record (NE Austria)[END_REF]. According to the correlations established with Greenland climate records, the micro-gleys IV-b and IV-d of profile KW-A 2015 could be contemporaneous of unit 6 and the diffuse horizon with oxidation stains of unit 7 at Amiens-Renancourt 1. In between at Remagen (Germany), recent high-resolution total organic carbon (TOC) measurements were performed along several cores drilled in a plateau loess sequence [START_REF] Fischer | Millennial-scale terrestrial ecosystem responses to Upper Pleistocene climatic changes: 4D-reconstruction of the Schwalbenberg Loess-Palaeosol-Sequence (Middle Rhine Valley, Germany)[END_REF]. Along the thickest of these cores (REM 3), TOC increases of 0.1 to 0.3 % characterised pedogenetic horizons. Based on correlations with the RP1 profile located on the edge of the plateau and dated by radiocarbon and optically stimulated luminescence, excursions 10 and 9 associated with tundra gleys could be correlated with interstadials GI 4 and GI 3 respectively. Three weaker (0.4-0.5 %) TOC excursions (8, 7 and 6) were measured between GI 3 and the Eltville tephra dated around 24.3-24.4 ka [START_REF] Zens | The Eltville Tephra (Western Europe) age revised: Integrating stratigraphic and dating information from different Last Glacial loess localities[END_REF]. Though only excursion 6 appears associated with a tundra gley, all constitute potential equivalents for Amiens-Renancourt and Nussloch micro-gleys, but require, as do the Krems-Wachtberg ones, direct radiocarbon dating to ensure precise correlations.

The comparison between the Nussloch GSI record and a few different ice, oceanic and continental proxy records of the Northern Hemisphere has highlighted the resemblance of their signals and suggests the occurrence of millennial-timescale variations between 28 and 20 ka. Their absence from the NGRIP δ 18 O record, though present in the dust record, indicates that climatic cycles of minor amplitude and/or limited to the mid-latitudes occurred during the first half of the Last Glacial Maximum [START_REF] Moine | The impact of Last Glacial climate variability in west-European loess revealed by radiocarbon dating of fossil earthworm granules[END_REF]. However, both proxy sensitivity and chronological model precision still hamper their discrimination. Presently, although these tenuous changes in the composition of the molluscan fauna may be considered as a response to local changes, their resemblance and contemporaneity in both sites located about 500 km apart, as well as the similar trends of associated loess grain size variations with NGRIP dust concentration ones (Fig. 7), rather support that they result from global mid-latitude interstadial-like climate oscillations during the early Last Glacial Maximum.

Age and context of the Human occupation

Gravettian open-air sites are rare in northern France [START_REF] Paris | La période du Gravettien dans la zone loessique du Nord de la France[END_REF], and some of them found in loess sequences have been discovered in brownish loam units at the base of the Weichselian Upper Pleniglacial sequence, e.g. at Havrincourt [START_REF] Antoine | Les séquences loessiques pléistocène supérieur d'Havrincourt (Pas-de-Calais, France) : stratigraphie, paléoenvironnements, géochronologie et occupations paléolithiques[END_REF]. However, despite the high resolution (5-cm-thick samples) of the molluscan records, the low sedimentation rate of these horizons prevents the precise reconstruction of associated palaeoenvironmental changes.

Amiens-Renancourt 1 is thus the first site for which we have both a precise stratigraphical location and a favourable context for the preservation of carbonated proxies. The 14 C ages obtained on various materials (charcoals, burnt bones, bones, earthworm calcite granules) support the correlation of the Amiens-Renancourt 1 archaeological layer with the GI-3 interstadial [START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland icecore records: refining and extending the INTIMATE event stratigraphy[END_REF]. Furthermore, after a precise description of the palaeoenvironmental evolution of the site, the relative position of the Human occupation within the succession of molluscan phases at the end of the "acceleration" sub-phase of phase II supports its occurrence at the end of the stadial-interstadial transition, during an optimum of vegetation diversification favoured by mild temperatures and moist conditions.

Conclusion

Our study is the first attempt to combine a high-resolution ( 5 In the future, such high-resolution studies of open-air sites will thus be crucial, not only to better constrain the millennial-timescale environmental dynamics, but also to specify the Human settlement dynamics in the cyclically changing environmental and climatic context of the end of the Last Glacial in the north-western side of the European periglacial domain.

Indeed, compared to studies carried out in caves, such a combination of palaeoenvironmental proxies and dating methods with high-resolution stratigraphy results in a better constrained chronology. Consequently, this type of high-resolution approach needs to be systematically applied in archaeological open-air sites presenting suitable characteristics such as a clear stratigraphical succession, no or minor reworking processes, a sufficient sedimentation rate, carbonated sediments, and an appropriate palaeontological content, as at Amiens-Renancourt 1.

Owing to the diversity of facies included in the Gravettian lithic techno-complex, the precise and respective chronological positioning of a maximum of well documented and typical archaeological layers from open-air sites in loess sequences appears as the best way to reconstruct the succession of Gravettian lithic trends that spread across the European Great Plain during the ten millennia preceding the Last Glacial Maximum.

  The relative abundances of dominant taxa indicate that the environmental conditions attributed to phase I differ slightly from one cycle to another. They can be rather dry (cycles C2 and C7), damp (cycles C3, C5 and C6), or well-balanced between the different components (dry, damp, palustral) of the micro-environmental mosaic (cycle C4). Although phase I molluscan faunas almost always reflect a discontinuous and poorly diversified vegetation cover, occasional occurrences of Vallonia costata indicate short phases of densification as during C2, C4 and C5.

  cm) molluscan study with earthworm calcite granule radiocarbon dating throughout a calcareous loess sequence encompassing two Dansgaard-Oeschger (D-O) cycles and one important Palaeolithic occupation layer. It strengthens and refines the scheme of (multi-)centennial environmental phases succeeding during both interstadials and stadials established in northern France and Nussloch (Rhine Valley, Germany), and especially the role of milder and more humid conditions that progressively set in from the end of stadials to interstadial optimums. It allows for the characterization of the environmental context of a unique archaeological layer and its exact positioning at the end of the stadial-interstadial transition of a D-O cycle, and the highlighting of interstadial-like environmental changes during the Last Glacial Maximum.

Figure 1 .

 1 Figure 1. Location of Amiens-Renancourt 1 site and of other sites mentioned in this study.Legends: ice caps (light blue), modern coast lines (dashed lines), palaeo-coastlines at -120 m (plain line) around 27 ka according to[START_REF] Clark | The Last Glacial Maximum[END_REF], elevation (level curves every 200 m from -120 m). Map realised with GMT 6[START_REF] Wessel | The Generic Mapping Tools version 6[END_REF] using a Mercator projection.

Figure 2 .

 2 Figure 2. Local topographical context of the Amiens-Renancourt 1 site. Altimetry in metres NGF. The asterisk indicates the position of the Renancourt 2000 test pit. Map realised with version 3.4 of QGIS software (http://www.qgis.org) using the IGN BD Alti® at 5 m.

Figure 3 .

 3 Figure 3. Pedostratigraphical record of Amiens-Renancourt 1 excavation with positions of all studied samples. The typology of pedological horizons follows the IUSS Working Group WRB (2015). H: upper profile; B: lower profile. Samples marked with a cross were not analysed due to the strongly reworked facies of the units concerned.

Figure 4 .

 4 Figure 4. Granulometry, chronology and chronostratigraphy of the Amiens-Renancourt 1 loess sequence and correlations with the Nussloch loess sequence. Ages are laterally spaced according to their respective depths, except those of the archaeological layer that are artificially scattered on both sides of 14 C age H: -70:-65 for a better readability. 14 C ages Lyon-632 and Lyon-633 come from another profile (see table 2 and part 4.2). Rejected ages are marked by a dashed contour; they include H3 not figured. GI: Greenland interstadials. ET: Eltville tephra; LS: Lohne soil. Correlations between the Nussloch pedostratigraphy and the NGRIP were established in Moine et al. (2017). Dashed lines highlight correlations that are not supported by 14 C ages.

Figure 5 .

 5 Figure 5. Diagrams of molluscan taxa absolute and relative abundances, and curves of several molluscan population parameters. Asterisks highlight successive specific abundance maxima.

Figure 6 .

 6 Figure 6. Correlations between molluscan fauna parameters and grain size index variations of Amiens-Renancourt 1 and Nussloch loess sequences. NGRIP data: Rasmussen et al. (2014).

Figure 7 .

 7 Figure 7. Correlation attempt between Amiens-Renancourt 1 molluscan phases and stadialinterstadial cycles. NGRIP data: Rasmussen et al. (2014). 7H (Sieben Hengste) data: Luetscher et al. (2015). The dashed vertical line symbolizes the equilibrium between the Mediterranean (depleted values) and the North Atlantic (enriched values) advection. NGRIP and 7H signals have been averaged to fit with the resolution (~100 years per sample) of Amiens-Renancourt 1 proxies.

Table 1 .

 1 OSL datings performed in Amiens-Renancourt 1 excavation. U, Th and K: elemental concentrations measured by gamma spectroscopy. W: Saturated water content. Dx: annual dose rates of α, β, γ and cosmic radiations, and equivalent dose. *The age of sample REN1 15-01 was recalculated with a more realistic water content from another sample that was physically close and taken on another profile of the site.

	Unit / Depth	Sample	U (ppm)	Th (ppm)	K (%)	W (%)	Dα (Gy/ka)	Dβ (Gy/ka)	Dγ (Gy/ka) Dcos. (Gy/ka) Dann. (Gy/a)	De (Gy)	Age (ka)
	7 / 3.65 m	REN1 14-03	2.72 ± 0.10	8.08 ± 0.10	1.16 ± 0.01	34.4 ± 3.4	0.40 ± 0.03	1.15 ± 0.05	0.75 ± 0.03	0.13 ± 0.01	2.43 ± 0.11	66.95 ± 1.41	27.6 ± 1.4
	8 / 4.17 m	REN1 13-01	2.86 ± 0.17	8.48 ± 0.17	1.15 ± 0.02	32.0 ± 3.2	0.43 ± 0.03	1.19 ± 0.06	0.78 ± 0.04	0.12 ± 0.01	2.52 ± 0.12	70.41 ± 1.84	27.9 ± 1.5
	8 / 4.17 m	REN1 14-04	3.26 ± 0.18	8.68 ± 0.20	1.14 ± 0.02	31.5 ± 3.2	0.47 ± 0.04	1.24 ± 0.06	0.83 ± 0.04	0.12 ± 0.01	2.65 ± 0.13	71.98 ± 1.82	27.2 ± 1.5
	9 / 4.40 m	REN1 14-01	3.00 ± 0.18	8.48 ± 0.17	1.15 ± 0.02	30.8 ± 3.1	0.45 ± 0.03	1.22 ± 0.06	0.80 ± 0.04	0.12 ± 0.01	2.59 ± 0.12	73.49 ± 1.63	28.4 ± 1.5
	11 / 5.10 m REN1 14-02	2.82 ± 0.18	8.68 ± 0.20	1.17 ± 0.02	30.6 ± 3.1	0.44 ± 0.03	1.21 ± 0.06	0.80 ± 0.04	0.11 ± 0.01	2.56 ± 0.12	79.25 ± 1.67	31.0 ± 1.6
	13 / 6.30 m REN1 15-01	2.61 ± 0.19	8.07 ± 0.08	1.18 ± 0.02	23.3 ± 2.3	0.43 ± 0.03	1.26 ± 0.05	0.80 ± 0.03	0.12 ± 0.01	2.61 ± 0.11	68.42 ± 1.86	26.2 ± 1.3
	13 / 6.30 m REN1 15-01*	2.61 ± 0.19	8.07 ± 0.08	1.18 ± 0.02	44.7 ± 4.5*	0.36 ± 0.03	1.07 ± 0.06	0.69 ± 0.04	0.12 ± 0.01	2.24 ± 0.13	68.42 ± 1.86	30.5 ± 2.0

Table 2 .

 2 Radiocarbon datings performed in Amiens-Renancourt 1 excavation and on a bone 860 found in Renancourt 2000 test pit. Ages were calibrated with the software Calib 7.0.4 (Stuiver

Table 3 .

 3 Molluscan taxa abundances of the Amiens-Renancourt 1 excavation. Ecological ) and undetermined (U). See table 5 for the description of molluscan phase types.

	865
	groups: half-forested (2), open (5), mesophilous (7), slugs (7'), hygrophilous (8), palustral (9),

866

aquatics (

10

867

Table 5 .

 5 Definition criteria of molluscan phases identified at Amiens-Renancourt 1. Symbols are bracketed when changes do not systematically occur.

	Phases (and sub-phases)	Abundance	Richness	Succinella obonga	(hygrophilous)	Pupilla alpicola	(palustral)	Pupilla muscorum (open	habitat)	Slugs (stable clayey	soil)	Trochulus hispidus	(mesophilous)	Vegetation-demanding	taxa (V. costata , C. r.	parvula, P. pygmaeum,	) A. arbustorum
	I: Stadial		MIN	(MIN)
	II: Stadial-interstadial	Start	↗	↗	↗ (↗)	↗	↗	↗	(↗)		( ↗)
	transition	Acceleration	↗↗↗	↗	(→ MAX) ↗↗↗	↗↗↗	↗↗↗	↗↗	(↗)		( ↗)
		Humid	MAX	↘	↘	↗(↗) MAX	(↗)	( ↗)				( ↘)
	III: Interstadial optimum
		Dry	MAX		↘(↘)	↘	(↗) MAX	↗	↗
	IV: Interstadial-stadial transition	↘↘(↘)		↘(↘)	↘(↘)	↘(↘)	↘(↘) MAX	(↘)

& Reimer, 1993) and the IntCal13 calibration curve[START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF]. 862

Acknowledgements and funding. We thank both the Poznan Radiocarbon Laboratory and the LMC14 laboratory that undertook 14 C dating. Radiocarbon dating was supported by a CNRS project "Projets Exploratoires Premier Soutien" program and by CNRS ARTEMIS calls for 14 C dating. The first author would like to thank Léa Roth for her help in sorting the mollusc shells. The authors also thank Barna Páll-Gergely and an anonymous reviewer for their helpful comments that greatly improved the initial version of the manuscript.

Data statement

The data that support the findings of this study are available from the corresponding author upon reasonable request. Table 4. Taxa ecological preferences synthesised after [START_REF] Boycott | The Habitats of Land Mollusca in Britain[END_REF], [START_REF] Ložek | Quartärmollusken der Tschechoslowakei[END_REF], [START_REF] Kerney | Die Landschnecken Nord-und Mitteleuropas[END_REF], Welter-Schultes ( 2012) and [START_REF] Horsák | Měkkýši České a Slovenské republiky[END_REF] and additional remarks. Taxa are listed by ecological groups, which are ordered by decreasing importance in the site.

Taxonomical remarks: (i) while internal shells of Limacidae and Milacidae can be distinguished, they cannot be identified at the species rank; (ii) according to their finely ribbed microsculpture, the two apices of Clausilia rugosa (Draparnaud, 1801) found at Amiens-Renancourt 1 were attributed to the sub-species C. rugosa parvula A. Férussac, 1807;(iii) valves and fragments of Pisidium were too few to allow for a reliable identification at the species rank; (iv) the single opercula attributed to this taxa cannot belong to another Bithynia species such as Bithynia leachii (Sheppard, 1823). Open, dry and sunny, calcareous stations (dry meadows, sand dunes).

It tolerates "earth bare of vegetation" and "a considerable range of diurnal temperature as well". [START_REF] Evans | Land snails in archaeology[END_REF] Vallonia costata (O.F. Müller, 1774) Varied open, dry and humid, stations with stones; rarely found in woods or swamps.

It "prefers more stable grassland". [START_REF] Kerney | A Middle Weichselian deposit at Halling, Kent[END_REF] Trochulus hispidus (Linnaeus, 1758)

Various moderately dry to humid stations (flood plains, marshy places, damp meadows, shrublands, light forest).

It is qualified as "relatively thermophilous"(*) and considered as an "early coloniser of damp broken ground before the development of a binding cover of vegetation"(**) (*) [START_REF] Kerney | Late-glacial deposits on the chalk of South-East England[END_REF]; (**) [START_REF] Kerney | A Middle Weichselian deposit at Halling, Kent[END_REF] Punctum pygmaeum (Draparnaud, 1801) fairly wet habitats with a developed vegetation cover (e.g. wood litter). Also in open stations and poor habitats offering a sufficient moisture or hiding places.

It is "characteristic of short-turf grassland and of open habitats where there is much bare ground"(*) and "often appear in places at an early successional stage" (**).

(*) [START_REF] Evans | Land snails in archaeology[END_REF]