Dating the landscape evolution around the Chauvet-Pont d’Arc cave

The Chauvet cave (UNESCO World Heritage site, France) is located in the Ardèche Gorge, a unique physical and cultural landscape. Its setting within the gorge—overlooking a meander cutoff containing a natural arch called the Pont d’Arc—is also remarkable. Investigating possible associations between sites’ physical and cultural settings, chronologies of human occupation, and access conditions has become a major theme in archeological research. The present study aims to reconstruct the landscape of the Pont d'Arc meander cutoff during the Upper Paleolithic, when humans were present in the Chauvet Cave. We used uranium-series and electron spin resonance analyses to date the formation of the Pont d’Arc natural arch in the Combe d’Arc meander cutoff, near the Chauvet Cave. Results show that the meander became totally cutoff between 108 and 138 ka (95%). Hence, the natural arch formed before the Upper Paleolithic and the first known human presence in the Chauvet Cave, dated to 37 ka cal BP. These results allowed us to reconstruct a key part of the landscape surrounding the Chauvet Cave when it was being used by Upper-Paleolithic societies.

www.nature.com/scientificreports/ the Ardèche River at the entrance to the Ardèche Gorges (Fig. 1). The concentration of Upper Paleolithic sites in such a specific landscape feature questions choice of the site frequentation by the prehistoric people [16][17][18] . An essential first step in conducting such analyses is to ascertain whether current features were already part of the landscape when a site was being used 9,11 . The present study aims to trace the evolution of this landscape in order to determine whether the meander cutoff and Pont d' Arc had already formed 37 ka cal BP, when the Chauvet cave was frequented by Upper Palaeolithic societies. The piercing of the natural arch allowed the river to bypass the meander 19 . We tried then to date the meander cutoff, as it would indicate when the natural arch formed. The first step was to carry out a geomorphological survey of the Ardèche gorges entrance. Because of its karstic context, the study focuses both the external morphological features (alluvial deposits) and sediments found in the proximate Châtaigniers cave system, located within the meander cutoff (Figs. 1, 2). The Châtaigniers cave was chosen because of its direct proximity with the Pont d' Arc, and for its range of altitude which corresponded both to the Pont d' Arc and the lower-level alluvial deposits heights 20 (compared to the other surrounding caves), ( Fig  2). We determined the surface and subsurface morphologies of the alluvial deposits by combining the results of topographical surveys with electrical resistivity data conducted inside the cutoff meander. Fieldwork investigations in the Ardèche canyon entrance led to determine three former river levels, which we labeled T1 (8 m above the current river base level), T2 (+15 m), and T3 (+30 m). Those observations were compared with the ones inside the Châtaigniers cave through a unified topographic and stratigraphic model. The comparison of the altitude of the deposits allowed their integration in a relative chronological model containing alternances of aggradation phases (coarse fluvial deposits in the river system and laminated clay deposits in the Châtaigniers cave) and erosion phases (river entrenchment and speleothem growth 20 . We then tried to date those respective phases. Because of the quartz-rich component of the Ardèche river sediments 19,21,22 , we used paleodosimetric methods (Electron Spin Resonance -ESR) to date the river deposits [23][24][25][26][27] . In cave environment, radiometric analyses (Uranium series -U-series) are used to date the speleothems growth phases [28][29][30][31] . The combination of dating methods in both river and cave environment provides independent constrains that can be discussed through topographic and chronological correspondences [32][33][34][35][36] , potentially better constraining landscape evolution during the Quaternary period. The use of ESR on fluvial sediments and U-series in karst systems provides absolute constrains for landscape evolution studies in the Middle and Late Pleistocene.
Similar approaches combining relative and absolute dating of geomorphological objects have been used successfully at numerous archaeological sites 11,14,37,38 .  www.nature.com/scientificreports/ Site setting. The Ardèche is one of the main tributaries of the Rhône river right bank and is located in the Massif Central eastern part (Cevennes mountain range). Its 2700 m 2 catchment represents at least one third of the Cevennes Mountains, which reach 1700 m a.s.l, and extend from South-West to North-East with a mean altitude of 1200 m a.s.l. The Ardèche upper catchment is incised into plutonic and metamorphic formations. It then crosses sandstone Trias formation and finishes its course in the Rhône river incising the middle valley Jurassic sedimentary rocks and finally, the Cretaceous sedimentary rocks in the Ardèche gorges in the shape of entrenched meanders 21 . While the upper two third of the catchment are formed by the impervious rocks, the lower valleys incise carbonate rock shelves which are prone to karstification 39 . The Mediterranean conditions affecting the Ardèche watershed make its hydrological regime prone to flood events 40 . The Velay lakes constitute the closest reference palynological sequence for the middle to late Quaternary period 41 and shows the impact of glacial and interglacial phases that may have affected the paleo-hydrological conditions in the Ardèche catchment. The Vallon-Pont-d' Arc plain is a floodplain located at the entrance of the Ardèche gorges which contains stepped alluvial deposits 19,22 , (Fig. 2). The lower fluvial levels are organized parallel to the current rock talweg that crops out regularly in the river bed. The relative height above the river (RH) of the deposits is consistent throughout the middle and lower Ardèche valley 22 . The lower levels are the most visible in the current landscape and we focus here on the three lower ones (T1, T2 and T3) which have been characterized through geomorphological mapping and outcrops 19 , (Fig. 2). At the Ardèche canyon entrance, alluvial deposits are mainly composed of coarse elements (pebbles and cobbles ranging between 5 and 50 cm) with locally sand lens or sand beds (medium sands with stratigraphic alluvial features). They are organized as it follows: the T1 level is located at + 8 m (RH) and is characterised by large cobbles and boulders. Granitic and basaltic large cobbles (0.1-0.2 m) dominate the petrographic pattern 19,22,42 . The T1 characteristics is similar to the current alluvial plain 19 , Fig. 2). The T2 level (+ 15 m RH) is also well visible in the three main Ardèche plains. It is characterised by large cobbles (about 10 cm), and a low basaltic content 19 . The petrographic composition evolution of river terraces is impacted by both the late Miocene volcanism (Coirons lava) and late Pleistocene one (Bas-Vivarais) which occurred in the Ardèche catchment 43 . The current floodplain and T1 level composition are basalt rich, which is not the case for  Supplementary Fig. S2). The T1, T2 and T3 altitudes are reported on the Châtaigniers cave topography for comparison with the speleothem and clay deposits location. Geomorphological map from 19  www.nature.com/scientificreports/ the T2 and T3 levels. Bas-Vivarais lava flows were deposited in the river talweg since between 80 and 30 ka and were subject to intense erosion 43 . The T3 level (+ 30 m RH) is characterized by slightly coarser cobbles than the T2 level (superior to 10 cm) and have an important spatial extent (Fig. 2). The sand matrix is also slightly more hardened although this was variable along the outcrops. In situ alteration was observed for about 20 % of the granite cobbles visible inside the T3 level at numerous outcrops 19 . This can be related with granite bisiallitisation into quartz sand 44,45 , as observed in other rivers in south of France 25,46 . The released quartz grains are millimetric to multi-millimetric in size. The Châtaigniers cave is located in the vicinity of the cutoff meander and in the altitudinal range of the Pont d' Arc arch and the T1, T2 and T3 fluvial levels 20 . Trapped speleothems were visible at the corresponding T1 (+ 8 m RH) and T3 (+ 30 m RH) heights. They were in connection with laminated clay deposits linked with former water table levels in the cave.

Results
Geophysical prospection. For the +15 m alluvial deposit (T2), boreholes and trenches inside the Combe d' Arc ( Fig. 3) revealed the presence of thin clay layers on top of layers of coarse sand and pebbles. The cobbles are linked to the last perennial flows through the meander before the cutoff became complete, while the clay is linked to flood events that occurred later in the cutoff meander 40 (Fig. 3). ERT values of 250 Ωm were interpreted as corresponding to the top cobble deposit inside the meander. Plotting these values gave an altitude of 84.7 ±3.3 m A.S.L (±1 σ SD) across the entire meander, which is 4 to 7 m below the current surface (Fig. 3). This buried cobble-rich alluvial body indicates the last stage of the meander perennial activity, and thus represents the cutoff ending phase.

U-series on speleothems.
We obtained U-series ages for samples from three stalagmites (Table 1). CHAT-03 is a well-laminated, 20-cm-long stalagmite containing clay laminations that suggest that calcite growth was almost synchronous with the horizontal clay infilling. U-series ratios revealed high levels of detrital thorium that can be attributed to contamination during regular floods or water table rising within the cave, thereby suggesting that the speleothem grew concurrently with the deposition of the clay deposits in the cave. The 230 Th/ 232 Th ratio is relatively low, which is in agreement with probable contamination from detrital 232 Th during the speleothem www.nature.com/scientificreports/ grow, which leads to more important age uncertainties, hence the reverse ages obtained for the speleothem growth (13.2 ±1,12 ka for the bottom and 15.7 ±2.9 ka for the top). The speleothem ages remain close to each other, and error ranges are slightly overlapped. Such age reversal is acceptable considering the time range aimed in the study. The CHAT-01 speleothem was sampled on the sides of the laminated clay infilling, in a lateral eroded part (Fig. 2). U-series delivered ages comprised between 7.25 ±1.95 and 2.34 ±1.54 ka. The relatively low 230 Th/ 232 Th ratio of CHAT-01 are also probably linked with detrital inputs (Supplementary Table S6). CHAT-04 is a 25-cm long stalagmite. The calcite appears to be free from clay deposits, suggesting that it grew well above the river base level, before being covered by clay sediments when the river level rose (Fig. 2). The good 230 Th/ 232 Th ratio show a probable absence of detrital contamination by high water table periods during the CHAT-04 speleothem growth (Supplementary Table S6). The top of the CHAT-04 speleothem gave a U-series age of 162.5 ±5 ka (CHAT-04B). This speleothem was later covered by clay sediments when the river level rose.
ESR on fluvial quartz sediments. We used electron spin resonance (ESR) analyses to determine the ages of quartz-rich sediment samples from the +30 m (T3) and +15 m (T2) alluvial levels. The samples analyzed were taken inside the Combe d' Arc meander cutoff and upstream from the meander, mainly on the Vallon Pont d' Arc plain, which is immediately upstream from the entrance to the Ardèche Gorges ( Fig. 2 and Supplementary  Fig. S2). Samples coming from the T3 level showed more important optical bleaching rates ranging between 60 and 90%, contrary to the T2 level which shows more common values around 50 %.
Preliminarly, Al and Ti-Li burial ages obtained on the six T3 level samples show an age repartition ranging between 179 and 511 ka. High bleaching rate affect all the samples of the T3 level. Samples taken in sandy matrix between pebbles show the highest bleaching rates (ARD 1801, 1806 and 1809). Apart from the ARD 1809 sample which shows important age discrepancy linked with analytic limitations, all the other T3 samples follow the Multi-Center approach (slower bleaching kinetics for the Al centers 47 ). Al ages show deviations ranging from 100 to 200 ka compared to the Ti-Li ages for ARD 1806, 1809, 1805 and 1808. Such deviation is however less important for ARD 1801 and 1802 samples which yield similar Al and Ti-Li ages within the range of 1 sigma SD. The high bleaching rates affecting all the T3 samples leads to probable age overestimation of the Al centers 27,48 , which makes them unreliable for dating the T3 level (See Supplementary Information, Dating section) and should be considered as maximum age estimations. Nevertheless, Ti-Li centers indicate an effective bleaching rate and can be considered reliable for dating purpose. Although the ARD 1801 and 1802 Al ages fall within the range of their respective Ti-Li ages at 1 sigma SD, they are not considered either for dating for homogeneity purpose. Most of the Ti-Li ages are within the same range (150-200 ka), except for ARD 1801 which delivers Middle-Pleistocene ages (around 500 ka).
To date the T2 level, two samples were taken directly inside the meander cutoff, in a 4-m-deep natural trench in the alluvial sands above the pebble surface (ARD 1803 and ARD 1804). The third sample was taken 10 km upstream, inside the cobble level (ARD 1807). Sediments from this level do not appear to be subject to optical bleaching, so we have no reason to suspect that the ages we obtained were overestimates (Supplementary Table S5). Some of the samples yield slightly older ages for Ti-Li centers (ARD 1803 and 1807). While such configuration is uncommon 49 , the young ages and the high sensibility of Ti-Li centers (low measured values) may lead to potential Ti-Li age overestimation. Nevertheless, all ages remain close within a range comprised within a 100-150 ka range regardless of the center. It thereby confirms the suitability of both Al and Ti-Li centers for dating and that they belong to the same alluvial deposit phase.

Discussion
Combining geomorphological mapping of the sedimentary units in and around the Ardèche River with a geophysical survey and absolute dating of sediment samples enabled to reconstruct the evolution of the Pont d' Arc meander from the Middle Pleistocene to the Holocene (Fig. 4). The following discussion examines the chronological and stratigraphic relationships between external and underground geomorphological features. Features are discussed from oldest to youngest.
At the same altitude than the T3 level, the CHAT-04 speleothem growth stopped at 162.5 ±5 ka and was later covered by laminated clay (Fig. 2). Even if coarse sediments were not discovered in the Châtaigniers cave system, that deposit is linked with a rising of the water level inside the Châtaigniers cave system 20 and is well above the known Holocene floods of the Ardèche river 40 . We included the date for when the CHAT-04 speleothem stopped growing as a terminus post-quem constraint 50 for the age of T3, as the speleothem could not have Table 1. U-series ages. All ages are given in ka, with a 2 σ SD. www.nature.com/scientificreports/ continued growing under the T3 water table. The T3 sedimentation phase occurred in all the river system and display large cobbles that are typical of high-energy hydrological conditions. Such processes might have impacted the bleaching rates of the sampled quartz sand 51,52 , but contamination by unbleached quartz grains coming from in situ weathered granite cobbles remains also possible. Most of the Ti-Li ages from the T3 level are comparable with the Optically Stimulated Luminescence (OSL) age (142.5 ±9.2 ka) obtained on fluvial deposits located at 30 m RH in the Ranc Pointu 2 cave 53 , (Fig. 2). Ranc Pointu 2 fluvial deposits also come from the Ardèche river. Considering that we are at the end of the river system (relatively low slope and confluence with the Rhône river), relative heights are comparable before and after the Ardèche Gorges, which allows the stratigraphical attribution of the Ranc Pointu 2 OSL age to the T3 level 22 . The Middle Pleistocene age of the ARD 1801 sample questions an eventual local reworking of fluvial deposit coming from more ancient levels, or a long-term stability of the T3 level during the middle Pleistocene period. Nevertheless, the addition of the OSL age with the ESR Ti-Li ages (n = 6) produced an estimated Bayesian age of between 136 and 176 ka (95%), with a mean value of 158 ka, that is, during Marine Isotope Stage (MIS) 6 (Supplementary Table S7). In the Combe d' Arc, subsequent erosion has removed most of the cobble layers that formed at the top of T3, but the meander still contains remnants of these deposits (Fig. 4). Their perched position indicates that they were eroded by the river that passed through the old meander. Because the Pont d' Arc natural arch and the T3 alluvial deposit are at almost the same altitude, the cutoff could not have started before the T3 level formed.
In order to constrain the T2 age, we used the Al and Ti-Li ages of the 1803, 1804 and 1807 ESR samples (n = 6) extracted from sand deposits that correspond to that sedimentation stage. T2 deposits contain few basalt fragments, whereas T1 deposits and the current alluvial plain contain a lot of basaltic material, probably derived from the Coirons lava (dated to between 8 and 5.5 Ma 54 ), and the Bas-Vivarais lava (dated to the late-Pleistocene 43 ). All ESR ages from the T2 alluvial formation indicate deposition prior to 91 ±13 ka, which is in good agreement with the dates of the last volcanic phases in the Ardèche catchment, around 80 to 35 ka. The Bayesian age obtained for the +15 m level (T2) ranges between 108 ka and 138 ka (95%), with a mean of 124 ka, which shows that this level formed during MIS 5d or MIS 5e (Supplementary Table S7). Absolute dating of the meander abandonment surface shows that the meander became definitively cutoff between 108 and 138 ka (95%), well before the Upper Palaeolithic frequentations (Fig. 5). Although the natural arch probably started to form before that time, the channel through the arch would have been no more than a few meters wide, a similar size to other cave drains within the Pont d' Arc bedrock 20 . Chemical and mechanical weathering, combined with fluvial and gravitational www.nature.com/scientificreports/ erosion, continued widening the arch until it reached its current size 14,55,56 . Once the meander had been cut-off completely, there would have been little flow in the former river bed, although it has been submerged by flood events during the Holocene and until 1890 40 . These flood events deposited the low-resistivity (<250 Ωm), clayrich sediments that form the upper part of the Combe d' Arc sediment sequence 40 (Fig. 2). Because the T1 alluvial sediments lie below the level of the meander cutoff (+8 m), and therefore do not occur within it, they must have been deposited after the meander became cutoff. The CHAT-03 speleothem grew on top of the corresponding +8 m deposit in the Châtaigniers Cave. Because these clay deposits are at the same altitude as the T1 alluvial sediments, we feel justified in attributing the U-series ages obtained on the speleothem to these sediments (Fig. 5). The stability and age of the + 8 m clay deposit is confirmed by the CHAT-01 ages which span most of the Holocene period. Then, floods affecting the Combe d' Arc meander did not seem to have consequences on the clay deposition in the Châtaigniers cave. Hence, the T1 level probably formed between 11 and 14 ka, that is, at the end of MIS 2, immediately after the last glacial maximum (Fig. 5 and Supplementary Table S7).

Conclusion
Modeling geomorphological changes to the entrance to the Ardèche Gorge and dating the meander cutoff enabled to reconstruct the area's landscape during the Upper Paleolithic. The well-constrained age of between 108 and 138 ka for the final meander cutoff yields evidence of the existence of the Pont d' Arc in the Palaeolithic landscape (Fig. 5). The natural arch was highly visible in the river, at the entrance of the Ardèche Gorges. Considering its location and the Upper-Palaeolithic site's concentration around the Combe d' Arc, we can state that it was unmissable by the Upper Palaeolithic societies. The arch was an element of the cultural landscape of the Upper Paleolithic societies, in the same way as the former Chauvet cave entrance 14 . It shows the need to take into account the surrounding landscape at larger scale, and its reconstruction to its state during the period in question, when studying an archaeological site. This work shows how an integrated geomorphological study of an archaeological site's surroundings can contribute to investigations of possible associations between the landscape and past human frequentations. The emphasis on the natural arch as part of the archaeological landscape during Geophysical tomography. Because existing data on the sediments in the Combe d' Arc 40 were limited to the uppermost 3 m and did not cover the entire meander, we conducted an electrical resistivity tomography survey of the whole site in 2017-2018. For this we used an ABEM multi-electrode system to obtain 44 electrical resistivity profiles involving a total of 2,111 electrode positions, spaced at 5-m intervals (Fig. 3). We measured each electrode's coordinates with a handheld kinematic Global Positioning System, which allowed us to determine positions to within 3 cm. We used parallelized E4D code 60 to perform a 3D Occam's inversion of the electrical resistivity data and selected the 10th iteration as the final electrical resistivity model, with an RMS of 3.3 (see Supplementary Fig. S1 for details of the inversion convergence). The results of this survey enabled us to correlate sediments in the Combe d' Arc with alluvial deposits elsewhere in the Ardèche catchment.

Sediment sampling.
We established an absolute timescale for the evolution of the river landscape by dating alluvial deposits and speleothems. Fluvial sediments were sampled for ESR on quartz dating at T2 and T3 levels. The T1 level was excluded from the sampling because it is often covered by historical flood deposits 40 , and the visible parts are usually in the middle of the river and thus prone to remobilization. The T2 level (ARD 1803, 1804 and 1807) was sampled in yellow middle to coarse grain sized sand lenses and layers located in between pebble layers. For the T3 level, we focused on the sand matrix in between pebbles. We avoided sampling next to weathered granite cobbles and could find middle to coarse grain sized sand lenses for ARD 1805, 1806 and 1808, contrary to ARD 1801, 1802 and 1809.
In the Châtaigniers cave system, speleothems trapped or in contact with clay deposits were sampled for U-series dating on a + 8 m RH well laminated clay level (CHAT-02 and CHAT-03) and inside a laminated clay level at + 30 m RH (CHAT-04). The top and bottom parts of the speleothems were sampled for U-series dating in order to obtain insight on the speleothems growth periods. We assume the sampled speleothems grew if they were above the water table. Their growth period and their stratigraphic organization with paleo water table indicators (i.e laminated clay deposits) or a corresponding altitude with sediments observed outside the cave system can provide constraints for discussing the age of fluvial deposits. Speleothems were chosen for their relative height www.nature.com/scientificreports/ above the river which corresponded to the T1 and T3 alluvial deposits ones. No speleothem corresponding to the T2 height was found inside the cave.
Dating. ESR analyses of quartz-rich T3 and T2 alluvial sediments, carried out at the Museum National d'Histoire Naturelle in Paris, provided burial ages for these sediment layers 51,[61][62][63][64] . We used the multiple paramagnetic centers approach to date the sample 47,[65][66][67] . Analytical uncertainties are reported as ±1 σ errors. We derived Al and Ti-Li ages for each sample (Table 2 and Supplementary tables S4 and S5). Three speleothem samples (CHAT-01-2018, CHAT-03-2018 and CHAT-04-2018) were sent to the Laboratoire des Sciences du Climat et de l'Environnement in Paris, for U-series dating. Analyses were performed on a Multicollector Inductively Coupled Plasma Mass Spectrometer according to a standardized protocol 68 . Analytical uncertainties are reported as ±2 σ errors ( Table 1). The dates obtained for CHAT-03-2018 allowed us to constrain the age of the +8 m (T1) river level. We then used Chronomodel software to convert these ESR Al and Ti-Li values along with U-series dates into a Bayesian model with ±2 σ uncertainties [69][70][71] . We entered prior assumptions, for example, the principles of river incision (level T3 is older than T2, which is older than T1) and of stratigraphic superposition (of speleothems and underground clay deposits), into the relative model in order to test the dating results and obtain final Bayesian ages for each river level (Supplementary Table S7).