Southern French Alps Research Articles

Denudation systematics inferred from in situ cosmogenic 10Be concentrations in fine (50–100 µm) and medium (100–250 µm) sediments of the Var River basin, southern French Alps

Abstract. Marine sedimentary archives are well dated and often span several glacial cycles; cosmogenic 10Be concentrations in their detrital quartz grains could thus offer the opportunity to reconstruct a wealth of past denudation rates. However, these archives often comprise sediments much finer (<250 µm) than typically analyzed in 10Be studies, and few studies have measured 10Be concentrations in quartz grains smaller than 100 µm or assessed the impacts of mixing, grain size, and interannual variability on the 10Be concentrations of such fine-grained sediments. Here, we analyzed the in situ cosmogenic 10Be concentrations of quartz grains in the 50–100 and 100–250 µm size fractions of sediments from the Var basin (southern French Alps) to test the reliability of denudation rates derived from 10Be analyses of fine sands. The Var basin has a short transfer zone and highly variable morphology, climate, and geology, and we test the impact of these parameters on the observed 10Be concentrations. Both analyzed size fractions returned similar 10Be concentrations in downstream locations, notably at the Var's outlet, where concentrations ranged from (4.02±0.78)×104 to (4.40±0.64)×104 atoms g−1 of quartz. By comparing expected and observed 10Be concentrations at three major river junctions, we interpret that sediment mixing is efficient throughout the Var basin. We resampled four key locations 1 year later, and despite variable climatic parameters during that period, interannual 10Be concentrations were in agreement within uncertainties, except for one upper subbasin. The 10Be-derived denudation rates of Var subbasins range from 0.10±0.01 to 0.57±0.09 mm yr−1, and spatial variations are primarily controlled by the average subbasin slope. The integrated denudation rate of the entire Var basin is 0.24±0.04 mm yr−1, in agreement with other methods. Our results demonstrate that fine-grained sediments (50–250 µm) may return accurate denudation rates and are thus potentially suitable targets for future 10Be applications, such as studies of paleo-denudation rates using offshore sediments.

Phenology of wood formation in larch (Larix decidua Mill.) trees growing along a 1000-m elevation gradient in the French Southern Alps

Spring temperature increase is the main driver of larch tree wood formation onset along a 1000-m elevation gradient in the Southern Alps, while its cessation is more probably controlled by water stress at the lowest elevation and photoperiod at higher ones. The survival of perennial plants depends on their adaptation to changing environment and specially temperature, which in trees is notably implemented through wood formation process. Our main objective is to understand how the phenology of wood formation is related to environmental factors and to temperature in particular. We monitored the xylogenesis of 60 larch trees, distributed in four stands along an elevation gradient of 1000 m in the French Southern Alps. Cambial activity started around mid-May at the lowest site (1350 m) and around mid-June at the highest one (2300 m), showing a delay of 5.4 days per °C. The onset of wall-thickening and mature phenophases followed the same linear trend with a delay of 5.2 and 3 days per °C, respectively. Phenophase cessations followed a parabolic trend with trees from the lowest site finishing their growth the first, while those from 1700 m finished the last. Our results show that the onset of xylem formation is mainly driven by spring temperature increase, while its cessation is more related to photoperiod, with water shortage being able to hasten it. Future climatic changes will most probably increase growing season length (but not necessarily wood production) and shift upwards the optimal elevation for larch growth in the Southern Alps.

Does higher place difficulty predict increased attachment? The moderating role of identity

In a contribution conducted in the French Southern Alps, Hinojosa et al. (2016) suggested that place attachment is relatively higher where it is difficult to live. We examine whether this figure holds in other environments bringing insight on a likely general tenet. A study using comparable survey data in two different ecosystems of Ecuador (the mountains and the subtropics) reveals, indeed, that higher place difficulty predicts increased attachment. Nevertheless, our results show that place difficulty is not significant when considered solely, that is, regardless of an additional element: place identity. In the Andean mountains, place attachment is found to be higher in more difficult areas only when place identity plays a role in individuals' views of their local community (their district). In the subtropical area of Intag, place difficulty is found to be a predictor of attachment regardless of place identity.

Accounting for permafrost creep in high-resolution snow depth mapping by modelling sub-snow ground deformation

Snow depth estimation derived from high-resolution digital elevation models (DEMs) can lead to improved understanding of the spatially highly heterogeneous nature of snow distribution, as well as help us improve our knowledge of how snow patterns influence local geomorphic processes. Slope deformation processes such as permafrost creep can make it challenging to acquire a snow-free DEM that matches the sub-snow topography at the time of the associated snow-covered DEM, which can cause errors in the computed snow depths. In this study, we illustrate how modelling changes in the sub-snow topography can reduce errors in snow depths derived from DEM differencing in an area of permafrost creep. To model the sub-snow topography, a surface deformation model was constructed by performing non-rigid registration based on B-splines of two snow-free DEMs. Seasonal variations in creep were accounted for by using an optimization approach to find a suitable value to scale the deformation model based on in-situ snow depth measurements or the presence of snow-free areas corresponding to the date of the snow-covered DEM. This scaled deformation model was used to transform one of the snow-free DEMs to estimate the sub-snow topography corresponding to the date of the snow-covered DEM. The performance of this method was tested on an active rock glacier in the southern French Alps for two surveys dates, which were conducted in the winter and spring of 2017.By accounting for surface displacements caused by permafrost creep, we found that our method was able to reduce the errors in the estimated snow depths by up to 33% (an interquartile range reduction of 11 cm) compared to using the untransformed snow-free DEM. The accuracy of the snow depths only slightly improved (root-mean-square error decrease of up to 3 cm). Greater reductions in error were observed for the snow depths calculated for the date that was furthest (i.e., the winter survey) in time from the snow-free DEM. Additionally, we found that our approach to scaling the deformation model has promising potential to be adapted for monitoring seasonal variations in permafrost creep by combining in-situ snow depth measurements with high-resolution surface deformation models.

River incision and migration deduced from 36Cl cosmic-ray exposure durations: The Clue de la Cerise gorge in southern French Alps

We estimate recent (20 ka) incision and lateral migration rate on a spectacular ~N-S striking gorge cross-cutting N-dipping limestones in a fold-and-thrust belt of the external Southern French Alps. Upstream of the gorge, the river makes a right angle turn and follows the limestone surface parallel to the bedding direction (~EW). Eighteen samples were gathered along two vertical river polished profiles and three others on the denuded north-dipping surface layer, in order to determine river incision and lateral migration timing and rates. Surface age exposures of polished river cliff and bedding surface have been obtained from 36Cl cosmogenic nuclide dating and allow us to constrain the rate of vertical incision and of surface denudation of the Jurassic limestone layer. Fifteen 36Cl exposure ages have been obtained and range from ~21 ka to ~3 ka. Samples clearly define two age/height linear trends, which slopes correspond to incision rates of 1 and 0.3 mm/yr, respectively. The transition from low to high incision rate occurs at about ~12 ka, i.e. during the Younger Dryas glacial phase. Data also indicate that the vertical incision of the N-S branch of the gorge is coeval with the lateral (northward) river migration of its E-W branch associated with the denudation of the limestones surface. Extrapolating incision rates deduced from 36Cl exposure ages through time suggests that the incision of the Clue and denudation of the Jurassic limestones started about 500–600 ka ago.

Debris-flow functioning and their contribution to sedimentary budgets: the Peynin subcatchment of the Guil River (Upper Queyras, Southern French Alps)

The Peynin catchment (15 km2) is prone to catastrophic floods (June 1957 – Recurrence Interval R.I.>100 yr), June 2000 (R.I.-30 yr) with serious damages to infrastructure and buildings located at the outlet. In this paper, PIT tags tracers and Vensim modelling software are used to better assess the sediment delivery unsteadiness, and more specifically to evaluate the respective role of geomorphological processes on sediment supply during flood event. For the last 20 years, our results highlight a significant variability in sediment delivery from a tributary to another one. According to our studies, we suppose that two torrential tributaries of the Peynin river, the Peyronnelle and Three Arbres subcatchments (<2 km2, representing <15% of the Peynin catchment area) are responsible of 80% of the sediments observed at the outlet of the catchment. Several processes take a part of these sediment transfers, but the efficiency of the sediment cascade in this catchment can be explained by a strong connectivity between sediment erosion area and the main channel of the catchment. Debris and torrential flows triggered during high intensity meteorological event are actually effectively coupled in space and time and guarantee an important sediment supply able to reload the downstream part of the sediment cascade. Recent climate trends, marked by extremes, suggest consequently more damaging events to come, in a context of increasing vulnerable assets

New insights on the chronology of medieval mining activity in the small polymetallic district of Faravel (Massif des Écrins, Southern French Alps) derived from dendrochronological and archaeological approaches

A large amount of well-preserved timbers was found during several archaeological excavations of the Faravel mining site (Southern French Alps, between 1950- and 2150 m a.s.l.). 232 of these timbers were sampled for dendrochronological analysis and 67% of them were dated. These 156 larch (Larix decidua Mill.) series, crossdated against existing reference chronologies, were averaged for a site chronology spanning from 777 to 1243. From this dataset, 33 timbers with (almost) complete sapwood allowed us to obtain tree felling years with seasonal resolution. The chronological distribution of these felling years highlights nine distinct mining phases that occurred between 1059 and 1243, revealing a discontinuous exploitation of the study site during the medieval period. In addition, the presence of late wood in the vast majority of complete samples, demonstrates that logging mainly occurred during late fall and early winter. These results, combined with historical, palynological and archaeological investigations, plead for short, seasonal, and low-intensity, mining campaigns, mainly carried out after the bulk of agropastoral activities using rudimentary techniques with limited impact on the forest cover.

Upgrading of an index-oriented methodology for consequence analysis of natural hazards: application to the Upper Guil catchment (southern French Alps)

Abstract. Vulnerability is a complex concept involving a variety of disciplines from both the physical and socio-economic sciences. Currently, two opposite trends exist: the physical approach in which vulnerability is analysed as a sum of potential impacts on elements at risk and the social approach in which vulnerability is mostly viewed as a combination of socio-economic variables determining people's ability to anticipate, cope with and recover from a catastrophic event. Finding a way to combine these two approaches is a key issue for a global vulnerability assessment. In this paper we propose to combine elements from these two approaches through the analysis of the potential consequences of a high-magnitude flood event (recurrence interval (RI) > 100 years) on human and material stakeholders. To perform our analysis, we choose to upgrade an existing index, the Potential Damage Index (PDI; Puissant et al., 2013), by including social criteria. The PDI was originally developed to assess the physical consequences of hazards on the elements at risk (people, building and lands). It is based on the calculation of three sub-indices representing different types of direct and indirect consequences: physical injury consequences (PIC), structural and functional consequences (SFC), indirect functional consequences (IC). Here, we propose to add a fourth sub-index representing the social consequences. This new sub-index, called social consequences (SC) is obtained by combining criteria derived from INSEE French census data and a risk-perception survey conducted in the field. By combining the four indices (PIC, SFC, IC and SC), we managed to create a new index called the Potential Consequences Index (PCI). The new PCI was tested on the Upper Guil catchment to assess the consequences of a high-magnitude flood event (RI > 100 years). Results of the PDI were compared with the PCI and show significant differences. The upgrade to the PDI method provided us with many inputs. The introduction of elements from social vulnerability added an extra dimension to the total consequence map. It allowed us to qualify the potential physical consequences (physical injury, structural and functional consequences) on elements at risk by considering the global resilience of local communities.

Distribution spatiale et estimation des volumes sédimentaires dans un bassin-versant de schistes lustrés : l’exemple du Peynin (Queyras, Alpes du Sud)

Des investigations geomorphologiques, geologiques et geophysiques ont ete realisees afin de localiser et d'estimer les volumes sedimentaires stockes dans le bassin-versant du Peynin d’une superficie d’environ 15 km², situe dans les Alpes francaises du Sud. Suite a un travail de terrain et de cartographie ayant permis l’identification et la caracterisation des stocks sedimentaires, deux modelisations statistiques des epaisseurs de sediments sont proposees. La premiere modelisation repose sur l’estimation du toit suppose du substratum rocheux a l’aide de fonctions polynomiales. La seconde modelisation s’appuie sur les observations de terrain par l’analyse de 46 coupes topographiques transversales et longitudinales couplee a une expertise geologique et geomorphologique des formations superficielles et des couches sous-jacentes. Pour contraindre les deux modeles, des mesures geophysiques (profils de sismique refraction) realisees durant l’ete 2014 ont ete integrees pour caler et valider les modeles d’epaisseur. Les volumes de sediments stockes dans le bassin-versant du Peynin ont ete estimes a respectivement 0,46 x 10³ m³ (estimation haute) et 0,13 x 10³ m³ (estimation basse). Rapportes a la surface du bassin-versant, ces volumes correspondent a des epaisseurs moyennes de sediments de l’ordre de ~30 m pour la premiere methode et ~9 m pour la seconde. Dans les deux cas, les depots d’anciens mouvements de terrain constituent le principal stock sedimentaire (~80 % des sediments stockes). Les autres stocks notables sont constitues par les depots d’eboulements, les tabliers d’eboulis et les cones torrentiels.

Modeling the precision of structure-from-motion multi-view stereo digital elevation models from repeated close-range aerial surveys

The accuracy of digital elevation models (DEMs) derived from structure-from-motion (SFM) multi-view stereo (MVS) 3D reconstruction is commonly computed for a single realization of model elevations. This approach may be adequate to estimate an overall measure of systematic error; however, it cannot provide a good estimation of measurement precision. Knowing measurement precision is crucial for measuring elevation surface changes observed by DEM comparisons. In this paper, we illustrate an approach to characterize spatial variation in the precision for SFM-MVS derived DEMs. We use a snow-covered surface of an active rock glacier located in the southern French Alps as the case study. A spatially varying precision estimate is calculated from repeated close-range aerial surveys for a single acquisition period by calculating the standard deviation per grid cell between the DEMs created for each flight repetition. Regression analysis using a generalized additive model (GAM) is performed to model the estimated precision and provide insights regarding how sensor, survey design and field site conditions may spatially influence the measurement precision. Additionally, we define how DEM error can be described differently depending on the available validation data. In our study image height above ground level and distance to ground control points had the greatest explanatory power for spatial variation in DEM precision. Image overlap, mean reprojection error and saturation were also useful for explaining spatially varying measurement precision of the DEMs. Field site characteristics, such as slope angle and shading, had the least importance in our model of precision. From a practical point of view, regression-modeled relationships between precision and image and site characteristics can be utilized to design future surveys with similar sensing platforms and site conditions for improved DEM precision.

Role of bioengineering structures made of willow cuttings in marly sediment trapping: assessment of three real-size experiments in the Southern French Alps

Improving knowledge on the ability of bioengineering structures made of willow cuttings to enhance efficient and sustainable sediment trapping in marly gullies in the Southern French Alps under a mountainous Mediterranean climate, to decrease sediment yield at their outlets, is a key issue today for the international scientific community working in geosciences and ecology. This study therefore aims to assess three real-size experiments (A, B and C) carried out between 2003 and 2013 in this environment. A total of 157 bioengineering structures using purple and white willow (Salix purpurea and Salix incana) cuttings–which have been shown to resprout and survive more than 2 years after their installation, corresponding to brush layers with brush mats on wooden sills (BLM), 1.2 m wide and 2 m long, installed on the floors of 33 experimental marly gullies, were monitored. The results showed that sediment trapping occurred upstream of the vegetation barriers from the 1st year onwards. Considering the depth of sediment trapped per experiment, the mean annual values reached 11.2 cm yr-1 after 3 years in experiment A, 7.7 cm yr-1 after 2–4 years in experiment C and 5.1 cm yr-1 after 5 years in experiment B. Occasionally, BLMs showed that they could trap up to 16 and 15 cm.yr-1 in experiments A and C, respectively. Considering the volumes of sediment trapped per experiment, the mean annual values reached 0.25, 0.14 and 0.08 m3 yr-1 in experiments C, A and B, respectively. Maximum values for one structure reached 1.94 m3 per year in experiment C. The significance of the volumes of trapped sediment and the sustainability of sediment trapping are discussed, and rules for bioengineering strategies are proposed.

A gravity-driven runoff and erosion model for sediment transfers at the catchment scale

The aim of this work is to simulate hydraulic transfers and upstream erosion sources in steep and erodible mountain watersheds with a physicallybased hydraulic model. In such environments, immature debris flows and shallow landslides can be the largest sources of sediments transported at the outlet. To simulate these phenomena, a gravity-driven erosion model and a 1D vertical infiltration model have been developed in the TELEMAC 2D numerical code. In this new erosion model, the motion of the granular flow is described with a fully dynamic system and a Coulomb-like bottom friction treatment, more adapted to the properties of the flow. The new model is first qualitatively evaluated on a theoretical test case: a steep plot with a slope break is used to evaluate the erosion and deposition dynamics of a single immature debris flow. Then, the model is confronted to field data on a real catchment (Draix, in the Southern French Alps). First, the infiltration model is successfully calibrated in order to simulate two different rain events. Then, the new erosion model is applied at the catchment scale. The numerical results show a very realistic behavior compared to the field observation, providing erosion in the upper parts of the hillslopes and deposition in the hydraulic network. This work opens promising perspectives, for example by coupling this new model with a classical and complementary velocity-driven model for the erosion, deposition and transfers in the hydraulic network.

10Be exposure dating of the timing of Neoglacial glacier advances in the Ecrins-Pelvoux massif, southern French Alps

Alpine glacier variations are known to be reliable proxies of Holocene climate. Here, we present a terrestrial cosmogenic nuclide (TCN)-based glacier chronology relying on 24 new 10Be exposure ages, which constrain maximum Neoglacial positions of four small to mid-sized glaciers (Rateau, Lautaret, Bonnepierre and Etages) in the Ecrins-Pelvoux massif, southern French Alps. Glacier advances, marked by (mainly lateral) moraine ridges that are located slightly outboard of the Little Ice Age (LIA, c. 1250-1860 AD) maximum positions, were dated to 4.25 ± 0.44 ka, 3.66 ± 0.09 ka, 2.09 ± 0.10 ka, c. 1.31 ± 0.17 ka and to 0.92 ± 0.02 ka. The ‘4.2 ka advance’, albeit constrained by rather scattered dates, is to our knowledge exposure-dated here for the first time in the Alps. It is considered as one of the first major Neoglacial advance in the western Alps, in agreement with other regional paleoclimatological proxies. We further review Alpine and Northern Hemisphere mid-to-high latitude evidence for climate change and glacier activity concomitant with the ‘4.2 ka event’. The ‘2.1 ka advance’ was not extensively dated in the Alps and is thought to represent a prominent advance in early Roman times. Other Neoglacial advances dated here match the timing of previously described Alpine Neoglacial events. Our results also suggest that a Neoglacial maximum occurred at Etages Glacier 0.9 ka ago, i.e. during the Medieval Climate Anomaly (MCA, c. 850-1250 AD). At Rateau Glacier, discordant results are thought to reflect exhumation and snow cover of the shortest moraine boulders. Overall, this study highlights the need to combine several sites to develop robust Neoglacial glacier chronologies in order to take into account the variability in moraine deposition pattern and landform obliteration and conservation.

Estimating chemical composition of atmospheric deposition fluxes from mineral insoluble particles deposition collected in the western Mediterranean region

Abstract. In order to measure the mass flux of atmospheric insoluble deposition and to constrain regional models of dust simulation, a network of automatic deposition collectors (CARAGA) has been installed throughout the western Mediterranean Basin. Weekly samples of the insoluble fraction of total atmospheric deposition were collected concurrently on filters at five sites including four on western Mediterranean islands (Frioul and Corsica, France; Mallorca, Spain; and Lampedusa, Italy) and one in the southern French Alps (Le Casset), and a weighing and ignition protocol was applied in order to quantify their mineral fraction. Atmospheric deposition is both a strong source of nutrients and metals for marine ecosystems in this area. However, there are few data on trace-metal deposition in the literature, since their deposition measurement is difficult to perform. In order to obtain more information from CARAGA atmospheric deposition samples, this study aimed to test their relevance in estimating elemental fluxes in addition to total mass fluxes. The elemental chemical analysis of ashed CARAGA filter samples was based on an acid digestion and an elemental analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectrometry (MS) in a clean room. The sampling and analytical protocols were tested to determine the elemental composition for mineral dust tracers (Al, Ca, K, Mg and Ti), nutrients (P and Fe) and trace metals (Cd, Co, Cr, Cu, Mn, Ni, V and Zn) from simulated wet deposition of dust analogues and traffic soot. The relative mass loss by dissolution in wet deposition was lower than 1 % for Al and Fe, and reached 13 % for P due to its larger solubility in water. For trace metals, this loss represented less than 3 % of the total mass concentration, except for Zn, Cu and Mn for which it could reach 10 %, especially in traffic soot. The chemical contamination during analysis was negligible for all the elements except for Cd, which has a very low concentration in dust. Tests allowed us to conclude that the CARAGA samples could be used to estimate the contents of nutrients and trace metals in the limits of loss by dissolution. Chemical characterization of CARAGA deposition samples corresponding to the most intense dust deposition events recorded between 2011 and 2013 has been performed and showed elemental mass ratios consistent with the ones found in the literature for Saharan dust. However, the chemical analysis of CARAGA samples revealed the presence of some anthropogenic signatures, for instance high Zn concentrations in some samples in Lampedusa, and also pointed out that mineral dust can be mixed with anthropogenic compounds in the deposition samples collected on Frioul. Results showed that the chemical analysis of CARAGA ashed samples can be used to trace the origins of elemental deposition. The elemental atmospheric fluxes estimated from these chemical analyses of samples from the CARAGA network of weekly deposition monitoring constitute the first assessment of mass deposition fluxes of trace metals and P during intense dust deposition events at the scale of the western Mediterranean Basin. The mass fluxes strongly depend on the distance from dust sources and the most intense events, while proximity from anthropogenic sources strongly impacted the masse fluxes of Zn and Cu at Lampedusa and Frioul.

The reproductive biology of the Rock Partridge Alectoris graeca saxatilis in the southern French Alps: first evidence of double-nesting behaviour

ABSTRACTCapsule: Double-nesting occurs frequently in Rock Partridges Alectoris graeca living in the southern French Alps.Aims: To investigate reproductive parameters of a Rock Partridge Alectoris graeca population.Methods: The reproductive behaviour of 62 radio-tagged birds was monitored during the breeding seasons 2012–16, to record breeding phenology, clutch sizes, hatching success, nest survival and parental care.Results: Double-nesting behaviour in the Rock Partridge was confirmed for the first time. Participation in incubation was similar for males and females (86% versus 70%). Clutch size was larger for nests incubated by males (11.0 ± 1.6 eggs) than by females (9.5 ± 1.2). Male nest survival rate (0.37) tended to be lower than female nest survival rate (0.62), although the difference was not significant. No significant difference was detected between male and female hatching success. Nest failures were caused by mammals taking the eggs (78%) or predation of the incubating parent (13%). Some circumstantial evidence suggests that occurrence of double-nesting behaviour could depend on previous winter and spring weather influencing the body condition of females. The reproductive biology of the Rock Partridge contrasts in some breeding traits with Red-legged Partridge and natural partridge hybrids, possibly due to climatic differences between habitats.Conclusion: Confirmation of double-nesting in Rock Partridges indicates that climatic constraints inherent to its mountain environment do not act as an impediment to this behaviour. Variation in weather conditions between years could influence the annual occurrence of double-nesting.

Wood14C Dating with AixMICADAS: Methods and Application to Tree-Ring Sequences from the Younger Dryas Event in the Southern French Alps

AbstractThe AixMICADAS facility is in part dedicated to research on radiocarbon (14C) calibration by means of various archives. For this purpose, we are improving upon the capacity to accurately date subfossil wood. In the current study, nine chemical pretreatment protocols are tested on six wood samples of known ages. The optimization based on14C ages,13C/12C ratios, carbon % and overall mass yield % leads us to favor the acid-base-acid-bleaching pretreatment (ABA-B). This efficient method is shown to provide a residue of holocellulose with optimal blanks equivalent to an age of 51,30014C BP with a standard deviation of 1500 yr based on 25 analyses. The seven wood samples from the Sixth International Radiocarbon Intercomparison (SIRI) are then analyzed as a further verification of the accuracy of our method. As a first scientific contribution, we studied two tree-ring sequences from subfossil pines (Barb12 and Barb17) collected in the southern French Alps. New14C analyses were performed at high resolution (every third year) and are shown to agree well with results obtained previously by high precision β-counting on CO2from large samples at lower resolution for Barb17 and accelerator mass spectrometry (AMS) data for Barb12. The new14C series are then matched to the Kauri and YDB chronologies: the new sequence of Barb12-17 tentatively corresponds to the interval between 12,836 and 12,594 cal BP within the Younger Dryas cold period. The14C comparison between the Barb12-17 sequence from France and the Kauri sequence from New Zealand allows calculating the14C Inter-Hemispheric Gradient (IHG), with an average value of ca. 57 yr. The IHG stayed relatively high throughout the studied period. Interestingly, the IHG exhibits a transient maximum value (ca. 100 yr) during the period of rapid Δ14C rise (12,750–12,720 cal BP), a behavior that could be due to a delayed response of the Southern Hemisphere.

Assessment of structural sediment connectivity within catchments: insights from graph theory

Abstract. To describe the sedimentary signal delivered at catchment outlets, many authors now refer to the concept of connectivity. In this framework, the sedimentary signal is seen as an emergent organization of local links and interactions. The challenge is thus to open the black boxes that remain within a sediment cascade, which requires both accurate geomorphic investigations in the field (reconstruction of sequences of geomorphic evolution, description of sediment pathways) and the development of tools dedicated to sediment cascade modeling. More precisely, the development of tools devoted to the study of connectivity in geomorphology is still in progress, although graph theory offers promising perspectives (Heckmann and Schwanghart, 2013). In this paper, graph theory is applied to abstract the network structure of sediment cascades, keeping only the nodes (sediment sources, sediment stores, outlet) and links (linkage by a transportation agent), represented as vertices and edges. From the description of the assemblages of sedimentary flows, we provide three main indices to explore how small-scale processes may result in significant broad-scale geomorphic patterns. The main hypothesis guiding this work is that the network structure dictates how sediment inputs from various sources interact at tributary junctions and finally at the outlet of a cascading system. First, we use the flow index to assess the potential contribution of each node to the sediment delivery at the outlet. Second, we measure the influence of each node regarding how it is accessible from both sediment sources and the outlet (using the Shimbel index). Third, we propose a new connectivity index named Network Structural Connectivity index (NSC) revealing whether the potential contribution of a node is lower or higher than expected from its location within the network. These indices are first computed for a conceptual sediment cascade network and then applied to a catchment located in the southern French Alps. We demonstrate that this index may be used to simulate sediment transfer and help in identifying the hotspots of geomorphic change. In the present case, we try to predict how a sediment cascade may be impacted by an edge disruption or a reconnection.

Recent, climate-driven river incision rate fluctuations in the Mercantour crystalline massif, southern French Alps

We present a new geomorphological analysis of the Tinée River tributaries in the southern French Alps based on numerical inverse and forward modelling of their longitudinal profiles. We model their relative uplift history with respect to the main channel, hence the incision rate history of this channel. Inverse models show that all tributaries have consistent incision rate histories with alternating high and low values. A comparison with global temperature curves shows that these variations correlate with quaternary climate changes. We suggest that during warm periods, a wave of regressive erosion propagates in the Tinée River, while its tributaries deeply incise their substratum to catch up with the falling base-level. We also show that the post 140 ka history of this landscape evolution is dominated by fluvial incision. We then perform forward models of river incision and simulate the incision of the Tinée River system over a time span of 600 ka. This model allows us to extract time and space incision rate variations of the Tinée River. With a background of a few mm.yr−1, incision rate can increase up to more than 1 cm yr−1 during short periods of time due to climatic oscillations. This result is compatible with published cosmogenic nuclide based dating, which evidenced incision rates from 0.2 to 24 mm yr−1. The part of the channel located between 12 and 20 km downstream from the source has undergone several periods of rapid incision rates, which could explain the steep hillslopes and the triggering of a landslide ∼10 ka ago.

Regional Scale Mapping of Debris-Flow Susceptibility in the Southern French Alps

This article describes a regional methodology of debris-flow susceptibility mapping. The implementation of this approach is based on statistical and GIS methods and makes it possible to identify catchments in the Southern French Alps, according to their morphometric (Melton index and local slope) and sediment supply conditions (active erosion zones in headwater catchments), that are prone to debris flows because of hydrometeorological forcing. This methodology has been validated on 50 sites with very high scores; the geomorphic response was correctly predicted for 80% of the sites. The map of susceptibility to the occurrence of debris flow has been integrated into the RhyTMME project’s Web platform. This platform also offers a visualisation of the rainfall’s spatial distribution (from meteorological radar data), enabling automatic real-time detection of the triggering of debris flow at the regional scale.

Un nouvel Episernus des Alpes françaises (Col., Ptinidae, Ernobiinae)

A new Episernus from the French Alps (Col., Ptinidae, Ernobiinae). Episernus taygetanus alpestris ssp. nov. is described from the French southern-Alps. The aedeagus is drawn and the identification key for the European Episernus is updated. This specimen was found by beating a Pinus sylvestris, close to the Izoard pass, where was also discovered a new locality for E. angulicollis Thomson, 1863.