Surface exposure dating and geophysical prospecting of the Holocene Lauvitel rock slide (French Alps)

Large rock falls and rockslides represent a risk for human communities in mountainous areas as they can cause fatalities and destruction of settlements and infrastructures. Assessing the associated hazard requires constraining the time frequency of such events. Since large rockslides are not common, estimating their frequency requires recording them over a long period of time. The Holocene period then appears as pertinent, which implies that rockslide features have to be dated with absolute chronology methods. This paper presents a characterisation and dating of the Lauvitel rockslide, one of the largest Holocene rockslides in the French Alps. Combining field observation with electrical tomography profiles performed on the rockslide deposit that constitutes the Lauvitel Lake dam allows estimating its volume at a minimum of 12?×?10⁶?m³. In addition, cosmic ray exposure dating using in situ-produced ¹⁰Be concentration measurements has been applied to date seven samples collected both on the main sliding surface and on blocks lying on the dam and further downstream. Ages obtained are consistent with a single large rockslide event, which occurred at 4.7?±?0.4 ¹⁰Be-ka and formed two distinct deposits. However, from a mechanical point of view, these clearly separated deposits could hardly result from a single movement. A comparison of their reach angles with those reviewed in the literature highlights that the lower deposit must result from rock avalanches larger than 10⁷?m³, while the upper one (the Lauvitel dam) must result from several events smaller than 10⁶?m³. In the context of hazard assessment for land use planning, these events can, however, be considered as a unique event.     

Multi-isotopic composition (δLi–δB–δD–δO) of rainwaters in France: Origin and spatio-temporal characterization

In the present work, the first results are reported for both Li and B isotope ratios in rainwater samples collected over a long time period (i.e. monthly rainfall events over 1 a) at a national scale (from coastal and inland locations). In addition, the stable isotopes of the water molecule (δD and δ¹⁸O) are also reported here for the same locations so that the Li and B isotope data can be discussed in the same context. The range of Li and B isotopic variations in these rainwaters were measured to enable the determination of the origin of these elements in rainwaters and the characterization of both the seasonal and spatio-temporal effects for δ⁷Li and δ¹¹B signatures in rainwaters. Lithium and B concentrations are low in rainwater samples, ranging from 0.004 to 0.292 μmol/L and from 0.029 to 6.184 μmol/L, respectively. δ⁷Li and δ¹¹B values in rainwaters also show a great range of variation between +3.2‰ and +95.6‰ and between −3.3‰ and +40.6‰ over a period of 1 a, respectively, clearly different from the signature of seawater. Seasonal effects (i.e. rainfall amount and month) are not the main factors controlling element concentrations and isotopic variations. δ⁷Li and δ¹¹B values in rainwaters are clearly different from one site to another, indicating the variable contribution of sea salts in the rainwater depending on the sampling site (coastal vs. inland: also called the distance-from-the-coast-effect). This is well illustrated when wind direction data (origin of air masses) is included. It was found that seawater is not the main supplier of dissolved atmospheric Li and B, and non-sea-salt sources (i.e. crustal, anthropogenic, biogenic) should also be taken into account when Li and B isotopes are considered in hydrogeochemistry as an input to surface waters and groundwater bodies as recharge. In parallel, the isotopic variations of the water molecule, vector of the dissolved B and Li, are also investigated and reported as a contour map for δ¹⁸O values based on compiled data including more than 400 δ¹⁸O values from throughout France. This δ¹⁸O map could be used as a reference for future studies dealing with δ¹⁸O recharge signature in relation to the characterization of surface waters and/or groundwater bodies.     

The Minorca Basin: a buffer zone between the Valencia and Liguro‐Provençal Basins (NW Mediterranean Sea)

Detailed analyses of seismic profiles and boreholes in the Valencia Basin (VB) reveal a differentiated basin, the Minorca Basin (MB), lying between the old Mesozoic Valencia Basin sensu strico (VBss) and the young Oligocene Liguro‐Provençal Basin (LPB). The relationship between these basins is shown through the correlation of four Miocene‐to‐present‐day megasequences. The Central and North Balearic Fracture Zones (CFZ and NBFZ) that border the MB represent two morphological and geodynamical thresholds that created an accommodation in steps between the three domains. Little to no horizontal Neogene movements are found for the Ibiza and Majorca Islands. In contrast, the counterclockwise movement of the Corso‐Sardinian blocks induced a counterclockwise movement of the Minorca block towards the SE along the CFZ and NBFZ, during the exhumation of lower continental crust in the LPB. This new understanding implies pure Neogene vertical subsidence in the VBss and places the AlKaPeCa northeastward of the present‐day Alboran Area.     

A statistical�Cdynamical scheme for reconstructing ocean forcing in the Atlantic. Part II: methodology, validation and application to high-resolution ocean models

A novel statistical?Cdynamical scheme has been developed to reconstruct the sea surface atmospheric variables necessary to force an ocean model. Multiple linear regressions are first built over a so-called learning period and over the entire Atlantic basin from the observed relationship between the surface wind conditions, or predictands, and the anomalous large scale atmospheric circulations, or predictors. The latter are estimated in the extratropics by 500?hPa geopotential height weather regimes and in the tropics by low-level wind classes. The transfer function further combined to an analog step is then used to reconstruct all the surface variables fields over 1958?C2002. We show that the proposed hybrid scheme is very skillful in reproducing the mean state, the seasonal cycle and the temporal evolution of all the surface ocean variables at interannual timescale. Deficiencies are found in the level of variance especially in the tropics. It is underestimated for 2-m temperature and humidity as well as for surface radiative fluxes in the interannual frequency band while it is slightly overestimated at higher frequency. Decomposition in empirical orthogonal function (EOF) shows that the spatial and temporal coherence of the forcing fields is however very well captured by the reconstruction method. For dynamical downscaling purposes, reconstructed fields are then interpolated and used to carry out a high-resolution oceanic simulation using the NATL4 (1/4°) model integrated over 1979?C2001. This simulation is compared to a reference experiment where the original observed forcing fields are prescribed instead. Mean states between the two experiments are virtually undistinguishable both in terms of surface fluxes and ocean dynamics estimated by the barotropic and the meridional overturning streamfunctions. The 3-dimensional variance of the simulated ocean is well preserved at interannual timescale both for temperature and salinity except in the tropics where it is underestimated. The main modes of interannual variability assessed through EOF are correctly reproduced for sea surface temperature, barotropic streamfunction and mixed layer depth both in terms of spatial structure and temporal evolution. Collectively, our results provide evidence that the statistical?Cdynamical scheme presented in this two-part study is an efficient and promising tool to infer oceanic changes (in particular those related to the wind-driven circulation) due to modifications in the large-scale atmospheric circulation. As a prerequisite, we have here validated the method for present-day climate; we encourage its use for climate change studies with some adaptations though.     

The application of an innovative inverse model for understanding and predicting landslide movements (Salazie cirque landslides,Reunion Island)

The prediction of landslide movement acceleration is a complex problem, among others identified for deep-seated landslides, and represents a crucial step for risk assessment. Within the scope of this problem, the objective of this paper is to explore a modelling method that enables the study of landslide function and facilitates displacement predictions based on a limited data set. An inverse modelling approach is proposed for predicting the temporal evolution of landslide movement based on rainfall and displacement velocities. Initially, the hydrogeology of the studied landslides was conceptualised based on correlative analyses. Subsequently, we applied an inverse model with a Gaussian-exponential transfer function to reproduce the displacements. This method was tested on the Grand Ilet (GI) and Mare-à-Poule-d’Eau (HB) landslides on Reunion Island in the Indian Ocean. We show that the behaviour of landslides can be modelled by inverse models with a bimodal transfer function using a Gaussian-exponential impulse response. The cumulative displacements over 7 years of modelling (2 years of calibration period for GI, and 4 years for HB) were reproduced with an RMSE above 0.9. The characteristics of the bimodal transfer function are directly related to the hydrogeological functioning demonstrated by the correlative analyses: the rapid reaction of a landslide can be associated with the effect of a preferential flow path on groundwater level variations. Thus, this study shows that the inverse model using a Gaussian-exponential transfer function is a powerful tool for predicting deep-seated landslide movements and for studying how they function. Beyond modelling displacements, our approach effectively demonstrates its ability to contribute relevant data for conceptualising the sliding mechanisms and hydrogeology of landslides.     

Voronoi grids conforming to 3D structural features

Flow simulation in a reservoir can be highly impacted by upscaling errors. These errors can be reduced by using simulation grids with cells as homogeneous as possible, hence conformable to horizons and faults. In this paper, the coordinates of 3D Voronoi seeds are optimized so that Voronoi cell facets honor the structural features. These features are modeled by piecewise linear complex (PLC). The optimization minimizes a function made of two parts: (1) a barycentric function, which ensures that the cells will be of good quality by maximizing their compactness; and (2) a conformity function, which allows to minimize the volume of cells that is isolated from the Voronoi seed w.r.t., a structural feature. To determine the isolated volume, a local approximation of the structural feature inside the Voronoi cells is used to cut the cells. It improves the algorithm efficiency and robustness compared to an exact cutting procedure. This method, used jointly with an adaptive gradient solver to minimize the function, allows dealing with complex 3D geological cases. It always produces a Voronoi simulation grid with the desired number of cells.     

Discrete modelling of rock-ageing in rockfill dams

The aim of this paper is to obtain, from a numerical study using the discrete element method, quantitative information on the behaviour of a rockfill dam, particularly the effect of rock ageing on the movements of the dam. A numerical model is defined at three different scales: the block scale, the elementary representative volume scale and the dam scale. The model uses the discrete element code PCF^2D, considering breakable clusters of 2D balls. Three rock-ageing laws, taking into account the time-evolving resistance for blocks, are proposed. The different parameters are determined from experimental data of laboratory tests performed on rock blocks. The relevance of the model seems acceptable, considering that the displacements obtained in the simulations are in the same order of magnitude as the displacements measured on the actual dam.     

Exploration of tektite formation processes through water and metal content measurements

Abstract– Impact events are a significant surface‐modifying process on solar system objects lacking frequent resurfacing by atmospheric or igneous processes. To better explore the effects of impacts on surface materials, we measured the water and trace element compositions of tektites from Da Lat, Vietnam. Fourier transform infrared spectroscopy was used for water measurements and laser ablation inductively coupled plasma mass spectrometry was used for trace element measurements. Consistent with previous investigations of tektites, we found that the samples are depleted in volatile metals (e.g., Zn, Pb) as well as in water compared with the average continental crust, although water contents are still extremely high for rocks melted at atmospheric pressure. While Zn and Pb concentrations are linearly correlated with each other, there is no correlation between H₂O and Zn or Pb contents. Combined with water contents of other tektites in the Australasian strewn field, our results demonstrate that the source impact occurred at a wet site in or off the coast of the Indochina peninsula.     

The symbolic and political appropriation of scales: A critical analysis of the Amazonian ranchers’ narrative

There are many examples of “local” discourses aiming to delegitimize environmental issues, demands, actions and governance by arguing that they pursue international goals, defined by international actors that do not meet local needs and frequently oppose them. We focus on the narrative held by large ranchers in the Eastern Amazon, which disqualifies the so-called top-down creation of protected areas that jeopardizes the “local development” these actors have, following the narrative, encouraged and sustained for years. Through the combination of an ontological and constructivist approach of scales, our contribution challenges this scalar construction questioning the “locality” of the large ranchers and showing that their role as “local developers” is actually rooted in regional/national/international scales. We then study the most important effect of the anti-environmental grand scalar narrative: its influence on regional and national decision makers. Well-conceived and publicized scalar scenarios are efficient in this context as in many others.