Géosciences Rennes

The NW-SE-trending Malta Graben is one of the main extensional structures of the Sicily Channel, whose tectonic evolution within the broader Africa-Eurasia convergent setting remains debated. We reconstruct the tectono-stratigraphic evolution of the Malta Graben since the Pliocene through the interpretation of seismic reflection profiles, integrated with bathymetric, geodetic, and seismological data. The Plio–Pleistocene succession is organized into syn-extensional sequences, which are bounded by unconformities and record the progressive development of the rift above a Miocene–Mesozoic basement. The structural architecture is dominated by high-angle normal faults and negative flower structures, which controlled the growth of the graben. Moreover, some normal faults remain active today, while some have been locally inverted, producing folding and a prominent seafloor bulge in the northern part of the Malta Graben. The geometry and distribution of these inverted structures indicate that contractional reactivation occurred in recent geological times, since the Upper Pliocene-Lower Pleistocene. We propose that these coeval extensional and contractional structures reflect differential foreland deformation style within the Sicily Channel in response to the Africa–Eurasia plate convergence. The Malta Graben is therefore a valuable natural laboratory for better understanding how foreland region responds locally to competing tectonic forces at major plate boundaries.

Tropical Dry Forests (TDFs) exhibit strong couplings between precipitation and vegetation phenology. TDFs are broadly defined as ecological formations characterized by -but not limited to- the presence of deciduous tree species, occurring in tropical regions where precipitation is highly seasonal, with a distinct dry season lasting several consecutive months (Miles et al., 2006), and mean annual rainfall ranging from 250 to 2000 mm (Murphy, 1986; Holdridge, 1969). This broad definition covers multiple bioclimatic types depending on authors, ranging from woodland savannah to moist semi-deciduous forests. A final agreement on their extent and definition may remain unattainable (Murphy, 1986; Blasco, 2000), and areas locally recognized as TDFs sometimes defy the most commonly used thresholds (Pando-Ocon, 2021).TDFs provide numerous critical ecosystem services including carbon sequestration, support for local livelihoods, maintenance of biodiversity through habitat provision, high levels of floristic endemism, and a buffering effect against desertification (Siyum, 2020; Mendes, 2025). Despite these vital services, TDFs have been described as one of the most threatened biomes worldwide, experiencing extensive loss, fragmentation, and degradation driven by agricultural conversion, fire, and other anthropic pressures, with less than one-third of original forest area remaining (Stan et al., 2024). However, these valuable ecosystems have long suffered from a lack of public interest and from limited attention in conservation policies and research (Santos et al., 2011).In this context, our study aims to assess and compare the dynamics of vegetation phenology, precipitation and their relationships across multiple TDF hotspots: Santa Rosa national park (Costa Rica), the Caatinga ecoregion (Brazil), Bandipur and Mudumalai national parks (Southwestern Ghats, India), and the Chizarira and Sijarira national parks within the Miombo and Mopane woodlands (Zimbabwe). These sites collectively span more or less pronounced gradients in rainfall seasonality, topography, edaphic conditions, tree density and forest composition. Using two decades (2003-2023) of MODIS NDVI time-series and CHIRPS precipitation data, we investigate inter and intra-site variability based on the visual interpretation of weekly mean NDVI and precipitation time-series over multiple points along a rainfall gradient, and compare metrics characterizing both phenological dynamics (amplitude of the vegetation index time-series and temporal phenometrics) and rainfall regimes (precipitation values and temporality of the rainy season). Additionally, we cross both times-series to assess their relationship (lag time between rainy season onset and vegetation response).Other drivers of phenology are mentioned as TDF definitions are not limited by the presence of deciduous vegetation, and phenological dynamics in these systems may be impacted by other factors such as access to groundwater or atmospheric moisture, floristic composition and stand age (Hasselquist et al., 2010; Cuba et al., 2017; Siyum, 2020; Parthasarathy et al., 2008).Overall, understanding and spatializing the links between phenology, environmental drivers and the associated plant functional traits in TDFs has important implications for the assessment of carbon flux and storage, the projection of ecosystem resilience and redistribution under climate change as well as accurate description of land-cover, ecotones and habitat connectivity (Li et al., 2024; Pereira Dos Santos et al., 2025; Ribeiro et al., 2025).

Distinguishing magmatic from hydrothermal processes in rare-metal granite systems is critical for understanding ore formation; however, zircons in these rocks are commonly affected by fluid-mediated modification, complicating the interpretation of both geochronological and geochemical signatures. We investigated zircon crystals from the Janchivlan rare-metal granite complex (Central Asian Orogenic Belt, Mongolia), a highly fractionated peraluminous system evolving from biotite-granite through graphic granite, amazonite-bearing to albite-lepidolite granite, with potential for Sn, W, Ta, and Li mineralization, as well as associated pegmatites. These lithologies record successive stages of magmatic differentiation and increasing fluid involvement.Zircon U-Pb dating yields concordant ages of 290 ± 2.1 Ma for pegmatite and 195 ± 2.1 Ma for biotite-granite, indicating that the pegmatites formed from a different magmatic event. Zircon single-grain ages from biotite and lepidolite granites define a discordia with lower intercepts at 220 ± 2.1 Ma and 195 ± 2.1 Ma, respectively, interpreted as Pb loss during hydrothermal alteration. This interval overlaps with the apatite U-Pb age of 213 ± 3.7 Ma, supporting hydrothermal activity at this time.Zircon REE patterns show a systematic evolution from biotite granite, characterized by (1) high ΣREE, moderate Eu/Eu* (~0.1–0.2), through graphic and amazonite granites with variable REE distributions and weak tetrad effects, to (2) lepidolite granite marked by LREE depletion, very low Eu/Eu* (

Mountainous landscapes often contain sediment-filled valleys that control ecosystem diversity, flood hazards, and the distribution of human populations. Valley-floor geometry has been shown to correlate with climate proxies such as discharge or drainage area and is also influenced by lithology and tectonic uplift. However, the relative importance of these controls remains poorly constrained, and no global dataset of valley geometry currently exists.Here, we focus on the automatic mapping of valley floors at the scale of entire mountain ranges. We compare two methods for extracting valley floors from digital elevation models, based on different conceptual definitions. The first is a geometric approach following Clubb et al. (2017, 2022), which defines valleys as low slope areas with a low relative elevation to the nearest river. The second is a new method that identifies valleys as floodplains using a simplified hydraulic model that locally distributes water across flat surfaces adjacent to channels. We evaluate and calibrate both methods by comparison with maps of alluvial cover in four catchments spanning a wide range of tectonic and climatic settings: the European Alps, Scottish Highlands, Pyrenees, and Taiwan. Both methods achieve comparable agreement with alluvial data, although the geometric method is more sensitive to calibration parameters. We compute valley width along the stream network by identifying valley centers and measuring the distance to valley margins, allowing us to quantify the scaling between valley width and drainage area. The scaling relationships of width with drainage area shows exponents ranging from 0.3 to 0.5, consistent with values reported in the literature, with an inter-catchment variability. We show that large-scale valley extraction also allows a more precise characterization of valley networks by identifying local deviations from width–area scaling by using a wideness index similar to the steepness index for channel gradient, and by extracting additional valley attributes such as slope and elevation. This study paves the way for a global analysis of valley morphology to better constraints its dependency to climate, tectonic and geological conditions the controls acting on it.

Glacial and paraglacial floods are among the most destructive natural hazards in high-mountain regions. These events result from cascades of processes, which rapidly transfer large amounts of water, sediment and energy across entire catchments. Their initiation typically occurs in remote, poorly instrumented areas, while impacts propagate far downstream, strongly limiting process-based understanding at the regional scale. Here, we present preliminary results from an ongoing analysis of glacial and paraglacial hazards in the Bhote Koshi catchment (Nepal), one of the best instrumented glacierized basins at the regional scale, with continuous seismic monitoring since 2016. This study is conducted within the framework of the French PEPR IRIMA program (project IRIMONT), which aims to improve the assessment and mitigation of natural hazards in mountain regions through integrated and interdisciplinary approaches.First, we focus on the July 2016 glacial lake outburst flood (GLOF). Seismic records of this event provide a unique opportunity to investigate its mechanics from initiation to far-field propagation. Preliminary analyses reveal distinct seismic signatures associated with different phases of the flood, characterized by systematic variations in amplitude, frequency content and phase coherence as a function of time and distance. These signatures indicate an exceptional capacity of the GLOF to mobilize large boulders, leading to seismic energy levels and inferred sediment transport that far exceed those observed during seasonal hydrological events. In parallel, we investigate the temporal evolution of slope instabilities in the Bhote Koshi catchment following the 2015 Gorkha earthquake. We apply unsupervised machine learning approaches to cluster seismic signals, identify recurrent signal families, and establish a baseline of background hydrological and geomorphic activity at the catchment scale. The seismic observations reveal sustained post-seismic landslide activity, with evolving signal characteristics reflecting the progressive relaxation of hillslopes modulated by hydrometeorological forcing. Overall, these preliminary results demonstrate the potential of environmental seismology, combined with data-driven approaches, to bridge the gap between local process understanding and regional-scale hazard assessment.

Basal shear stresses exerted by river flow control the capacity of river to erode and transport sediment. Material properties (e.g. lithology, grain size) modulate how basal shear stress translates into morphological change. Quantifying the spatial variability of basal shear stress is therefore essential to assess fluvial erosion processes and to infer the tectonic and climatic forcings recorded in landscape morphology. Direct and systematic measurement of the basal shear stress in rivers is not feasible at large scales, making numerical hydrodynamic modelling the primary tool for its estimation. However, applications beyond the reach scale remain computationally prohibitive due to (i) the need for high-resolution topography to resolve channels, banks, and bars, and (ii) the numerical cost of solving the Shallow Water Equations (SWEs), which require small time steps to propagate changes induced and complex solvers. Here, we present a novel numerical framework that substantially reduces the computational cost of hydrodynamic modelling for morphometric analysis, enabling simulations over large, high-resolution DEMs and ranges of hydrological conditions. The approach reformulates the SWEs into a simplified stationary scheme, linearizing algorithmic complexity, and allowing scalable computations. In addition, we employ GPU-accelerated, graph-based flow accumulation algorithms to compute discharge efficiently. Together, these developments reduce computation time by up to three orders of magnitude compared to conventional hydraulic modelling approaches. The method is implemented in the pyfastflow package within the TopoToolbox ecosystem. We apply it to more than 100 watersheds in the Mendocino Triple Junction (California, USA), a region characterized by strong spatial gradients in tectonic uplift. Hydrodynamics are computed for five hydrological states constrained by precipitation data, spanning low flow to flood conditions. We quantify spatial variations in river width and shear stress and show that these metrics capture complementary temporal signatures of uplift timing and magnitude. Basin-wide shear stress responds quickly to uplift onset but exhibits a significantly delayed response during relaxation, whereas channel width displays a more variable and spatially contrasted transient signal upstream of the onset.

Aquifers may naturally contain undesirable and toxic trace elements, known as geogenic contaminants. The presence of these micropollutants poses a major challenge for groundwater management, and has health, economic, and environmental consequences.Areas with high concentrations, either elevated or exceed the guideline value of water standards, are generally identified through the analysis of groundwater data. However, it should be possible to predict these occurrences based on their presence in the solid matrices of aquifers and their mobilization controlled by the physicochemical conditions of the water.To better predict the occurrence of inorganic natural pollutants, we have developed a methodology, based on an integrated approach to better understand the distribution of these elements in aquifers, the conditions controlling their presence, and their evolution in groundwaters.The methodology combines two complementary approaches: i) predictions of geogenic elements content in rocks, including their speciation, using a source to sink methodology are cross-correlating with ii) hydrogeochemistry to identify water-rock interaction processes and the potential mobility of the elements according to physicochemical conditions (e.g., redox conditions). This makes it possible to predict the spatial distribution of geogenic elements (e.g. As, Se, F, etc.) as well as the processes of mobilization in groundwater.Using a geographic information system (GIS), this study compares predicted occurrences using a source-to-sink (S2S) approach with available data including a large dataset on groundwater quality data (ADES data base, a national database publicly available). By interpretating the chemical composition of water, geochemical modelling via PHREEQC and geological data, it is possible to confirm but also to contribute to the S2S modelling.The study focuses on aquifers linked to the Massif Central (France). Geological source-to-sink paleomaps and drilling data are used to correlate the availability in sedimentary deposits of elements such as arsenic (As), selenium (Se), fluorine (F), with the main periods of erosion, transfer, deposit and remobilisation between the end of the Cretaceous and the Miocene. Arsenic, in particular, is studied in various geological layers with a focus on its speciation and mobilization in confined aquifers such as the Beauce calcareous confined aquifer (Southern Paris Basin).This approach presents a real interest in terms of groundwater quality, as it helps to anticipate water quality degradation linked to groundwater exploitation in aquifer impacted by the natural presence of geogenic metals.This work is part of the PEPR OneWater DEESAC project (France 2030), illustrating a transdisciplinary approach combining geology, geophysics, geochemistry, and hydrogeology.

The early Eocene long-term warming was punctuated by relatively short (50 to 200 kyr) and abrupt warming events, which are used as analogues to understand current anthropogenic global warming. Among these hyperthermal events, the Eocene Thermal Maximum 2 (ETM2) corresponds to an orbitally paced release of 2,600 to 3,800 Gt of carbon at 54.1 Ma. It has primarily been identified in deep oceanic settings, while terrestrial and coastal records of this event remain scarce. Indeed, the ETM2 has only been identified in shallow marine settings in a few locations (Arctic, USA Atlantic coast, Egypt, India, and New Zealand), hindering a full understanding of its environmental impact.Here, we present a high-resolution multi-proxy record from a newly drilled 25-m-long core in southwest Belgium (Mons Basin), at a paleolatitude of ~40°N, combining Gamma-ray spectrometry, mineralogy (XRD bulk-rock and clays, TEM, grain-size) and organic geochemistry (δ13Corg, Rock-Eval®, and palynofacies). Sedimentological interpretation indicates a siliciclastic tidal shallow marine environment (a few tens of meters water depth). Using an age model based on nannofossils, dinocysts and cyclostratigraphy, the ETM2 is recorded over approximately 2.5 m by a ~1‰ negative carbon isotope excursion (CIE) located within the NP11 nannofossil biozone. The peak of this CIE corresponds, with a slight delay, to an increase in carbonate content and nannofossil abundance, suggesting enhanced primary productivity related to an intensified hydrological cycle and higher nutrient inputs during the ETM2. An increase in detrital input is also suggested by the transition to coarser grain size. After a progressive decline in kaolinite, illite, and chlorite contents in the clay fraction, smectite becomes the dominant clay mineral during the CIE, possibly pointing to a transgressive event in relation with the ETM2, as also suggested by palynofacies and dinocyst assemblages.This study presents the first high-resolution record of the ETM2 in the coastal environments of the southern North Sea Basin, preserved in the Mons Basin. In these settings, the ETM2 is associated with a deepening trend, possibly related to sea-level rise, as well as with increased primary productivity and detrital inputs, which point to an enhanced hydrological cycle.

Le gui, Viscum album, parasite de nombreuses espèces arborées, incluant certaines essences majeures des forêts françaises. L’Inventaire forestier national collecte, depuis 2008, des données concernant la présence et l’abondance de boules de gui sur les arbres des placettes inventoriées annuellement. En nous appuyant sur ces observations, nous présentons ici un état des lieux du gui dans les forêts françaises. Plus particulièrement, nous nous intéressons à la répartition géographique du parasite ainsi qu’à son évolution temporelle. Nous explorons également les facteurs, tant environnementaux que propres aux peuplements ou aux arbres individuels, favorisant la présence du gui. Messages clés : Trois sous-espèces de gui sont présentes en France hexagonale et en Corse, qui se distinguent par les essences ligneuses parasitées : gui des feuillus, sur tout le territoire ; gui du pin, en forte proportion dans les Alpes ; gui du sapin, particulièrement présent dans les forêts vosgiennes. Le Peuplier cultivé, le Robinier faux-acacia et le Pin sylvestre sont, en termes de taux d’incidence, les principales essences hôtes du gui en France ; le Sapin pectiné, les Saules, les Petits érables et les arbres fruitiers étant d’autres essences fréquemment guitées. Les arbres présentant une proportion importante de branches mortes dans la partie supérieure de leur houppier sont ceux sur lesquels le gui a été observé le plus fréquemment.