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Fruit d’une collaboration entre paléoclimatologues et experts de l’information scientifique, le thésaurus de paléoclimatologie décrit et structure pour la première fois près de 2000 concepts de la discipline, en français et en anglais. Destiné à offrir à la communauté scientifique un vocabulaire de référence, il facilite le partage des données de recherche et des productions scientifiques en paléoclimatologie.

International audience

The greater Paris area has some 12 million inhabitants and 48 heating network production units that exploit the heat capacity of a 1.5 km deep aquifer, the Bathonian limestone. This is one of the most productive aquifers in the world for district heating, with an annual output of about 1.7 TWh of energy. The current challenge for Paris is to triple the number of heating networks using geothermal energy so as to reduce dependence on fossil fuels (40% in 2020 in France). As no detailed geothermal reservoir model is available, drilling involves geological risk. A recent well, drilled at Grigny (20 km south of Paris), turned out to be a failure in transmissibility because the bed of permeable limestones it tapped into was very thin (meter-scale). The main aim of this study is to create a digital database and a 3D geological model of this aquifer to minimize geological risks and optimize the location of future geothermal operations around Paris. By compiling data from 168 wells, a high-resolution 3D geological model of 360 km3 size is constructed (about 40 km x 50 km x 0.2 km), made up of 12.2 million cells and displaying sedimentary facies, sequence stratigraphy, porosity (Φ) and permeability (k). About 20% of the oolitic and bioclastic facies are of good reservoir quality (Φ > 13% and k > 350 mD), especially in two targeted, high-quality reservoir sequences. These facies of interest probably correspond to giant dunes and a shoal/barrier prograding from east to west. In these facies, permeable zones are generally 4 m thick and form patches of 1600 m x 1100 m, on average, elongated perpendicular to the depositional slope. 2D and 3D maps of temperature, salinity, porosity, transmissivity, and permeability allow us to understand the areas of interest for geothermal exploration, as demonstrated around Grigny. This model helps us to apprehend better the heterogeneous character of the reservoir for geothermal prospection and to reduce the risk of future doublets during well implantation. Detailed local models may be extracted to anticipate better the implantation of new doublets in areas with already densely spaced existing wells.

Extensive fieldwork at the Cotacachi-Cuicocha Volcanic Complex (CCVC, North of Ecuador) resulted in a new collection of geological data including cartography, chronology, petrography, geochemistry, and morphology. This volcanic complex is formed by a central volcano (Cotacachi: 4939 m asl, current bulk volume of 56 ± 4 km3), several peripheral domes, and a 3 km wide volcanic caldera (Cuicocha: 4.2 ± 0.1 km3). CCVC comprises three stratigraphic members: The first, Cotacachi Basal, represents the initial phase of construction, which started at 173 ± 4 ka with a basal andesitic lava flow succession (∼500 m-thick) including isolated basaltic-andesitic lavas (Verde Tola unit; NE: 113 ± 6 ka, SE: 133 ± 9 ka), the construction of some peripheral amphibole-bearing andesitic domes, such as Muyurcu and Loma Negra (138 ± 4 ka and

Jurassic carbonate strata in the eastern Paris Basin exhibit several generations of faults, tension gashes and stylolites. Although their relative chronology can sometimes be determined according to cross-cutting relationships, the duration of major deformation phases and their influence on fluid flow and carbonate cementation are still uncertain. This contribution aims to clarify the timing of brittle deformation and associated calcite cementation. Tension gashes filled by calcite in Jurassic carbonates were sampled in outcrops and boreholes and dated through U–Pb geochronology. Almost all the sampled fractures were cemented during the Cenozoic period. Continuous deformation spread from c. 50 to 30 Ma. Tension gashes oriented N10° to N20° dated at 48–43 Ma show the main Pyrenean contractional stage. A second set of calcites were dated at c. 35–33 Ma and document a Late Eocene – Oligocene extension. A transition from the compressional to the extensional regime is expressed by tension gashes dated between 43 and 35 Ma. Finally, tension gashes oriented N150° to N175°, dated between 32 and 18 Ma, may result from the propagation of the horizontal stress generated by the Alpine orogen or by late Pyrenean deformation. Clumped isotope thermometry on five samples revealed both low crystallization temperatures (from 27 to 53 °C) and the meteoric origin of calcite-precipitating fluids. Our research therefore documents a continuous fracturing from Ypresian to Rupelian times, and less expressed brittle deformation during the Miocene period.

We have measured Cd/Ca ratios of several benthic foraminiferal species and studied benthic foraminiferal assemblages on two cores from the northern Indian Ocean (Arabian Sea and northern Bay of Bengal, BoB), in order to reconstruct variations in intermediate-water circulation and paleo-nutrient content since the last deglaciation. Intermediate water Cdw records estimated from the benthic Cd/Ca reflect past changes in surface productivity and/or intermediate–bottom-water ventilation. The benthic foraminiferal assemblages are consistent with the geochemical data. These results suggest that during the last deglaciation, Cdw variability was primarily driven by changes in intermediate-water properties, indicating an enhanced ventilation of intermediate–bottom water masses during both Heinrich Stadial 1 and the Younger Dryas (HS1 and YD, respectively). During the Holocene, however, surface primary productivity appears to have influenced Cdw more than intermediate water mass properties. This is evident during the early Holocene (from 10 to 6 cal ka) when benthic foraminiferal assemblages indicate that surface primary productivity was low, resulting in low intermediate-water Cdw at both sites. Then, from ∼ 5.2 to 2.4 cal ka, surface productivity increased markedly, causing a significant increase in the intermediate-water Cdw in the southeastern Arabian Sea and the northeastern BoB. The comparison of intermediate-water Cdw records with previous reconstructions of past Indian monsoon evolution during the Holocene suggests a direct control of intermediate-water Cdw by monsoon-induced changes in upper-water stratification and surface primary productivity.

Interpreting the ages of supergene mineralogical phases in laterite is complex because they consist of polycrystalline mixtures of different phases at the microscopic scale that could be crystalized at different epochs. Among the geochronometers, the (U-Th)/He method on hematite and goethite is more often used, but ages can be difficult to interpret due to phases mixing. To resolve this issue, this study proposes a methodology for performing detailed mineralogical analysis of hematite and goethite single grains prior to their dating using the (U-Th)/He method. Strictly non-destructive mineralogy of single grains is not achievable by classical tools, such as conventional powder XRD (X-ray diffraction; requiring at least some milligrams of powder) or SEM (scanning electron microscopy; that can contaminate the grain by coating or fixing). Therefore, we performed X-ray diffraction patterns of single grains using high-flux X-ray beams from both a rotating anode (XRD_rotat) laboratory diffractometer and a synchrotron beamline (XRD_synch) and compared the results in order to design a method based on XRD_rotat only. For this purpose, two samples from the pisolitic facies of a Brazilian ferruginous duricrust (Alto Paranaíba region, Minas Gerais State, Brazil) were chosen because they presented a usual heterogeneity. Rietveld refinements of the XRD patterns obtained from both XRD_rotat and XRD_synch yielded similar results for the weight percentage ratio of the main phases and mean coherent domain sizes and less similar results for Al substitution rates, thus validating the XRD_rotat approach. No beam damage was observed when increasing X-ray exposure time, neither on XRD patterns nor on (U-Th)/He ages. Hence, sub-millimeter, undisturbed grains can be used to analyze the mineralogy of ferruginous duricrusts by XRD_rotat with a short exposure, and the same grains can subsequently be dated by (U-Th)/He geochronology analysis. The (U-Th)/He dating of pisolitic core and cortex grains also provided meaningful ages: they revealed two evolution phases of the ferruginous duricrust, which occurred at or before the Oligocene for the pisolitic core and middle Miocene for the pisolitic cortex, agreeing with the previous model for the development of pisolites. The mineralogy of single grains selected for dating is helpful for discussing the crystallization ages, and the high-flux XRD approach may be applied to other supergene mineral parageneses used for absolute dating.

International audience; During serpentinite rock weathering, Ni is concentrated in the regolith owing to residual and secondary enrichment, forming a Ni-rich deposit under tropical conditions. This study presents geochemical, isotopic, and mineralogical data for Ni from the Serra do Puma Complex (SPC) weathering profile in Carajas (Brazil). The Ni fluxes, redistribution, and Ni isotopic fractionation magnitudes were quantified along the entire weathering profile: rock, saprock, lower and upper saprolite, and limonite. The results show that chlorite and serpentine are the primary Ni-scavenging phases in saprolite, whereas Fe oxyhydroxides are the main Ni-hosting minerals in the limonite unit. The mass balance model confirmed a global Ni gain at the weathered profile scale, with larger Ni enrichment in the upper saprolite. The isotopic dataset obtained in this study contributes greatly to the knowledge of the current Ni cycle on regional and global scales. The detailed insights into Ni isotopes in the SPC, coupled with chemical and mineralogical composition, allow for the first time a Ni mass balance and Ni isotopic values for an entire weathering profile in an Amazonian context. The Ni isotopic profile agrees with the preferential retention of light Ni isotopes in the residual material relative to the parent rock during weathering processes, with the Δ60Ni limonite-saprock of −0.72‰. In the SPC, major isotopic fractionation was notably recorded during saprolitization, while limonitization was accompanied by an overall Ni loss without significant Ni isotopic fractionation. As already observed in previous studies, heavier Ni isotopes, preferentially leached during weathering processes, can be further mobilized downward and lost from the profile or incorporated in secondary minerals in the saprolite and limonite sections. The Ni isotopic mass balance model indicated that the Ni loss from the weathered serpentinite profile was isotopically heavy, in agreement with the isotopically heavier composition of the dissolved load of Amazonian rivers. The isotopically light Ni pool, associated with the significant Ni gain encountered in the upper saprolite, is notable in the SPC weathering profile and confirms the existence of a light Ni isotope reservoir in the continent.

International audience; The knowledge acquired on the exhumation of the Pyrenean chain and the evolution of the adjacent foreland basins makes this Alpine-type domain a good laboratory to better constrain a full sediment routing system in a compressive context and to apprehend the driving processes controlling the sediment routing in space and time. This integrated approach aims at enhancing our basin mastering approach as well as improving our predictions of reservoir properties. This Source-to-Sink study seeks to understand the evolution of sedimentary routing from the Source (orogenic relief, craton, basin recycling) through the transfer zone (peripheral or internal to the basin) to the final sink (flexural basin, deep turbiditic margin). Within this new cartography, we propose to compile the data over the entire peri-Pyrenean domain. We produced large scale quantitative and qualitative maps to better observe and interpret the tectonic, climatic and surface processes impacts of the SRS behavior. These maps include kinematic reconstructions of the Iberian-European-Mediterranean system, restored sequential cross-sections, history/magnitude of exhumation by thermochronology, source tracking, characterization of weathering and erosion surfaces, synthesis of the major structures activity, paleogeographic reconstructions, analysis of sedimentary geometries and transport directions as well as the quantification of volumes preserved in the basins. Their interpretation is combined with a time representation along the routing system, linking classical basin wheeler diagram representation to source erosion and lithologies to obtain a continuous view on the sediment journey. The time steps chosen for these 5 maps account for the different stages of tectono-sedimentary evolution of the peri-Pyrenean system at the early-, syn- and post-orogenic stages. The compilations carried out compare exhumed domains and sedimentation zones in terms of fluxes and volumes and make it possible to map the routing systems and point out the main drivers for the surface evolution during the construction/destruction cycle of an orogen. Research work financed and carried out as part of the BRGM-TOTAL Source-to-Sink program

Arctic regions are highly impacted by the global temperature rising and its consequences and influences on the thermo-hydro processes and their feedbacks. Theses processes are especially not very well understood in the context of river–permafrost interactions and permafrost degradation. This paper focuses on the thermal characterization of a river–valley system in a continuous permafrost area (Syrdakh, Yakutia, Eastern Siberia) that is subject to intense thawing, with major consequences on water resources and quality. We investigated this Yakutian area through two transects crossing the river using classical tools such as in–situ temperature measurements, direct active layer thickness estimations, unscrewed aerial vehicle (UAV) imagery, heat transfer numerical experiments, Ground-Penetrating Radar (GPR), and Electrical Resistivity Tomography (ERT). Of these two transects, one was closely investigated with a long-term temperature time series from 2012 to 2018, while both of them were surveyed by geophysical and UAV data acquisition in 2017 and 2018. Thermodynamical numerical simulations were run based on the long-term temperature series and are in agreement with river thermal influence on permafrost and active layer extensions retrieved from GPR and ERT profiles. An electrical resistivity-temperature relationship highlights the predominant role of water in such a complicated system and paves the way to coupled thermo-hydro-geophysical modeling for understanding permafrost–river system evolution.

Air masses loaded with mineral dust and originating from the Sahara arrive frequently in Europe, which has multiple impacts on global and regional cycles. However, the occurrence of these processes may further accelerate in the future in response to climate change, and more knowledge is therefore required on the characteristics of the particles transported during these massive dust transport and deposition episodes. Furthermore, questions arise regarding the content of this dust in radionuclides, in relationship with the atmospheric nuclear bomb testing conducted around the world between the 1950s and the 1970s in general, and those tests conducted by France in the Sahara in the early 1960s in particular.The Saharan dust episode that took place from 13th to 16th March 2022 led to the occurrence of dense dust deposition across multiple European countries, which raised concerns among the population regarding the potential radioactivity content of this dust. To address this question with a representative sample set, a participative science campaign to collect dust across Europe was launched on Twitter on 17th March 2022. Thanks to this initiative, 110 dust samples could be collected along a transect from Southern Spain to Austria.This unique sample bank was regrouped at University Paris-Saclay, France, to conduct a set of physico-chemical analyses on a selection or on the totality of these dust samples including particle size, colourimetry, mineralogy and fallout radionuclides.Backward trajectories of air masses that have led to these deposits were calculated, and this analysis confirm their potential origin from Algeria. 137Cs was detected in all dust samples, with variable activity concentrations. A strong relationship was found between the particle size of the analysed particles and the 137Cs activity concentrations, which is consistent with the literature on this topic. Particle size was found to decrease with increasing distances from the source. The colour and mineralogy analyses demonstrated that the dust collected in Austria showed different properties than those samples collected in Spain, France, Luxembourg and Germany, which likely indicates that this material did not fully consist of Saharan dust deposited during the March 2022 episode. Accordingly, the following interpretations did not take the properties of Austrian dust into account.The mineralogical analyses confirmed the potential origin of the dust from the Maghreb region, including a vast area in Southern Morocco and Southern Algeria. In contrast, the analysis of plutonium isotopic ratios (240Pu/239Pu) and 137Cs/239+240Pu activity ratios, which provide diagnosis tools to investigate the source of artificial radionuclides, in a selection of dust samples collected between Southern Spain and Luxembourg showed that the dust signature was consistent with that of the global fallout largely dominated by the nuclear tests conducted by the USA and the Soviet Union. The 137Cs contained in the dust transported and deposited during this episode was therefore very likely not associated with the French nuclear tests conducted in the early 1960s in Sahara.In the future, elemental geochemistry analyses will provide additional information on their source provenance. All results will also be published in open-access database and disseminated to the public.

International audience; This contribution presents an application of the conceptual model of ’aggradational succession’ (i.e., the sedi-mentary record deposited in response to sea-level rise during the glacial terminations) to a series of geologicalsections of the Paleo-Tiber delta cropping out along the Via Aurelia near Rome, Italy. The geochronologicalconstraints provided here through 40Ar/39Ar dating of volcanic layers intercalated within the sedimentary de-posits of the MIS 13 and MIS 11 aggradational successions resulted in some remarkable outcomes:1) we reconstruct an independent chronology of the Mediterranean sea-level oscillations 450 through 380 ka,in good agreement with the δ18O and the Red Sea relative sea level curve chronology;2) the Glacial Termination V is here bracketed in the interval 423.1 ± 4.4–438.7 ± 1.2 ka;3) we show that the aggradational successions of MIS 11 and MIS 13 display similar spatial/geometric featuresdespite the significantly different amplitude of their Oxygen isotope records, challenging the regression modelslinking absolute values of benthic foraminifera δ18O and ice-volume;4) we show the relevance of a climatic-stratigraphic approach in dating coastal-to-fluvial sedimentarycontexts;5) we provide indirect age constraints with the precision of a few ka for two important palaeoanthropologicaland archaeological sites in central Italy bearing Acheulean lithic industries and human remains.

Biomineralization integrates complex physical and chemical processes bio-controlled by the living organisms through ionic concentration regulation and organic molecules production. It allows tuning the structural, optical and mechanical properties of hard tissues during ambient-condition crystallisation, motivating a deeper understanding of the underlying processes. By combining state-of-the-art optical and X-ray microscopy methods, we investigated early-mineralized calcareous units from two bivalve species, Pinctada margaritifera and Pinna nobilis, revealing chemical and crystallographic structural insights. In these calcite units, we observed ring-like structural features correlated with a lack of calcite and an increase of amorphous calcium carbonate and proteins contents. The rings also correspond to a larger crystalline disorder and a larger strain level. Based on these observations, we propose a temporal biomineralization cycle, initiated by the production of an amorphous precursor layer, which further crystallizes with a transition front progressing radially from the unit centre, while the organics are expelled towards the prism edge. Simultaneously, along the shell thickness, the growth occurs following a layer-by-layer mode. These findings open biomimetic perspectives for the design of refined crystalline materials. STATEMENT OF SIGNIFICANCE: Calcareous biominerals are amongst the most present forms of biominerals. They exhibit astonishing structural, optical and mechanical properties while being formed at ambient synthesis conditions from ubiquitous ions, motivating the deep understanding of biomineralization. Here, we unveil the first formation steps involved in the biomineralization cycle of prismatic units of two bivalve species by applying a new multi-modal non-destructive characterization approach, sensitive to chemical and crystalline properties. The observations of structural features in mineralized units of different ages allowed the derivation of a temporal sequence for prism biomineralization, involving an amorphous precursor, a radial crystallisation front and a layer-by-layer sequence. Beyond these chemical and physical findings, the herein introduced multi-modal approach is highly relevant to other biominerals and bio-inspired studies. ispartof: ACTA BIOMATERIALIA vol:142 pages:194-207 ispartof: location:England status: published

Monitoring active layer dynamics is critical for improving the understanding of near-surface thermal and hydrological processes in the cryosphere. This study presents the laboratory test of a low-cost ground-penetrating radar (GPR) system within a laboratory experiment of active layer freezing and thawing monitoring. The system is an in-house-built low-power monostatic GPR antenna coupled with a reflectometer piloted by a single-board computer (SBC) and was tested prior to field deployment. The correspondence between the frozen front electromagnetic (EM) reflection and temperature allowed us to test the ability of the system to closely monitor the frozen front and bottom of the active layer reflection.

International audience; The current rate and magnitude of temperature rise in the Arctic are disproportionately high compared to global averages. Along with other natural and anthropogenic disturbances, this warming has caused widespread permafrost degradation and soil subsidence, resulting in the formation of thermokarst (thaw) lakes in areas of ice-rich permafrost. These lakes are hotspots of greenhouse gas emissions (CO2 and CH4), but with substantial spatial and temporal heterogeneity across Arctic and sub-Arctic regions. In Central Yakutia (Eastern Siberia, Russia), nearly half of the landscape has been affected by thermokarst processes since the early Holocene, resulting in the formation of more than 10,000 partly drained lake depressions (alas lakes). It is not yet clear how recent changes in temperature and precipitation will affect existing lakes and the formation of new thermokarst lakes. A multi-decadal remote sensing analysis of lake formation and development was conducted for two large study areas (~1200 km2 each) in Central Yakutia. Mask Region-Based Convolutional Neural Networks (R-CNN) instance segmentation was used to semi-automate lake detection in Satellite pour l’Observation de la Terre (SPOT) and declassified US military (CORONA) images (1967–2019). Using these techniques, we quantified changes in lake surface area for three different lake types (unconnected alas lake, connected alas lake, and recent thermokarst lake) since the 1960s. Our results indicate that unconnected alas lakes are the dominant lake type, both in the number of lakes and total surface area coverage. Unconnected alas lakes appear to be more susceptible to changes in precipitation compared to the other two lake types. The majority of recent thermokarst lakes form within 1 km of observable human disturbance and their surface area is directly related to air temperature increases. These results suggest that climate change and human disturbances are having a strong impact on the landscape and hydrology of Central Yakutia. This will likely affect regional and global carbon cycles, with implications for positive feedback scenarios in a continued climate warming situation.

Riparian wetlands are environments of high ecological interest as biodiversity reactors, natural river purifiers and carbon sinks. Among the numerous various types of wetlands, peatlands are sensitive environnments that rely on the brittle balance between high primary production and waterlogging conditions. Seasonal water table fluctuations can initiate major changes in their structure. The objective of this thesis is to understand the hydrological processes of these specific environments, at the site scale, and to seize how they affect the biogeochemical cycles that take place within the peat body.The method developped in this work to adress this issue is multidisciplinary: high frequency water table monitoring (10min) using about 10 observation wells distributed over the 4,6ha of the studied wetland, bimonthly groundwater sampling within this piezometer network for major ions and water and sulfate stable isotopes analyses, geophysical investigations using the electrical resistivity tomography method (ERT), analyses of undisturbed soil samples, and hydrological modelling using the numerical tool HydroGeoSphere (HGS). The coupling of all these methods led to understand the processes at the origin of the observed water table dynamics: they are the consequences of the vegetation cycles, with intensive growth from June to October, and are controlled by the lateral flow, in particular that of the river, which is boosted by a high permeable surface layer. These dynamics are behind the undergoing pedological changes observed in soil samples from the periodically unsaturated zone by stimulating the organic matter biodegradation, increasing at the same time the groundwater mineralization, and lead to a bi-layer system with different properties. Water levels at maximum drawdown thus act as a pedological, hydrological and geochemical boundary within the peat body.; Les zones humides ripariennes sont des environnements au potentiel écologique important que ce soit dans le maintien de la biodiversité, de la qualité des cours d'eau qui les longent ou encore comme réservoirs de carbone. Parmi la typologie de ces environnements très diversifiés, les tourbières sont des milieux fragiles qui reposent sur l'équilibre entre conditions hydrologiques saturées en eau et production primaire importante. Des variations de niveaux d'eau répétées peuvent être à l'origine de leur transformation. L'objectif principal de cette thèse est de comprendre le fonctionnement hydrologique de ce type d'environnement particulier, à l'échelle d'un site pilote présentant de fortes variations piézométriques saisonnières, et d'en comprendre les implications sur les cycles biogéochimiques.L'approche développée dans ce travail pour répondre à cette problématique est multidisciplinaire : suivi piézométrique à haute fréquence (10min) sur un réseau constitué d'une dizaine de puits d'observation installés de façon distribuée sur les 4,6ha de la zone humide, campagnes de prélèvements géochimiques bimestrielles sur ces ouvrages pour analyse des ions majeurs, des isotopes stables de l'eau et des sulfates, prospections géophysiques par tomographie de résistivité électrique, analyse d'échantillons non-remaniés issus de carottages, et enfin modélisation hydrologique sur un transect 2D à l'aide du code HydroGeoSphere (HGS). Le couplage de ces différentes méthodes a permis de comprendre les processus qui pilotent les dynamiques de nappes observées sur le site : elles ont pour moteur le cycle annuel de croissance intensive de la végétation entre juin et octobre, et sont contrôlées par des apports d'eau latéraux provenant notamment de la rivière et circulant rapidement dans une couche de surface caractérisée par une forte conductivité hydraulique. Ces dynamiques sont à l'origine d'une transformation pédologique de la zone de battement de nappe par la stimulation des processus de dégradation de la matière organique, qui a pour effet l'augmentation de la minéralisation des eaux souterraines, et participe à la distinction de deux horizons aux propriétés très différentes. Les niveaux piézométriques en basses eaux jouent ainsi le rôle de frontière à la fois pédologique, hydrologique et aussi géochimique.

In the context of global warming, palaeoclimatic records at different temporal and spatial scales are essential to understanding climate processes and mechanisms. Chilean Patagonia (41°S to 56°S), crossed by the Andes from north to south, represents a major topographic barrier to oceanic and atmospheric circulation. It is the only region that intercepts the entire westerly wind belt (SHW) and therefore represents a key area for the study of palaeoenvironmental changes in the Southern Hemisphere as well as for the understanding of ocean-atmosphere mechanisms and their interactions between the middle and high latitudes of the Southern Hemisphere. In this perspective, the main objectives of this thesis are: (1) to carry out analyses of diatom assemblages in deep marine sediments collected along the southern Chilean margin from three ocean cores located along a latitudinal transect between 41°S and 49°S since the last glacial, (2) to assess their relationship to regional palaeohydrological changes. The analysis of diatom assemblages reflects the proximity of the marine cores to the coastal Patagonian margin. Throughout the period studied, the percentages of coastal benthic species decrease from north to south while those of subantarctic oceanic species, which are absent in the north, increase towards the south. Beyond the very important influence of a more coastal environment for the diatom assemblages, the study of the 3 cores made it possible to highlight regional variations linked to changes in the various currents and the influence of the different water masses. During the last glacial period, the diatom assemblages of the 3 studied cores do not show a significant presence of species associated with the subantarctic-polar group, so that no proximity of the subantarctic front (SAF) to our sites is observed during this period. Throughout the period recorded in the cores, the macronutrient-rich diatom taxon (Chaetoceros spp.) was dominant with the highest percentages during the deglaciation. This increase is associated either with the resumption of the Southern Ocean upwelling either the greater influence of nutrient-rich equatorial subsurface water (ESSW). Finally, during the Holocene, the increased presence of coastal planktonic diatoms suggests the influence of continental waters leading to an increased supply of micronutrients and low salinities. For the most oceanic core, this period is marked by a strong increase in open ocean subantarctic species (F. kerguelensis, A. tabularis), driven by an intensification of the circumpolar current (ACC). Both changes would be the result of SHW intensification.; Dans le contexte du réchauffement climatique, les données paléoclimatiques à différentes échelles temporelles et spatiales sont essentielles pour appréhender les mécanismes et les processus climatiques. La Patagonie chilienne (41°S à 56°S), traversée par les Andes du nord au sud, représente une contrainte topographique majeure sur la circulation océanique et atmosphérique. C'est la seule région qui intercepte toute la ceinture des vents d'ouest et représente donc une zone clé pour l'étude des changements paléoenvironnementaux dans l'hémisphère sud et la compréhension des mécanismes océan-atmosphère et de leurs interactions entre les moyennes et hautes latitudes de l'hémisphère sud. Dans ce contexte, les objectifs principaux de cette thèse sont : (1) de réaliser des analyses d'assemblages de diatomées sur des sédiments marins profonds collectés le long de la marge sud-chilienne à partir de trois carottes océaniques localisées selon un transect latitudinal entre 41°S et 49°S de la dernière période glaciaire à l'Holocène, (2) d'évaluer leurs liens avec les changements paléohydrologiques régionaux. L'analyse des assemblages de diatomées reflète la proximité des carottes marines à la côte de la marge Patagonienne. Tout au long de la période étudiée, les pourcentages des espèces côtières et benthiques diminuent du Nord vers le Sud tandis que ceux des espèces océaniques subantarctiques, inexistants au nord, augmentent vers le sud. Au-delà de l'influence très importante d'un environnement plus côtier pour les assemblages de diatomées, l'étude des 3 carottes a permis de mettre en évidence des variations régionales liées aux changements des différents courants et de l'influence des différentes masses d'eau. Pendant la dernière période glaciaire, les assemblages de diatomées des 3 carottes étudiées ne montrent pas une présence importante d'espèces associées au groupe subantarctique - polaire, de sorte qu'aucun rapprochement du front subantarctique (SAF) de nos sites n'est observé pendant cette période. Tout le long de la période étudiée dans les carottes, le taxon de diatomées associées à une importante présence de macronutriments (Chaetoceros spp.) était dominant avec les plus forts pourcentages au cours de la déglaciation. Cette augmentation est associée soit à la reprise de l'upwelling Austral soit à la plus grande influence de l'eau équatoriale de sub-surface (ESSW) riche en nutriments. Enfin au début de l'Holocène, l'augmentation relative des diatomées planctoniques côtières suggère l'influence des eaux continentales qui conduit à un apport accru de micronutriments et des faibles salinités. Pour la carotte plus océanique, cette période est marquée par une forte augmentation des espèces subantarctiques d'océan ouvert (F. kerguelensis, A. tabularis), apportées par une intensification du courant circumpolaire (ACC). Ces deux effets s'expliquent par une intensification des vents d'ouest.

International audience

Deep atmospheric convection in the Indo-Pacific Warm Pool (IPWP) represents a major source of heat and moisture, thereby affecting the global climate, but its past changes remain debated. Here, we present sub-millennial clay mineralogy and elemental records spanning the last 40 kyr from within the IPWP. From these data, we infer millennial-scale fluctuations in precipitation, with generally lower precipitation during Heinrich Stadials 1-4, corresponding to El Niño-like conditions. Higher precipitation coincided with the warm interstadials, accompanied by La Niña-like conditions. Moreover, our record indicates the lowest precipitation occurred during the late Holocene, supporting the hypothesis of a stronger-than-modern Walker circulation during the Last Glacial Maximum. In combination with other proxy records and TraCE-21 modelling results, we recognize a distinct spatial heterogeneity of precipitation within the IPWP, resulting from the dominant influences of the ENSO-like system and migration of the Intertropical Convergence Zone in the eastern and western IPWP, respectively. Key Points We present sub-millennial clay mineral and elemental records from the Indo-Pacific Warm Pool spanning the last 40 kyr Our records indicate higher precipitation during La Niña-like conditions and lower precipitation during El Niño-like conditions Different patterns and controls on precipitation variability between the eastern and western Indo-Pacific regions are revealed Plain Language Summary Tropical rainfall systems in the Indo-Pacific Warm Pool (IPWP) play a significant role in transferring energy and moisture. However, their millennial-scale variabilities in precipitation during remains poorly understood. Here, we provide sub-millennial sediment mineralogy and elemental reconstructions spanning the last 40,000 years from a sediment core in the IPWP. We infer that variability in precipitation induced by atmospheric convection in the IPWP is the main factor controlling the variability in our data through time. Hence, we interpret our records to reflect sub-millennial fluctuations in precipitation, with lower precipitation during cool stadial periods and higher precipitation during warm interstadials. Such variability corresponds to the changing zonal state of the tropical Pacific atmosphere-ocean system, in a pattern similar to the modern-day. Additionally, our record indicates that precipitation was weaker during the late Holocene than during the Last Glacial Maximum (LGM), suggesting that the Pacific atmospheric zonal circulation was stronger during the LGM than today. Furthermore, a model-data comparison reveals distinct spatial heterogeneity in the precipitation changes, with precipitation in the eastern IPWP (West Pacific) likely controlled mostly by the tropical Pacific atmosphere-ocean system, whereas precipitation in the western IPWP (Indian Ocean) was driven by latitudinal movement of the Intertropical Convergence Zone.

Le projet Relev s’appuie sur une approche interdisciplinaire en intégrant des compétences en risques naturels (géographie, génie civil, géologie), aménagement du territoire (urbanisme, architecture, génie urbain) et sciences humaines (psychologie, sociologie, histoire), pour permettre d’améliorer la gestion de la reconstruction des territoires et des populations à la suite de catastrophes naturelles.Le projet se concentre sur le relèvement en cours des îles de Saint-Martin et Saint-Barthélemy, à la suite des ouragans Irma et Maria en septembre 2017. L’approche s’est articulée autour de deux temporalités d’étude pour mieux comprendre l’évolution des stratégies de développement de ces territoires exposés aux catastrophes naturelles : d’une part, une analyse historique des stratégies d’aménagement ante-Irma (de 1947 à 2017), d’autre part un retour d’expérience interdisciplinaire post-Irma (2017-2022) pour suivre la reconstruction de ces territoires. Ce retour d’expérience s’est focalisé plus particulièrement sur :• La reconstruction physique des infrastructures endommagées (bâtiments, infrastructures et réseaux techniques),• Le redémarrage des activités du tourisme et des PME en incluant les conséquences de la crise sanitaire de la Covid-19,• Le relèvement humain du point de vue individuel et collectif (état de stress, solidarité, vie sociale, autonomie économique, habitat, etc.),• La réorganisation de la gouvernance locale.L’objectif est de s’appuyer sur une approche intégrée pour contribuer à l’émergence de nouveaux savoirs sur l’efficacité des stratégies de reconstruction post-catastrophe et de renforcer de manière cohérente la résilience des territoires face aux catastrophes naturelles,et par là même, l’adaptation au changement climatique. Les enseignements du projet Relev documentent la gestion de la phase de reconstruction post-catastrophe à Saint-Martin et Saint-Barthélemy, et soulignent la pertinence de planifier cette phase selon des modalités proches de celles déjà adoptées pour anticiper et préparer la gestion de crise. L’analyse interdisciplinaire a permis de mieux comprendre les liens de causalité entre la gouvernance de ces 2 îles, le relèvement psychosociologique des sinistrés, la reconstruction des infrastructures endommagées, la reprise des activités touristiques et l’histoire de l’urbanisation d’îles exposées aux risques naturels.En amont de futures catastrophes, les enseignements du projet alimenteront également l’élaboration d’outils et de méthodes pour anticiper la gestion de la phase de reconstruction post-catastrophe. L’approche interdisciplinaire initiée dans le projet Relev nous incite à poursuivre cet effort de rapprochement et d’échange entre les disciplines à l’avenir, notamment sur des sujets mêlant enjeux environnementaux et sociétaux.

International audience; Current mass balance and meteorological surveys of Mera glacier located about 30 km south of Mount Everest in Nepal show the dominant role of Asian monsoon precipitation on interannual mass balance variability while temperature controls the altitude of snow-rain threshold. As these observations on mass balance variability only explore the recent decades, studies on paleo glacial extents are useful to investigate the long-term climate forcing on glacier evolution. To do so, we investigated the long-term evolution of the debris-free Mera glacier and a neighbouring small debris-free South Khare glacier. Fifty-one 10Be CRE ages were obtained from samples collected on moraine boulders and roches moutonnées. 10Be CRE ages of the boulders span from the end of the Lateglacial (19.0–11.7 ka) to the Little Ice Age (∼0.6–0.1 ka). The oldest dated moraine in this study was observed at the base of South Khare glacier with an age of 13.6 ± 0.5 ka. The two glaciers subsequently experienced their largest Holocene extent in the Early Holocene with moraines dated to 11.0 ± 0.3 ka at the base of Mera glacier and 10.8 ± 0.5 ka at the base of South Khare glacier. We did not observe any moraine from the Mid-Holocene. During the Late Holocene several glacier advances were recorded around 2.3 ± 0.2 ka, 1.5 ka and then during the last centuries at Mera glacier and around 2.8 ± 0.6 ka, and during the Little Ice Age at South Khare glacier. To explore the links between long-term Nepalese glacier changes and climate, we used oceanic and terrestrial Indian Summer monsoon reconstructions and temperature and precipitation output from two transient global climate models TraCE and LOVECLIM. These climate data outputs were corrected by a reconstruction of the Atlantic Meridional Overturning Circulation (AMOC) over the Holocene and its associated climatic impacts. We also used sensitivity experiments from the IPSL (Institut Pierre Simon Laplace) model to discuss the possible influence of horizontal resolution, land hydrology, vegetation and runoff on changes in Asian summer monsoon. Importantly, we show this long-term Nepalese glacier pattern does not perfectly conform neither to the Indian monsoon precipitation that is documented from terrestrial and marine records nor to temperature and precipitation changes simulated by the models. While the maximum glacier extent in the Early Holocene corresponds to enhanced precipitation documented by proxies and models, the Late Holocene glacier advance remains puzzling. We claim that new paleo glacier records and improved climate simulations are necessary to get a better understanding of past glacier changes and the associated climate dynamics, which might be crucial to gain confidence in both glacier and climate future evolutions.

International audience; The Climate Graduate School of the Institut Pierre-Simon Laplace (CGS-IPSL) iscurrently developing a series of seven courses online for Master students. On these sevencourses of 3 ECTS each, three have a strong focus on the ocean: Dynamics of the Ocean andAtmosphere; Contemporary Biogeochemical Cycles; Study of Paleo-climates. Each course is ledby a pair of university teachers specialists in their field accompanied by a pedagogical andgraphic team in order to design all resources specifically adapted to graduate students andonline education. Indeed, our first goal is to open these courses as self-paced learning underthe IPSL Learning Management System (Moodle) to students who will be joining CGS-IPSLmasters without having all the prerequisites so that they can update their academicbackground. These courses could also be open to second year CGS IPSL Master students whoaimed at acquiring credits on a secondary theme not necessarily developed in their master (e.g.on biogeochemistry for students registered in an ocean – climate master). Finally all the e-resources developed will be made available at least to the educational community of the CGS-IPSL that include several Universities in and around Paris (Sorbonne Universite, Paris Saclay,Universite Versailles Saint Quentin, Université Paris Est Créteil...) in order to reuse theseresources for higher education either on-line or face-to-face courses. The courses will beavailable in both French and English so that international students can have access to these e-resources.

International audience; Niobium (Nb) is one of the most immobile elements during supergene weathering, widely used for mass-balance calculations, despite elusive information on the mechanisms controlling its dynamics in the critical zone. Here, a multiscale approach, from weathering profile to atomic-scale, is developed to monitor Nb speciation along a thick (about 50 m) lateritic regolith formed over the Pitinga pluton (Amazonas, Brazil). In the A-type parent granite, Nb is mainly hosted in ilmenite (1.5-2 wt% Nb 2 O 5) and, to a lesser extent, in rutile (2-3 wt% Nb 2 O 5). A quantitative assessment of the average Nb speciation from the parent rock to the upper horizons has been carried out by combining spectroscopic and spatially-resolved chemical techniques. The contribution of Nb-bearing Ti oxides (2-6 wt% Nb 2 O 5) and Fe oxides (0.1-0.3 wt% Nb 2 O 5) in the average Nb speciation increases with the degree of weathering and reaches 80% in the most altered horizon. This unusual Nb speciation results from the nature of the primary Nb carriers, less resistant to weathering than common Nb ore minerals such as pyrochlore. X-ray absorption spectroscopy demonstrates that Nb released from the weathering of ilmenite substitutes for Fe and Ti in goethite and Ti oxides, respectively, providing atomic-scale evidence of the high affinity of Nb for these phases. Elemental mobility of Nb, Ta and Sn followed by geochemical mass-transfer calculations evidences lateral transport from Nb-Sn-enriched laterites developed over the surrounding Nb-enriched facies of the Pitinga granite to the studied profile. Elements such as Ti, Zr and Hf, often considered immobile, are leached or redistributed in the studied profile. Our results demonstrate Nb remobilization during intense weathering. Although Nb scavenging by secondary Fe and Ti oxides limits Nb mobility at the mineral scale, this work questions the unrestricted use of Nb as a chemical invariant during surficial alteration processes.

International audience; The early Cambrian Takab iron ore deposit is situated in the northern part of the Sanandaj-Sirjan zone, western Iran. It consists of banded, nodular and disseminated magnetite hosted in folded micaschists. Trace element and Fe and O isotopic experiments reveal various hydrothermal precipitation environments under reduced to slightly oxidizing conditions. Disseminated magnetite has high Ti (945-1940 ppm) positively correlated with Mg + Al + Si, and heavy Fe (+0.76 to +1.86‰) and O (+1.0 to +4.07‰) isotopic compositions that support a magmatic/high-T hydrothermal origin. Banded magnetite has low Ti (15−200 ppm), V (≤100 ppm), Si and Mg (mostly ≤300 ppm) and variable Al. The ∂ 56 Fe values vary from −0.2‰ to +1.12‰ but most values also support a magmatic/high-T hydrothermal origin. However, variable ∂ 18 O (−2.52 to +1.22‰) values provide evidence of re-equilibration with lower-T fluid at ~200-300 • C. Nodular magnetite shows high Mn (≤1%), and mostly negative ∂ 56 Fe values (average, −0.3‰) indicative of precipitation from an isotopically light hydrothermal fluid. Re-equilibration with carbonated rocks/fluids likely results in a negative Ce anomaly and higher ∂ 18 O (average, +6.30‰). The Takab iron ore deposit has, thus, experienced a complex hydrothermal history.

Dissolved rare earth elements (REEs) and neodymium isotopes (ϵNd) have been jointly used to evaluate water mass mixing and lithogenic inputs in the ocean. As the largest marginal sea of the West Pacific, the South China Sea (SCS) is an ideal region for reconstructing past hydrological changes. However, its REE and ϵNd distributions and underlying controlling mechanisms remain poorly understood. On the basis of four seawater profiles spread across the SCS, this study presents dissolved REE concentrations and ϵNd data under summer condition to better understand the processes that potentially influence changes in these parameters and their marine cycling. The results show high concentrations of REEs and large variations in ϵNd (−6.7 to −2.8) in surface water, likely caused by the dissolution of riverine and marine particles. Comparison with published data from samples taken during the winter of different years in this and previous studies suggests a possible seasonal variability of middle REE enrichment. The SCS deep water shows a narrow ϵNd range from −4.3 to −3.4, confirming the dominant presence of the North Pacific Deep Water in the deep SCS. The intermediate water in the central SCS is characterized by a more negative ϵNd signal (–4.2 to –3.4) than that found in its counterpart in the West Pacific (–3.5 to –2.8), indicating alterations by deep water through three-dimensional overturning circulation from the northern to southern SCS below ~500 m. The contributions of external sources could be quantitatively estimated for the SCS in terms of Nd. The dissolution of particles from the SCS surrounding rivers (0.26–1.3 tons/yr in summer; 5.6–29 tons/yr in winter) and continental margins (2–12 tons/yr in summer; 23–44 tons/yr in winter) may play an important role in providing additional Nd to the SCS surface water.

International audience; In urban environments, diachronic evolution of water quality can be reconstructed using geochemical analysis of urban secondary carbonate deposits (USCDs), from urban underground structures, similar to speleothems from natural caves. The use of the radiocarbon bomb peak to build their precise chronology was recently tested in two Paris-area urban sites (France). In this study, new samples from contrasted environments in the Paris region were sampled in order to test the sites’ effects on the radiocarbon signal recorded: under wood, under a fountain, in underground aqueducts, in the south and north of Paris. We compared the post-bomb atmospheric radiocarbon record with the one measured at the top of USCDs, and estimated the dead carbon proportion (DCP), between 0 and 40%. USCDs fed by water with a rapid transfer through thin soil (Versailles fountain) had the lowest DCP ( 14 C very close to atmospheric one). Highest DCP were found for USCD from deep underground quarry under urban wood, and intermediate ones for USCDs fed by the waters of perched aquifers. These data support the use of radiocarbon as chronometer for USCDs in contrasted urban contexts, and show that it can be used to determine carbon transport and sources, an important parameter for pollution reconstruction.

The formation and differentiation of the crust of Mars in the first tens of millions of years after its accretion can only be deciphered from incredibly limited records. The martian breccia NWA 7034 and its paired stones is one of them. This meteorite contains the oldest martian igneous material ever dated: ~4.5 Ga old. However, its source and geological context have so far remained unknown. Here, we show that the meteorite was ejected 5–10 Ma ago from the north-east of the Terra Cimmeria—Sirenum province, in the southern hemisphere of Mars. More specifically, the breccia belongs to the ejecta deposits of the Khujirt crater formed 1.5 Ga ago, and it was ejected as a result of the formation of the Karratha crater 5–10 Ma ago. Our findings demonstrate that the Terra Cimmeria—Sirenum province is a relic of the differentiated primordial martian crust, formed shortly after the accretion of the planet, and that it constitutes a unique record of early crustal processes. This province is an ideal landing site for future missions aiming to unravel the first tens of millions of years of the history of Mars and, by extension, of all terrestrial planets, including the Earth.

International audience; Le SNO RENOIR est un réseau de mesures à long terme des teneurs en isotopes de la molécule d'eau (d18O et d2H) dans les précipitations (pluie, neige) à l'échelle de la France métropolitaine et ultramarine. Il est constitué de 33 stations de prélèvement des eaux de pluie réparties sur tout le territoire métropolitain et ultramarin. Cet observatoire national de la composition isotopique des précipitations a pour but de partager des données essentielles sous forme de chroniques isotopiques au pas de temps mensuel avec toute la communauté scientifique.

International audience

International audience; The Paleolithic site of Kalinga, in the Cagayan River basin in the Philippines, is associated with the oldest known human occupation traces of the archipelago and dated to 709 ± 68 ka by our research group. This work presents the geomorphological and geochronological study carried out in the region of the site. After the discovery of several surfaces paleontological remains and lithic productions, the region was the subject of numerous excavations from the 1970s, but the chronological framework remained to be specified. In the 2010s, on the Kalinga site, lithic tools and a rhinoceros carcass showing butchery traces were discovered undisturbed in the sedimentary levels, dated afterwards by our team in 2019. Despite these dates, the geomorphology and geochronology of the area are still little known. This work aims to clarify them through the use of ESR dating methods on quartz and 40Ar/39Ar on plagioclase. The sedimentary deposits of the Cagayan Basin are mainly composed of fluvial sands partly formed from minerals of volcanic origin. This composition allows the application of these two absolute dating methods. Our results reveal a human occupation before 900 and after 400 ka.

40Ar/39Ar geochronology constraints to aggradational phases and grain size variations show that the two large gravel beds occurring in the sedimentary filling of the Liri fluvial-lacustrine basin (central Italy) recorded the occurrence of deglaciation events synchronous within uncertainties with global meltwater pulses at ca. 450 and 350 ka. In particular, we find a precise match between the ages of gravel deposition and the occurrence of moderate sea-level rise events which anticipate those more marked during the glacial termination V and IV in the Red Sea relative sea level curve, as already verified by data from the Tiber River catchment basin. Such correspondence suggests that gravel deposition is facilitated by melting of Apennine mountain range glaciers, which provide the water transport energy and a surplus of clastic input to the rivers draining the mountain regions and flowing into the Tyrrhenian Sea. Therefore, the thick gravel beds intercalated in the sedimentary filling of the catchment basins of the major rivers in central Italy may be regarded as an equivalent proxy of large deglaciation events, similar to the ice-rafted debris in northern Atlantic. Consistent with this hypothesis, we also show the close correspondence between the occurrence of particularly mild (warmer) minima of the mean summer insolation at 65° N and these early aggradational phases, as well as with other anomalous early sea-level rises occurring c. 750 ka and 540 ka at the onset of glacial termination VIII and VI, and 40 ka at the onset of the so-called Heinrich events.

Some mollusc shells are formed from an amorphous calcium carbonate (ACC) compound, which further transforms into a crystalline material. The transformation mechanism is not fully understood but is however crucial to develop bioinspired synthetic biomineralization strategies or accurate marine biomineral proxies for geoscience. The difficulty arises from the simultaneous presence of crystalline and amorphous compounds in the shell, which complicates the selective experimental characterization of the amorphous fraction. Here, we use nanobeam X-ray total scattering together with an approach to separate crystalline and amorphous scattering contributions to obtain the spatially resolved atomic pair distribution function (PDF). We resolve three distinct amorphous calcium carbonate compounds, present in the shell of Pinctada margaritifera and attributed to: interprismatic periostracum, young mineralizing units, and mature mineralizing units. From this, we extract accurate bond parameters by reverse Monte Carlo (RMC) modeling of the PDF. This shows that the three amorphous compounds differ mostly in their Ca–O nearest-neighbor atom pair distance. Further characterization with conventional spectroscopic techniques unveils the presence of Mg in the shell and shows Mg–calcite in the final, crystallized shell. In line with recent literature, we propose that the amorphous-to-crystal transition is mediated by the presence of Mg. The transition occurs through the decomposition of the initial Mg-rich precursor into a second Mg-poor ACC compound before forming a crystal.

International audience; Partagé entre la Jordanie et l'Arabie saoudite, le système aquifère du Saq-Ram (520 000 km²) constitue l'une des principales ressources en eau de la péninsule arabique. Largement exploité depuis les années 80 pour soutenir le développement d'une agriculture irriguée intensive, cet aquifère multicouche, principalement gréseux, est aujourd'hui un des systèmes aquifères les plus surexploités au monde, caractérisé par des cônes de rabattement de plusieurs centaines de kilomètres. Afin d'évaluer la durabilité de cette ressource aquifère, le projet WAO (Past, present and future Water resources in Al-Ula Oasis) combine méthodes satellitaires et géochimiques pour contraindre les différentes composantes du bilan hydrique de l'hydrosystème : prélèvements, recharge-actuelle et passée-, retour d'irrigation… A l'échelle régionale, les données satellitaires gravimétriques (GRACE) ont permis de contraindre les variations du stock d'eau souterraine pour la période 2002-2019. En s'appuyant sur une estimation des prélèvements des eaux souterraines, de la recharge artificielle (principalement liée aux excédents d'irrigation) et des sorties naturelles du système, le calcul du bilan hydrologique dérivé de GRACE donne une estimation moyenne de la recharge naturelle sur les deux dernières décennies de 1,6 ± 1,2 mm.an-1 pour l'ensemble du Saq-Ram, correspondant à 2,9 ± 2,2% de la pluviométrie moyenne interannuelle (i.e. 55 mm.an-1). Ce taux de recharge est nettement supérieur à ceux d'aquifères arides sahariens similaires (i.e. 1,7 ± 1,2% des pluies), obtenus avec la même approche gravimétrique. Une explication plausible est que les plateaux volcaniques qui couvrent une surface conséquente du système aquifère du Saq-Ram favorisent l'infiltration des eaux pluviales. Ne couvrant que 8 % du domaine du Saq-Ram, ces massifs basaltiques contribueraient à environ 40 % de la recharge naturelle totale. Pour conforter/confronter ces résultats obtenus à l'échelle du système Saq-Ram, une étude a été menée plus localement sur une zone de recharge située dans la région d'Al Ula. Un large panel d'outils géochimiques (majeurs, gaz anthropiques (CFCs, SF 6) et nobles (Ne, Ar)) et isotopiques (dD, d 18 O, 14 C, 36 Cl, 87 Sr/ 86 Sr) a été déployé pour déterminer l'origine (naturelle ou artificielle) et l'âge de la recharge de l'aquifère du Saq-Ram et des aquifères alluviaux de la région. Si les traceurs comme les CFCs et SF 6 ou 36 Cl indiquent une recharge active pour les aquifères alluviaux, les traces d'une recharge récente du Saq-Ram dans cette zone restent peu évidentes, d'autant plus que les retours d'irrigation, aisément tracés par l'augmentation de la salinité, sont bien visibles dans les eaux souterraines prélevées au droit des zones agricoles. Les seules traces de recharge récente sont données par la composition isotopique des eaux des sources qui sourdent au contact entre les grès et le plateau basaltique qui surplombe la vallée d'Al Ula. Leur composition est marquée par un mélange entre les précipitations actuelles et les eaux du Saq-Ram dont la signature est significativement appauvrie. Cette signature très distincte des eaux du Saq-Ram indique une recharge lors de périodes caractérisées par un régime de précipitations distinct de l'actuel. La présence de 14 C dans tous les échantillons et la relative constance du rapport (36 Cl/Cl), preuve de l'absence de décroissance significative de ce radionucléide, permettent de restreindre cette recharge très probablement à la période humide africaine holocène.

The Arabian Sea (AS) is one of the most productive oceanic regions in the world due to several monsoon-related processes that can increase nutrients contents in the euphotic zone. Previous studies of the imprint of oceanic Primary Productivity (PP) in AS sediments yielded diverse results depending on the studied area and the chosen proxies, with unprecise paleoclimatic interpretations. Here, we provide multi-decennial PP and coastal upwelling dynamic records off northern Oman over the last 23 kyr, based on the analysis of coccoliths from sediment core MD00-2354. Our results have been compared with previous paleoenvironmental records as well as new modelling data to get precise paleoclimatic interpretations. We document higher PP and weaker coastal upwelling during the Last Glacial Maximum relative to the Holocene, and significant millennial-scale variations over the last deglaciation corresponding to the fluctuations of the Atlantic Meridional Overturning Circulation strength. Higher PP and weaker upwelling are found during cold stadials, while lower PP and stronger upwelling during the warm interstadial. We propose that the increases of PP were driven by increased bioavailable nutrient content in surface waters under both stronger winter monsoon conditions that strengthened the convective mixing, and higher aeolian inputs. Over the Holocene, stronger upwelling and slightly lower PP are found during the Early-Mid Holocene, when higher summer insolation triggered stronger summer monsoon. At that time, the lower PP was probably the result of restricted advection of eutrophic summer upwelling seawater under negative wind stress curls and less aeolian inputs. Key Points Past primary productivity and upwelling in the NW Arabian Sea are reconstructed by coccolith assemblages from a marine sediment core Primary productivity was relatively high during the cold periods compared to the warm periods of the last 23 kyr Primary productivity changes were mainly controlled by winter monsoon through changes in convective mixing and by aeolian inputs Plain Language Summary Ocean Primary Productivity is the production rate of organic carbon from inorganic carbon by ocean phytoplankton through photosynthesis, in which nutrients and sunlight are necessary. In the tropical oceans, this rate is usually limited by the availability of nutrients in the sunlit ocean, which is controlled by physical processes such as seawater upward motion, ocean mixing, and dust storms. All these processes play a significant role in the northwestern Arabian Sea today. It is surrounded by arid lands that provide the highest amount of airborne material in the world and it is influenced by the Indian monsoon system that drives seawater upward motion and mixing in summer and winter, respectively. However, not much is known about their impact on Primary Productivity in the past. This work aims to reconstruct changes of Primary Productivity in the northwestern Arabian Sea over the past twenty thousand years and understand by which process(es) they are impacted, based on a model-data comparison approach. We’ve found that Primary Productivity was higher during cold periods than during warm periods. The changes were mainly controlled by both, winter monsoon through changes in convective mixing, and dust storms through changes in nutrient-enrich aeolian inputs.

International audience; Major eruptions in the Andes are mainly characterized by the emission of large volumes of gas and volcanic ash. The plume may reach the stratosphere and be transported by winds. In Ecuador, the prevailing winds are westward, and the volcanic ash, also called tephra, is transported towards the Pacific Ocean. Vallejo (2011) studied tephra layers present in marine terraces and proposed a first draft of the identification of their source in the Cordillera. However tephras deposited onshore may have been remobilized, eroded or covered by younger deposits. There are therefore several interests in studying tephras recorded in marine sediments : it allows to (1) know the age and source of major eruptions whose products reached the coast, (2) know the distribution of fallout from each major eruption, (3) estimate the volume of largest events, (4) participate to better assess the current volcanic hazard, and (5) provide temporal constraints on continental or marine sediments that cannot be dated by radiocarbon. To study the tephra layers recorded in marine sediments I have described the glass morphology and the mineralogy of tephra beds. To determine the source of the eruptions, I have used data published onshore (e.g., Hidalgo et al., 2008; Hall et Mothes, 2008; Robin et al., 2010). As each volcano has its own geochemical signature when we combine major, trace and isotope data, I compared them with geochemical analyses performed on distal tephra recorded in marine sediments. To determine the age of distal tephra beds, we have performed 14C ages on foraminifera present above and below the bed for tephra emitted during the past 50 ka, and d18O and biostratigraphy for the older deposits. In this presentation, I first presented our results obtained on cores collected during Amadeus (2005) and Atacames (2012) oceanic campaigns along the Ecuadorian margin, and recently published (Bablon et al., 2022). We have sampled 28 tephra layers, from coring sites that cover 5 degrees of latitude, from the southern half of Ecuador to southern Colombia. We observed four main lithofacies: isolated lenses that typical of bioturbation, layers with sharp and sometimes diffuse contacts in the upper part that correspond to primary deposits of tephra fallout, and successions of thin laminated layers that correspond to tephra layers reworked by turbidity currents, and that have not been sampled. Volcanic glass shards have various morphologies depending on the density of vesicles and their deformation, such as block-shaped glass without bubbles, pumice-shaped glass, or glass with completely elongated bubbles. The glass morphology of each tephra allows us to propose a first correlation of layers between each marine core. Concerning the geochemistry, glasses are mostly rhyolitic and belong to the low potassic series typical of the volcanic front, and to high potassic series typical of the eastern cordillera. This distinction between the eastern and western cordillera is also found in their trace element signature. To identify the source volcano, we used Sr and Pb isotopes. On The compositional fields of the volcanoes products overlap little and thus allows us to refine the correlations. We show that distal marine tephra come from the Cerro Machin in Colombia, and from Pichincha, Atacazo, and Cotopaxi volcanoes in Ecuador. Together with the determination of the sources, the radiocarbon dating of sediments allowed us to show that the oldest tephras belonged to the about 8 ka eruption of Cotopaxi, and the youngest correspond to the 10th century eruption of Pichincha. Using the spatial distribution of tephra, we made isopach maps of the fourth major eruptions of Pichincha, Atacazo and Cotopaxi, and we estimated their volumes. They vary between 1.3 and 6 km3, which corresponds to volcanic explosivity indexes of 5, thus eruptions which would be particularly destructive today. A perspective for our work is to study of turbidite beds present in the Holocene cores along the coast. As such deposits are emplaced during major earthquakes, and we can therefore use their correlation to identify past seiscally active areas.Unfortunately, some major Holocene eruptions described in the Cordillera and constitute stratigraphic markers have not been recorded in marine sediments. Tephra layers may have been destructed during drilling such as the very young 700 BP eruption of Quilotoa, they may have been dispersed by ocean currents, or tephra were not present in the cores du to a restricted distribution of deposits, for example for the 3000 BP eruption of Cuicocha (Vallejo, 2011).In the second part of my presentation, I focused on another case study, performed at ODP site 1239, above the Carnegie Ridge. This core is much deeper as it reaches 500 m, and sediments deposited about 10 Myr ago. It contains 24 tephra layers, and we focused on the thickest, 18 cm-thick at 7 m deep (Schipboard Scientific Party, 2003; Bablon et al., 2020). The main volcanic structure that could be the source of such a thick deposit is the Chalupas caldera, located in the Eastern Cordillera, near Cotopaxi volcano. About 50 km southwest of the caldera, we sampled the ignimbrite and dated the glass shards at 216 +/- 5 ka using the K-Ar dating method applied on glass shards. In order to verify if the ignimbrite and marine tephra of ODP Site 1239 belong to the same eruption, we have compared their geochemistry, and we have shown that their major and trace element contents are very close. To check the reliability of this land-sea correlation, we have also compared their ages. Variations of d18O are related to climate changes linked to the Earth's orbital forcing. We then used d18O data available to know the age of sediments as a function of depth. The stratigraphic position of the tephra layer corresponds to the 7d isotopic stage that occured at 220 ka, in agreement with our K-Ar age obtained onland. We then have shown that the 216 +/- 5 ka eruption of Chalupas is the largest of the Quaternary in northern Andes, with products that reached more than 1000 km from their source. Our land-sea correlation also allow to provide an independant temporal constraint to the regional d18O records.

International audience; Unlike the richly documented rock art of the Sahara or southern Africa, that of sub-Saharan Africa often remains poorly known. With 117 inventoried sites (including 20 decorated caves), the Lovo Massif contains one of the largest concentrations of rock sites in the entire region, representing more than 5700 images. Dating this rock art is a real challenge, and only a few direct dates of drawings made with charcoal have been obtained so far (Heimlich et al., 2013). In this paper, we present the first chronology for the red geometric paintings of the Lovo Massif. We used uranium-series and 14C on secondary carbonates deposited above a series of geometric paintings at the M'Bubulu rock art site, in order to propose a minimum age for these motifs. The cross-dating shows evidence of a close system behavior for these secondary carbonate layers, thereby validating the ages obtained. They can thus be used to constrain the chronology of the red geometric paintings, which were made before the seventeenth century. Thus, this art could be attributed to a specific Kongo ritual.

International audience; Volcano sector collapses are catastrophic events that can mobilize huge volumes of material and cause changes in the magmatic plumbing system, leading to variations in growth rate and/or composition. Dating pre-historic volcanic debris avalanche deposits is challenging. Geological materials directly recording avalanche formation and amenable to radiometric dating are rare, and, in the case of Holocene events, the applicable radiometric dating techniques are scarce. Socompa volcano suffered the largest collapse event in the Central Volcanic Zone of the Andes, producing one of Earth's most spectacular and best-preserved volcano avalanche deposits. We apply multiple dating techniques to unravel the timing of Socompa's collapse and gain insights into its pre- and post-collapse eruptive history. The age of the collapse event is constrained by a 14C age of 6,200-6,400 BP of a paleosol buried by the avalanche, and by a post-collapse lava flow dated at 5.91 ± 0.43 ka by zircon double-dating. Bayesian age sequence modeling integrating these ages determines that the collapse occurred at 6.18-0.64+0.28 ka. Four zircon eruption dates and one unspiked K-Ar age between 69.2 ± 6.0 and 22.1 ± 1.9 ka constrain the age of the youngest stage of activity before the collapse. The ages, together with paleosurface modeling and volume calculations, allow estimating growth rates for the young pre-collapse and post-collapse stages of ~ 0.2-0.3 km3/kyr and ~ 0.5-2 km3/kyr, respectively, indicating a significant increase in activity after the collapse event. The collapse may have triggered a new growth phase or accelerated an ongoing one and was likely associated with a Plinian eruption that produced widespread pumice fallout. The pre- and post-collapse lavas have similar compositions and zircon crystallization age distributions, suggesting that the same or a similar magma reservoir was tapped before and after the collapse. Thus, huge collapses such as Socompa's event can promote increased volcanic activity as a consequence of the unloading effect, but the overall plumbing system may not be affected enough to show significant variations in erupted compositions, at least transiently. Our results highlight the efficacy of the zircon double-dating method for dating very young felsic lavas and for constraining the age of debris avalanche deposits. This is particularly relevant in the Andean Central Volcanic Zone and other regions with arid climates, where organic material is rare and hence 14C dating is often unfeasible. Furthermore, the post-collapse zircon eruption age of 5.91 ± 0.43 ka is the youngest radiometric age yet obtained for a lava flow in the southern Central Volcanic Zone, highlighting the youth of volcanic activity at Socompa, and confirming its status as a Holocene active volcano.

Several analytical techniques are available to study underground calcite samples (U/Th, U/Pb dating, chemical and isotopic analyses, morphoscopy...). This leads to an increase of the number of samples taken from the caves. However, the conservation of the underground environment, a limitedlandscape impact and efficient documentation of the sampling are all elements that militate in favour of the development of adapted coring techniques. These techniques, presented here, cover three types of sampling: (i) diameter 4 to 10 mm, length 65/80 mm adapted to stalagmite bases; (ii) diameter 28 to 32 mm, length 0.3 to 1 m adapted to small floors and stalagmite bases; (iii) diameter 80 to 112 mm, length 0.3 to 2 m adapted to stalagmite floors and flowstones. A technical study group of the CNRS "Milieux Souterrains et Karsts" trade network offers here a synthesis of feedback and tests carried out in the field.The constraints of coring (protected cave), site accessibility (deep karst), extraction (conditioning) according to the type of sampling and their documentation are discussed. The final objective is to propose a methodological guide adapted to the different issues and situations encountered underground., Le développement de techniques analytiques permettant d'étudier des échantillons de calcite souterraine (datations U/Th, U/Pb, analyses chimiques et isotopiques, morphoscopie…) amène à multiplier les prélèvements d'échantillons sous terre. Cependant le souci de conservation du milieu souterrain, les enjeux d'un impact paysager limité, la nécessité d'une documentation efficace du prélèvement sont autant d'éléments qui militent pour le développement de techniques de carottage adaptées. Ces techniques, présentées ici, couvrent trois types d'échantillonnage : (i) diamètre 4 à 10 mm, longueur 65/80 mm adapté aux bases de stalagmites ; (ii) diamètre 28 à 32 mm, longueur 30 cm à 1 m adapté aux petits planchers et bases de stalagmites ; (iii) diamètre 80 à 112 mm, longueur 0,3 à 2 m adapté aux planchers et coulées stalagmitiques. Un groupe d'études techniques du réseau de métiers CNRS « Milieux Souterrains et Karsts » propose ici une synthèse des retours d'expérience et de tests menés sur le terrain. Les contraintes du carottage (grotte protégée), d'accessibilité au site (karst profond), d'extraction (conditionnement) en fonction du type d'échantillonnage et de leur documentation sont discutées. L'objectif final est de proposer un guide méthodologique adapté aux différents enjeux et situations rencontrées sous terre.

Laterites are deep stratified weathering profiles which form under tropical and subtropical climatic conditions commonly found at the Earth’s continental surface. Although they are very widespread in (sub)tropical areas, many aspects of their genesis and evolution are still not well constrained. Previous studies have shown that they can be very old (Millions of years) and are therefore records of past climate, but little is known about their temporal evolution, notably in relation with climatic and geodynamic changes, due to the small quantity of age data and the difficulty to date this material. The Guiana shield is of particular interest to study laterites as this area remained tectonically stable and in tropical latitudes since the Cretaceous, favoring the formation of very deep lateritic profiles. In this work, two areas were studied: the northeastern rim of the Guiana shield in French Guiana and the eastern rim of the shield in the Brazilian state of Amapá. In both areas, deep (>70 m) lateritic profiles developed on top of Paleoproterozoic basement rocks. The lateritic profiles were studied using mineralogical, geochemical and geochronological tools, notably (U-Th)/He dating and micro X-ray diffraction of supergene hematite and goethite and electron paramagnetic resonance spectroscopy (EPR) of kaolinite. In order to better understand the signification of hematite and goethite (U-Th)/He ages, a theoretical study on the He retention in these supergene minerals has been performed. The results allow to propose correction factors for the (U-Th)/He ages and to better apprehend the signification of these ages.Results from the lateritic-bauxitic duricrust of Kaw mountain in French Guiana indicate that weathering started at least during the Oligocene and was favored during several episodes in the Middle Miocene and the Late Neogene. Coupling of geochronological, geochemical and mineralogical data allows to distinguish between a period of ferruginous lateritic conditions and another one with bauxitic conditions indicating an important change in the weathering conditions. Comparison with other lateritic covers in northeastern French Guiana reveals that these periods also effected other sites in the area and that climate was the main driving force for the Late Neogene bauxitization. In contrast, results from northern Brazil record weathering since the Late Cretaceous and indicate very discrete intervals of weathering at ca. 30 and 12 Ma. The Late Neogene bauxitization event recorded in French Guiana is absent in the studied samples and the therefore very well constrained (U-Th)/He ages, coupled to high resolution micro-XRD analyses, allow to establish a detailed precipitation chronology of the supergene iron (oxyhydr)oxide phases. The results of the EPR analyses of purified kaolinites from two studied profiles indicate that kaolinite in the saprolites of the profiles formed probably several Myr to tens of Myrs ago. At Kaw, in French Guiana, EPR analyses of structural Fe impurities reveal that the kaolinites of the saprolite formed under near identical conditions over 70 m depth. In Amapá, the presence of rather young kaolinite at the top of the studied profile indicates relatively recent rejuvenation of kaolinite at the top but older kaolinite was preserved inside the duricrust as well as in the lower, saprolitic, part of the profile. Finally, this thesis shows that a detailed combination of geochronological results coupled to mineralogical and geochemical analyses improves our understanding of tropical weathering processes and laterite formation by placing mineralogical and geochemical processes into a temporal framework., Les latérites sont d’épais profils d’altération supergène se formant dans des conditions climatiques tropicales et subtropicales communément rencontrées à la surface de la Terre. Bien que très répandues dans les zones (sub)tropicales, beaucoup d’aspects de leur genèse et évolution ne sont pas bien contraints. Des études précédentes ont montré que les latérites peuvent être très anciennes, à l’échelle des millions d’années, et qu’elles constituent donc des archives des climats passés. Néanmoins, la dynamique de l’altération au cours du temps, associée aux forçages climatiques et géodynamiques est encore une question peu explorée, compte-tenu de la difficulté à dater les latérites et les processus de latéritisation. Le bouclier guyanais présente un intérêt particulier pour étudier les latérites car cette région a été tectoniquement stable et à des latitudes tropicales depuis le Crétacé. Des profils latéritiques profonds (>70 m) du bord nord-est (Guyane Française) et est (état brésilien de l’Amapá) du bouclier guyanais ont été étudiés en combinant des approches minéralogiques, géochimiques et géochronologiques, incluant notamment la datation (U-Th)/He sur hématite et goethite, et la spectroscopie de résonance paramagnétique électronique (RPE) sur kaolinite. Afin de mieux comprendre la signification des âges (U-Th)/He sur goethite et hématite, un travail théorique sur la rétention de l’He dans ces minéraux supergènes a été effectué. Les résultats permettent ainsi de proposer des facteurs de corrections des âges (U-Th)/He. En ce qui concerne les oxydes et hydroxydes de fer, les analyses de la cuirasse ferrugineuse-bauxitique de la montagne de Kaw en Guyane Française indiquent que l’altération a débuté au moins à l’Oligocène et qu’elle a été intense au cours de plusieurs épisodes du Miocène moyen et du Néogène supérieur. Le couplage des données géochronologiques, géochimiques et minéralogiques permet de distinguer des périodes de latéritisation ferrugineuse et de latéritisation bauxitique, impliquant un changement important des conditions d’altération. La comparaison avec d’autres couvertures latéritiques du nord-est du bouclier guyanais montre que ces périodes ont aussi affecté d’autres sites dans la région et que le climat était le facteur le plus important pour la bauxitisation du Néogène tardif. Par contre, les cuirasses ferrugineuses latéritiques du nord du Brésil enregistrent une altération tropicale depuis la fin du Crétacé et indiquent deux épisodes discrets à environ 30 et 12 Ma. L’évènement de bauxitisation du Néogène tardif détecté en Guyane Française est absent dans ces échantillons et les âges (U-Th)/He, qui sont très reproductibles, permettent d’établir une chronologie de précipitation des phases d’oxydes et d’oxyhydroxydes de fer supergènes et de discuter de la durée des processus d’altération. Les analyses par RPE ont été effectuées sur les kaolinites purifiées des deux profils étudiés. Une datation préliminaire suggère que les kaolinites des saprolites se sont formées il y a plusieurs millions ou dizaines de millions d’années. À Kaw, l’analyse par RPE du fer en impureté montre que les kaolinites de la saprolite se sont formées dans des conditions identiques sur presque 70 mètres de profondeur. En Amapá, on observe un rajeunissement relativement récent des kaolinites au sommet du profil, tandis que des kaolinites plus vieilles ont été préservées dans la cuirasse et dans la partie plus basse, saprolitique, du profil. L’analyse du fer structural montre des conditions de formation de kaolinite contrastées dans ce profil. Ainsi, cette thèse montre qu’une approche géochronologique couplée aux analyses minéralogiques et géochimiques améliorent notre compréhension des processus d’altération tropicale, de formation et d’évolution des latérites, en les caractérisant et en les plaçant dans un cadre temporel.

Recent evidence has shown that Arctic regions have warmed about twice as much as elsewhere on the planet over the last decades, and that high-latitude periglacial processes and hydrological systems are deeply impacted by rising temperatures. The study presents recent results concerning the fluvial dynamics of a large periglacial river, the Lena River. The Lena river drains a large basin (2.9 million km²) entirely occupied by permafrost (77% with a deep and continuous permafrost). We had previously demonstrated that in the Lena basin, the climate change induces important increase in the river water discharge, that destabilizes the fluvial bed. Here we focus on the fluvial islands dynamics by examining islands with permafrost and islands without permafrost, such comparison being considered as a good indicator of the sensitivity of the hydrosytem to climate change. Island changes are precisely examined: morphological parameters of about 100 islands are surveyed using a GIS on seven series of aerial photographs and satellite images of a 100 km-long reach, for the 1967 – 2017 period. Furthermore, data obtained on several monitored islands allow to analyze the control factors.Field surveys and monitoring of islands shows that within a zone of thick and continuous permafrost, the Lena floodplain is far from being thermally and geomorphologically homogenous: the floodplain rather consists of a juxtaposition of seasonally frozen islands and permanently frozen islands. First, the analysis demonstrates that the two types of islands present different dynamics in terms of erosion and sedimentation. A major change is observed for islands with permafrost at the beginning of the 21st century: they clearly underwent a stronger erosion for the last twenty years. During the same period, numerous small and non-frozen islands have been formed. Second, the morphological parameters are analyzed with respect to factors that play a major part: water discharge (duration of bar-full, bankfull and flood discharges, number and season of flood peaks…) and temperature of the river water and of the island ground. The submersion of islands during flooding do not deeply modify the thermal regime within the island ground. However, the duration of the discharge exceeding the bar-full level can induce a marked erosion of island bank, especially in summer. Thus, our study highlights the complexity of large river responses across Arctic periglacial environments.

During Earth’s history, geosphere-biosphere interactions were often determined by momentary, catastrophic changes such as large explosive volcanic eruptions. The Miocene ignimbrite flare-up in the Pannonian Basin, which is located along a complex convergent plate boundary between Europe and Africa, provides a superb example of this interaction. In North Hungary, the famous Ipolytarnóc Fossil Site, often referred to as “ancient Pompeii”, records a snapshot of rich Early Miocene life buried under thick ignimbrite cover. Here, we use a multi-technique approach to constrain the successive phases of a catastrophic silicic eruption (VEI ≥ 7) dated at 17.2 Ma. An event-scale reconstruction shows that the initial PDC phase was phreatomagmatic, affecting ≥ 1500 km2 and causing the destruction of an interfingering terrestrial–intertidal environment at Ipolytarnóc. This was followed by pumice fall, and finally the emplacement of up to 40 m-thick ignimbrite that completely buried the site. However, unlike the seemingly similar AD 79 Vesuvius eruption that buried Pompeii by hot pyroclastic density currents, the presence of fallen but uncharred tree trunks, branches, and intact leaves in the basal pyroclastic deposits at Ipolytarnóc as well as rock paleomagnetic properties indicate a low-temperature pyroclastic event, that superbly preserved the coastal habitat, including unique fossil tracks.

International audience

International audience; Thick regoliths developed under tropical climate, namely, laterites, resulting from long-term and pronounced geochemical and mineralogical rearrangement of the parent rock in response to environmental changes. Little information is available on the timing of laterite and bauxite formations, especially on the chronology of the main weathering episodes responsible for lateritic cover formation on the Guiana shield. For this purpose, we focused on both lateritic and bauxitic duricrusts developed over the Paleoproterozoic Greenstone Belt in the Brownsberg, Suriname. The duricrust samples have a relatively simple mineralogy (i.e., goethite, gibbsite, hematite, and kaolinite) but reveal, when observed at a microscopic scale, a complex history of formation with multiple episodes of dissolution/reprecipitation. The (U-Th)/He dating of 179 Fe-oxides subsamples shows that duricrusts sampled at the top of the Brownsberg plateau have ages ranging from

Between the Last Glacial Maximum and the mid-Holocene, the Mediterranean Sea experienced major hydrological changes. The deposition of the last sapropel, S1, during the Early Holocene is a consequence of these changes. In order to cause anoxia in the Eastern Mediterranean Sea (EMS) bottom water, a long preconditioning period of a few thousand years would need to occur throughout the deglaciation prior to S1. It is generally believed that this freshwater was of North Atlantic origin, later supplemented by the African Humid period (AHP). Here, we investigate another potentially important source of freshwater to the EMS: the Fennoscandian ice sheet (FIS) meltwater, running into the Caspian and Black Seas. A few scenarios of continental hydrologic perturbation have been developed to drive a high-resolution Mediterranean Sea general circulation model. We demonstrate that, during the last deglaciation, FIS meltwater flowing into the Black Sea reduced surface salinity and ventilation over the main convection areas in the EMS. By including continental hydrological changes, a more consistent framework is produced to characterize the hydrology of the Mediterranean Sea during the last deglaciation and the Early Holocene.

In the Western Alps, the Penninic frontal thrust (PFT) is the main crustal-scale tectonic structure of the belt. This thrust transported the high-pressure metamorphosed internal units over the non-metamorphosed European margin during the Oligocene (34–29 Ma). Following the propagation of the compression toward the European foreland, the PFT was later reactivated as an extensional detachment associated with the development of the High Durance extensional fault system (HDFS). This inversion of tectonic displacement along a major tectonic structure has been widely emphasized as an example of extensional collapse of a thickened collisional orogen. However, the inception age of the extensional inversion remains unconstrained. Here, for the first time, we provide chronological constraints on the extensional motion of an exhumed zone of the PFT by applying U–Pb dating on secondary calcites from a fault zone cataclasite. The calcite cement and veins of the cataclasite formed after the main fault slip event, at 3.6 ± 0.4–3.4 ± 0.6 Ma. Cross-cutting calcite veins featuring the last fault activity are dated at 2.6 ± 0.3–2.3 ± 0.3 Ma. δ13C and δ18O fluid signatures derived from these secondary calcites suggest fluid percolation from deep-seated reservoir at the scale of the Western Alps. Our data provide evidence that the PFT extensional reactivation initiated at least ∼ 3.5 Myr ago with a reactivation phase at ∼ 2.5 Ma. This reactivation may result from the westward propagation of the compressional deformation toward the external Alps, combined with the exhumation of external crystalline massifs. In this context, the exhumation of the dated normal faults is linked to the eastward translation of the HDFS seismogenic zone, in agreement with the present-day seismic activity.

The eastern part of the Souss Basin (Morocco) contains several Mn deposits in the Tasdremt district. ThreeMn orebodies occur withinthe Cenomanian-Turonian dolostones and the Senonian (Coniacian to Maastrichtian) detrital series, the main orebody being located atthe boundary between them. The Mn ores consist of coronadite group minerals, mostly coronadite and hollandite, in a karstifieddolostone. New field observations, petrographic analyses, and geochemical data define the Tasdremt deposits as a karst-hostedaccumulation (11–60 wt.% Mn), particularly enriched in Ba (1.5–8.2 wt.%) and Pb (1.0–5.0 wt.%) with poor contaminations in Al,Fe, and P. This study shows that the ore-forming process is similar to that occurring in the Imini C3 level, located ~ 100 kmto the northeast.Such similarities with the high-grade pyrolusite-bearing ore suggest that the Tasdremt deposit is a lateral equivalent of the Iminideposits. However, the scarcity of pyrolusite in Tasdremt results in lower Mn grades, the Tasdremt ores being considered an aborted/incomplete system in comparison with the Imini deposits. 40Ar/39Ar geochronology of K-bearing Mn oxides yields late Cretaceousages, defining three phases at ~ 91.5 Ma, ~ 77.5–82 Ma, and ~ 65–67 Ma. Although the source of metals remains hypothetical,mineralizing fluids were carried by O2-free groundwater that mixed with O2-rich shallow meteoric waters at the Tasdremt depositionalsite. The dissolution of the host dolostones and the karst environment have provided suitable conditions for the precipitation of Mnoxides, causing the coeval increase of pH and Eh, respectively. The Early Atlasic deformation during the Late Cretaceous is associatedwith mineralization events and was responsible for creation of low stand reliefs from Tasdremt to Imini. This period enabledkarstification and mineralization. Connecting the Tasdremt deposits to other African Mn deposits is difficult since the latter consistof laterite resting above PaleoproterozoicMn protores, and consequently formed under different conditions from karst-hosted deposits.It is likely that other Mn occurrences formed along the Atlas belt in similar settings.

Ocean circulation plays a central role on climate regulation. The paleoceanographic studies of the last decades have allowed to better document the variations in the production of the North Atlantic Deep Water (NADW). However, the role of intermediate water (IW) masses through time remains to be documented and is highly controversial. Indeed, some studies have highlighted the increased contribution of the Antarctic Intermediate Water (AAIW) in all ocean basins during the cold events recorded in the North Atlantic [1] while others suggest their absence [2]. Moreover, during the last deglaciation, the Southern Ocean played a fundamental role in the Carbon transfer from the deep ocean to the atmosphere via the increased upwelling associated to the AAIW production. In order to reconstruct the dynamics of IW masses, to better understand the relationships between variations in ocean circulation in the Atlantic and in the Southern Ocean, and the impact of these changes on the global carbon cycle during Termination I, we use two marine sediment cores from the Porcupine basin MD01-2461 (1153m) and the Iberian margin SU92-28 (997m). We combine the study of benthic foraminifera assemblages sensitive to variations in their environment (nutrient content, oxygen), and different geochemical proxies such as elemental ratios (Mg/Ca, Sr/Ca, Cd/Ca, Ba/Ca, B/Ca, Li/Ca and U/Ca), stable isotopes (δ18O and δ13C) and Neodymium isotopes records (eNd). On core SU92-28, past changes in the benthic foraminiferal content exhibit strong differences in the paleo-environments, with different ecological conditions from the LGM to the Holocene, as well as during the YD and H1 events. These differences are also observed in the δ13C, oxygen concentrations and elemental ratios records obtained from Uvigerina peregrina (or U.mediterranea), Cibicidoides mundulus and Melonis affinis. Changes in the Nd record allow to distinguish changes in the IW mass sources, reflecting the balance between Northern and Southern contributions. Future analysis (e.g., 14C reservoir ages) and the comparison with core MD01-2461 records will help to better constrain the North-South connections in the Atlantic Ocean at IW depths, and their impact on global climate changes.[1] Ma et al. (2019) Geochemistry, Geophysics, Geosystems, 20(3), 1592-1608[2] Gu, S., et al. (2017). Paleoceanography, 32, 1036-1053.

International audience; The dynamics of tropical weathering through time with the formation and long-term evolution of lateriteassociated with climatic and geodynamic forcing is still a poorly explored issue. To better access this, wefocused on lateritic-bauxitic duricrusts from the well-explored, 300 m high Kaw mountain in northeasternFrench Guiana. Macroscopically homogeneous Fe (oxyhydr)oxides-rich subsamples were separated from10 bulk samples. Bulk- and subsamples were investigated by mineralogical and geochemical analyses and(U-Th)/He geochronology. The samples show a large heterogeneity on the macro- and microscopic scalereflecting different stages of duricrust formation and evolution through processes of dissolution and(re)precipitation of Fe (oxyhydr)oxides. The 284 (U-Th)/He ages obtained for goethite -(Fe, Al)OOH andhematite -Fe2O3 range from 30.5  3.1 to

co-auteur étranger; International audience; The youngest volcanism of the Ecuadorian Volcanic Front (Western Cordillera) is mainly dominated by highly explosive events, including the growth and violent destruction of lava domes, and the formation of thick pyroclastic sequences. Deposits associated with such eruptive dynamics have been identified at Iliniza, a compound volcano located in the Western Cordillera with a poorly defined evolutionary history. We present the first K-Ar ages of Iliniza volcano combined with stratigraphic data, numerical reconstructions, and geochemical analyses, providing a new perspective on its evolution. Our results show that Iliniza volcano is much younger than previously proposed. The Iliniza twin-peaked shape is the result of the superposition of two andesitic to dacitic stratovolcanoes. (1) The North Iliniza (NI) edifice was constructed by two lava successions and an intermediate satellite vent, showing a short range of mainly effusive activity between 123 ± 6 and 116 ± 2 ka. (2) The South Iliniza (SI) edifice began its construction through the Lower SI stage when massive lavas dated at ~45 ka formed a basal cone. During the Upper SI stage, the uppermost part of this edifice was destroyed by the highly explosive Jatuncama phase (VEI 5) leaving a 30–40 m-thick ignimbrite sequence. The subsequent extrusion of several dacitic lava domes reconstructed the South Iliniza summit at around 35 ka. The Terminal SI stage corresponds to the emission of several andesitic lavas between 31 ± 4 and 25 ± 3 ka. The Iliniza eruptive activity extended into the Late Pleistocene and Holocene with the extrusion of the Tishigcuchi lava dome, and the emission of the Pongo lava flow dated at 6 ± 4 ka. Based on the proposed eruptive history, we suggest that a revised volcanic hazard assessment of the potentially active Iliniza volcano is required.

International audience; In order to advance methodologies used in the investigation of Hyporheic Zone (HZ) mixing processes, this article combines experimental and modelling tools to follow a tracer injected into the river and infiltrating into the HZ. A highly concentrated sodium chloride solution was injected into the river; (i) the river conductivity, (ii) the riverbed resistivity by Electrical Resistivity Tomography (ERT) and (iii) vertically distributed chloride concentrations within the HZ were monitored. Both ERT and concentration measurements showed an infiltration depth of the tracer of 35 cm, and a partial recovery after injection, which was faster within the superficial layer that was found to be more resistive according to the ERT initial image. The modelling approach used the HydroGeoSphere code to model the coupling between river surface flows and HZ groundwater flows and transport processes. The model set-up involved a 50 cm high existing riverbed step, a vertical contrast in HZ saturated hydraulic conductivity and the aquifer discharge flux. Fitting the vertical chloride profile, the adjusted values were 5 × 10−2 m s−1 for the saturated hydraulic conductivity of the first highly permeable layer below the riverbed, and 4 × 10−6 m s−1 for the aquifer discharge flux. The bottom layer saturated hydraulic conductivity was found to be at least 10 times lower than the value within the first layer. Numerical simulations showed that the two main parameters controlling the mixing within the HZ were the groundwater discharge and the saturated hydraulic conductivity first sediment layer of the riverbed. The riverbed step was found to be less significant here compared to these two parameters. The combination of experimental and modelling tools allowed us to quantify the aquifer discharge flux, which is complicated to investigate in the field without any model. Results of this study showed that combining modelling with ERT and vertically distributed chloride sampling allows the quantification of the main factors controlling the hyporheic exchange.