Observations and Modelling of Coastal Dune Dynamics Along the Gironde Coast, France
Considered as reservoirs of biodiversity, coastal dunes also represent natural barrier against coastal flooding and large source of sediment to mitigate coastal erosion. Dynamics of coastal dunes are forced and controlled by marine, aeolian and biological processes. A better understanding of the interactions between all these processes based on field observations or numerical modelling is crucial to define management strategies that aim to develop the resilience of coastal dune against sea level rise. The analysis of multi-annual topographic data collected along the Gironde coast in SW France show a strong landward migration of the coastal dunes caused by strong wind events and a decrease in vegetation cover. The same data were also used to calibrate and validate a numerical model, AeoLiS, that simulated Aeolian sediment transport. This model showed good performance to reproduce the landward migration of non-vegetated dune.
(. vol. 41, pp. 141-146, 05/03/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BRGM
Holocene climatic changes in the Kerguelen archipelago (South Indian Ocean) based on marine and lacustrine palaeoclimatic archives
Climatic variability in the Southern Hemisphere is largely controlled by the latitudinal position of the Southern Hemisphere Westerly Winds (SHW), whose migration influences precipitation, temperature, and Antarctic upwelling. This study presents the results of analyses of two lacustrine sediment cores from Lake Armor, located on the subantarctic Kerguelen Islands (49 • 15′S, 69 • 10′E), within the SHW belt. Lipid biomarkers (Glycerol Dialkyl Glycerol Tetraethers, n-alkanes, and their hydrogen isotopes) were used to reconstruct mean annual air temperature above freezing (MAF) and humidity conditions. These records are compared with a high-resolution diatom-based summer sea surface temperature (SST) reconstruction from marine core MD11-3353, situated 150 km southwest of Lake Armor. In the late glacial and Early Holocene, our results reveal a period of warm air temperature, comparable to current values and very warm sea surface temperature, 5°C above the current values. Around 9000 cal a BP, an abrupt transition occurred, marked by a cooling of 5°C in SST and 1.5°C in MAF, interpreted as a northward migration of the SHW and associated oceanic fronts. The Mid-to-Late Holocene period is characterized by pronounced MAF variability, including a notably warm interval between 3000 and 2000 cal a BP, when n-alkane dD suggests the prevalence of wetter conditions. Since ~250 cal a BP, a southward migration of the SHW has produced a 2.5°C rise in MAF. Our findings are overall consistent with previous studies from the Indian Ocean, but permit us to go a step further as by comparing SSTs and air temperatures. This suggests that SST is not a reliable predictor of air temperature on the Kerguelen Islands, particularly during the Early Holocene. We hence argue that Kerguelen air temperature is predominantly controlled by the position of westerly winds, as an indicator of reorganisations in air mass trajectories.
(Quaternary Science Reviews. vol. 375, n° 0277-3791, pp. 109753, 01/03/2026)
EDYTEM, USMB [Université de Savoie] [Université de Chambéry], CNRS, Fédération OSUG, LGL-TPE, ENS de Lyon, UCBL, INSU - CNRS, UJM, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, MIO, IRD, AMU, INSU - CNRS, UTLN, CNRS, LOCEAN-VOG, LOCEAN, MNHN, IRD, INSU - CNRS, SU, CNRS, IPSL (FR_636), ENS-PSL, UVSQ, CEA, INSU - CNRS, X, CNES, SU, CNRS, UPCité, LOCEAN-VALCO, LOCEAN, MNHN, IRD, INSU - CNRS, SU, CNRS, IPSL (FR_636), ENS-PSL, UVSQ, CEA, INSU - CNRS, X, CNES, SU, CNRS, UPCité, UiB, IP, INSU - CNRS, CNRS, JAMSTEC, INEE-CNRS, CNRS, INSU - CNRS
Origin and evolution of giant comet marks along the North Atlantic Deep-Water flow on the Demerara plateau
The Demerara Plateau, located in the equatorial Atlantic, is particularly well-suited for recording the activity of the Deep Western Boundary Current (DWBC), which transports North Atlantic Deep Water (NADW) southward into the Atlantic basin. This current, active between 1500 and 3500 m depth, constitutes the deep part of the global thermohaline circulation and plays a crucial role in climate regulation. The Demerara Plateau is remarkable for the abundance and wide distribution of comet mark-type sedimentary structures, which can reach several kilometres in length. These hydrodynamic bedforms, interpreted as erosional features associated with strong bottom currents, are currently used as proxies for deep currents velocities, with minimum formation thresholds estimated between 0.60 and 0.75 m/s according to the literature (Rebesco et al., 2014; Werner et al., 1980). The DIADEM (Dive At DEMerara) oceanographic cruise (Basile and Loncke, 2023) enabled detailed investigation of one such structure using a combination of complementary tools with the aim of better understanding their functioning and evolution with in-situ observation, physical records, and sampling. Those new data were also combined with formerly acquired high-resolution seismic data, allowing a new vision of these bedforms their formation, and their evolution over time. The main findings are the following: (1) comet-marks localize on carbonate mass transported blocs outcropping on the seafloor; (2) Clearly show a polyphase evolution of the bedforms of comet mark presenting alternations of erosion and sedimentation phases. The bedforms therefore record long-term variations in bottom current activity with an alternation of intense hydrodynamic events (erosion) and quieter hydrodynamic periods (deposition) through time, reflecting a complex hydrodynamic history; (3) At present, comet-marks recorded a significant decrease in current velocity within the erosional zones located inside the comet tails associated with fined-grained sediments infill in this area, highlighting the need for cautious interpretation of such features as direct indicators of present-day current intensity; (4) Finally, a result that was not necessarily anticipated but documented by exploring those bedforms with the Nautile submersible, comet-marks host quite important benthic and epibenthic biodiversity with a wide variety of associated species.
(Marine Geology. vol. 493, n° 0025-3227, pp. 107717, 01/03/2026)
CEFREM, UPVD, INSU - CNRS, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LOCEAN-CYBIOM, LOCEAN, MNHN, IRD, INSU - CNRS, SU, CNRS, IPSL (FR_636), ENS-PSL, UVSQ, CEA, INSU - CNRS, X, CNES, SU, CNRS, UPCité, LOCEAN-VALCO, LOCEAN, MNHN, IRD, INSU - CNRS, SU, CNRS, IPSL (FR_636), ENS-PSL, UVSQ, CEA, INSU - CNRS, X, CNES, SU, CNRS, UPCité, LOPS, IRD, IFREMER, INSU - CNRS, UBO EPE, CNRS, GEO-OCEAN, UBS, IFREMER, INSU - CNRS, UBO EPE, CNRS, ENTROPIE [Réunion], IRD, UR, CNRS, ISTerre, IRD, INSU - CNRS, USMB [Université de Savoie] [Université de Chambéry], CNRS, Fédération OSUG, UGA
Multi-annual evolution of coastal dunes: Transition from fixed to transgressive dunes state
Most of coastal dunes located in temperate latitudes, especially in the Northern Hemisphere, are relatively stable. However, along the Gironde coast (SW France) substantial dune remobilization has been observed over the last decade following major marine erosion events during the 2013-2014 winter. This study is based on the analysis of a robust dataset including (i) 10 high-resolution Digital Terrain Models (DTMs) derived from airborne LiDAR surveys conducted over a 12-year period (2011-2023) and (ii) 7 Satellite-derived Digital maps of dune vegetation cover derived from Sentinel-2 satellite images acquired between 2017 and 2023. These morphological and biological parameters are linked to forcing parameters derived from observed wind data, to provide a comprehensive analysis of coastal dune changes related to the transition from vegetation-fixed dunes to the development of transgressive dunes. For the first time, morphological and vegetation dynamics are explored over a large spatial scale (tens of km), covering a range of initial dune morphology and sediment supply.
Dunes have transitioned from stable to transgressive states primarily driven by sediment stoss slope recycling process (cannibalism) across a gradient of alongshore variable dune sediment budget, ranging from slightly negative to notably positive (+10 to 15 m 3 /m/yr), Along this coast, transgressive dunes defined as dune migrating via similar stoss and lee slope migration rates, have tripled in number over the last 10 years (reaching ≈ 15 km or 17.3 % of the studied coast). At the center of the Gironde coast where dunes are heavily remobilised, the lee slope of the dune translates landward at a rate of several meters to more than 10 m/year. In the following years, dunes will probably continue to migrate and remobilise across a broader scale if no re-stabilization management plan is implemented.
(CATENA. vol. 264, n° 0341-8162, pp. 109787, 01/03/2026)
BRGM, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CEFREM, UPVD, INSU - CNRS, CNRS, ONF
Drastic changes in tidal hydrodynamics following seagrass decline and their seasonal variations in a shallow lagoon
Over the past decades, seagrasses have drastically declined worldwide, reducing their capacity to regulate flow conditions. Intertidal species have been particularly affected by this decline, yet there is limited understanding of how intertidal seagrass loss influences hydrodynamics in shallow coastal lagoons. In this study, we use a 3D flow-vegetation model that accounts for vegetation effect on mean and turbulent flow, as well as flow-induced leaf bending, to investigate how tidal hydrodynamics respond to seasonal and multi-decadal changes in intertidal seagrass characteristics. The model is applied to the Arcachon lagoon (France), colonized by extensive Zostera noltei and Zostera marina meadows. This study reveals that a short-leaf and flexible seagrass species such as Zostera noltei can regulate tidal hydrodynamics throughout the lagoon due to the presence of broad and dense meadows on the tidal flats. In summer, seagrass decline leads to a significant increase in the 75th percentile in bottom flow velocities (+100 %) on the tidal flats, but to a decrease in the channels (−20 %). However, in winter, the response of tidal hydrodynamics to the reduction in seagrass coverage is far less pronounced. Comparison of simulated scenarios reveals that the multi-decadal decline of Zostera meadows with summer characteristics and the seasonal loss between summer and winter lead to modifications in tidal-flow parameters (current velocities, tidal asymmetry, high-tide water level) of a comparable magnitude. These changes in hydrodynamics likely enhance suspended sediment concentration, reducing light availability, contributing to further seagrass loss, and modifying sediment management for stakeholders due to enhanced siltation in channels.
(Coastal Engineering. vol. 205, n° 0378-3839, pp. 104948 (34p.), 01/03/2026)
LERAR, COAST, IFREMER, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Seasonal dynamics of cable bacteria in an estuarine intertidal mudflat subject to green tides: Implications for the foraminiferal community and test preservation
As many coastal areas draining intensive agricultural activities, Ledano estuary mudflats (French Brittany) experience Ulva proliferation, causing green tides. We studied the seasonal dynamics of sulphoxidizing cable bacteria from April 2019 to July 2020 using microsensors (O 2 , pH, H 2 S). The activity of these filamentous bacteria, called electrogenic sulphur oxidation (eSOx), results in strong acidification and in pore-water CO 3 2depletion in the first few centimeters of sediment (within the suboxic zone). Living and dead benthic foraminiferal assemblages were studied in July 2020 to observe the effects of eSOx on the calcareous meiofauna and their shell preservation in the sediment. eSOx was patchy on the mudflat but persistent throughout the year. It contributed up to 45% of oxygen consumption during the algal mat decay, and exceeded 100% during the flooding period suggesting stimulation by nitrate inputs. The corrosive effect was maximal in July 2020 (ΔpH ~ 1.7, [CO$_3^{2-}$]calc < 10 μM). The living foraminiferal community was sparse and nearly monospecific, dominated by the calcareous species Haynesina germanica, probably due to green tides coupled with eSOx-driven acidification. However, living specimens of H. germanica showed no signs of advanced dissolution of their shell suggesting a biological capacity to survive in such environmental acidification, potentially linked to their photosynthetic capability. In contrast, the dead assemblages displayed greater diversity despite a loss of about 20% of the calcareous shell recording with depth due to the synergetic effect of low salinity and eSOx. Overall, this study shows that green tides strongly influence cable bacteria activity and then, sedimentary biogeochemical processes in eutrophic coastal environments.
(Journal of Sea Research (JSR). vol. 210, n° 1385-1101, pp. 102683, 01/03/2026)
LPG, UM, UA, INSU - CNRS, CNRS, Nantes univ - UFR ST, Nantes Univ, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
A new open boundary condition for Boussinesq-type models, applied to irregular wave fields
We present a novel approach to handle open boundary conditions for a Boussinesq-type wave model coupled with the nonlinear shallow water equations. Traditional methods for managing open boundaries — such as sponge layers and source functions — are computationally intensive and require ad hoc calibration. To address this, we reformulate the Boussinesq equations as a system of conservation laws with nonlocal flux and a rapidly decaying source term. This reformulation is adapted to generate waves at the boundary of the numerical domain, from surface elevation data in situations where both incoming and outgoing waves are present. The proposed numerical scheme employs a MacCormack prediction-correction strategy combined with finite volume and finite difference methods, preserving key physical properties and ensuring stability. Comparison with laboratory experiments demonstrates that our approach avoids boundary reflection issues. In particular, it is able to accurately reproduce infragravity waves associated with a random wave field propagating over a sloping beach. This work opens important perspectives for improving phase-resolving coastal wave models, with the aim of forecasting complex random wave conditions in natural environments.
(Ocean Modelling. vol. 201, n° 1463-5003, pp. 102713, 21/02/2026)
INSA Toulouse, INSA, Comue de Toulouse, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, IMB, UB, Bordeaux INP, CNRS
Multi-centennial internal variability in the North Atlantic could drive additional warming over Europe
Europe has experienced abnormal warming over the recent decades. Model-based studies highlight that multi-centennial internal variability of the North Atlantic can strongly affect European temperatures. However, the limited availability of high-resolution proxy records has hindered observational assessment of the existence and amplitude of such variability in the real climate system. Here, we compile annual-to-decadal proxy-based Holocene reconstructions, instrumental observations, and climate model simulations to demonstrate the existence of this multi-centennial variability mode and quantify its amplitude. We show that this mode is closely tied to the internal variability of the Atlantic Meridional Overturning Circulation. Its temporal evolution explains part of the observed 20th century variability, and a shift towards a positive phase in the late 1990s can explain the recent amplified warming over Europe. When its amplitude is constrained by observations, this internal variability may enhance anthropogenic warming in Northern Europe by up to 30% over 2000-2035.
(Nature Communications, n° 2041-1723, 11/02/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, VUB, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, MERMAID, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, RHUL
Microplastic transport and trapping in a highly turbid, tide-dominated estuary
Microplastics are an emerging pollutant in aquatic systems, with estuaries acting as key zones for their retention and transformation. However, limited field observations, complex estuarine hydrodynamics and diverse particle properties hinder a comprehensive understanding of microplastic transport and fate, limiting accurate risk assessment and evaluation of environmental impacts. The objective of this work is to better understand the physical processes governing the transport and trapping of microplastics in macrotidal turbid estuaries, using the Gironde estuary (SW France) as a case study. The methodology of this work is mainly based on a hydro-sedimentary numerical model coupled with a Lagrangian particle tracking model. This approach is complemented with in-situ observation data. A comprehensive review of process-based modelling approaches used to study microplastic dynamics in estuaries was first conducted to assess various parameterization strategies, identify key challenges, and offer recommendations and future directions to advance microplastic modelling strategies in estuaries. Building on insights from this review, the relative influence of estuarine physical processes on microplastic transport was examined through sensitivity scenarios using different release configurations. The results identify the shoreline interaction by beaching– refloating dynamics as a key process for buoyant particles, while resuspension and vertical mixing modulate the transport and vertical distribution of non-buoyant particles. Microplastic-sediment interactions, such as flocculation and temporary trapping in bottom sediments, play an important role in enhancing particle retention within the estuary. Model results also show that hydrodynamic processes alone can significantly trap microplastics, with seasonal variability modulating the intensity and location of trapping. Elevated river discharge during the spring season enhances seaward transport, particularly for buoyant particles, whereas in summer, microplastics are more likely to be retained, with denser particles accumulating near the estuarine turbidity maxima (ETM) in the tidal rivers. This accumulation forms a water-column estuarine microplastic maximum (EMPM), sustained by net upstream transport driven by tidal pumping. In-situ observations in the water column support these findings, confirming the presence of strong near-bottom microplastic concentrations in summer in the Garonne tidal river, particularly during strong flood and ebb current velocities, with a dominance of high-density fibrous particles. Model simulations also indicate that floating particles are consistently trapped along a frontal line near the main channel, generating a surface EMPM. In the upper estuary, this line of particle accumulation follows the primary convergence zone produced by the combined effects of tidal currents and estuarine bathymetry. However, in the middle estuary, the accumulation line shifts to a secondary convergence zone due to the combined effect of morphological features and the alternance between convergence and divergence patterns over the tidal cycle. Sensitivity tests confirm that baroclinic effects play a significant role in shaping frontal convergence, with sediment-induced water density modulating its strength. Overall, the results highlight that tide-dominated, highly turbid estuaries act as efficient microplastic retention zones due to the combined influence of tidal hydrodynamics, sediment-microplastic interactions, and morphological features.
(03/02/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Storm Wind Flow and Sediment Dynamics on Adjacent Coastal Dunes With Contrasting Morphologies During One Storm Event
This study presents the first high‐resolution spatial and temporal analysis of wind flow, sediment transport and topographic evolution under simultaneous storm conditions across two morphologically contrasting beach‐dune systems, characterized by a gently sloping dune face (11°) and a steep, scarped dune face (36°). Results demonstrate that the dune slope strongly controls near‐surface wind acceleration, the development of secondary airflow structures (amplitude, spatial positions), and the continuity of sediment transport pathways. Over the gentle slope, airflow accelerates progressively up the stoss face, promoting sustained, landward‐directed sediment fluxes across the entire beach–dune system and enabling efficient sediment recycling. In this configuration, beach‐derived contributions account for only 12%–15% of the total sediment flux. In contrast, the steep scarp induces flow deceleration and separation at the dune toe, limiting sediment transfer from the beach and favoring seaward‐directed transport associated with secondary vortices at the crest. These contrasting airflow organizations result in fundamentally different storm responses. The gently sloping dune undergoes landward translation with minimal net volume change, whereas the scarped dune experiences dominant marine erosion, leading to a 4 m retreat of the dune front and a sediment loss of ∼30 m 3 m −1 . A new conceptual model of storm‐driven airflow over contrasting dune morphologies is proposed, illustrating how inherited dune slope governs airflow structure and circulation patterns. Overall, these results identify inherited dune morphology as a primary control on airflow organization, sediment pathways, and dune resilience during extreme events.
(Journal of Geophysical Research: Earth Surface. vol. 131, n° 2169-9003, 02/02/2026)
CEFREM, UPVD, INSU - CNRS, CNRS, BRGM, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, ONF