Polar Front System Variability and Its Control on Export Rain Ratio Over the Past 800 ka: Implication for Atmospheric p CO 2 Changes
Little is known about long-term variability in the organic carbon to carbonate carbon export rain ratio, despite its key role in the global carbon cycle. Here, we reconstruct glacial-interglacial changes in the sedimentary rain ratio over the Southern Ocean, using micropaleontological (coccoliths and foraminifera) and geochemical (CaCO 3 , Total Organic Carbon (TOC), δ 13 C, C/N) records from sediment core MD04-2718, located in the Polar Front Zone (PFZ, Indian sector), complemented with published data sets from across the Subantarctic Zone (SAZ). We show that sedimentary CaCO 3 primarily reflects the export of biogenic calcium carbonate by calcifying phytoplankton and zooplankton, while TOC captures the export of phytoplanktonderived organic carbon. The sedimentary TOC/CaCO 3 ratio thus serves as a robust proxy for past variations in the balance between organic and inorganic carbon export and hence, of the export rain ratio. Our results indicate higher rain ratios during glacial periods, driven by enhanced organic carbon export, as colder conditions and intensified iron-rich dust inputs stimulated diatom productivity. In contrast, lower rain ratios during interglacials reflect strengthened biogenic carbonate export, as warmer conditions and elevated macronutrient supply from reinvigorated Southern Ocean upwelling supported coccolithophore and foraminifera blooms. These shifts reflect an ecological seesaw between silicifying and calcifying phytoplankton modulated by changes in westerly wind intensity and the position of the Polar Front. The negative correlation between rain ratios from the SAZ-PFZ and atmospheric pCO 2 suggests that enhanced organic carbon export during glacials likely promoted deepocean carbon sequestration, modulating atmospheric CO 2 levels.
Plain Language Summary Marine phytoplankton uses sunlight to convert atmospheric carbon dioxide (CO 2 ) into organic matter, which helps to store carbon in the ocean. However, certain plankton species produce calcium carbonate shells, a process that reduces this carbon storage by increasing CO 2 levels in surface waters. The balance between organic carbon and carbonate export-known as the "rain ratio"-is an important indicator of how marine life influences atmospheric CO 2 . In this study, we reconstructed changes in the rain ratio over the past 800,000 yrs from sediment samples collected in the Southern Ocean. We found that during cold glacial periods, a higher rain ratio corresponds to increased productivity of plankton with silica-based shells. During warmer interglacial periods, a lower rain ratio reflects dominance by plankton with calcium carbonate shells. These shifts between plankton communities are driven by changes in ocean conditions-cold, iron-rich waters favor silica-forming plankton, while warmer, nutrient-rich waters promote calcifiers. These environmental changes are linked to the strength of westerly winds and movements of ocean fronts. Our findings suggest that biological productivity in the Southern Ocean has played an important role in regulating Earth's carbon cycle and atmospheric CO 2 levels over long climate cycles.
(Paleoceanography and Paleoclimatology. vol. 41, n° 2572-4525, 01/03/2026)
GEOPS, INSU - CNRS, CNRS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, NIOZ, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, EPHE, PSL, UB, CR2P, MNHN, SU, CNRS, ESE, CNRS, UNIBE, UNIL
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
Metal pollution and floral abundance influence the diversity of flower visiting insects
Soil pollution with metals is a worldwide problem with negative effects on ecosystem health. Metals are persistent and can accumulate in the food web, with potential toxic effects for most organisms. Some metal-tolerant plants can accumulate metals in their aboveground parts, including the pollen and nectar of their flowers. Therefore, insects visiting these flowers are exposed to these toxic elements. They can also be influenced by the plant species turnover observed along pollution gradients. Surprisingly, the impact of soil metal contamination on pollinating insect diversity has rarely been studied. To fill in this gap, we carried out a study in a former mining valley in the French Pyrenees with metal contamination by Zn, Pb and Cd. We estimated the richness and community composition of insects visiting the flowers of metal-tolerant and metal-intolerant plant species in 96 plots along metal pollution gradients. Insect richness did not change along the pollution gradient and was similar between metal-tolerant and metal-intolerant plant species. On the contrary, insect community composition changed along the gradient and differed between plant types. We also found that insect species richness increased with the flower abundance of the focal plant. This study showed that flowers of metal-tolerant plants, including hyper-accumulator species, are visited by several insect species and harbour a distinct pollinator community. The ingestion of metal-rich nectar or pollen may have lethal or sub-lethal effects for these flower-visiting insects. Further experimental studies are needed to determine if metal-rich floral resources act as an ecological trap for some of these species.
(Arthropod-Plant Interactions. vol. 20, n° 1872-8855, pp. 16, 26/02/2026)
BioGeCo, UB, INRAE, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
On the relations between morphotectonics, seismicity, perennial and sporadic hydrothermalism on the volcanic island complex of Milos (Greece)
The volcanic island complex of Milos, situated ∼200 km north of the Hellenic subduction zone, records the interplay of volcanic, tectonic, and gravitational processes within multidirectional, polyphased rift systems that developed throughout the Mio-Plio-Quaternary. Its active and seismogenic fault network hosts both perennial and transient hydrothermal venting zones, offering a natural laboratory to examine fluid circulation within a volcanic arc built on stretched continental crust. This study refines the relationships between magmatism, seismicity, and hydrothermal activity, with implications for local geological hazards. Two new morpho-tectonic sketches are presented. The first places the Milos volcanic center within the structural framework of the Aegean microplate; the second illustrates the spatial organization of the Milos tectono-hydrothermal-volcanic system. These reconstructions integrate detailed physiographic analyses (relief, slope), field observations, and multidisciplinary datasets on seismicity, hydrothermalism, and mineralization. At the scale of the Aegean microplate, (i) NE-SW faults are attributed to extensional deformation along the Mid-Cycladic Lineament; (ii) E-W faults relate to the opening of the Milos, Cretan, and Christiana basins, parallel to the mid-Miocene West Cycladic Detachments; (iii) NW-SE faults correspond to the Myrtoon Basin and Gulf of Milos openings, the latter being closed to the south by the Fyriplaka volcano; and (iv) N-S faults controlling the Zephyria graben and the eastern margin of Milos are continuous with Cretan fault systems and are parallel both to the Eocene trans-Cycladic thrust in the region and to magnetic anomalies in the subducted Tethyan oceanic crust south of the Hellenic subduction zone. At the archipelago scale, seismicity, hydrothermal venting, alteration zones, and phreatic explosions are concentrated in the hanging wall of the Achivadolimni fault, along or near the intersections of the N-S Zephyria and NW-SE Fyriplaka grabens, directly above the inferred magma chamber. Microearthquake hypocentres, likely linked to fluid-induced fault dilation, occur beneath the Gulf of Milos (∼7 km) and Fyriplaka volcano (∼5 km). Their distribution suggests a genetic link between the intersecting grabens, cooling of isotherms beneath the gulf by descending seawater, surface hydrothermal manifestations, and tectonic earthquakes (e.g., Mw 5.3 Milos, 1992). A shallow hydrothermal convection loop (<3 km) probably overlies a magmatic reservoir where fluids accumulate beneath a self-sealed, low-permeability cap, possibly corresponding to a thermal brittle-ductile transition (370-450 °C). Co-seismic ruptures may locally breach this barrier, inducing "arterial fault" behaviour along the Achivadolimni fault. Pulsating hydrothermal activity and historical phreatic eruptions are interpreted as surface expressions of transient injections of over-pressurized magmatic fluids, leading to decompression, phase transitions, and fluid
(Bulletin de la Société Géologique de France. vol. 197, n° 0037-9409, 22/02/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LGL-TPE, ENS de Lyon, UCBL, INSU - CNRS, UJM, UJM EPE, CNRS, LGENS, INSU - CNRS, CNRS, ENS-PSL, PSL, I2M-BX, UB, CNRS, INRAE, NKUA
The Dual Climate Role of Seagrass Meadows in Arcachon Bay
Vegetated coastal ecosystems such as seagrass meadows are increasingly recognized as key contributors to climate regulation. Acting as blue carbon sinks, they exhibit carbon burial rates up to 30–50 times higher than those of terrestrial forest soils, thereby storing large amounts of organic carbon in their sediments. However, these benefits may be partly offset by the release of greenhouse gases (GHG), particularly methane (CH₄) and nitrous oxide (N₂O), which possess a much higher global warming potential than CO₂. Comparing seagrass meadows to bare sediments helps identify their specific role as blue carbon ecosystems. In this study, we assessed carbon accumulation rates (CAR) together with in situ fluxes of CH₄ and N₂O in Zostera noltii seagrass meadows and adjacent bare sediments within Arcachon Bay, France, which hosts Europe’s largest Z. noltii meadows. Our results demonstrate that seagrass areas exhibit CAR values that are 2.5 times higher, highlighting their superior carbon burial capacity. However, GHG emissions from seagrass meadows were 3.5 times higher than those from bare sediments, offsetting 28.7 ± 0.3% of the estimated net climate benefit provided by carbon burial across the bay. Hydrodynamic conditions influence this balance: sheltered areas promote organic matter accumulation but also amplify GHG emissions. Despite these trade-offs, Z. noltii meadows still provide a net climate benefit 2.2 times greater than bare sediments, emphasizing their significant contribution to climate regulation. These findings highlight the dual nature of seagrass meadows as both carbon sinks and GHG sources. Incorporating GHG flux estimates into blue carbon assessments will improve global model predictions and guide better conservation and restoration strategies.
(Modeling Earth Systems and Environment, n° 2363-6203, 21/02/2026)
LEM, UCBL, ENVL, VAS, CNRS, INRAE, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UMR Eco&Sols, Cirad, IRD, INRAE, Institut Agro, UNIMI
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