The marine biogeochemical cycling of trace elements (TEs) at ocean margins is still poorly understood. Unravelling TE sources and exploring their exchange between the coastal environment and the open ocean remain poorly explored. Here, we examined the dissolved Co, Ni, Cu, Zn, and Pb concentration distributions in the waters of the northern part of the Western Mediterranean (WMED), namely the continental shelf of the Gulf of Lion (GoL) and the North Gyre (NG). In GoL waters, median values (with interquartile ranges, IQR) were 135 pM (110-463 pM) for Co, 3.94 nM (3.73-4.52 nM) for Ni, 2.70 nM (2.31-3.73 nM) for Cu, 4.73 nM (3.60-6.44 nM) for Zn, and 117 pM (102-161 pM) for Pb. In NG waters, they were 37 pM (35-46 pM) for Co, 4.60 nM (4.52-4.65 nM) for Ni, 1.79 nM (1.77-1.85 nM) for Cu, 5.43 nM (5.05-5.95 nM) for Zn, and 94 pM (92-98 pM) for Pb. Our results show that in the GoL, Co, Ni, Cu, and, to a lesser extent, Zn, are negatively correlated with salinity, indicating significant riverine inputs. In the NG, Ni presents a nutrient-like behavior, whereas other TEs are still enriched in upper waters. Zn and Pb distributions suggest the influence of atmospheric deposition with substantial human imprints. The estimated TE transfers in this system disclose the importance of dense water formation in the export of TEs from the continent to the offshore deep waters of the WMED.

Dense shelf water cascading (DSWC) in the northwestern Mediterranean Sea occurs due to the densification and overflow of shelf water in winter. This process ventilates the deep basin with waters that can be denser than those generated by open-ocean convection. Between 1987 and 2021 (i.e., 35 winter periods), shelf water reached depths greater than 1000 m on only 9 occasions, and open questions remain regarding the triggering factors and inhibitors for these deep DSWC events. Combining atmospheric and ocean reanalysis data, we found that, at interannual scales, the East Atlantic (EA) mode of climate variability is more strongly associated with the thermal components of the atmospheric forcing: air temperature, wind, and sea surface temperature, contributing to a strong connection between the negative phase of EA, and an increased heat loss and buoyancy loss. At shorter synoptic to subseasonal time scales, the dense shelf water formation in winter is linked to the local northerly winds and cold air outbursts. As important as the thermal atmospheric forcing, the evaporation-precipitation balance and river runoff play a key role in facilitating or preventing dense shelf water formation and cascading. Besides, the temperature of surface shelf water at the beginning of winter can be critical. In the years after an intense DSWC event, the favorable preconditioning by advected dense water and reduced intermediate stratification enhances deep cascading. These insights are meaningful for how the atmospheric and oceanic signals from a changing climate propagate into the deep Mediterranean Sea, which ultimately could affect the thermohaline circulation.

Coccolithophores are important components of marine phytoplankton and are found to be useful indicators of the environmental conditions of the upper water column. In this study, we investigate coccolithophore abundance and composition in the Cretan Sea and South Cretan area (Eastern Mediterranean), and their relation to prevailing hydrodynamic conditions during late February/early March 2019. Results showed that total coccolithophore abundance ranged from 26.3 & times; 102 to 258.8 & times; 102 coccospheres L-1, averaging at 135.8 & times; 102 coccospheres L-1. Among the 45 identified species, the opportunistic Emiliania huxleyi was the most dominant, representing 89% of the coccolithophore assemblage. In the Cretan Sea, this species showed relatively homogeneous abundances throughout the upper 100 m depth of the water column; however, towards the Rhodes Cyclone, where a weak stratification had started, and the mixed layer was relatively shallow, higher abundances were found at depths shallower than 50 m. Syracosphaera molischii co-occurred with Emiliania huxleyi, whereas Rhabdosphaera clavigera, Syracosphaera pulchra, and Syracosphaera mediterranea were also present but in lower abundances, reflecting the influence of warm, salty Levantine Surface Water. Based on the morphological analysis, Emiliania huxleyi was mostly represented by heavily calcified forms consistent with winter-spring patterns in the Aegean Sea. The observation of signs of dissolution with high relative abundances of etched/corroded coccospheres indicates the sensitivity of Emiliania huxleyi to the prevailing circulation pattern during the 2019 mixing event within the Rhodes gyre.

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.

This study examines conductivity-temperature-depth (CTD), acoustic Doppler current profiler (ADCP) and optical data collected during the PERLE-3 cruise in March 2020 between the surface and 2000 m depth over an east-west section of the Tyrrhenian Sea in the Mediterranean. The focus is on the impact of double-diffusion processes, in particular salt fingering, on the distribution and dynamics of particulate and dissolved matter. Staircases form at the interface between the warm, saline Eastern Intermediate Water (EIW) and the colder, less saline Tyrrhenian Deep Water (TDW) in the centre of the basin in a region of weak hydrodynamic activity. The results show that thermohaline staircases formed by salt fingering significantly influence particle sedimentation and biogeochemical cycling in deep ocean environments by altering vertical flux patterns. These density steps create distinct vertical layers that act as physical barriers, slowing the descent of particles and facilitating their retention and aggregation. The retention of fine particles at density gradients promotes the formation of larger aggregates, thereby affecting the particle size distribution. The staircases also affect dissolved matter by creating pronounced concentration gradients of oxygen and nutrients, which may influence microbial activity and nutrient fluxes.

Shifts in the phytoplankton assemblage induced by environmental changes have significant implications for carbon cycling and marine food webs, but remain poorly constrained across spatiotemporal scales. Here, we investigate the effects of rising sea surface temperatures and increased stratification on the phytoplankton composition and size in the northwestern Mediterranean Sea (2010-2019) using two sediment trap series: one in the oligotrophic Ligurian Sea and the other in the deep convection zone of the Gulf of Lion. We apply deep learning image analysis to quantify phytoplankton particle fluxes, size distributions, and relative assemblages, focusing on coccolithophores, diatoms, and silicoflagellates. Our results show a general decline of phytoplankton fluxes to the seafloor, mirroring the decrease in vertical mixing in the water column. Both sites show a shift toward phytoplankton species adapted to stratified and nutrient-depleted conditions, although with contrasting patterns. In the Ligurian Sea, deep-dwelling coccolithophore species become dominant, while in the Gulf of Lion, summer-associated siliceous species, including large diatoms and silicoflagellates, show an increase. These contrasted trends, which likely result from differences in nutrient inputs and pH changes in the surface between the two sites, have implications for the efficiency of carbon export pathways at depth. Specifically, the increasing dominance of smaller phytoplankton in the Ligurian Sea leads to a reduction in carbon burial efficiency, while in the Gulf of Lion, the enhanced contribution of larger diatoms may sustain higher export and burial rates in the future.

Regional studies on the origin and fate of organic carbon (OC) in marine sediments are scarce due to limited spatial data coverage and the complex interplay among biological, physicochemical, and geological processes that can influence OC content and geochemical signatures on different spatial scales. Yet, such studies are vital to constrain global carbon inventories for ocean sediments. To shed light on the controls on the origin, distribution, and fate of sedimentary OC in continental margins and adjacent deep-sea basins, we investigate the geochemical and sedimentological characteristics of organic matter (OM) in the semi-enclosed Western Mediterranean Sea. Here, we analyze 149 core-top samples from the Western Mediterranean Sea and the adjacent Atlantic Ocean sector and explore the spatial distribution of OC content, OC-ẟ13C, OC-Δ14C, C/N, grain size, and mineral surface area, among others. Most geochemical parameters depict a clear SW-NE gradient between the westernmost and the easternmost basins. This gradient reverses in the Gulf of Lions (NW Mediterranean). Thus, OC is younger and of primarily marine origin in samples from the Atlantic sector and the Alboran Sea (SW Mediterranean). In the Algerian Basin, the Balearic Sea, and the Algero-Provencal Basin the influence of terrestrial OC input increases towards the NE characterized by the presence of highly 13C- and 14C-depleted (aged) sedimentary OC. Finally, samples from the Gulf of Lions show a larger influence of fresh and young OC compared to other northeastern basins. The interplay between marine primary productivity and delivery of terrestrial OC is the main factor that determines the observed gradient. Primary productivity decreases from the southwestern basins towards the NE and increases again northeasternmost basin, the Gulf of Lions. By contrast, the terrestrial OC carbon delivered by rivers and channeled to the deeper basin by canyons has an increasing influence on sedimentary OC toward the NE. When explored from a sedimentological context, our results reveal that lateral transport of OC and OM protection by mineral surfaces potentially act as secondary controls on the OC fate in surface sediments of the Western Mediterranean Sea. This integrated study contributes to a better knowledge of the interplay of biological, chemical, and hydrological factors that influence the amount and geochemical characteristics of sedimentary OC in the land-sea continuum and the deeper ocean, a fundamental consideration to constraining global carbon inventories.

The Demerara Plateau in the Equatorial Atlantic is a transform marginal plateau (TMP) where the Deep Western Boundary Current (DWBC) transports North Atlantic Deep Water (NADW) to the South Atlantic. This current, circulating in the depth between 1300 and 3500 m, represents the deep part of the thermohaline circulation. It also forms hundreds of sedimentary structures along its path, looking like giant flute-casts and called 'comets’. Those comets can reach 3 km in length and theses field of 'comets' more than ten kilometers large. Nautile dives, AUV acquisitions (equipped with Multibeam Sounder SMF, Doppler current meter ADCP, Sediment Sounder) and a mooring, implemented during the DIADEM campaign (N/O Pourquoi Pas?, January-February 2023, DOI : 10.17600/18000672), first allow to document the dynamics of the current DWBC in this equatorial domain, its spatial and temporal variability. Furthermore, investigated the numerous associated sedimentary systems associated with this current, as the "comets" forming giant erosion structures. Two AUV bathymetric surveys and four Nautile dives have helped to better understand the location of these hydrodynamic structures. They are located along outcrops of intensely tilted and fractured carbonated rock, probably associated with an ancient sliding mass. Nautile data coupled with photogrammetry are also used to reconstruct the outcrops of these carbonate blocks and to characterize their deformation. AUV ADCP data (hydrodynamics) acquired in parallel highlight the difference in current intensity between the comet head, where the current has a much greater magnitude than in the comet tail, which appears more sedimented. Measurements of currents and turbidity recorded at the mooring deployed upstream of the comet over 17 days of recordings clearly demonstrate the effect of the semidiurnal tide in the high-frequency variability of currents and sediment resuspension. Taken together, these geomorphological, oceanographic and sedimentary parameters provide a clearer picture of those complex seafloor sedimentary structures that seem to result from the interaction of the DWBC with remobilized carbonated outcrops. Our observations also suggest that those comets initially formed under higher hydrodynamic conditions than those recorded today.

Underwater gliders equipped with current profilers and optical turbidity sensors offer a low-energy solution for high-resolution measurements of currents, suspended particle properties, and sediment transport in coastal waters. Because the spatial structure of hydrosedimentary processes often changes on short time scales (hours to weeks), especially in coastal areas, validating the distribution of glider observations is required to assess our capacity to represent hydrosedimentary processes. Here we propose to validate in a shelf tide-dominated environment, both (i) glider-based currents, and (ii) glider-based acoustic backscatters and optical turbidities in full resolution delayed mode, using in situ collocated and synchronous ancillary observations. The deployed glider system correctly measures the periodic pattern of the tidal current, with a RMSD of O(3 cms-1), demonstrating the system's ability to accurately capture tidal variability. Glider optical turbidities highly correlate with the ancillary observations (R2 up to 0.83). They also correlate well with their glider acoustic counterpart for most of the campaign period (R2=0.76), allowing an estimation of suspended particulate matter concentrations from acoustic measurements. Hence, the glider could observe not only the presence of bottom nepheloid layers of several mgL-1 but also residual fluxes of the order of 1 gm-1s-1 on the shelf. These results highlight the potential of gliders for quantifying sediment fluxes and advancing our understanding of coastal hydrosedimentary processes.

The Levantine Intermediate Water (LIW) plays a crucial role in the Mediterranean thermohaline circulation, significantly influencing the regional climate and ecosystems through mixing with under- and above-lying waters. Our study aims to explore the spatial and temporal evolution of LIW, tracing the journey of this water mass from its formation in the Levantine Basin to its arrival in the western Mediterranean Sea. While previous studies have confirmed some aspects of the evolution of the LIW along its trajectory, our approach stand out by using an extensive in situ database composed by a compilation of different data sets, extending on the entire Mediterranean Sea. This dataset incorporates CTD profiles from ship cruises, gliders, floats, and XBT measurements during 2018-2023 period. This data will be used to map LIW pathways and dispersion patterns, to offer insights into its interactions with neighboring water masses. Furthermore, our investigation focuses on LIW's evolution over years, with a specific emphasis on identifying signals of climate change in particular areas. Preliminary findings indicate notable shifts in LIW properties, encompassing temperature, salinity, and nutrient content, hinting at potential climate-induced alterations. These observations contribute significantly to our understanding of the broader implications of climate change on Mediterranean water masses and ecosystems.

Dense shelf water cascading (DSWC) is a key oceanographic process in transferring energy and matter from continental shelves to deep ocean areas. Although intense DSWC (IDSWC) events have received most attention due to their large impacts, mild DSWC (MDSWC) events are the most frequent in the northwestern Mediterranean and are expected to become more common under climate change. However, their dynamics, particularly in the Cap de Creus Canyon, have been less comprehensively described and compared to strong-winter events. This study investigates MDSWC in the Cap de Creus Canyon during the mild winter of 2021–2022, examining shelf-canyon transports of both dense shelf waters and suspended particulate matter (SPM). Observations from the FARDWO-CCC1 multiplatform cruise in March 2022 revealed the presence of cold, dense, and turbid shelf waters, enriched in dissolved oxygen and chlorophyll a, on the continental shelf adjacent to the canyon. These waters cascaded into the canyon head and progressed further into the canyon along its southern flank to ∼ 390 m depth. Estimated water and SPM transports during this event were 0.7 Sv and 105 metric tons (t), respectively, at the continental shelf. Within the canyon, transports were 0.3 Sv and 105 metric tons in the upper section, while mid-canyon transports were lower (0.05 Sv and 104, respectively), indicating that most dense shelf waters likely remained confined to the shelf area and upper canyon. During this event, dense shelf waters were transported ∼ 30 km from the shelf into the canyon. Our results show that significant transport of dense shelf waters (260 km3) and suspended sediment can occur in the Cap de Creus Canyon during MDSWC events under mild winters, also contributing significantly to the formation of Western Intermediate Water (WIW) in the canyon. The Mediterranean Sea Physics reanalysis data indicate that the cascading season lasted from late-January to mid-March 2022, with several shallow cascading pulses throughout this period. Peak transport occurred in mid-March associated with an eastern storm, which likely intensified MDSWC in the canyon. Our study reinforces the idea that dense shelf water transports exhibit marked interannual variability, even under mild winters.

Zooplankton-microbial interactions play crucial roles in epipelagic ecosystem functions. The distinct west-to-east gradients and complex circulation patterns in the Mediterranean Sea, combined with the ubiquity of pelagic copepods, provide an ideal model to study the ecological processes driving host-associated microbial spatial distribution. Here, we characterized and compared the copepod-associated microbial metacommunities (CAMC) with those from seawater microbial metacommunities (SMC). Copepod-associated microbial metacommunities displayed spatial dissimilarity between the western and eastern basins, while SMC exhibited similar microbial compositions. The within-basin similarity observed in CAMC was associated with connectivity by the surface currents. Ecological drift explained most of CAMC variability, likely as a response to the restricted co-dispersal of the hosts with their microbes, which presented low prevalence and abundance. Seawater microbial metacommunities displayed higher homogenizing dispersal, with widely distributed generalist taxa. We conclude that CAMC better reflect cross-basin gradients and connectivity patterns than SMC, suggesting that CAMC may serve as a useful proxy for studying microbial biogeography.

Dense shelf water cascading (DSWC) is an oceanographic process that occurs when dense shelf water overflows over the shelf edge downslope toward the deep sea. Monitored in the northwestern Mediterranean by moorings since 1993 in the Lacaze-Duthiers Canyon and since 2005 in the Cap de Creus Canyon, numerical modeling with reanalysis extends this timeline further into the past. This study investigates a regional reanalysis (1987-2021) validated against mooring observations at 750-1000 m depth. The reanalysis successfully reproduces observed intense DSWC (IDSWC) events from 1999, 2000, 2005, 2006, 2012, 2013, and 2018 while identifying one previously unreported event in 1987 and detecting no IDSWC between 1988 and 1998. The reanalysis effectively matches 84 % of observed IDSWC days within the same week and 56 % on the exact date. Instead of assimilating IDSWC events from mooring observations to resolve the cascading process, the model relies solely on the seawater density on the shelf and revealed the seawater properties along the canyon that caused IDSWC. This work highlights the importance of high-resolution reanalyses in investigating the impacts of mesoscale processes on larger scales in the deep ocean.

Pteropods are a group of cosmopolitan holoplanktic gastropods that produce an aragonite shell and play an important role in both marine ecosystems and geochemical cycles. In addition to being affected by anthropogenic impacts that include warming and changes in carbonate system parameters, the Mediterranean Sea is considered to be understudied concerning pteropods dynamics and abundances. This work aims to document the modern spatial and temporal distributions of pteropods populations in the Northwestern and Central Mediterranean Sea (Gulf of Lions and Strait of Sicily), respectively. We present data from two sediment-trap records that cover the timeframe between early 1996 and early 2004 for the Gulf of Lions and late 2013 to late 2014 for the Sicily Strait. A total of 843 pteropod shells and 18 different species were identified. Limacina inflata, Creseis virgula and Creseis clava were the most abundant species in the Gulf of Lions, while in the Sicily Strait, C. conica replaced C. clava as the most abundant species. These taxons represented around 70% of the total individuals identified in both sites. Overall, our results suggest a greater pteropod abundance in the Gulf of Lions than in the Sicily Strait, most likely due to enhanced food conditions. In the Gulf of Lions, maximum fluxes occurred in autumn (32.5% of the annual pteropod fluxes registered in October), while in the Sicily Strait peak fluxes occurred in winter (30.5% of the annual pteropod fluxes registered in January). Comparison of temporal changes pteropod fluxes with satellite sea surface temperature (SST), and chlorophyll-a concentration suggest a possible positive effect of high algal accumulation and cool water conditions in the Strait of Sicily on the main pteropod groups. In turn, no clear relationships between pteropod groups, SST and chlorophyll-a were identified in the Gulf of Lions, highlighting the effect of salinity and carbonate system parameters. Overall, and despite the limitations associated with the use of sediment traps for pteropod population monitoring, the consistency of our results with the literature supports the use of sediment traps as useful tools for documenting the diversity and temporal distribution of pteropods.

Abstract. This study examines CTD, ADCP and optical data collected during the PERLE-3 cruise in March 2020 between the surface and 2000 m depth over an east-west section of the Tyrrhenian Sea in the Mediterranean. The focus will be on the impact of double diffusion processes, in particular salt fingering, on the distribution and dynamics of particulate and dissolved matter. The staircases develop at the interface between the warm, salty Levantine Intermediate Water (LIW) and the colder, less salty Tyrrhenian Deep Water (TDW) in the centre of the basin with low hydrodynamic energy. The results show that thermohaline staircases formed by salt fingering significantly influence particle sedimentation and biogeochemical cycling in deep ocean environments by altering vertical flux patterns. These density steps create distinct vertical layers that act as physical barriers, slowing the descent of particles and facilitating their retention and aggregation. Retention of fine particles at density gradients promotes the formation of larger aggregates, affecting particle size distribution. The staircases also affect dissolved matter by creating pronounced concentration gradients of oxygen and nutrients, which can influence microbial activity and nutrient cycling.

The Beagle Channel is a 300-km long passage connecting the Pacific and Atlantic Oceans at similar to 55 degrees S, where glaciers and river streams meet subantarctic waters. Here we present the first evaluation of downward fluxes and composition of particulate matter in the channel. Settling particle fluxes were collected by sequential sediment traps deployed in two contrasting areas: one in the western part of the channel, corresponding to an early post-glacial environment (site A) and a second, fully deglaciated, river-dominated environment (site B) in the eastern part. In early summer, fluxes at both sites are driven by organic matter produced in spring, with peak organic carbon fluxes of 289 and 413 mg C m(-2) d(-1) at sites A and B, respectively (C:N ratios of 7.3 and 6.3, respectively). During winter, the fluxes of fecal pellets, particulate organic carbon (POC) and particulate nitrogen (PON) were at their minimum. At site A (integrated annual POC flux of 74 g C m(-2) yr(-1)), seasonality was weak and the flux was driven by ballast material (>95% of total particle flux) of glacial origin year-around, which also promotes the POC export. According to isotopic and taxonomic analyses performed at site A, the low seasonality in the organic component of the flux appears to be mainly related to autochthonous production of nano- and pico-phytoplankton during autumn and winter, later replaced by microphytoplankton fluxes during spring and summer. At site B, ballast material accounted for <60% of total mass flux and the POC flux showed a marked seasonality with a well-defined maximum after the spring phytoplankton bloom. Regarding the contribution of zooplankton, fecal pellets of appendicularians dominated at the western sector of the channel (site A) while Munida gregaria pellets dominated the flux at the eastern site (site B). This work is a contribution to ongoing efforts to unveil the physical and biogeochemical variables driving the biological carbon pump and the land-sea connections in this high-latitude ecosystem threatened by climate change.

Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties with respect to understanding the ocean carbon cycle and its link to global change (ocean carbon sinks and sources, ocean acidification) and ultimately finding carbon-based solutions or mitigation procedures (marine carbon removal). We present a database of more than 44 400 AT and CT observations along with basic ancillary data (spatiotemporal location, depth, temperature and salinity) from various ocean regions obtained, mainly in the framework of French projects, since 1993. This includes both surface and water column data acquired in the open ocean, coastal zones and in the Mediterranean Sea and either from time series or dedicated one-off cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ±4 µmol kg−1 for both AT and CT. The data are provided in two separate datasets – for the Global Ocean and the Mediterranean Sea (https://doi.org/10.17882/95414, Metzl et al., 2023), respectively – that offer a direct use for regional or global purposes, e.g., AT–salinity relationships, long-term CT estimates, and constraint and validation of diagnostic CT and AT reconstructed fields or ocean carbon and coupled climate–carbon models simulations as well as data derived from Biogeochemical-Argo (BGC-Argo) floats. When associated with other properties, these data can also be used to calculate pH, the fugacity of CO2 (fCO2) and other carbon system properties to derive ocean acidification rates or air–sea CO2 fluxes.