Rhone River Mouth Research Articles

Sediment dynamics in the Gulf of Lion (NW Mediterranean Sea) during two autumn–winter periods with contrasting meteorological conditions

A simulation based on a hydro-sedimentary model was conducted for the period between summer 2010 and spring 2012 in the Gulf of Lion (northwestern Mediterranean Sea) to understand the spatial and temporal variability of sediment transport, erosion and deposition on the continental shelf and slope. Datasets of both simulated and observed current, temperature and suspended matter from the shelf and the Cap de Creus Canyon which is the main export route towards the continental slope, were first compared to assess the reliability of the simulation. The simulation shows the massive sediment accumulation near the Rhone River mouth (∼56 % of the inputs), the accretion along the mid-shelf mud belt, and the impact of dense shelf water cascading on sediment resuspension and erosion inside the Cap de Creus Canyon. The two studied autumn–winter periods were strongly contrasted in terms of meteorological conditions and subsequent impacts on the sediment dynamics. During the first period (2010–2011) dominated by marine storms, the shelf sediment underwent strong changes, the Rhone River sediment load accumulated in a relatively small area, stock of littoral sands moved to the inner shelf (20–40 m) while inner shelf fine particles fed the mid-shelf mud belt and the upper Cap de Creus Canyon. During the second period (2011–2012) with very little marine wind and a particularly cold winter, sediment on the shelf underwent little change except for a southwestward growth of the Rhone River prodeltaic deposit. Sediment from the southwestern end of the shelf as well as from the upper Cap de Creus Canyon was flushed toward deeper reaches by dense shelf water cascading. Cascading also had a more moderate impact in the various canyons incising the continental shelf. Our work supports the view of an unbalanced sedimentary system, with a deficit mainly over the inner shelf, whose main driver is probably the strong and fast reduction of particulate matter inputs from the Rhone River (by a factor of 4 in less than one century).

Suspended sediment flux at the Rhone River mouth (France) based on ADCP measurements during flood events.

Suspended sediment distribution and fluxes were estimated within the dominant channel at the mouth of the Rhone River for two annual flood events. The estimates were based on ADCP acoustic backscatter intensity and using calibration and post-processing methods to account for the grain-size distribution (GSDs). The fluxes were very similar to those obtained from suspended sediment measurements based on surface sampling at an automated station located 35 km upstream. Suspended sediment concentrations (SSC) and GSDs showed little variation along the channel cross-section, except for a graduate suspension that appeared at the maximum of discharge, corresponding to velocities lower than 1 m s-1 near the bottom. However, without post processing to account for the GSD, an under-estimation of 10% was observed during the two flood periods. The two flood events (12 November 2012 and 29 November 2012), separated by only 2 weeks, had clear differences in suspended sediment fluxes (SSF) and SSC during the peak of the river discharge, with twice more flux during the first, respectively, 925,226 and 430,879 tons of SSF.

Occurrence of α, ω-dicarboxylic acids and ω-oxoacids in surface waters of the Rhone River and fluxes into the Mediterranean Sea

The Mediterranean Sea is a semi-enclosed marine environment surrounded by densely populated areas. This ecosystem is under strong anthropogenic pressure at present. Riverine waters are important input pathways of water-soluble organic compounds that potentially contribute to the dissolved organic carbon (DOC) pool. Here, we report the first ever measurements of bi-functional carboxylic acids such as α, ω -dicarboxylic acids (or diacids) and related polar compounds ω -oxoacids (or ketoacids) (BCAs=α, ω-dicarboxylic acids+ω -oxoacids), along with the DOC levels in Rhone River waters. Surface water samples were collected from February 2006 to June 2009 approximately 50km above the Rhone River mouth, which is the main supplier of freshwater to the Mediterranean Sea. The BCA concentrations averaged 32.4±15.3µgl−1, and exhibited a wide range of values from 13.2µgl−1 (Spring 2008) to 71.2µgl−1 (winter 2007). The contribution of carbon from BCAs to the DOC pool (BCA-C) accounted for 0.28 to 1.42% of the DOC. Although no seasonal trend was evident in the studied period, our results showed that the highest BCA concentrations did not always follow high water discharges. α,ω -dicarboxylic acids are the most abundant compound class (20.7±10.6µgl−1), which is followed by ω -oxoacids (11.7±6.2µgl−1). The Rhone River was estimated to deliver between 1.37 and 16.4 t d−1 of BCA to the Gulf of Lions. Assuming a mean water discharge of ∼1790m3s−1, a broad estimate suggests a loading of 750–4000 t yr−1 BCAs to the northwestern Mediterranean Sea. Our results indicated that glyoxylic acid (ωC2) was the most abundant BCA followed by oxalic acid (C2di) and fumaric acid (trans configuration of unsaturated C4 diacid). This result indicates that there is a different molecular distribution between the aquatic and atmospheric compartments, including rainwaters and aerosols, for which previous studies highlighted a predominance of oxalic acid, which was followed by malonic and/or succinic acid. Runoff and riverine biological process (rather than photochemical oxidation reactions) are hypothesized to control the BCA occurrence and molecular distribution in the Rhone River and thus their inputs to the coastal NW Mediterranean Sea, although the environmental implications of this stock of BCA are still unknown.

Live–dead comparison of benthic foraminiferal faunas from the Rhône prodelta (Gulf of Lions, NW Mediterranean): Development of a proxy for palaeoenvironmental reconstructions

Dead benthic foraminiferal faunas (> 150 μm) from the Rhone prodelta (Gulf of Lions, NW Mediterranean) were analysed at 41 stations (15–100 m water depth) sampled in June 2005 and September 2006, and compared to the living faunas investigated during previous studies at the same stations. The comparison between dead and living assemblages enhances the understanding of taphonomic processes that may modify the composition of the dead faunas in this area. We observed a loss of individuals from living to dead assemblages of species characterised by a fairly fragile test and therefore more prone to fragmentation or dissolution (e.g., Bolivina alata, Quinqueloculina tenuicollis). Allochthonous dead and/or live specimens may be transported to some parts of the prodelta, particularly the shallowest sites where hydrodynamic processes (i.e., river flood, storm swells, longshore currents) are more intense. These specimens may originate from relict deltaic structures (e.g., Elphidium spp. from the lobe of Bras de Fer) or from surrounding areas (e.g., Ammonia beccarii forma beccarii from the river). Opportunistic species (e.g., Bulimina marginata, Cassidulina carinata) characterised by high reproductive rates have much higher relative abundances in the dead than in the living fauna. Cluster analyses based on dead foraminiferal assemblages divide our study area into four main thanatofacies directly related to distinct local environmental conditions prevailing in the prodelta. Close to the river mouth, Ammonia beccarii forma beccarii and Ammonia tepida are found in sediments subject to a high riverine influence (i.e., bottom currents, high organic and inorganic material input of continental origin). Elphidium species are abundant in the silty-sandy relict deltaic lobe west of the river mouth which is characterised by strong longshore currents that disturb the benthic environment. Nonion fabum, Rectuvigerina phlegeri and Valvulineria bradyana are found along the coast west of the Rhone River mouth, in the area defined as the “river plume” thanatofacies. In the more stable and deeper prodeltaic area, species known to feed on fresh phytodetritus (e.g., Bulimina aculeata/marginata, C. carinata, Hyalinea balthica) dominate the faunas. Since only minor variations in species relative abundances and spatial distributional patterns are observed between the living and the dead faunas, we consider that our thanatofacies have not been influenced by substantial transport of dead tests. This suggests that fossil benthic foraminifera can provide a reliable tool for investigating the development of the palaeo-Rhone prodelta.

Analysis of riverine suspended particulate matter fluxes (Gulf of Lion, Mediterranean Sea) using a synergy of ocean color observations with a 3‐D hydrodynamic sediment transport model

AbstractThe export of riverine suspended particulate matter (SPM) in the coastal ocean has major implications for the biogeochemical cycles. In the Mediterranean Sea (France), the Rhone River inputs of SPM into the Gulf of Lion (GoL) are highly variable in time, which severely impedes the assessment of SPM fluxes. The objectives of this study are (i) to investigate the prediction of the land‐to‐ocean flux of SPM using the complementarity (i.e., synergy) between a hydrodynamic sediment transport model and satellite observations, and (ii) to analyze the spatial distribution of the SPM export. An original approach that combines the MARS‐3D model with satellite ocean color data is proposed. Satellite‐derived SPM and light penetration depth are used to initialize MARS‐3D and to validate its predictions. A sensitivity analysis is performed to quantify the impact of riverine SPM size composition and settling rate on the horizontal export of SPM. The best agreement between the model and the satellite in terms of SPM spatial distribution and export is obtained for two conditions: (i) when the relative proportion of “heavy and fast” settling particles significantly increases relative to the “light and slow” ones, and (ii) when the settling rate of heavy and light SPM increases by fivefold. The synergy between MARS‐3D and the satellite data improved the SPM flux predictions by 48% near the Rhone River mouth. Our results corroborate the importance of implementing satellite observations within initialization procedures of ocean models since data assimilation techniques may fail for river floods showing strong seasonal variability.

Distribution of organochlorine compounds in superficial sediments from the Gulf of Lion, northwestern Mediterranean Sea

Superficial sediments from Cap de Creus to the Rhone Delta, in the Gulf of Lion, Northwestern Mediterranean Sea, including the mid-shelf mud belt and the continental slope were collected between 2005 and 2008 to assess the levels, main sources and distribution patterns of organochlorine pollutants. Discharges from the Rhone River are the main source for all these compounds around the area. The spatial distribution of organochlorine pollutants was also related to their physicochemical properties and to sediment grain size and composition. The concentrations of polychlorobiphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDD and DDE), and the chlorobenzenes (CBzs) – pentachlorobenzene (PeCB) and hexachlorobenzene (HCB) – decreased westwards along the mid-shelf mud belt. In contrast, hexachlorocyclohexane isomers (HCHs), namely lindane (γ-HCH), followed another concentration pattern suggesting a different transport mode. The major concentrations of organochlorine compounds were observed off the Rhone River mouth, in the prodelta, where PCB, DDT and CBz concentrations reached 38, 29 and 8.3ngg−1, respectively. These average concentrations in the mid continental shelf were two to ten times lower than those found in a study performed about 20years ago, albeit in almost all the sites the values of PCBs and DDTs still exceed the NOAA’s Sediment Quality Guidelines. In contrast, the concentrations in the continental slope were nearly the same as 20 years ago, which may evidence that even most of these compounds were banned decades ago, their background concentrations associated to diffuse pollution have not decreased in the deep continental margin.

Mixing of Rhône River water in Lake Geneva (Switzerland–France) inferred from stable hydrogen and oxygen isotope profiles

Summary Depth profiles were sampled at different locations throughout Lake Geneva on a monthly and seasonal basis over the course of 2 years and analysed for their stable hydrogen and oxygen isotope compositions. The isotopic compositions indicate an isotopic stratification in the metalimnion during summer and fall. This is related to mixing of Rhone River water, which in summer is dominated by snow and glacier melt waters, and lake water, with the latter having a homogenous isotopic composition. The observed interflow layer is 7–15 m thick and can be traced by the distinct stable isotope composition of the water for about 55 km throughout the lake as well as into shallow bay regions. Depth of the interflow layer close to the Rhone River mouth is similar to those previously described based on echo-soundings and turbidity profiles of sediment dispersion. In contrast to previous descriptions of the interflow within Lake Geneva, the stable isotope compositions allow for direct, natural tracing of the Rhone River water even in cases where the turbidity and conductivity measurements do not indicate such an interflow. In addition, the method allows for a quantification of the Rhone River and lake water in the interflow with the fraction of Rhone River water within the interflow estimated to be up to 37% in summer. The isotopic composition further indicates different vertical mixing processes within the two lake basins of Lake Geneva, related to the density gradients and local stability within the water column. The method may be applicable to other lakes in catchments with large differences in the topography as water that originates from high altitudes or glaciers has a distinct oxygen and hydrogen isotope composition compared to other sources of water originating at lower altitudes and/or from direct precipitation over the lake. Stable isotope measurements thus improve the understanding of the circulation of water within the lake, which is fundamental for an evaluation of the water residence times, dissolved pollutant and nutrient transport as well as oxygenation.

Pattern of Sediment Transport in a Microtidal River Mouth Using Geostatistical Sediment-Trend Analysis

We perform a grain-size trend analysis (GSTA) to investigate sediment-transport mechanisms in the Grand Rhone microtidal mouth during various hydroclimatic conditions. The objective is to determine the efficiency of this method to explain the directions of residual transport pattern and mode of distribution of the fluvial sediment input in an environment that is very difficult to equip with instrumentation during extreme events. The known biases of this sedimentological approach are reduced by using enriched geostatistical processing and choosing periods of sampling subject to unequivocal hydrodynamic forcings. The modeling results and grain-size distributions of surface sediments clearly show three vector fields during periods of low river discharge. The first vector field corresponds to an upstream zone under fluvial influence. The second field is characteristic of a mixing zone, confined inside the river mouth and marked by heterogeneous sedimentation. This sector is interpreted as the confrontation zone between alluvial and marine dynamics. Farther offshore, the vectors are oriented towards the river and at counter slope. In deeper zones, the vectors reflect the sediment-transport generated by wave refraction at the top of the mouth bar. At the delta front (bathymetry from −5 m to −20 m), the vectors are interpreted as representing a lag deposit. The periods of medium and extreme floods are associated with storms. The marine dynamic regime generates a longshore drift current, which brings sediments onto the mouth bar. During medium flood events, only the inner part of the mouth bar is highly influenced by the river. During extreme flood events, the river influences both the eastern part of the mouth and the wave regime in the central part. Consequently, the GSTA method shows that, even during flood events, the river solid discharge is not transferred towards the adjoining upper shoreface. In the context of a wave-dominated delta, the distribution of surface sedimentation in a microtidal mouth remains controlled mainly by the swell, even in periods of flood when the intensity of river dynamics is exacerbated. Finally, all these results are similar to the known hydro-sedimentological behaviours of the Rhone River mouth. Nevertheless, GSTA results seem not self-sufficient to explain all the processes, but this method proposes an alternative approach to traditional methods used to measure bed-load sediment transfer during extreme events.

MICROHABITAT SELECTION OF BENTHIC FORAMINIFERA IN SEDIMENTS OFF THE RHONE RIVER MOUTH (NW MEDITERRANEAN)

The microhabitats and the composition of living benthic foraminiferal faunas in the sediments deposited off the Rhone River mouth are directly influenced by the Rhone River input. In this shallow-water environment (20–98 m water depth), the vertical distribution of the species is not well defined, probably due to the low penetration of oxygen into the sediment. We show the existence of two types of species: “predominantly superficial” taxa showing a density maximum in the topmost sediment layer, and “potentially/ predominantly infaunal taxa” that are also frequent in the topmost sediment, but which show considerable densities in anoxic deeper sediment layers as well. In the area that is strongly influenced by river input (near the river mouth and in the southwestern direction, following the river plume), the fauna is composed mostly of “potentially/predominantly infaunal” species adapted to a higher contribution of terrestrial organic matter, generally of lower quality, and to the low penetration of oxygen into the sediment. This fauna is composed of species tolerant to strong environmental stress (e.g., Nonionella turgida , Nonion scaphum , Rectuvigerina phlegeri , and Valvulineria bradyana ). The stations less influenced by fluvial input (located south and east of the river mouth) are composed of species that colonize oxygenated interstices in the upper centimeter of sediment. The dominant species are mainly “predominantly superficial” taxa (e.g., Cassidulina carinata , Bulimina aculeata ), which are known to react quickly to provisions of labile organic matter. The correlation between these two types of faunas and the two prevalent environmental factors suggests that the vertical distribution of living foraminifera in front of the Rhone River mouth is primarily controlled by the quality of the organic matter and less by the quantity of the organic matter and depth of oxygen penetration into the sediment.

Spatial and temporal variations of plutonium isotopes (238Pu and 239,240Pu) in sediments off the Rhone River mouth (NW Mediterranean)

The dispersion and fate of the Rhone River inputs to the Gulf of Lions (Northwestern Mediterranean Sea) have been studied through the spatial and temporal distributions of plutonium isotopes in continental shelf sediments. Plutonium isotopes ((238)Pu and (239,240)Pu) are appropriate tracers to follow the dispersion of particulate matter due both to their high affinity for particles and their long half-lives. In the Rhone River valley, plutonium isotopes originate from both the weathering of the catchment basin contaminated by global atmospheric fallout, and the liquid effluents released from the Marcoule reprocessing plant since 1961. This work presents a first detailed study on (238)Pu and (239,240)Pu distributions in sediments from the Rhone prodelta to the adjacent continental shelf, since the decommissioning of Marcoule in 1997. The vertical distribution of Pu isotopes has been analysed in a 4.75 m long core sampled in 2001 at the Rhone mouth. Despite this length, plutonium is found at the last 10 cm, manifesting the high sedimentation rate of the prodeltaic area and its ability for trapping fine-grained sediments and associated contaminants. The highest (238)Pu and (239,240)Pu concentrations reached 1.26 and 5.97 Bq kg(-1) respectively and were found within the layer 280-290 cm. The (238)Pu/(239,240)Pu activity ratios (AR) demonstrated an efficient and huge trapping of the Pu isotopes derived from Marcoule. The fresh sediments, located on the top of the core, show lower plutonium activity concentrations and lower (238)Pu/(239,240)Pu ratios. This decrease is in close relation with the shut down of the Marcoule reprocessing plant in 1997. In 2001, plutonium isotopes were also analysed in 21 surface sediments located offshore and concentrations ranged from 0.03 to 0.17 Bq kg(-1) for (238)Pu and from 0.33 to 1.72 Bq kg(-1) for (239,240)Pu. The (238)Pu/(239,240)Pu AR ranged from 0.24 close to the river mouth to 0.06 southwards, indicating the decreasing influence of the Marcoule releases (global fallout AR 0.03-0.05 and Marcoule AR 0.30). This is in good agreement with the main direction spread of the Rhone River plume and the bottom current. This dataset has been compared to those obtained in the same area in 1984 and 1990 in order to follow the time trend in Pu concentrations. This comparison highlights the decrease with time in plutonium concentrations close to the Rhone River mouth, but further away this reduction is not so evident.

The significance of pollen signal in present-day marine terrigenous sediments: The example of the Gulf of Lions (western Mediterranean Sea)

Terrigenous marine and continental sediments constitute pollen reservoirs able to fulfil the lack of Pleistocene data extracted from autochtonous lacustrine sediments for palaeovegetation reconstructions. Nevertheless, it is essential to constrain the signification of the pollen signal in such still under-exploited sediments. This article introduces a study carried out on the shelf of the Gulf of Lions. It aims at (1) highlighting the transport of pollen grains and the signification of the record in that area as well as at (2) discussing the pollen transport in various sedimentary contexts. The present study was carried out on surface water of the Rhone River, sea water and surface sediments of the shelf of the Gulf of Lions. The pollen content of the Rhone River and of its plume is in agreement with the season of sampling. On the contrary, the other samples of sea water did not contain any pollen grain. The samples from the surface sediments of the shelf are very rich in pollen grains and evidence the absence of sorting by currents except for Pinus. Indeed, its relative abundance increases when the fluvial impact decreases. The pollen signal (except Pinus) records the phytogeographic changes linked to the different drainage basins. The riparian woodland is over-represented at the Rhone River mouth. In front of the Pyreneo-languedocian rivers, Mediterranean xerophytes pollen grains are more abundant. From these results and from others taken in the literature, a model of pollen transport is built: all thin terrigenous sediments contain pollen grains essentially transported by rivers while autochthonous sediments (i.e., found in closed lakes or marine pelagic sediments) contain pollen grains brought by wind. Small closed lakes (or peat bogs) pollen spectra document local floras while those from pelagic marine sediments document floras from several hundred square kilometres.

The Rhone river dilution zone present in the northeastern shelf of the Gulf of Lion in December 2003

Acoustic doppler current profiler (ADCP), CTD and thermosalinograph data from the Gulf Of Lion Time Series (GOLTS) cruise of December 2003, and corresponding AVHRR images, show the unusual presence of the Rhone river dilution zone far east from the Rhone river mouth. It is the first time this event is studied with simultaneous hydrological and current data. This dilution zone extends as far as 5.27°E in longitude (45 km from the Rhone river mouth). At longitude 5.13°E (37 km from the Rhone river mouth), the dilution zone is 40 m deep and spreads over 0.075° latitude (8 km). It is due to an eastward current present there throughout the ADCP-detected range (12–120 m). The analysis of moored ADCP time series reveals that such eastward currents occur there about 18% of the time and that diluted waters from the Rhone reach the Station d’Observation Fixe (SOFI) site between 3.9% and 8.4% of the time. This December 2003 event is the consequence of the combined effects of a storm with east winds and the presence of freshwater along the coast.

Potential plutonium remobilisation linked to marine sediment resuspension: First estimates based on flume experiments

In coastal environments, resuspension is a key issue regarding contaminant remobilisation from the sediments. A first attempt to quantify both sedimentary bed erosion and related plutonium isotope remobilisation from eroded particles was carried out through flume experiments. Erosion fluxes under controlled hydrodynamic stresses and their resulting plutonium remobilisation were measured for cohesive sediments sampled at several locations (8 to 97 m depth) near the Rhone river mouth (North-western Mediterranean Sea). Surface sediments were characterised in terms of plutonium content ( 238Pu and 239,240Pu) and particle size distribution. Laboratory resuspension experiments were performed under realistic hydrodynamic conditions close to those prevailing in coastal zones. Critical shear stresses of erosion ranged between 0.068 and 0.087 N m − 2 whereas erosion rate varied within a factor of 3 (57–176 g m − 2 h − 1 ). After 1 hour of resuspension, the plutonium activity increased in the seawater particulate phase by factors ranging from 2 to 14, depending on the site at which the sediment was sampled. Plutonium fluxes ranged between 2 and 56 mBq m − 2 h − 1 for 239,240Pu depending on location. The highest fluxes were measured for sediments from the deepest site, where the highest Pu activities and mud content were found. During our resuspension experiments, no significant increase was found in dissolved Pu activities. These laboratory experiments have provided a unique opportunity to investigate the behaviour of plutonium at the sediment-water interface in a coastal environment. They emphasised the importance of sediment resuspension in plutonium remobilisation and its possible dispersion on continental shelves.

Glauconitization processes in the northwestern Mediterranean (Gulf of Lions)

Two sedimentary profiles, at 18 km and at 125 km from the Rhone River mouth were selected to study the steps of glauconitization inside green (glaucony) grains, in a temperate marine environment. The proximal site is characterized by a greater content of alpine minerals (chlorite, illite) than the distal one. In the proximal site, pellets at a younger level demonstrate a largely inherited mineral composition. In the older level, representing the most advanced stage of glauconitization, there is a mixed-layer glauconite-nontronite (G-N) phase with 20% glauconite-80% nontronite in dark green grains. In the distal site, more evolved glaucony has up to 92% glauconite in the G-N. In spite of this difference, the glauconitization process developed similarly in the two sites. The relatively low Fe content in the mud-matrix does not reflect the concentration of this element inside the grains. During diagenesis, most of the inherited minerals disappear, making way for neoformed mixed-layer glauconite-nontronite.

Seasonal and interannual variations (1996–2000) of the coastal waters east of the Rhone river mouth as indicated by the SORCOM series

Between 1996 and 2000, 79 cruises have been achieved following always the same transect between Marseilles and the Rhone river’s mouth. While cruising, salinity, temperature and fluorescence of surface waters (1.5 m) were recorded every minute, in association with precise GPS position. The whole set of data (i.e. 128 measured points per cruise) constitutes the Sorcom series whose synthetic results are presented here. Despite the high spatial and temporal variability encountered between two successive cruises, the regularity and duration of the sampling strategy permitted to define coherent spatial zonation and temporal evolutions. In the Cape Couronne vicinity the dynamic of the eastern extension of the dilution plume clearly changes. Westward, waters are frequently under its influence, but more sporadically to the east, with a less pronounced gradient. In few cases, salinity lowering has been measured up to the gulf of Marseilles. In these two areas, which can be divided in two sub zones, seasonal evolutions are different. In the eastern area (Cape Couronne–Marseilles), the classical rhythm prevailing over marine waters is observed but with earlier phytoplanktonic bloom than usually reported. In the western part, and as getting closer to the Rhone river mouth, a weaker seasonal variability is observed for the phytoplanktonic biomass then salinity and temperature differences remain high. The better knowledge of the eastern dilution plume is of important socio-economical concern due to the numerous population living on this coast and the presence of pollutants in the river waters.

Phosphate in the sediments of the Gulf of Lions (NW Mediterranean Sea), relationship with input by the river Rhone

We investigated P-input by the Rhone river into the Mediterranean Sea taking into account P trapped in the surface sediment of the Gulf of Lions. Total phosphate concentration was determined every cm in the upper 10 cm-layer of sediments sampled at 11 stations in the Gulf of Lions during two cruises (March 1998 and January 1999). Two low downward gradients, one East–West and another North–South, with distance to the Rhone river mouth were found. Except at one station, total phosphate concentration in surface sediments was found to be constant with depth down to 10 cm. Values for individual stations ranged between 400 and 700 μg g−1 with an average value of 547 μg g−1 (st. dev. = 63 μg g−1) for the whole gulf. The low variability in total-P concentration in sediments is in contrast to the large variability in suspended matter load of the river Rhone and suggests the dominance of authigenic P removal mechanisms in P burial. The total P-pool in the upper 10 cm-layer of the sediments in the gulf was estimated at 562 kt, with about 80% trapped into the shelf and 20% into the slope. Annual P-deposition was estimated as 7.2–12.4 kt y−1, from the P-pool in the sediment and the sedimentation rates. This is equivalent to a previous estimation of the river Rhone input, estimated to be about 6.5–12.2 kt y−1. As the Rhone is the major river flowing into the Mediterranean Sea, total P in surface sediments of the Gulf of Lions should be taken into account in P-budgets at the scale of the Mediterranean Sea.

Surface suspended matter off the Rhone river mouth from visible satellite imagery

The quantification of surface Total Suspended Matter (TSM) off the Rhone river mouth is investigated using SPOT-XS and Landsat-TM images collected in November 1994. The method of satellite data calibration from in situ measurements is discussed. It is based on TSM concentration measurements through filtration of surface water samples and on radiometric data collected with a 256-channel spectroradiometer in the range 400–1100 nm. Empirical relationships between reflectance and TSM concentration are established by linear regression analysis of the data. These relationships are then applied to the image data corrected from atmospheric and surface effects. A good agreement, within a factor of less than two, was found on derived TSM concentration values as compared with sea truth data. The results for three different river flows are presented: a quasi-normal river discharge, a flood situation and an intermediate situation. The TSM corresponding concentration maps are presented and discussed.

Long-Core Profiles of 137 Cs, 134 Cs, 60 Co and  210 Pb in Sediment near the Rhone River (Northwestern Mediterranean Sea)

Various anthropogenic radionuclides and210Pb were analyzed in a 4.3-m-long core, sampled near the Rhone River mouth in March 1991, to evaluate the extent of industrial releases that accumulate in this area. The whole core was significantly marked by radionuclide inputs from the nuclear facilities located along the river (137Cs,134Cs,60Co). Irregular profiles in natural and artificial radionuclides should be related to variations in their respective inputs from the Rhone River to the Mediterranean Sea. Minimum concentrations were found during high flow periods. Using both the137Cs/134Cs profile in the core and the range of this ratio in Rhone waters, mean apparent accumulation rates were estimated to range between 37 cm yr−1 and 48 cm yr−1. This core would then represent a sedimentary record over a 7–10 year period. However, the presence of a signal from the Chernobyl accident, which occurred on April 26, 1986, was not clearly observed in the core. Inventories of both artificial and natural radionuclides were greater than expected from atmospheric inputs. The increased sedimentation occurring in close vicinity to the mouth of the Rhone River is thus responsible for trapping of elements transported by the river to the Mediterranean Sea. In this area, inventories of artificial radionuclides are well in excess of aerial deposition from Chernobyl and atmospheric weapons tests and are linked primarily to industrial releases.

Biogenic aromatic hydrocarbon geochemistry in the Rhone river delta and in surface sediments from the open North-Western Mediterranean sea

The origin, evolution and transport of biogenic polycyclic aromatic hydrocarbons (PAH) were studied in the Rhone river delta and the Gulf of Lions (North-western Mediterranean). Sediments and riverborne suspended particulate matter were collected and analysed for PAH using a combination of high performance liquid chromatography (HPLC), gas chromatography (GC) and GC/mass spectrometry (GC/MS). Special attention was given to aromatic hydrocarbons from natural sources: retene and its precursors related to abietic acid found in the resins of conifers, and pentacyclic triterpenoids derived from α - and β -amyrins, constituents of terrestrial plants. Retene concentrations varied in the following ranges: 15–59 ng g −1 in river-borne particulate material, 20–70 ng g −1 in deltaic sediments, 3–16 ng g −1 in open-sea sediments. Retene precursor concentrations were generally higher than those of retene in the delta, whereas the inverse was observed in open-sea sediments. In these sediments the low concentrations or the absence of precursors suggested a predominant transport of pre-formed retene by aerosols and fine particles issued from the Rhone river. In the deltaic area, both retene and its precursors are transported by river waters from the forest runoff of the drainage basin. Transformation of precursors to retene occurred during transport by river waters and/or fast after deposition in surface deltaic sediments. Pentacyclic triterpenoid concentrations varied from 54 to 296 ng g −1 in deltaic sediments and from 0 to 21 ng g −1 in open-sea sediments. Concentrations of tetrahydrochrysenes (3,3,7-THC and 3,4,7-THC) decreased with increasing distance from the Rhone river mouth. The presence of THC in the riverine particulate matter (10–105 ng g −1 ) indicated that the degradation of terrestrial organic matter by microbial activity had begun before the deposition of particulate matter in the surface sediment. A good correlation was observed between ΣTHC and δ 13 C ( r 2 = 0·89; n = 9), which suggests that the THC tracer information was representative of natural continental organic carbon inputs.