Northwestern Mediterranean Margin Research Articles

Sources, Transport, and accumulation of Mercury in the northwestern Mediterranean margin sediments during the Industrial Era and influence of turbiditic events

Sources and pathways of the Hg accumulated in the sediments of the Gulf of Lion (GoL) and its adjacent marine areas (Northwestern Mediterranean) have been explored using sediment grab samples, sediment cores, and sediment trap samples. The main source of Hg along this margin is the Rhône River, whose suspended sediments settle mainly in the prodelta area but also along the mid-shelf, then reaching the continental rise via wave resuspension and cascading of dense shelf waters. Seaward, these riverine particles are mixed with carbonated ooze conveyed to the bottom by the biological pump. The Hg is enriched in surface sediments of the GoL with decreasing concentrations westward and seaward from the Rhône prodelta to the continental rise. Dated cores from the Rhône prodelta give access to riverine sediments deposited over the last 400 years and show that the Hg concentration time trend resembles the evolution of coal consumption in France, peaking during in the 1960s. Similar trends were observed in sediment cores collected along the GoL slope and submarine canyons along with the preservation of traces of erosion and sediment instability events. Seaward, on the continental rise, the Hg concentration distribution suggests the deposition of fine planktonic-derived material and particles episodically advected from the shelf during deep cascading pulses. Anthropogenic Hg accumulated in GoL and continental rise sediments during the Industrialized Era is ∼ 150 Mg (tons), two-thirds of which are buried in the Rhône prodelta area. Significant correlations are found between Hg and organic matter in GoL sediments, but the relationships differ between areas and are disrupted by the inputs of anthropogenic Hg and by the Hg availability in the Rhône River watershed during the pre-industrial period. The HgT availability in waters appears to be the limiting factor to the Hg enrichment of the particulate organic matter. Monomethyl mercury (MMHg), which represents on average 0.3 % of the total Hg, was positively correlated to total Hg. Their distributions suggest in situ MMHg formation and a more effective net Hg methylation for surface sediment or particles collected in traps compared with particles buried in the sedimentary column.

Sources and exchanges of mercury in the waters of the Northwestern Mediterranean margin

Ocean margins are focal regions in terms of mercury (Hg) exchanges between the continent and the open sea. The aim of this paper is to describe the distribution and partition of Hg between the gaseous, dissolved and particulate phases in the waters of the Northwestern Mediterranean (NWM) margin, in order to assess the Hg sources and exchanges within the continuum between the continental shelf (Gulf of Lions) and the open sea (Northern Gyre).Mean (±standard deviation) of total Hg species (HgT) concentrations in unfiltered water samples were 1.52±1.00pmolL−1 (n=36) in the inner shelf, 1.09±0.15pmolL−1 (n=30) along the slope, and 1.10±0.13pmolL−1 (n=99) in the Northern Gyre. The dissolved phase (<0.45µm) average concentrations were 0.80±0.47pmolL−1 (n=37) in the inner shelf, 0.93±0.20pmolL−1 (n=4) along the slope and 0.84±0.10 (n=20) pmolL−1 in the Northern Gyre. The particulate fraction of Hg decreased very strongly seaward from around 60% on the shelf to 10–25% above the Northern Gyre. Very low dissolved HgT concentrations occurred in the inner shelf water, consistent with the results of incubation experiments, which demonstrated that shelf water is very efficient in both production and release of dissolved gaseous Hg into the atmosphere. In the North Gyre waters column HgT presents a distribution pattern inverse to that of dissolved oxygen, and a slight Hg enrichment in the deep layer (Western Mediterranean Deep Water).The Hg from the open sea water is the largest Hg input to the Gulf of Lions (∼5.7kmolyr−1), whereas inputs from the riverine source account for ∼3.4kmolyr−1 and atmospheric deposition for less than 0.5kmolyr−1. The Hg accumulated in the sediments of the shelf is ∼4.5kmolyr−1, including 0.6–1.7kmolyr−1 in the Rhône prodelta sediments. The evasion to the atmosphere represents a Hg flux of ∼2.6kmolyr−1.

The open sea as the main source of methylmercury in the water column of the Gulf of Lions (Northwestern Mediterranean margin)

Despite the ecologic and economical importance of coastal areas, the neurotoxic bioaccumulable monomethylmercury (MMHg) fluxes within the ocean margins and exchanges with the open sea remain unassessed. The aim of this paper is to address the questions of the abundance, distribution, production and exchanges of methylated mercury species (MeHgT), including MMHg and dimethylmercury (DMHg), in the waters, atmosphere and sediments of the Northwestern Mediterranean margin including the Rhône River delta, the continental shelf and its slope (Gulf of Lions) and the adjacent open sea (North Gyre).Concentrations of MeHgT ranged from <0.02 to 0.48pmolL−1 with highest values associated with the oxygen-deficient zone of the open sea. The methylated mercury to total mercury proportion (MeHgT/HgT) increased from 2% to 4% in the Rhône River to up to 30% (averaging 18%) in the North Gyre waters, whereas, within the shelf waters, MeHgT/HgT proportions were the lowest (1–3%). We calculate that the open sea is the major source of MeHgT for the shelf waters, with an annual flux estimated at 0.68±0.12kmola−1 (i.e., equivalent to 12% of the HgT flux). This MeHgT influx is more than 80times the direct atmospheric deposition or the in situ net production, more than 40times the estimated “maximum potential” annual efflux from shelf sediment, and more than 7times that of the continental sources. In the open sea, ratios of MMHg/DMHg in waters were always <1 and minimum in the oxygen deficient zones of the water column, where MeHg concentrations are maximum. This observation supports the idea that MMHg could be a degradation product of DMHg produced from inorganic divalent Hg.

Origin and accumulation of trace elements in sediments of the northwestern Mediterranean margin

Continental margins receive natural and anthropogenic trace elements (TEs) from direct atmospheric deposition of aerosols onto the sea surface and from advection of riverine suspended particles and/or resuspended sediments from the continental shelf/slope. When the margin is incised by submarine canyons, as for example in the Northwestern Mediterranean Sea, most of these particles are preferentially transferred via these topographic features towards their final repositories in the abyssal plain. The Gulf of Lions (GoL) shelf receives the largest particulate riverine input to the Western Mediterranean, with its associated chemical contaminants originating from the industrialized and urbanized Rhone Valley. Sediment samples (grabs, cores and moored traps) collected in the Cap de Creus (CdC) Canyon and its adjacent areas at the Southwestern exit of the GoL were analyzed to explore the origin, dispersion, transfer and accumulation of a suite of TEs (Ag, Cd, Co, Cr, Cu, Ni, Pb, Zn and V) from the GoL shelf to the adjacent continental rise. Distributions of Cu, Cr, Ni, Pb, Zn and V in the surface sediments of the shelf confirm their terrigenous origin in association with clay minerals, whereas Ag and Cd are more associated with organic matter (OM). All these TEs are anthropogenically enriched in the Rhone prodelta sediments. Anthropogenic influence remains clearly discernible in the GoL shelf surface sediments for Ag, Pb and Zn. Hydrodynamical resuspension and sorting of shelf sediments occur at the head of the CdC Canyon during dense shelf-water cascading events. During these events, the material collected in moored sediment traps contains a higher coarse carbonate fraction slightly impoverished in TEs compared to the clays of the nepheloid layer and the organically-rich particles deposited before and at the end of the cascading period. Upper and middle canyon sediments are characterized by high sedimentation rates (~0.2cmyr−1) of fine clay material. Conversely, sediments from the lower continental slope and rise exhibit low sedimentation rates (~0.06cmyr−1) and receive carbonaceous planktonic detritus from the water column. At the lower continental slope, coarse material includes foraminifers and pteropods, whereas at the continental rise finer planktonic-derived material is more abundant. Both in the CdC Canyon and in its adjacent lower continental slope/rise sediments, Co, Cu, Cr, Ni and V are associated with clay, whereas Ag, Cu and Pb are preferentially associated with OM. Cadmium, Cr, and Zn are also associated with OM in canyon sediments. Carbonaceous plankton appears to be especially efficient for scavenging Ag, whereas, Cr, V, Zn and Pb are diluted by biogenic carbonates. An authigenic Mn fraction is enriched with Co and Ni. Lead and Zn concentration levels and vertical profile patterns, along with Pb stable isotopic ratios, indicate that significant parts of Pb and Zn are of anthropogenic origin. A sediment chronology based on 210Pb dating reveals that Pb anthropization, mainly from gasoline additives, culminated between 1960 and 1980, being the current concentrations >40% lower than 30years ago. A similar distribution is observed for Zn, which originates mainly from combustion processes; but the reduction of Zn contamination amounts to only 20% during the same period. The largest anthropogenic Pb accumulation occurs in the middle part of CdC Canyon, with an inventory of 200μgcm−2. At the most distal part of the continental rise anthropogenic Pb accumulation within the first ~10cm below the surface sediment is estimated around 10μgcm−2, which is similar to the direct atmospheric deposition estimate.

Vertically stacked Gilbert-type deltas of Ventimiglia (NW Italy): The Pliocene record of an overfilled Messinian incised valley

Overfilled incised valleys develop when the rate of sediment supply outpaces the rate of accommodation. An overfilled incised valley presents simple or compound valley-fill architecture, depending on the depth of the valley incision, compared with the height reached by the following sea-level rise. The Ventimiglia incised valley, exposed on the Ligurian coast, north-western Mediterranean margin, presents a spectacular example of compound incised-valley fill, developed in perennial “overfill” conditions. The valley was subaerially incised during the Messinian Salinity Crisis and rapidly flooded by the sea at the beginning of Pliocene, then filled by eleven coarse-grained Gilbert-type deltas during Early–Middle Pliocene time. The basal Messinian unconformity is locally paved with subaerial scree breccias and bioclastic shallow-marine sandstones, and blanketed by bathyal marls. These deposits record the lowstand, transgressive and early-highstand systems tracts of the first valley-fill sequence. The subsequent progradation of Gilbert-type deltas occurred in four stages, or depositional sequences, separated by transgressive marine-marl intervals. Within each depositional sequence, the deltaic bodies display offlapping architecture, recording falling shoreline trajectory, downward shifts in facies, and overall forced regression. The water depth and accommodation in the inundated coastal valley was gradually decreasing with time. The reduced accommodation allowed the youngest deltas to prograde out to the shelf edge, triggering mass collapses and subsequent filling into the newly created slump scars. Some of the deltas probably acted as “canyon-perched deltas” and supplied sediment to the deep-water slope and floor of the Ligurian Basin. The vertical stacking of Gilbert-type deltas is usually attributed, in tectonically active basins, to fault-related subsidence pulses. In Ventimiglia, the accommodation was created by high-frequency eustatic sea-level rises that, probably accompanied by climate controlled reductions in sediment supply, temporarily outpaced uplift, leading to the development of multiple cycles of infill.

Spatio-temporal variations of biomass and abundance in bathyal non-crustacean megafauna in the Catalan Sea (North-western Mediterranean)

The spatio-temporal variations in diversity, biomass and abundance of bathyal invertebrates (excluding decapod crustaceans, which have been analysed elsewhere) from the North-western Mediterranean margin are described. The upper canyon (∼450 m), middle slope (∼650 m) and lower slope (∼1,200 m) habitats were investigated throughout the year. The first two sites are visited daily by a specialised commercial fisheries’ fleet, while the deeper site has not been impacted by fishing activities. A total of 140 species from 12 phyla were collected from the 3 study areas. Of these, the Mollusca, Echinodermata, Polychaeta and Cnidaria were the most speciose groups. The patterns of species diversity (H′) and evenness (E) were similar in time and space. The upper canyon and middle-slope sites were dominated by echinoderms and molluscs, while the lower-slope site was dominated by cnidarians and sponges. Multifactorial ANOVA showed significant spatio-temporal differences in the biomass and/or abundance only in echinoderms, molluscs, cnidarians and polychaetes. The faunal differences observed were explained by habitat type, but not by season. The role of submarine canyons and commercial fishing on shaping the diversity and biomass of bathyal Mediterranean invertebrates is discussed.

210Pb sedimentation rates from the Northwestern Mediterranean margin

A total of 83 cores were collected in the Gulf of Lions continental margins and analysed for 210Pb xs (excess 210Pb) in order to understand sedimentation patterns. Apparent sedimentation rates (ASR) range from 0.65 cm year −1 in the vicinity of the Rhône River mouth to 0.01 cm year −1 in the deep basin. Except for the prodelta area, rates decrease with depth linearly with the water depth. On the slope, ASR do not differ between canyons and open slope, except for the western area where the rates are slightly higher in the Lacaze–Duthiers canyon compared to its adjacent, open slope. In the canyon and open slope areas, mass accumulation rates determined from 210Pb xs profiles (0.10 and 0.08 g cm −2 year −1, respectively) are in good agreement with particulate fluxes calculated from 5 years of near-bottom sediment trap data, even when the trap particle fluxes and the apparent accumulation rates are overestimated in response to resuspension and bioturbation effects. However, differences in sediment trap data, between west and east portion of the slope, are not observed in the sedimentation rates calculated with 210Pb xs. The outer shelf area may have an important role in trapping sediment but it is not sufficiently documented. Sediment surface mixed layer depths decrease with water depth, with a mean value for the whole margin of 8±6 cm. 210Pb xs inventories in the sediment are systematically higher than the net 210Pb export flux estimated from the above water column. Over the margin, the ratio between accumulated 210Pb and available 210Pb is about 3, suggesting boundary scavenging effects and advective transport.

Neutral monosaccharides in surface sediments of the northwestern Mediterranean Sea

This paper is focused on carbohydrates, an abundant chemical class in marine organic matter, mostly found as polysaccharides. An experiment showed that the use of H 2SO 4 1 M (95 °C, 4 h) yielded a good compromise between recovery and degradation of all neutral monosaccharides from two kinds of organic material: surface sediments and trapped particles. Twenty samples of surface sediments (0–0.5 cm) were collected in various physiographic and bathymetric locations on the Northwestern Mediterranean margin. Neutral monosaccharides were determined by an ionic chromatography system with electrochemical detection (HPAEC–PAD). Differences between total neutral monosaccharides (NM TOT) and total carbohydrates, determined by a spectrophotometric method on the same samples, brought further assumptions on the nature of the carbohydrate pool. Total neutral monosaccharides contents varied from 0.46 to 1.04 mg g −1 and their proportions donated to the organic carbon pool (NM TOT–C/OC) barely reached 5%. The spatial distribution of NM TOT and organic carbon (OC) clearly showed that organic inputs, originating from the shallower epicontinental area, spread along canyon axis while undergoing dilution processes. In spite of the high physiographic and bathymetric diversity, surface sediments from most stations were characterized markedly by the homogeneity in the neutral monosaccharide compositions. Two stations, E21 (Ebro prodelta) and EC36 (Balearic slope) showed, however, distinct signatures with high relative abundances of glucose and galactose, respectively. Glucose was found to trace the continental inputs, whereas galactose characterized the marine inputs. The relative “freshness” of the organic material at station EC36, assumed by the high proportion of hexose neutral monosaccharides, confirms the dominance of phytoplankton remains at this station. This work showed that the use of monosaccharides as tracers of the biological source and fate of organic compounds is deeply linked to the quality of the organic material. The monosaccharide signature is only preserved in samples of surface sediments where inputs of fresh organic material are higher than the refractory one.

Sedimentary mechanisms and underlying tectonic structures of the northwestern Mediterranean margin, as revealed by comprehensive bathymetric and seismic surveys

The tectonic structures, lithological contrasts and sedimentary processes of the northwestern Mediterranean margin are revealed by a comprehensive survey, including recently acquired Seabeam and seismic profiling data. Mass-wasting by gravity flow processes occurs over this entire area. Gravity slides are concentrated on the thick Plio-Quaternary formations located in the interchannel areas. They can lead to surprisingly steep walls, and contribute to the widening of the canyons. The contrasts between these superficial formations and the underlying Neogene formations are morphologically well expressed. The slide products, sometimes directly observed as voluminous erratic blocks, can seal the deep-sea channels, and cause their migrations. Areal extent of these slides can be very large especially with respect to the small size of the basin. However, the turbidity currents are the main gravity flow processes leading to the construction, during the Quaternary glacial epochs, of the Rhône deep-sea fan and other sedimentary ridges. They are responsible for the formation of particular bedforms such as circular depressions into the valley floors or sediment waves on the interchannel areas. The wide extent of the gravity flow processes can be explained by the particular geological setting of this margin, which is a mountainous borderland with steep slopes and rapid deposition of undercompacted sediments. The general instability is favored by recent tectonic movements, linked with the Alpine setting, and especially in the deepest areas, by the lubricating effects of the underlying Messinian salt layer. Under the loading effect of the sedimentary cover, this salt layer crops out around the Rhône deep-sea fan as a peripheral belt of diapirs having shapes controlled by deep tectonic structures.

Quantitative distribution of deep-sea meiobenthos in the northwestern Mediterranean (Gulf of Lions)

The distribution of metazoan meiofauna abundances, in relation to environmental factors and “food indicators”, was examined in 29 surface sediment samples taken between 672 and 2367 m in five canyons on the northwestern Mediterranean margin in summer and autumn 1986. Almost all the parameters measured decreased significantly, with increasing water depth. Meiofauna densities were dominated by nematodes (92.4%), and ranged from 36 to 1005 individuals 10 cm −2. They were positively correlated with sediment water content (30–70%), organic carbon (2.7–7.8 mg g −1) and nitrogen (0.3–1.1 mg g −1) concentrations, chloroplastic pigment levels (0.35–8.29 μg g −1) as well as viable bacterial counts [9 × 10 −3 −3 × 10 −5 Colony Forming Units (CFU) ml −1], and microbial total uptake (5.8–50.6% after 4 h) and respiration (2.3–19.0%) of 14C-glucose. They were only poorly or were not correlated with sediment granulometry (56.7–97.0% < 40 μm), total microbial uptake (7.1–51.0% after 4 h) or respiration (2.0–24.0%) of 14C-glutamic acid. Principal Component Analysis was used to describe and evaluate associations of parameters and samples. The quantitative distribution of meiofauna along the margin is well reflected by the distribution of “food indicators”; this can be explained by “distance from land masses” and “water depth”. In the Gulf of Lions, meiofaunal abundance decreases more rapidly with increasing depth than in other oceans, because the margin includes two different systems: (1) the upper slope, which is comparatively rich and active due to continental and coastal (high primary productivity) influence, related to rivers that induce downslope transport of organic-rich fine sediment; (2) the lower slope and basin mainly fuelled by the low offshore productivity, combined with relative high degradation rates in the warm (13°C from 100 m, to the seabed) waters. The comparison of summer and autumn samples revealed the existence of temporal variability (meiofauna abundances doubled from summer to autumn over the entire area), related to seasonal inputs of organic matter into the benthos.

Fluxes and budget of organic matter in the benthic boundary layer over the northwestern Mediterranean margin

Sediment traps were deployed at depths of 26 and 645 m at two stations on the continental margin of the Gulf of Lions (northwestern Mediterranean). During the same period, surficial sediments were sampled by box corer. The material collected by bottom sediment traps and in corresponding surface sediments was analysed for total organic carbon, hydrolysable organic carbon, nitrogen, sugars, amino acids and lignin-derived compounds. Seasonal variations in organic inputs and the difference between particles from bottom layers and sediment were compared. For the continental shelf station, the annual averages of organic compound fluxes were found to be: 552 mg m −2 d −1 (orgC), 183 mg m −2 d −1 (N), 283 mg m −2 d −1 (hydrolysable orgC), 181 mg m −2 d −1 (Ceq. glucose) and 478 mg m −2 d −1 (amino acids). These values would have to be reduced by half if the large fluxes of autumn, due to resuspension during storm events, were excluded. For the slope, the average annual fluxes were evaluated as: 92.7 mg m −2 d −1 (orgC); 9.4 mg m −2 d −1 (N); 74.1 mg m −2 d −1 (hydrolysable orgC); 11.8 mg m −2 d −1 (Ceq.glucose); and 68.2 mg m −2 d −1 (amino acids). The values obtained for material trapped over the shelf are 4–7 times (orgC and amino acids) and 15–19 times (sugars and nitrogen) higher than for the slope. In contrast, the content in organic compounds of surficial sediments on the slope is 2–3 times higher than that of the shelf deposits. Budgets of orgC transformation at the sediment-water interface were based on calculations which include bottom orgC fluxes, sedimentation rates and orgC content for the first centimetre of deposits. For the continental shelf area, 5.3 g m −2 y −1 have accumulated and 16.7 g m −2 y −1 are mineralized. For the canyon and adjacent slope, the figures are 0.4 and 0.6 g m −2 y −1, respectively. Over the upper adjacent slope, the major part of organic matter is transported by advective processes, which contribute to the sediment interface, especially by chanelling through the canyon axis. Advective inputs of fresh organic material are maximized in late winter-early spring. Nearly 50% of the continental shelf organic matter is assumed to be exported from the shelf, to the adjacent slope.