André Monaco

Particle fluxes and ecosystem response on a continental margin: the 1985–1988 Mediterranean ECOMARGE experiment

Abstract The first experiment of the ECOMARGE programme (ECOsystemes de MARGE continentale) was initiated in 1983–1984, in the Gulf of Lions (northwestern Mediterranean Sea). The objectives of the ECOMARGE—I experiment were: to quantify the transfer of particulate matter, in general, and of organic carbon, in particular, from its introduction to and formation in the waters of the continental shelf—to its consumption or sedimentation on the shelf or its transfer to the slope and deep sea; and to understand the processes involved in that transfer, consumption and sedimentation together with their variability in space and time. The results of that experiment, from 1983 to 1988, are presented in this Special Issue. The highlights of the results are summarised in this paper. These results indicate that, of the particles formed in the waters of the continental shelf and those introduced by rivers, some are deposited as sediments on the shelf. A portion is transported offshore, however, to the slope and deep sea. The Rhoˆne River, in the northeastern part of the study area, is the major source of continental material; this is transported to sea in a benthic nepheloid layer and, mostly, alongshore to the southwest. Here, it largely leaves the shelf through the canyons, especially the Lacaze-Duthiers Canyon. In the offshore waters, particle concentrations and distributions show surficial, intermediate and benthic nepheloid layers. These turbid structures increase towards the southwest, corresponding to the seaward shift of the front between the coastal waters and the Liguro-Provencal cyclonic gyre, a major forcing function in the Gulf of Lions. Considering the source and fate of particles (largely biogenic from the euphotic zone and abiogenic from deeper waters) a layered system is described, which is emphasized by the concentrations of natural and artificial elements and compounds. Of the flux of particles to the Lacaze-Duthiers Canyon, on a decadal scale, about 30% (as a minimum) is estimated to be stored as sediment; the remainder is transported down-canyon, towards the deep sea. The temporal variability of processes affecting this net seaward transport, of both biogenic and abiogenic material, is from hours, days to seasonal, and probably interannual, time scales. The response of the system to these variations is rapid, with pulses of increased discharge of particles from the adjacent shelf being detected in sediment traps in the Lacaze-Duthiers Canyon in less than 16 days (the temporal resolution of the traps). Based upon the study of tracers of particulate matter and environmental factors (i.e. river discharge and climatic conditions), it appears that the contribution from the Rhoˆne River and its adjacent area is maximal during the winter; at this time, the flow of the Liguro-Provencal Current also increases. In contrast, the maximum relative contribution of the adjacent southwesterly area to the flux in the Lacaze-Duthiers Canyon occurs in summer, during storm events.

Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)

An analysis of sedimentological, sediment trap and hydrological data was performed to investigate the transport and fate of particulate matter in the Gulf of Lions. The sedimentological properties outline the major sedimentary units of the shelf (Rhone prodelta, mid-shelf mud belt, outer shelf) and slope. The geometry of these sedimentary units and the southwestwards increase of particulate fluxes on the slope highlight the influence of the general cyclonic circulation on the dispersion of land-derived particulate matter. Considering the known input and output terms, budgets of particulate matter and organic carbon in the Gulf of Lions are proposed. Inputs were river supply, atmospheric deposition and primary production; outputs were sediment burial and advective export on the slope. Degradation of particulate organic carbon in the water column and at the sediment-water interface was also estimated. Mass and POC budgets were balanced within uncertainties. Nevertheless, these results illustrate the difficulty in establishing budgets based on elements estimated at different time scales, for which the system is not necessarily in a steady state.

Seasonality and composition of particulate fluxes during ECOMARGE—I, western Gulf of Lions

As part of the ECOMARGE program (ECOsystemes de MARGE continentale), sequential sediment traps were deployed on the continental margin of the Gulf of Lions, in the northwestern Mediterranean Sea. Two sites located in the southwestern part of this region were selected for the first phase of the experiment: inner shelf (bottom depth: 27 m) and continental slope (Lacaze-Duthiers Canyon, bottom depth: 650 m). The canyon was selected as representative of the continental slope, because canyons comprise more than 50% of the slope area. Total mass, organic matter, opal, carbonate and siliciclastic residue fluxes were measured biweekly at 50, 100, 300 and 600 m in the upper part of the Lacaze-Duthiers Canyon (from July 1985 to April 1986) and for longer periods (2 weeks to 2 months) at 10 and 25 m on the shelf (from May 1985 to June 1986). Mass fluxes increased generally with depth, reaching values as high as 20,000 mg m−2 d−1 in the 600 m slope (canyon) trap; this indicated lateral transport of biogenic and abiogenic particulate matter, from local (adjacent shelf and upper slope waters) and distant origin (Rhoˆne River). Shelf-slope particulate transfer attested by the paracontemporaneity of high flux events on the shelf and the slope and by the continuity of the shelf benthic nepheloid layer and intermediate nepheloid layers over the canyon: this rapid and took generally less than 16 days (trap sample resolution time). Particle fluxes and compositions defined a layered system, in which the surface layers< 100m) were essentially characterized by biogenic material (organic matter and opal), and deep layers chiefly characterized by the collection of elastic material (carbonate and siliciclastic material). Significant temporal increases in total mass and constituent fluxes showed two frequencies: a low seasonal frequency, which was related to seasonal variations of the Liguro-Provencal Current and to the winter increase of the Rhoˆne River and other coastal river discharges; variations at higher frequencies, which were related to various impulse events such as summer internal waves, autumn and winter storms and spring nutrient enrichment. The role of the Lacaze-Duthiers Canyon in the particulate transfer across the continental margin is discussed within the particular context of this advective system. Although the importance of local, intra-canyon resuspension of bottom sediments could not be estimated, it seems that the high suspended particulate matter concentrations and particle fluxes observed are essentially related to the fact that this canyon acts as a natural trap which collects particles from the entire Gulf of Lions. Comparison of the flux necessary to sustain the unsupported210Pb inventory in the canyon sediments, with the mean210Pb flux measured by traps and of210Pb-determined mass accumulation rates with total mass fluxes indicate that the canyon partly acts as a modern sediment depocenter of the particles swept into it from the shelf and the slope; also partly as a modern conduit of particles to the deep basin. Several biological mechanisms are probably responsible for the rapid settling of particles onto the slope (within the canyon); these may, thereby, reduce lateral transfer from the shelf to the open ocean and constitute a kind of “biological barrier”.

Origin and variability of downward biogeochemical fluxes on the Rhone continental margin (NW mediterranean)

A one year study of downward particle fluxes conducted in the northwestern Mediterranean Sea is presented. Two mooring lines equipped with sediment traps and current meters were deployed at around 1000 m depth on the northeastern continental slope of the Gulf of Lions, one inside the Grand-Rhone canyon and the other outside on the adjacent open slope. Mean total mass fluxes increased slightly with trap depth inside the canyon, a feature quite typical of fluxes in continental margin environments. The near-bottom trap inside the canyon collected more material than its counterpart deployed at equivalent depth on the open slope, indicating a preferential transport of material within the canyon. Major biogeochemical constituents (organic and inorganic carbon, opal, and siliciclastic residue) revealed a marked difference in particle composition between the sub-surface (80 m) and deeper traps, suggesting the existence of at least two sources of material. The two shallower traps showed a clear biological signal: flux peaks were related to periods of surface biological production, especially perceptible in summer and autumn. The particulate matter trapped at deeper levels in the canyon and on the open slope was characterized by a more stable composition with a major lithogenic contribution, originating from sedimentary material most probably resuspended on the upper- or mid-slope. The seasonal variability was dominated by the summer/winter alternation; the latter period was characterized by a weak stratification of the water column and an enhanced current variability favoring vertical exchanges. The present results are compared with those obtained previously in the Lacaze-Duthiers canyon on the southwestern side of the Gulf of Lions. The comparison shows strong differences between the NE entrance and the SW exit of the gulf, with respect to the general along-slope circulation of water masses, both in terms of intensity of particulate fluxes and transport processes.

Sediment dispersal and accumulation on the continental margin of the Gulf of Lions: sedimentary budget

Abstract An integrated study of particle transport, deposition and storage was carried out over the continental margin of the Gulf of Lions (northwestern Mediterranean Sea). The dispersal system of fine-grained sediment is discussed and a sedimentary budget of sediment transfer and accumulation is established. Sediment cores were collected from the continental shelf and slope, along the ECOMARGE—I transect. Accumulation rates were measured using radiochemical techniques. Sediment traps were deployed, from 1985 to 1986, at two sites: on the continental shelf (27 m) and in the Lacaze-Duthiers Canyon (645 m). This study revealed that the most important sedimentary processes on this margin are high seasonal variability of lithogenic particle fluxes and lateral advection. The major fluctuations are related to storm events, to an increase in river discharge and the intensity of the regional current. Flux gradients increase exponentially toward the bottom, in shallow and deep water. However, in the deeper traps at the two sites annual lithogenic fluxes decrease seaward, from 25 to 3.5 g m −2 day −1 . Across this margin, sediment accumulation rates ranged from 2.9 g m −2 day −1 , on the middle of the prodeltaic area, to 0.16 g m −2 day −1 at 645 m in the canyon. Smectite was found to be a natural tracer for characterizing an annual cycle of sediment input, deposition, resuspension and shelf-slope exchange processes. At the shelf site, two intra-annual cycles of sedimentation and erosion of the surface sediments were recognized. Comparison between trap deposition rates and bottom sediment accumulation rates implies that the prodelta area acts as a zone of storage (11%) for the silty material and as a temporary sink for fine-grained sediment, that is eroded from the area in winter. In the canyon, variations in deposition rates, smectite content, and grain size are related to morphological features and the various modes of transport that control sedimentation. The discontinuity in the flux gradient between surface and deep layers, and the discrepancy between long-term accumulation rates based on 14 C dating and trap deposition rates suggest that fluxes in the deeper layer in the canyon are in fact essentially controlled by settling from the intermediate nepheloid layers detached from the shelf break and by along-bottom transport; also that resuspension has a longterm impact on the entire canyon maintenance. On the other hand, comparing near-bottom trap 210 Pb fluxes to the fluxes required to support the excess 210 Pb inventories in the sediments leads to the conclusion that much (70%) of the particulate flux passes through the canyon to the deep sea on this short time scale (

Downward fluxes of settling particles in the deep Cretan Sea (NE Mediterranean)

Abstract During the CINCS project (Pelagic–benthic Coupling IN the oligotrophic Cretan Sea—NE Mediterranean), a single mooring with two sediment traps (at 200 and 1515m water depth) and two current meters was deployed in the southern Cretan Sea margin at a depth of 1550 m. A second mooring deployed at the 500 m station was lost, as a result of fishing activities. The duration of the study was 12 months (November 1994 to November 1995) with sampling intervals of 15 or 16 days. The traps were retrieved, serviced and the sedimented material was collected every 6 months. In total, 48 samples were collected (24 from each trap) throughout the study period and fluxes of total particulate mass, opal, organic matter, carbonates, and lithogenic component were measured. Natural radionuclides (210Po and 210Pb) were determined for all trap samples. Total mass flux and the fluxes of four major constituents increased with depth, the total mass flux reaching values of nearly 550 mg m−2 d−1 at 1515 m and 187 mg m−2 d−1 at 200 m depth, following the same patterns observed in other experiments (ECOMARGE, SEEP-I, SEEP-II). The mean annual mass fluxes were 209 and 49.8 mg m−2 d−1 at the near bottom and near surface trap respectively. This suggests that lateral transport of particulate matter is of importance in the area. Total mass fluxes at the two depths were characterized by different seasonal fluctuations, although a general decreasing trend was observed from the I (winter) to the II (summer) deployment at both depths. This was mainly a result of reductions in aluminosilicate inputs during the summer dry period. At 200 m depth carbonates were more important during winter, because of a large carbonate input consisting mainly of coccoliths of Emiliania huxleyi, while during the summer decreased fluxes of carbonates and aluminosilicates resulted in a reduction of the mass flux. In contrast, at 1515 m depth the lithogenic component was the dominant component during the winter deployment, indicating a terrigenous input. During the summer period the decrease in mass flux was strongly effected by the decrease in aluminosilicates. There was a diminution in the organic carbon content with a concomitant increase in total mass flux, which, together with the almost negligible increase in the annual 210Pb activity with depth and the increase of 210Po activity with depth could be interpreted as indicating a contribution of resuspended material to the input at 1515 m. The complex mesoscale circulation of the Cretan Sea, consisting of a cyclone (east)–anticyclone (west) system, controls particle transfer in the area. This hydrodynamic system seems to move water masses towards the southern Cretan Sea margin, and consequently carry materials from the open sea to the upper slope and shelf.

Modern pollen deposition in the Rhone delta area (Lagoonal and marine sediments), France

The investigation identifies the two main types of pollen sedimentation in the Rhone delta, and presents synthetic pollen spectra in terms of ecological groups in order to predict the pollen source areas. Two different domains are studied: the lagoonal zone occupied by the Vaccares where atmospheric pollen imput predominates and the Rhone mouth area (river delta and prodelta) where fluvial pollen influx prevails. The reflection of local and regional vegetation in the surface sediment pollen spectra is carefully discussed with reference to atmospheric pollen records and to water sample analyses. This work is designed to help the interpretation of past pollen spectra and paleovegetation and through them also paleoclimate reconstructions.

Stable isotopes (13C/12C and 15N/14N) insettling organic matter of the northwestern Mediterranean Sea: biogeochemical implications

Within the framework of the High Frequency Flux (HFF) experiment (MATER programme), time-series sediment traps have been deployed for two months on the continental slope off Marseilles to measure downward particle fluxes at a high frequency sampling rate (two and six days). Combined isotopic analyses of carbon and nitrogen have been performed on selected samples. Both isotopic tracers have been used for the first time on organic material of the Mediterranean Sea (Gulf of Lions) to determine the main biological sources and to address the biogeochemical processes that affected this material. Settling particles were characterised by very low values of delta N-15 (near 0 parts per thousand) and delta C-13 (near -24.5 parts per thousand) that indicate the existence of a mixed material with two sources that differ according to the considered element (C or N). The terrestrial source slightly dominates the carbon pool because of its higher C:N ratio, whereas the nitrogen pool may mostly originate from N-2-fixing cyanobacteria (delta N-15 = 0 parts per thousand, low C:N ratio). These preliminary data suggest that dissolved atmospheric N, may act as a significant new nitrogen source in the Mediterranean Sea.

Valencia Fan (northwestern Mediterranean): Distal deposition fan variant

Abstract The Valencia Fan, one of the larger deep-sea depositional systems in the western Mediterranean Sea, developed in the large depression between the Valencia Trough and the Balearic Basin Plain. Six main lithoseismic units are identified and sedimentary processes inferred from 6000 km of sparker profiles. This fan is largely formed by channelized and irregularly stratified units. The sedimentary processes controlling the development of these units include channelized sediment flows that evolve downfan into sheet flows. Three fan depositional provinces are differentiated on the basis of the relative proportions of lithoseismic units and the inferred sedimentary processes. Regularly stratified seismic units predominate in the non-fan environments. These units are dominated by fine-grained deposits resulting from hemipelagic settling and overbank flows from turbid currents. Distal flows from the continental slopes of the Iberian Peninsula and Balearic Islands also contribute materials for the development of these environments. The wavy units flanking the upper fan may have resulted from migrating sediment waves or limited mass-displacement of the stratified units, while the transparent units are attributed to extensive mass-flow. The Valencia Valley is largely an erosional feature across which sediments from several source areas bypassed to the distal, deep-sea depositional system of the Valencia Fan. Deposition begins at the mouth of the valley beyond which there is a break in slope. The fan migrated upslope and filled the Valencia Valley from the distal sectors. This evolution has been controlled by the interplay between sedimentary processes active in the fan growth and large-scale geological factors related to the structural framework of the study area.

210Pb, manganese and carbon: indicators of focusing processes on the northwestern Mediterranean continental margin

Abstract Organic carbon contents, 210 Pb data, and organic and oxidised forms of manganese in sediments are used for reconstructing the shelf and deep-sea transport of particulate matter and for evaluating early diagenesis conditions. The latter are used as indicators of focusing processes on the northwestern Mediterranean continental margin. Twelve different physiographical sites were sampled between 400 and 2000 m in the Pyrenean and Golfe du Lion canyon axes and on their open-slopes. In the canyon axes, which indent the Pyrenean slope, 210 Pb activities show a regular exponential decrease with sediment depth: computed sedimentation rates range from 170 to 198 cm 10 −3 yr. For an identical regular exponential decrease in the Golfe du Lion canyon, the sedimentation rate is lower (119 cm 10 −3 yr). On the lower open slopes and deep-sea fans, rapid deposition (turbidites and surficial sliding) occurs onto sediments that were deposited regularly at a rate of about 175 cm 10 −3 yr. A deposit enriched in organic carbon is identified at mid-slope between 500 and 1300 m. In this area, surface sediments of the canyon axes are richer in organic carbon than at the adjacent open slope sites (0.9% against 0.6% dry weight). Canyon axes are more active than open-slopes as channels for organic matter transfer. In the axial channel of the Pyrenean canyon, organic carbon decreases exponentially in the uppermost 12 cm. In the Golfe du Lion canyons, only the upper 2 cm of sediment are enriched in organic carbon. This is related to lower sedimentation rates in the Golfe du Lion area as compared to the Pyrenean area. On the upper and lower open slopes, the organic carbon contents decrease rapidly within 1–2 cm of sediment. There is no exponential evolution profile and the residence times at the sediment-water interface are longer than in the canyons. The highest surface Mn concentrations are also found in the middle of the continental slope. The average is 64 μg g −1 for Mn associated with the organic phase (Mn(org)) and 1400 μg g −1 for (Mn(Ox)). As for organic carbon contents, canyons are areas of high concentrations. The lowest values (4 μg g −1 for Mn(org) and 495 μg g −1 for Mn(Ox)) are found for the lower open-slope (1200–1700 m) where there is active surficial remobilisation and gravity mass movements. Moreover, the vertical distribution of Mn has allowed us to identify sites located at depths of about 1000 m where the redox discontinuity is located in the first few centimetres and where the (Mn(Ox)) and the (Mn(org)) profiles display a positive gradient from the deepest layers to the sediment-water interface. The distribution of the various parameters was caused by three different mechanisms: (1) bottom advection of particulate matter transferred from the continental shelf deposits to the slope along the canyon axes; (2) scavenging of the dissolved phases of metals and (3) sedimentary focusing of the particles in a “favoured” area of the continental slope—a mid-slope depocenter. These data contribute to our knowledge of the role of continental margins in the global recycling of carbon and other materials.