X. Durrieu de Madron

Flow variability in the Gulf of Lions during the MATER HFF experiment (March-May 1997)

Abstract Hydrological and current meter data were gathered during the High Frequency Flux Experiment that took place in the Gulf of Lions from March to May 1997, within a 20×40-km experimental box over the shelf edge and the continental slope offshore of Marseille. The data set has been supplemented and jointly analysed with sea surface temperature images to characterise flow variability with a particular regard on the mesoscale effects associated to the regional circulation. The current meter observations showed a southwestward flow characteristic of the Northern Current. It is strongest near the shelf break, constrained to flow along topography near the bottom but much less polarised over the upper and mid-slope. Mesoscale variability shows up as fluctuations around 3.5- and 7-day periods at 250- and 650-m depth, and around 7-day periods at 1230-m depth. The Northern Current, as observed on satellite images, forms a 30-km-wide stream with meanders displaying length scales longer than 60 km. These meanders are observed to embrace smaller structures, which are responsible for the intense mesoscale activity recorded in current meter data. Both the hydrological observations and current meter data down to 650-m depth match the superficial structures. Two major flow patterns are observed during the experiment: (i) the core of the Northern current flows south of the experimental site during March and early April, with transitory eddies moving over the experimental site; (ii) following the traverse of a westward-propagating large meander on mid-April, the Northern Current remains over the experimental site. We provide evidence that the synoptic current variability observed in the upper layer may be related to baroclinic instability. A distinct near-bottom current variability is also documented on the mid-slope. Bottom trapped topographic waves are proposed as the mechanism which produces the along-slope deep flow variability.

Hydrographic structure and nepheloid spatial distribution in the Gulf of Lions continental margin

Abstract A general hydrographic and nephelometric survey of the Gulf of Lions margin was undertaken, under autumn conditions. The distribution of suspended material along the margin during this experiment is controlled, at least in part, by the following factors: (a) the hydrography of the shelf-slope waters, i.e. the nepheloid layers follow the isopyenals; and (b) the cyclonic circulation of the water masses (the Liguro-Provencal Current and, in particular the proximity and depth of the Levantine Intermediate Water circulation). On the northeastern part of the margin along the slope, the seaward extension of the nepheloid layers is sharply bounded and is concentrated in canyon heads by the general water circulation. In the southwestern part, the decreasing depth of the major flow of the general circulation and its increased distance seaward from the upper slope allow the seaward and downward expansion of the nepheloid structures. The suspended material extending offshore is swept away and diluted by the general circulation. The stepwise increase toward the southwest, in suspended particulate contents in the slope waters between the northeastern and southwestern ends of the Gulf of Lions, is assumed to be due mainly to inputs from the shelf through the canyons. The Marseille canyon, at the northeastern part of the Gulf of Lions margin, is influenced less by the Rhoˆne and other rivers of the shelf. The influence of the Rhoˆne is seen first at the longitude of the Rhoˆnes canyons. In the southwestern part of the Gulf, the Bourcart and Lacaze-Duthiers canyons are areas through which the suspended material, originating from the whole shelf, passes.

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.

Dense shelf water cascading in the northwestern Mediterranean during the cold winter 2005: Quantification of the export through the Gulf of Lion and the Catalan margin

Dense shelf water cascading in the northwestern Mediterranean Sea during winter 2005, which was shown to cause large erosion in the canyons and to influence deep benthic ecosystem, was investigated using numerical modeling validated with temperature and current observations. Intense dense water formation took place on the Gulf of Lion and Catalan shelves. Dense shelf water was transferred to the deep basin through three pathways. The Cap de Creus canyon in the western Gulf of Lion already identified as a huge pathway was shown to export about 1000 km3 of dense water during two months. The Palamos and Blanes canyons located on the Catalan margin were shown to be important pathways for water formed locally and for water transiting from the Gulf of Lion. After the cascading period, dense shelf water was transported mostly toward the Balearic Sea.

Observations of open‐ocean deep convection in the northwestern Mediterranean Sea: Seasonal and interannual variability of mixing and deep water masses for the 2007‐2013 Period

We present here a unique oceanographic and meteorological data set focus on the deep convection processes. Our results are essentially based on in situ data (mooring, research vessel, glider, and profiling float) collected from a multiplatform and integrated monitoring system (MOOSE: Mediterranean Ocean Observing System on Environment), which monitored continuously the northwestern Mediterranean Sea since 2007, and in particular high-frequency potential temperature, salinity, and current measurements from the mooring LION located within the convection region. From 2009 to 2013, the mixed layer depth reaches the seabed, at a depth of 2330m, in February. Then, the violent vertical mixing of the whole water column lasts between 9 and 12 days setting up the characteristics of the newly formed deep water. Each deep convection winter formed a new warmer and saltier “vintage” of deep water. These sudden inputs of salt and heat in the deep ocean are responsible for trends in salinity (3.3 ± 0.2 × 10−3/yr) and potential temperature (3.2 ± 0.5 × 10−3 C/yr) observed from 2009 to 2013 for the 600–2300 m layer. For the first time, the overlapping of the three “phases” of deep convection can be observed, with secondary vertical mixing events (2–4 days) after the beginning of the restratification phase, and the restratification/spreading phase still active at the beginning of the following deep convection event.

Circulation and distribution of suspended matter in the Sporades Basin (northwestern Aegean Sea)

Abstract A hydrographic survey, comprising 85 stations with measurements of temperature, salinity and turbidity, was performed in the semi-enclosed margin of the Sporades Basin (northwestern Aegean Sea) in June 1987. The data have been used to investigate the shelf-slope suspended matter exchanges in relation to the hydrology. The circulation, derived from hydrographic data, produces complex eddy patterns. The suspended matter distribution, determined from light scattering measurements, indicate a relationship between the suspended matter and the density-related flow field. Transport of suspended matter, supplied by rivers, is traced through the bottom nepheloid layer, mostly over the inner shelf and along the western coastline. There is however some evidence of transfer of suspended matter from the shelf towards the upper slope. The mid-depth nepheloid layer along the slope is thought to relate to advection of terrigenous material by topographically-controlled currents, and its seaward extension is inhibited by a strong anticyclonic circulation in the deep basin.

Comparison of sediment resuspension measurements in sheared and zero-mean turbulent flows

There are multiple processes at different scales that produce turbulent mixing and sediment lift off in the ocean bottom. Basic aspects of buoyancy and stratification in turbulent flows are used to investigate the lift off of sediments and we describe here some of the innovative techniques used in the laboratory. The present paper describes some of the work that our group has performed within the European Land Ocean Interaction Studies (ELOISE). Further, in a fully detailed paper, a presentation of all the experimental work as well as their results will be submitted. We present in this paper two laboratory experiments with the results of sediment lift off under sheared and zero-mean flow turbulence. We compare actual seabed sediments extracted from the Gulf of Lyon. Comparison is made between the boundary Reynolds stress derived from turbulent r.m.s. velocity, u 0 ; in the zero-mean flow turbulence, and from the friction velocity, u� (Shields critical parameter) in the shear-induced turbulence flow at the sediment laden bottom boundary layer. There is an increase in the sediment lift-off as boundary Reynolds stress increases, whereas there are important differences between zero-mean and shear-dominated boundary flows. Lower boundary Reynolds stresses are required in zero-mean flows to lift off a sediment bed of equal characteristics than in shear-induced flows, indicating the dominant role of r.m.s. velocity fluctuations at the sediment bed. r 2001 Published by Elsevier Science Ltd.

Deep sediment resuspension and thick nepheloid layer generation by open-ocean convection

The Gulf of Lions in the northwestern Mediterranean is one of the few sites around the world ocean exhibiting deep open-ocean convection. Based on 6-year long (2009-2015) time series from a mooring in the convection region, shipborne measurements from repeated cruises, from 2012 to 2015, and glider measurements, we report evidence of bottom thick nepheloid layer formation, which is coincident with deep sediment resuspension induced by bottom-reaching convection events. This bottom nepheloid layer, which presents a maximum thickness of around 2000 m in the center of the convection region, probably results from the action of cyclonic eddies that are formed during the convection period and can persist within their core while they travel through the basin. The residence time of this bottom nepheloid layer appears to be less than a year. In-situ measurements of suspended particle size further indicate that the bottom nepheloid layer is primarily composed of aggregates between 100 and 1000 µm in diameter, probably constituted of fine silts. Bottom-reaching open ocean convection, as well as deep dense shelf water cascading that occurred concurrently some years, lead to recurring deep sediments resuspension episodes. They are key mechanisms that control the concentration and characteristics of the suspended particulate matter in the basin, and in turn affect the bathypelagic biological activity

Comparison of Models for Assessing the Radiological Impacts of Deep-Sea Disposal of Radioactive Wastes

The radionuclides dispersion from the low-level waste drums dumped in the North Atlantic NEA site has been assessed by three independent box models developed in UK, in USA and in France within the international “Co-ordinated Research and Environmental Surveillance Programme”.