Xavier Durrieu de Madron

Flushing submarine canyons

The continental slope is a steep, narrow fringe separating the coastal zone from the deep ocean. During low sea-level stands, slides and dense, sediment-laden flows erode the outer continental shelf and the continental slope, leading to the formation of submarine canyons that funnel large volumes of sediment and organic matter from shallow regions to the deep ocean1. During high sea-level stands, such as at present, these canyons still experience occasional sediment gravity flows2–5, which are usually thought to be triggered by sediment failure or river flooding. Here we present observations from a submarine canyon on the Gulf of Lions margin, in the northwest Mediterranean Sea, that demonstrate that these flows can also be triggered by dense shelf water cascading (DSWC)—a type of current that is driven solely by seawater density contrast. Our results show that DSWC can transport large amounts of water and sediment, reshape submarine canyon floors and rapidly affect the deep-sea environment. This cascading is seasonal, resulting from the formation of dense water by cooling and/or evaporation, and occurs on both high- and low-latitude continental margins6–8. DSWC may therefore transport large amounts of sediment and organic matter to the deep ocean. Furthermore, changes in the frequency and intensity of DSWC driven by future climate change may have a significant impact on the supply of organic matter to deep-sea ecosystems and on the amount of carbon stored on continental margins and in ocean basins.

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.

High resolution modeling of dense water formation in the north-western Mediterranean during winter 2012-2013: Processes and budget

The evolution of the stratification of the north-western Mediterranean between summer 2012 and the end of winter 2013 was simulated and compared with different sets of observations. A summer cruise and profiler observations were used to improve the initial conditions of the simulation. This improvement was crucial to simulate winter convection. Variations of some parameters involved in air - sea exchanges (wind, coefficient of transfer used in the latent heat flux formulation, and constant additive heat flux) showed that the characteristics of water masses and the volume of dense water formed during convection cannot be simply related to the time-integrated buoyancy budget over the autumn - winter period. The volume of dense water formed in winter was estimated to be about 50,000 km 3 with a density anomaly larger than 29.113 kg m -3 . The effect of advection and air/sea fluxes on the heat and salt budget of the convection zone was quantified during the preconditioning phase and the mixing period. Destratification of the surface layer in autumn occurs through an interaction of surface and Ekman buoyancy fluxes associated with displacements of the North Balearic front bounding the convection zone to the south. During winter convection, advection stratifies the convection zone: from December to March, the absolute value of advection represents 58 % of the effect of surface buoyancy fluxes.

Transformation of PBDE mixtures during sediment transport and resuspension in marine environments (Gulf of Lion, NW Mediterranean Sea)

Polybromodiphenyl ethers (PBDEs) in superficial sediments from the Gulf of Lion were studied. They were largely predominated by BDE 209 (98.7% of all PBDEs) indicating that the main source of these pollutants was the commercial mixture deca-BDE. This compound and the less brominated BDE exhibited a southwestward decreasing concentration gradient following the dominant marine currents and bottom relief, e.g. the Mud Belt, the submarine canyons and the Open Continental Slope. All PBDEs exhibited statistically significant correlations confirming the common origin. However, a progressive transformation of the dumped BDE 209 was identified showing a depletion paralleled by increases of the less brominated BDEs (from 8.6% to 22%). These less brominated compounds were accumulated at about 100-140 km away from the Rhone prodelta, e.g., at the end of the submarine canyons, evidencing that these transformation compounds can be accumulated at long distances from the dumping sites in the marine system.

Scales and dynamics of submesoscale coherent vortices formed by deep convection in the northwestern Mediterranean Sea

Since 2010, an intense effort in the collection of in situ observations has been carried out in the northwestern Mediterranean Sea thanks to gliders, profiling floats, regular cruises, and mooring lines. This integrated observing system enabled a year-to-year monitoring of the deep waters formation that occurred in the Gulf of Lions area during four consecutive winters (2010–2013). Vortical structures remnant of wintertime deep vertical mixing events were regularly sampled by the different observing platforms. These are Submesoscale Coherent Vortices (SCVs) characterized by a small radius (∼5–8 km), strong depth-intensified orbital velocities (∼10–20 cm s−1) with often a weak surface signature, high Rossby (∼0.5) and Burger numbers O(0.5–1). Anticyclones transport convected waters resulting from intermediate (∼300 m) to deep (∼2000 m) vertical mixing. Cyclones are characterized by a 500–1000 m thick layer of weakly stratified deep waters (or bottom waters that cascaded from the shelf of the Gulf of Lions in 2012) extending down to the bottom of the ocean at ∼2500 m. The formation of cyclonic eddies seems to be favored by bottom-reaching convection occurring during the study period or cascading events reaching the abyssal plain. We confirm the prominent role of anticyclonic SCVs and shed light on the important role of cyclonic SCVs in the spreading of a significant amount (∼30%) of the newly formed deep waters away from the winter mixing areas. Since they can survive until the following winter, they can potentially have a great impact on the mixed layer deepening through a local preconditioning effect.

Research activities in the Gulf of Lion (NW Mediterranean) within the 1997–2001 PNEC project

Abstract The French national program on the coastal environment Programme National Environnement Cotier (PNEC) represented during the last decade a major support for the pluridisciplinary oceanographic research in the Gulf of Lion. During the second phase of the program, that lasted from 1997 to 2001, the main objective for this worksite was to establish an annual budget of carbon and associated biogenic elements to characterise the role of this region as source and the sink of elements. This introductory paper provides an overview of the research framework and recapitulates the major scientific outcomes.

Ecosystem effects of dense water formation on deep Mediterranean Sea ecosystems: an overview

Natural episodic events, such as gravity flows, submarine landslides, and benthic storms can determine severe modifications in the structure and functioning of deep-sea ecosystems. Here, we report and compare the ecosystem effects produced by dense water formation events that occurred in the Gulf of Lions (NW Mediterranean) and the Aegean Sea (NE Mediterranean). In both regions, the rapid sinking of cold dense waters, driven by regional meteorological forcings, results in important immediate modifications that can be summarised in: (i) increased organic matter content in the deep basin; (ii) diminished benthic abundance; and (iii) changes of benthic biodiversity. At longer time scale the analysis reveals, however, different resilience times in the two regions. The Gulf of Lions is characterized by a very fast (months) recovery whereas the Aegean Sea shows much longer (45 years) resilience time. New long-term studies are further needed to identify the potential effects that changes in the duration, intensity and frequency of episodic events could have on the structure, biodiversity and functioning of the deep Mediterranean Sea under environmental and climate change scenarios.

Multiscale Observations of Deep Convection in the Northwestern Mediterranean Sea during Winter 2012–2013 Using Multiple Platforms

During winter 2012–2013, open‐ocean deep convection which is a major driver for the thermohaline circulation and ventilation of the ocean, occurred in the Gulf of Lions (Northwestern Mediterranean Sea) and has been thoroughly documented thanks in particular to the deployment of several gliders, Argo profiling floats, several dedicated ship cruises, and a mooring array during a period of about a year. Thanks to these intense observational efforts, we show that deep convection reached the bottom in winter early in February 2013 in a area of maximum 28 ± 3 109 m2. We present new quantitative results with estimates of heat and salt content at the subbasin scale at different time scales (on the seasonal scale to a 10 days basis) through optimal interpolation techniques, and robust estimates of the deep water formation rate of 2.0 ± 0.2 Sv. We provide an overview of the spatiotemporal coverage that has been reached throughout the seasons this year and we highlight some results based on data analysis and numerical modeling that are presented in this special issue. They concern key circulation features for the deep convection and the subsequent bloom such as Submesoscale Coherent Vortices (SCVs), the plumes, and symmetric instability at the edge of the deep convection area.

Open‐ocean convection process: A driver of the winter nutrient supply and the spring phytoplankton distribution in the Northwestern Mediterranean Sea

This study was a part of the DeWEX project (Deep Water formation EXperiment), designed to better understand the impact of dense water formation on the marine biogeochemical cycles. Here, nutrient and phytoplankton vertical and horizontal distributions were investigated during a deep open-ocean convection event and during the following spring bloom in the Northwestern Mediterranean Sea (NWM). In February 2013, the deep convection event established a surface nutrient gradient from the center of the deep convection patch to the surrounding mixed and stratified areas. In the center of the convection area, a slight but significant difference of nitrate, phosphate and silicate concentrations was observed possibly due to the different volume of deep waters included in the mixing or to the sediment resuspension occurring where the mixing reached the bottom. One of this process, or a combination of both, enriched the water column in silicate and phosphate, and altered significantly the stoichiometry in the center of the deep convection area. This alteration favored the local development of microphytoplankton in spring, whereas nanophytoplankton dominated neighboring locations where the convection reached the deep layer but not the bottom. This study shows that the convection process influences both winter nutrients distribution and spring phytoplankton distribution and community structure. Modifications of the convection spatial scale and intensity (i.e. convective mixing depth) is likely to have strong consequences on phytoplankton community structure and distribution in the NWM, and thus on the marine food web.

Impact of an intense water column mixing (0-1500m) on prokaryotic diversity and activities during an open-ocean convection event in the NW Mediterranean Sea

Open-ocean convection is a fundamental process for thermohaline circulation and biogeochemical cycles that causes spectacular mixing of the water column. Here, we tested how much the depth-stratified prokaryotic communities were influenced by such an event, and also by the following re-stratification. The deep convection event (0-1500 m) that occurred in winter 2010-2011 in the NW Mediterranean Sea resulted in a homogenization of the prokaryotic communities over the entire convective cell, resulting in the predominance of typical surface Bacteria, such as Oceanospirillale and Flavobacteriales. Statistical analysis together with numerical simulation of vertical homogenization evidenced that physical turbulence only was not enough to explain the new distribution of the communities, but acted in synergy with other parameters such as exported particulate and dissolved organic matters. The convection also stimulated prokaryotic abundance (+21%) and heterotrophic production (+43%) over the 0-1500 m convective cell, and resulted in a decline of cell-specific extracellular enzymatic activities (-67%), thus suggesting an intensification of the labile organic matter turnover during the event. The rapid re-stratification of the prokaryotic diversity and activities in the intermediate layer 5 days after the intense mixing indicated a marked resilience of the communities, apart from the residual deep mixed water patch.