Circulation, transport and bottom boundary layers of the deep currents in the Brazil Basin

Zonal and meridional hydrographic sections obtained for the South Atlantic Ventilation Experiment are used to study the circulation patterns and estimate the transports of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the Brazil Basin. The NADW Deep Western Boundary Current (DWBC) appears to be a relatively large (≈ 800 km wide by 2 km thick), double core current, separated by counterflowing recirculation. It appears to split, branching seaward at the Cape Saõ Roque near 5S and again at the Columbia-Trinidad Seamount Chain at 21S. As a result of this latter bifurcation, the NADW DWBC flow in the southern basin decreases significantly. In the southern part of the basin, the AABW DWBC is a relatively broad (≈ 1000 km), thin (≈ 700 m) flow which hugs the bottom of the continental rise. The densest waters that compose the core of the AABW DWBC eventually separate from the DWBC in the northern part of basin as they are topographically diverted to the east. The southward return flow at the eastern edge of the AABW DWBC and a northward flow in the eastern part of the basin suggest a meandering meridional recirculation of AABW in the interior of the basin. In the north central part of the deep basin there is a cyclonic abyssal gyre with a large component of Weddell Sea Deep Water (WSDW). The along-isobath movement of the DWBCs over the sloping bottom drives cross-slope advection of the bottom boundary layer. The up-slope advection of denser water within the NADW DWBC is believed to set up a slippery bottom layer, while the bottom layer associated with the down-slope advection of lighter water within the AABW DWBC is estimated to be only partially slippery. Geostrophic transports of heat, salt and mass are used to estimate mixing in the AABW flow in the Brazil Basin. The rates at which heat and salt mix are characteristic of diapycnal turbulent mixing. The mixing processes appear to be more active along the western boundary

Bilan des expérimentations dans les dispositifs d'Irobo et de Mopri en forêt dense de Côte d'Ivoire après 12 et 14 ans de mesures

La forêt dense de Côte d'Ivoire couvrait 15 millions d'hectares au début du siècle, environ 2,8 millions d'hectares en 1990. Il est donc nécesaire de trouver des solutions à cette sur-exploitation de la forêt en aménageant celle-ci dans le "domaine forestier permanent" de manière à assurer une production soutenue voire même améliorée, au moyen d'une gestion contenue et réaliste, appuyée sur des règles simples d'aménagement. C'est pour mettre au point ces règles que trois dispositifs d'essais ont été mis en place à partir de 1976 par la SODEFOR : Mopri, Tene et Irobo. Le bilan présenté dans ce rapport concerne Irobo et Mopr

Impact of oceanic floods on particulate metal inputs to coastal and deep-sea environments: A case study in the NW Mediterranean Sea

An exceptional flood event, accompanying a marine storm, was investigated simultaneously at the entrance and the exit of the Gulf of Lion's hydrosystem (NW Mediterranean) in December 2003. Cs, Cr, Co, Ni, Cu, Zn, Cd and Pb signatures of both riverine and shelf-exported particles indicate that continental inputs and resuspended prodeltaic sediments were intensively mixed with resuspended sediments from middle/outer shelf areas during advective transport. As a result, particles leaving the Gulf of Lion inherited the mean signature of shelf bottom sediments, exporting anthropogenic Pb and Zn out into the open sea. When assessing the particulate metal budget in relation with the event, it appears that the output fluxes accounted for between 15% and 60% of the input fluxes, depending on the element and the period of reference. This trend is also observed for annual budgets, which were drawn up by compiling the data from this study and the literature. Results evidenced that, except some element fluxes during extreme output scenario, outputs never counter-balance the inputs. In its current functioning, the Gulf of Lion's shelf seems to act as a retention/sink zone for particulate metals. Regarding anthropogenic fluxes, the contribution of the oceanic flood of December 2003 to the mean annual scenario is considerable. Environmental impacts onto coastal and deep-sea ecosystems should therefore tightly depend on both the intensity and the frequency of event-dominated sediment transport

Interaction of dense dense shelf water cascading and open-sea convection in the Northwestern Mediterranean during winter 2012

Durrieu de Madron, Xavier ... et al.-- European Geosciences Union General Assembly 2013, 7-12 April, Vienna, AustriaThe winter of 2012 experienced peculiar atmospheric conditions that triggered a massive formation of dense water on the continental shelf and in the deep basin of the Gulf of Lions. Multi-platforms observations enabled, with an unprecedented resolution, a synoptic view of dense water formation and spreading at basin scale. Five months after its formation, the dense water of coastal origin created a distinct bottom layer up to few hundreds of meters thick over the central part of the NW Mediterranean basin, which was overlaid by a layer of newly formed deep water produced by open-sea convection. These observations highlight the role of intense episodes of both dense shelf water cascading and open-sea convection to the alteration of the characteristics of the NW Mediterranean deep watersPeer Reviewe

Impact of natural (waves and currents) and anthropogenic (trawl) resuspension on the export of particulate matter to the open ocean: Application to the Gulf of Lion (NW Mediterranean)

Accepted manuscript version. Published version available at https://doi.org/10.1016/j.csr.2008.02.002. Licensed CC BY-NC-ND 4.0.Modern sediment deposits on continental margins form a vast reservoir of particulate matter that is regularly affected by resuspension processes. Resuspension by bottom trawling on shelves with strong fishing activity can modify the scale of natural disturbance by waves and currents. Recent field data show that the impact of bottom trawls on fine sediment resuspension per unit surface is comparable with that of the largest storms. We assessed the impact of both natural and anthropogenic processes on the dispersal of riverborne particles and shelf sediments on the Gulf of Lion shelf. We performed realistic numerical simulations of resuspension and transport forced by currents and waves or by a fleet of bottom trawlers. Simulations were conducted for a 16-month period (January 1998–April 1999) to characterise the seasonal variability. The sediment dynamics takes into account bed armoring, ripple geometry and the cohesive and non-cohesive characteristics of the sediments. Essential but uncertain parameters (clay content, erosion fluxes and critical shear stress for cohesive sediment) were set with existing data. Resuspension by waves and currents was controlled by shear stress, whereas resuspension by trawls was controlled by density and distribution of the bottom trawler fleet. Natural resuspension by waves and currents mostly occurred during short seasonal episodes, and was concentrated on the inner shelf. Trawling-induced resuspension, in contrast, occurred regularly throughout the year and was concentrated on the outer shelf. The total annual erosion by trawls (5.6×106 t y−1, t for metric tonnes) was four orders of magnitude lower than the erosion induced by waves and currents (35.3×109 t y−1). However the net resuspension (erosion/deposition budget) for trawling (0.4×106 t y−1) was only one order of magnitude lower than that for waves and currents (9.2×106 t y−1). Off-shelf export concerned the finest fraction of the sediment (clays and fine silts) and took place primarily at the southwestern end of the Gulf. Off-shelf transport was favoured during the winter 1999 by a very intense episode of dense shelf water cascading. Export of sediment resuspended by trawls (0.4×106 t y−1) was one order of magnitude lower than export associated with natural resuspension (8.5×106 t y−1). Trawling-induced resuspension is thought to represent one-third of the total export of suspended sediment from the shelf. A simulation combining both resuspension processes reveals no significant changes in resuspension and export rates compared with the sum of each individual process, suggesting the absence of interference between both processes.</p