Particle flux in the Rainbow hydrothermal vent field (Mid-Atlantic Ridge): Dynamics, mineral and biological composition

In order to provide information about the export and the distribution of hydrothermal particulate material to the surrounding deep ocean, four moorings were deployed in the vicinity of the hydrothermal Rainbow vent field (Mid-Atlantic Ridge, 36°14′N, 2250 m depth). The first mooring was a sediment trap with a current meter deployed at 2 m from a chimney of the Rainbow vent field and 1.5 m above the bottom (a.b.) for 16 days. It represented the reference for the initial composition of particles produced by the vent. The total mean mass particle flux (6.9 g m-2 d-1) was distinctly higher than the flux measured at the shallower hydrothermal vents on the MAR segment. This particulate flux showed a high temporal variation at the scale of a few days and was characterized by a high concentration of sulphur (17.2%) and copper (3.5%) and a very low concentration of organic carbon (0.14%). Several hundred bivalve larvae belonging to the hydrothermal mytilid Bathymodiolus azoricus were collected in this trap at the beginning of the experiment. The density of larvae decreased strongly at the end, indicating a patchiness distribution or a discontinuous reproduction of this species. The other three moorings, including sediment traps, current-meters and thermistor chains, were deployed for 304 days at different distances and altitudes from the Rainbow vent field. The mean speed of the current in the rift valley was low (6 cm s-1) and was oriented toward the north. The total mean particle mass flux measured with the five sediment traps varied little, from 10.6 to 25.0 mg m m-2 d-1, and displayed temporal variations which are typical of deep-sea environments with seasonal changes in the overlying production. However, in the trap at 500 m from the vents 150 m a.b., the presence of the hydrothermal plume can be observed: the sulphur, iron and copper concentrations of particles were significantly higher compared to the particles sampled in the pelagic reference trap. The plume composition was about 50% hydrothermal particles and 50% pelagic particles and its upper limit reached 300 m a.b. at this distance. In the traps at 1000 m from the vents, the elemental composition of particles was similar to the pelagic particles and we assume that these traps were not in the plume during the experiment. The zooplankton obtained in the long-term trap samples revealed high density variations in relation to the distance from the vent site. The nutrient enrichment around the hydrothermal area and the abundance of free living bacteria explain these variations in zooplankton density

Antarctic bottom water flow through the Vema fracture zone

During the October-November 1977 R/V "Jean Charcot" cruise, bathymetric and hydrological surveys of the Eastern Vema Fracture Zone were carried out in order to moor current meters on the silliocated in the area, with the aim of evaluating the bottom water flow through this passage in the Mid-Atlantic Ridge. The sill zone in fact comprises a succession of three secondary sills situated between 40052'W and 41°02'W at about 10048'N, at depths ranging from 4600 to 4700 m. Westward of the sills, there is a thick (750-950 m) bottom water layer of Antarctic origin. The potential temperature found at the bottom is 1. 30°C. In the sill zone, the homogeneous layer is thinner and has a potential temperature of 1.40°C. Eastward of the sills, the homogeneous layer is 1000 m thick, with e "-' 1. 51°C. Five current meter moorings were deployed in this bottom layer: one was placed directly on the sills, one slightly to the east, two slightly to the west and the fIfth 200 km further west. Current speeds increase from west to east. Mean speeds range from "-' 3 cm/sec. at the western moorings to "-' 9 cm/sec. on the sills, reaching 25 cm/sec. at the eastern mooring, where the maximum speed was found to be 42 cm/sec. On and eastward of the sills, the current direction fluctuates slightly between east and north-east. These results confIrm the eastward flow of Antarctic bottom water with a strong intensifIcation above the sills.Au cours de la campagne du NO « Jean Charcot» en octobre-novembre 1977, une reconnaissance bathymétrique et hydrologique dans l'est de la zone de fracture Vema a été effectuée dans le but de mouiller des courantomètres autour du seuil situé dans cette zone, afm d'évaluer le passage d'eau de fond à travers ce passage dans la dorsale médioatlantique. Ce seuil est en fait une succession de trois seuils secondaires entre 40052'W et 41°02'W à environ 10048'N à des profondeurs de 4600 à 4700 m. A l'ouest des seuils, on trouve une couche d'eau de fond d'origine antarctique de 750 à 950 m d'épaisseur, avec une température potentielle sur le fond de l,30°C. Sur la zone des seuils, la couche homogène est moins épaisse et la température potentielle est l,40°C. A l'Est des seuils, la couche de fond a 1 000 m d'épaisseur, avec e"-' 1 ,51°C. Cinq lignes de courantomètres ont été mouillées dans cette couche d'eau de fond: une sur les seuils, une juste à l'Est, deux juste à l'Ouest et la dernière à 200 km à l'Ouest. Les vitesses mesurées augmentent d'Ouest en Est. Les vitesses moyennes sont "-' 3 cm/s à l'Ouest, "-' 9 cm/s sur le seuil et atteignent 25 cm/s à l'Est du seuil où la vitesse maximum est 42 cm/s. Sur le seuil et à l'Est du seuil, les directions du courant varient peu entre l'Est et le Nord-Est. Ces résultats confirment le passage vers l'Est d'eau de fond antarctique avec intensification de l'écoulement au niveau des seuils

Dynamique et hydrologie de la couche profonde dans l'Atlantique Nord-Est. Campagnes EDYLOC

Ce document est une présentation des mesures courantométriques et hydrologiques recueillies au cours des campagnes EDYlOC 81 et 82. L'objectif était l'étude des courants et de leur variabilité spa­tiale et temporelle dans la couche de fond associée à l'étude de la structure hydrologique dans la même zone. Dans ce but, un ré­seau de six mouillages portant chacun deux courantomètres a été mouillé en mai 1981 au voisinage du point 47°N, 14°30°W; ceux-ci ont été relevés en avril 1982. Lors de cette seconde campagne, des mesures de néphélométrie ont été effectuées au cours de profils bathysonde

Biweekly current oscillations on the continental slope of the Gulf of Guinea

International audienceCurrent meter measurements have been carried out for 3 years on the continental slope of the Gulf of Guinea, near 7.5ring operatorS off the Angola coast. Currents in a water depth of 1300 m over the continental slope show a remarkable biweekly oscillation, bottom intensified, and with currents oriented nearly parallel to the isobaths. With a peak-to-peak amplitude reaching 20–Click to view the MathML source at 30 m above the bottom, this signal is the most energetic at sub-inertial frequencies. Simultaneous measurements deeper on the continental rise (in a water depth of 4000 m) show a more complex signal dominated by lower frequencies, and with less clear polarization. Simple linear topographic wave theories are compared to the observations. A combination of coastal trapped waves with cross-slope mode 3–5 could be consistent with the observed currents. A three-dimensional 1/6ring operator model suggests the existence of modes trapped to the slope, although with lower amplitude than observed

Remotely forced biweekly deep oscillations on the continental slope of the Gulf of Guinea.

International audienceCurrent meter measurements on the continental slope of the Gulf of Guinea (at 7°20′S and 1300 m depth) have revealed biweekly oscillations of the currents, bottom intensified and oriented along the bathymetry. We develop a three-dimensional primitive equation model of the Gulf of Guinea to study the oscillations and their forcing mechanism. The high resolution (1/12°) regional model reproduces remarkably well the main characteristics of the deep currents on the continental slope. Experiments with different forcings demonstrate that the biweekly variability at 1300 m depth is remotely forced by equatorial winds. Deep Yanai waves generated by the wind propagate eastward along the equator. Upon reaching the African coast, the energy propagates poleward in both directions as coastal trapped waves. The selection of the dominant biweekly period is explained by the absence of equatorial waves with westward group velocities in that frequency band. Contrary to a previous hypothesis involving tidal forcing, our interpretation is coherent with the significant interannual variability of the biweekly energy

Modélisation haute-résolution des courants dans le Golfe de Guinée (étude des oscillations bimensuelles)

Dans le Sud du Golfe de Guinée, vers 740 S, des observations de courant le long de la pente continentale ont mis en évidence des oscillations bimensuelles orientées le long de la bathymétrie. Ces oscillations, très énergétiques et intensifiées près du fond, dominent le signal. Un modèle de circulation générale océanique (NEMO/OPA) haute résolution du Golfe de Guinée est construit afin d étudier ces oscillations et leur mécanisme de forçage. Une étude des données de courant permet d écarter la marée comme mécanisme de forçage et le modèle, forcé par le vent, reproduit les différentes caractéristiques des oscillations. Différents jeux de conditions de forçage permettent d explorer l origine de la variabilité. Les simulations numériques mettent en évidence l origine équatoriale des oscillations. Des ondes de Yanaï, forcés par les vents équatoriaux, se propagent vers l Est du bassin puis se propagent en ondes côtières vers le Nord et vers le Sud. La théorie des ondes équatoriales permet d expliquer la focalisation de l énergie autour de la période 15 jours. Pour la bande de période autour de 15 jours, il n existe pas d ondes équatoriales se propageant vers l Ouest. Ainsi toute l énergie bimensuelle se propageant vers l Est est transmise en ondes côtières. Le modèle haute résolution du Golfe de Guinée et un modèle linéaire de l Atlantique équatorial permettent d étudier la structure spatiale et la variabilité basse fréquence des ondes. La prédominance des modes baroclines élevés est établie pour expliquer la signature en profondeur des oscillations et l atténuation du signal vers le sud. L étude de la variabilité temporelle des ondes à partir des données de courant met en évidence des intermittences du signal à 15 jours, reproduites en partie par le modèle du Golfe de Guinée mais absentes dans le modèle linéaire. Nous n avons pas pu établir de corrélation simple entre l intermittence des ondes et celle des vents équatoriaux générateurs de ces ondes.Currentmeter measurements on the continental slope of the Gulf of Guinea (near 740 S) point out a biweekly oscillation of the currents, bottom intensified and oriented along the bathymetry. Those energetic oscillations dominate the signal. A three-dimensional high-resolution primitive equation model (NEMO/OPA) of the Gulf of Guinea has been developed to study the oscillations and their forcing mechanism. The data does flot support the tide as a forcing mechanism and the wind-forced regional model reproduces biweekly oscillations, which fit the data quite well. Experiments with different forcings have been run to explore the origin of the biweekly variability. Those experiments highlight the equatorial origin of the oscillations forced by equatorial winds. At the equator, wind-forced Yanaï waves propagate eastward, reach the coast and propagate poleward in both directions. The dominant period of 1 5-day can be explained by equatorial waves theory. In the equatorial Atlantic, there are no 15-day waves propagating westward, thereby all the incident energy reaching the African coast propagates poleward via coastal-trapped waves. The spatial structure and the temporal variability of the biweekly waves are examined using the regional model and a linear mode of the equatorial Atlantic. High baroclinic modes explain the bottom intensification and the decrease of the signal to the South, because of the fast dissipation of the highest modes. Intermittency of the biweekly signal is underlined by wavelet analysis of the data and partially reproduced by the regional model but not by the linear model. Although being forced by equatorial winds, the temporal variability of the oscillations is not directly correlated with the variability of the equatorial winds.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Hydrothermal-vent alvinellid polychaete dispersal in the eastern Pacific .1. Influence of vent site distribution, bottom currents, and biological patterns

Deep-sea hydrothermal-vent habitats are typically linear, discontinuous, and short-lived. Some of the vent fauna such as the endemic polychaete family Alvinellidae are thought to lack a planktotrophic larval stage and therefore not to broadcast-release their offspring. The genetic evidence points to exchanges on a scale that seems to contradict this type of reproductive pattern. However, the rift valley may topographically rectify the bottom currents, thereby facilitating the dispersal of propagules between active vent sites separated in some cases by 10s of kilometers or more along the ridge axis. A propagule flux model based on a matrix of intersite distances, long-term current-meter data, and information on the biology and ecology of Alvinellidae was developed to test this hypothesis. Calculations of the number of migrants exchanged between two populations per generation (N-m) allowed comparisons with estimates obtained from genetic studies. N, displays a logarithmic decrease with increasing dispersal duration and reaches the critical value of 1 after 8 d when the propagule Aux model was run in standard conditions. At most, propagule traveling time cannot reasonably exceed 15-30 d, according to the model, whereas reported distances between sites would require longer lasting dispersal abilities. Two nonexclusive explanations are proposed. First, some aspects of the biology of Alvinellidae have been overlooked and long-distance dispersal does occur. Second, such dispersal never occurs in Alvinellidae, but the spatial-temporal dynamics of vent sites over geological timescales allows short-range dispersal processes to maintain gene flow

Polychaete diversity at tropical Atlantic deep-sea sites: environmental effects

This study assesses how differences in nutrient flux and bottom currents affect the diversity and trophic structure of deep-sea polychaete communities from the tropical northeast Atlantic at 20 degrees to 21 degrees N latitude. Faunal assemblages were studied from 3 sets of USNEL box cores (0.25 m(2)) taken at depths of 1700, 3100 and 4600 m. In terms of primary productivity, the cores were taken beneath water which is eutrophic, mesotrophic and oligotrophic, respectively. Total polychaete abundance, abundance of dominant species, and faunal similarity and diversity were compared among the 3 sites. Polychaete abundance decreased with increasing depth. Community structure reflected environmental characteristics as well as the influence of coastal upwelling. Polychaete diversity showed a parabolic distribution, with depth peaking at about 2000 m. Species richness observed at the eutrophic site was generally higher than observed at other deep-sea regions of comparable depth at temperate latitudes. This points to regional variation in processes contributing to diversity patterns of deep fauna. We suggest that the diversity pattern observed results from non-equilibrial interactions between production and disturbance in the form of current energy and bioturbation. Diversity patterns observed in the EUMELI sites seem best explained by the intermediate productivity theory. The rate of production encountered at the eutrophic site may overcome much of the impact of physical and biological disturbances, favouring a diverse species assemblage