Éditeur(s) : Yale University
Résumé : Data from a hydrographic section carried out in January-March 1994 offshore from the eastern coast of South America from 50S to 10N, are used to quantify the full-depth exchanges of water between the western boundary currents and the ocean interior. In the upper and intermediate layers, the westward transport associated with the southern branch of the South Equatorial Current was 49 Sv at the time of the cruise. The transports of the central and northern branches in the upper 200 m were 17 Sv and 12 Sv, respectively. After subtraction of the parts that recirculate in the subtropical, subequatorial, and equatorial domains, the fraction of the South Equatorial Current that effectively contributes to the warm water export to the North Atlantic is estimated at 18 Sv. The poleward boundary of the current southern branch is at 31S through the whole thickness of the subtropical gyre, but the latitude of the northern boundary varies from 7 degrees 30'S at the surface to 27S at 1400 m depth. The estimated latitude of its bifurcation into the Brazil Current and North Brazil Undercurrent also varies downward from about 14S at the surface to 28S at a depth of 600 m.In the North Atlantic Deep Water, eastward flows exceeding 10 Sv are observed at 3 degrees -4 degrees of latitude in both hemispheres, at 10S, and at 34S-30S. Between 4S and 17S, a net westward flow with an estimated transport of 19 Sv reinforces the southward deep western boundary current. Cyclonic circulations of Antarctic Bottom Water along the western boundaries of the Argentine and Brazil basins have amplitudes of 15 Sv and 13 Sv, respectively, exceeding those of the interbasin exchanges. The net alongshore transport of this water mass between the hydrographic section and the continental slope reverses to a southward direction from 13S to 27S, probably in relation with an eastward shift of the equatorward near-bottom boundary current at these latitudes.
Journal of Marine Research (0022-2402) (Yale University), 2000-11 , Vol. 58 , N. 6 , P. 1007-1039

Droits : 2000 Yale University
http://archimer.ifremer.fr/doc/2000/publication-803.pdf
DOI:10.1357/002224000763485782
http://archimer.ifremer.fr/doc/00000/803/

Éditeur(s) : Copernicus Gesellschaft Mbh
Résumé : The distributions of the bio-essential trace element dissolved cobalt (DCo) and the apparent particulate Co (PCo) are presented along the GEOTRACES-A02 deep section from 64(o) N to 50(o) S in the western Atlantic Ocean (longest section of international GEOTRACES marine environment program). PCo was determined as the difference between total cobalt (TCo, unfiltered samples) and DCo. DCo concentrations ranged from 14.7 pM to 94.3 pM, and PCo concentrations from undetectable values to 18.8 pM. The lowest DCo concentrations were observed in the subtropical domains, and the highest in the low-oxygenated Atlantic Central Waters (ACW), which appears to be the major reservoir of DCo in the western Atlantic. In the Antarctic Bottom Waters, the enrichment in DCo with aging of the water mass can be related to suspension and redissolution of bottom sediments a well as diffusion of DCo from abyssal sediments. Mixing and dilution of deep water masses, rather than scavenging of DCo onto settling particles, generated the meridional decrease of DCo along the southward large-scale circulation in the deep western Atlantic. Furthermore, the apparent scavenged profile of DCo observed in the deep waters likely resulted from the persistence of relatively high concentrations in intermediate waters and low DCo concentrations in underlaying bottom waters. We suggest that the 2010 Icelandic volcanic eruption could have been a source of DCo that could have been transported into the core of the Northeast Atlantic Deep Waters. At intermediate depths, the high concentrations of DCo recorded in the ACW linearly correlated with the apparent utilization of oxygen (AOU), indicating that remineralization of DCo could be significant (representing up to 37% of the DCo present). Furthermore, the preferential remineralization of phosphate (P) compared to Co in these low-oxygenated waters suggests a decoupling between the deep cycles of P and Co. The vertical diffusion of DCo from the ACW appears to be a significant source of DCo into the surface waters of the equatorial domain. Summarizing, the dilution due to mixing processes rather than scavenging of DCo and the above-mentioned remineralization could be the two major pathways controlling the cycling of DCo into the intermediate and deep western Atlantic.
Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2014 , Vol. 11 , N. 6 , P. 1561-1580

Droits : Author(s) 2014. CC Attribution 3.0 License.
http://archimer.ifremer.fr/doc/00241/35223/33727.pdf
DOI:10.5194/bg-11-1561-2014
http://archimer.ifremer.fr/doc/00241/35223/

Éditeur(s) : Université de Bretagne Occidentale
Résumé : The objective of the present work is to study the formation of the passive continental margins of the Central Segment of the South Atlantic, most particularly the Congo and Angola margins. We propose a combined approach, which integrates structural constraints based on geological cross-sections (based on seismic data) and global constraints based on plate kinematic reconstructions. The structural study is based on : i) MCS and refraction data collected during the ZaiAngo programme (a joint project conducted by Ifremer and Total) ; ii) proprietary, industrial seismic data (courtesy of Total) from the Angola margin and iii) on all available seismic lines from the Africa and Brazil conjugated margins, between Walvis Ridge and the Equatorial Fracture Zones. Based on theses data, three structural domains (continental, transitional and oceanic) have been defined, the major characteristics of which are : Crustal thinning occurs abruptly, mostly below the continental slope, over a lateral distance of less than 50 km. The top of the crust deepens as the Moho shallows. Only a few extensional structures are observed ; tilted blocks are very few (one or two, depending on the profile), found only on the upper part of the slope and sealed by a discordance prior to salt deposition. The transitional domain is characterized by the existence of a pre-salt basin lying over a thin crustal layer. No tilted blocks are observed in this domain and reflectors within the pre-salt sediment series are parallel to the base of the Aptian salt, over distances greater than 100 km, precluding the possibility of any significant deformation that would imply large horizontal motions. Two types of crust are observed in the transitional domain. "Type I" crust is found below the undeformed pre-salt sediment series located below the eastern part of the basin ; it is characterized by an upper layer of thickness greater than 5 km and a abnormal velocity layer (7.2 - 7.6 km/s), up to 6 km thick. "Type II" crust is less than 5 km thick and found below the salt compressive front that affects the western part of the basin. The salt cover is continuous (no erosion surface is observed), from the continental shelf to the western termination of the basin. Salt was not deposited in a confined environment (like in the Mediterranean), but in a shallow water, lagunal environment. This imposes the zero-level and constrains the paleo-bathymetry at the time of salt deposition, which dates the latest stage of margin formation. Understanding the formation of a margin cannot be approached without studying the homolog margin. Therefore, it is of major importance to reconstruct the closure of the ocean bordered by these homolog margins and take into account the constraints imposed by the kinematic reconstructions on the lateral motions of the lithospheric plates. In order to assess the relative position of the plates at the ocean closure (prior to crustal thinning), a global study was thus performed, integrating all geophysical and geological constraints, in the ocean and on land. The role of african intra-plate deformation and its limits and their consequences have been thoroughly studied. To juxtapose the margins of the central segment of the Southern Atlantic, it is all the margins bordering the Equatorial Atlantic that need to be adjuste precisely. The kinematic study of this last region shows that the reconstruction obtained are reliable, unambiguous with a quantifiable precision The best fitting poles (obtained using the PLACA software), show that it is impossible to close the margins beyond the superposition of the salt fronts, from the Angola and Brazil margins. The geological cross-sections based on seismic data from the homolog margins indicate that a 330 km wide basin with thin (< 12 km) crust was present at the time of the fit. This basin cannot result from horizontal movement related to pure stretching or simple shear, or any model implying conservative volume. This conclusion is consistent with the existence of presalt reflectors parallel to the salt layer wich extends to the platform: the formation of the pre-salt basin must be related to vertical motions. The scenario that we propose for the evolution of the Congo-Angola margin consist in four stages: the first phase corresponds to extensional deformation limited to the few tilted blocks observed on the upper part of the slope. During the second phase, the main crustal thinning occurs, vertical motions prevailes, resulting in the formation of the continental slope and in the subsidence of the basin. The third phase corresponds to the first stress striction: deformation is concentrated in a limited section of the basin, which corresponds to the salt compression front. A proto-oceanic crust is formed, probably composed of thinned continental crust intruded by mantle material. The second stress striction corresponds to the finale phase, resulting in oceanisation senso stricto. The evolution described shows that we can not apply conservative models for margin formation (such as McKenzie and Wernicke or any of their avatars). In order to explain this thinning, one should investigate non-conservative models (implying geochemical transformation, small scale convection, intrusion...) such as those proposed in marginal or continental basins with no horizontal movments.
Ce travail de thèse aborde la formation des marges continentales passives dans le segment central de l'océan Atlantique Sud (plus particulièrement au Congo et en Angola), en intégrant une étude en coupe (étude structurale à partir des coupes sismiques) et une étude en plan (étude cinématique). L'étude structurale de la marge a été réalisée à partir des données de sismique réflexion et réfraction de la campagne Zaïango et d'une compilation de données sismiques réflexion existantes sur toutes les marges africaine et brésilienne entre les zones de fracture équatoriales et la ride de Walvis. L'interprétation de ces données a permis d'individualiser la structure de la marge en trois domaines : continental, transitionnel et océanique et de déterminer quelques points majeurs sur la structuration de la marge. L'amincissement est abrupt, localisé dans la zone de pente continentale et restreint à 50 km. La marge montre peu de structures distensives : seuls un ou deux blocs basculés sont observés en haut de pente continentale. Le domaine transitionnel est caractérisé par la géométrie particulière de la sédimentation anté-salifère, l'absence de blocs basculés et la faible épaisseur de croûte. La couche sédimentaire anté-salifère montre des réflecteurs plans jusqu'à la base du sel, continus sur 100 km, éliminant toutes possibilités de déformation du socle pendant et après son dépôt. La croûte du domaine transitionnel peut-être divisée en deux types : une croûte de type I sur laquelle se déposent les sédiments non déformés, et une croûte de type II sur laquelle se superposent les limites du « front compressif salifère » bien exprimé dans les séries postsalifères. Enfin le sel, que l'on observe depuis la plate-forme jusqu'au bassin profond, ne se dépose pas dans un bassin confiné (comme en Méditerranée) mais à un niveau proche de 0 m (ressemblant probablement à un dépôt de type lagunaire) et donne la paléo-bathymétrie au moment de son dépôt qui marque la fin de la période de formation de la marge. La compréhension de la genèse d'une marge ne peut être approchée sans son homologue. Cette simple constatation, cette évidence, montre toute l'importance que l'on doit apporter à la reconstruction cinématique initiale de l'océan qui borde ces marges homologues et aux contraintes imposées par les reconstructions cinématiques sur les mouvements horizontaux des plaques lithosphériques. Afin d'étudier la position des marges au moment de cette fermeture, c'est-à-dire avant amincissement, une étude globale intégrant l'ensemble des données disponibles, géophysiques et géologiques, océaniques et continentales, a été réalisée. Le rôle de la déformation intraplaque africaine, ses limites et leurs conséquences a, en particulier, été l'objet d'une attention poussée. Pour juxtaposer les marges du segment central, ce sont toutes les marges de l'océan Atlantique Equatorial qui doivent être ajustées précisément. L'étude cinématique réalisée de la région équatoriale montre que l'on obtient une reconstruction fiable et sans ambiguïté, avec une précision que l'on peut quantifier. Les pôles issus de cette étude (et calculés avec le Logiciel PLACA) indiquent qu'il est impossible d'obtenir une fermeture plus serrée que celle qui conduit à la superposition des fronts salifères brésilien et angolais : les coupes issues de la sismique réflexion des deux marges indiquent qu'il subsiste un bassin aminci, large de plus de 330 km et dont la croûte n'excède jamais 13 kilomètres d'épaisseur. La formation de ce bassin ne peut résulter de mouvements horizontaux, ce qui exclut un amincissement par étirement (pure stretching) ou par l'existence d'une faille de détachement (simple shear) ou par quelque modèle conservatif que ce soit. Cette constatation corrobore l'observation de la présence d'horizons anté-salifère parallèles, entre eux et au sel, couche salifère que l'on retrouve sur la plate-forme : la création de ce bassin anté-salifère ne peut être que liée à un mouvement vertical. Le schéma d'évolution que nous proposons à partir des données structurales et des contraintes cinématiques présente quatre étapes : le premier stade correspond à une phase de déformation distensive limitée aux quelques rares blocs basculés observés en haut de pente continentale. C'est durant la deuxième étape que se déroule la phase d'amincissement principal, les mouvements verticaux prévalent, aboutissant à la formation de la pente continentale et à la subsidence du bassin. La troisième phase correspond à une première striction des contraintes : la déformation se concentre sur une partie réduite du bassin, coïncidant avec le front salifère compressif. Une proto-croûte océanique se forme, probablement composée de croûte continentale amincie et intrudée de matériel mantellique. La seconde striction correspond à la phase finale de formation de la marge et aboutit à l'océanisation sensu stricto. L'étude cinématique et la description de l'évolution de la marge à partir des données sismiques montre donc que l'on ne peut envisager l'application d'un modèle de genèse des marges avec conservation de volume (type McKenzie ou Wernicke et leurs avatars) : pour expliquer l'amincissement du bassin, il faudrait probablement nous intéresser aux modèles non-conservatifs (impliquant transformation, convection à petite échelle, ...) qui sont déjà invoqués pour la formation des bassins marginaux ou continentaux, sans mouvements horizontaux.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2003/these-82.pdf
http://archimer.ifremer.fr/doc/00000/82/

Éditeur(s) : Elsevier Science Bv
Résumé : The opening of the Equatorial and South Atlantic Oceans is still a matter of debate, particularly as concerns the locations of the intraplate deformation. We propose here a critical review of the kinematic models published since Bullard et al., 1965, based on a series of constraints: new interpretation of the magnetic anomalies, seafloor isochrons, flow lines, fracture zones, continental and oceanic homologous structures and radiometric dating of igneous rocks. All of these models present numerous unexplained misfits (gaps, overlaps and misalignments). We present here a new evolution of the Equatorial and South Atlantic Ocean from the tightest reconstruction to Chron C34. This new model confirms the hypothesis of a northward propagation of the South American deformation proposed by Eagles, but rejuvenates slightly the ages for this propagation and refines the plate reconstructions. In particular, we highlight the role of the kinematic "buffer" Santos block, located between the salty Aptian Central segment in the North and the Volcanic Hauterivian Austral segment in the South. The new initial fit presented in this study represents the tightest reconstruction that could be obtained and constitutes the base canvas on which the problem of the continental margin genesis should be addressed.
Earth Science Reviews (0012-8252) (Elsevier Science Bv), 2010 , Vol. 98 , N. 1-2 , P. 1-37

Droits : 2010 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00000/11160/7883.pdf
DOI:10.1016/j.earscirev.2009.08.001
http://archimer.ifremer.fr/doc/00000/11160/

Éditeur(s) : Amer Geophysical Union
Résumé : In the Atlantic Ocean, the northward export of warm surface water is compensated by a southward flow of cold North Atlantic Deep Water (NADW). The NADW is transported southward along the American continental margin within the Deep Western Boundary Current (DWBC). Some tracer and float observations have shown that part of the DWBC water flows eastward along the equator. Here we present three meridional velocity sections which give an instantaneous image of the top-to-bottom zonal circulation along the equatorial Atlantic. They reveal the presence of Equatorial Deep Jets (EDJs) between 1 degrees 30'N and 1 degrees 30'S, alternating eastward-westward currents with short vertical scale, surrounded by columns of eastward currents (the Extra Equatorial Jets or EEJs) at 2 degreesN and 2 degreesS. In addition to direct velocity measurements, tracer distributions give indications of water-mass feeding of the EDJs and EEJs by the DWBC.
Geophysical Research Letters (0094-8276) (Amer Geophysical Union), 2001-03 , Vol. 28 , N. 5 , P. 819-822

Droits : 2001 AGU
http://archimer.ifremer.fr/doc/00000/10369/9567.pdf
http://archimer.ifremer.fr/doc/00000/10369/

Éditeur(s) : Public Library Science
Résumé : Like hydrothermal vents along oceanic ridges, cold seeps are patchy and isolated ecosystems along continental margins, extending from bathyal to abyssal depths. The Atlantic Equatorial Belt (AEB), from the Gulf of Mexico to the Gulf of Guinea, was one focus of the Census of Marine Life ChEss (Chemosynthetic Ecosystems) program to study biogeography of seep and vent fauna. We present a review and analysis of collections from five seep regions along the AEB: the Gulf of Mexico where extensive faunal sampling has been conducted from 400 to 3300m, the Barbados accretionary prism, the Blake ridge diapir, and in the Eastern Atlantic from the Congo and Gabon margins and the recently explored Nigeria margin. Of the 72 taxa identified at the species level, a total of 9 species or species complexes are identified as amphi-Atlantic. Similarity analyses based on both Bray Curtis and Hellinger distances among 9 faunal collections, and principal component analysis based on presence/absence of megafauna species at these sites, suggest that within the AEB seep megafauna community structure is influenced primarily by depth rather than by geographic distance. Depth segregation is observed between 1000 and 2000m, with the middle slope sites either grouped with those deeper than 2000m or with the shallower sites. The highest level of community similarity was found between the seeps of the Florida escarpment and Congo margin. In the western Atlantic, the highest degree of similarity is observed between the shallowest sites of the Barbados prism and of the Louisiana slope. The high number of amphi-atlantic cold-seep species that do not cluster according to biogeographic regions, and the importance of depth in structuring AEB cold-seep communities are the major conclusions of this study. The hydrothermal vent sites along the Mid Atlantic Ridge (MAR) did not appear as "stepping stones" for dispersal of the AEB seep fauna, however, the south MAR and off axis regions should be further explored to more fully test this hypothesis.
Plos One (1932-6203) (Public Library Science), 2010-08 , Vol. 5 , N. 8 , P. 1-11

Droits : © 2010 Olu et al.
http://archimer.ifremer.fr/doc/00011/12207/8975.pdf
DOI:10.1371/journal.pone.0011967
http://archimer.ifremer.fr/doc/00011/12207/

Éditeur(s) : Elsevier
Résumé : A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600-1050dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200-2050dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW` mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e.g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7Sv across 16 degrees N between the Antilles islands and the Mid-Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADW east of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow. (C) 2009 Elsevier Ltd. All rights reserved.
Deep Sea Research Part I: Oceanographic Research Papers (0967-0637) (Elsevier), 2009-10 , Vol. 56 , N. 10 , P. 1615-1632

Droits : 2009 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6815.pdf
DOI:10.1016/j.dsr.2009.06.002
http://archimer.ifremer.fr/doc/00000/6815/

Éditeur(s) : Copernicus Gesellschaft Mbh
Résumé : Relict dune fields that are found as far south as 14 N in the modern-day African Sahel are testament to equatorward expansions of the Sahara desert during the Late Pleistocene. However, the discontinuous nature of dune records means that abrupt millennial-timescale climate events are not always resolved. High-resolution marine core studies have identified Heinrich stadials as the dustiest periods of the last glacial in West Africa although the spatial evolution of dust export on millennial timescales has so far not been investigated. We use the major-element composition of four high-resolution marine sediment cores to reconstruct the spatial extent of Saharan-dust versus river-sediment input to the continental margin from West Africa over the last 60 ka. This allows us to map the position of the sediment composition corresponding to the Sahara Sahel boundary. Our records indicate that the Sahara Sahel boundary reached its most southerly position (13 N) during Heinrich stadials and hence suggest that these were the periods when the sand dunes formed at 14 N on the continent. Heinrich stadials are associated with cold North Atlantic sea surface temperatures which appear to have triggered abrupt increases of aridity and wind strength in the Sahel. Our study illustrates the influence of the Atlantic meridional overturning circulation on the position of the Sahara Sahel boundary and on global atmospheric dust loading.
Climate Of The Past (1814-9324) (Copernicus Gesellschaft Mbh), 2013 , Vol. 9 , N. 3 , P. 1181-1191

Droits : Author(s) 2013. CC Attribution 3.0 License. Access Climate of
http://archimer.ifremer.fr/doc/00181/29240/27635.pdf
DOI:10.5194/cp-9-1181-2013
http://archimer.ifremer.fr/doc/00181/29240/

Éditeur(s) : Gauthier-Villars
Résumé : The wind, temperature and current data collected during the joined French-USA FOCAL/SEQUAL programme carried in the equatorial Atlantic band in 1983 and 1984, allowed for the first time a simultaneous study of both the equatorial and coastal (5-degrees-N) upwellings along 4-degrees-W. At the equator, the decreases of both the surface temperature and the depth of the thermocline in boreal summer are correlated with the zonal component of the local windstress in the period band 1-2 months. This corresponds to the time required by the thermocline to lift up at 0-4-degrees-W in the presence of an easterly wind. The local wind cannot however explain the entire vertical variability of the thermal structure at 0-4-degrees-W. The correlation function shows in addition that the temperature there is highly correlated to the wind recorded at St Peter and St Paul Rocks (SPP) in the period band 0-2 months. This time scale is the one required by the Kelvin and Rossby waves to propagate along the equator through equatorial wave guide dynamics. The influence of these waves seems particularly important when considering the upward displacements of the thermocline both in February-March when the winds abruptly relax and in November-December when the wind at 0-4-degrees-W is southwesterly with no secondary negative maximum in the wind record at 1-degrees-N-29-degrees-W (SPP), contrary to the climatological wind field. The computation of the different terms of the heat equation applied to an homogeneous surface layer shows that the cooling is entirely due to vertical advection and eddy diffusion. The cooling would be more effective without the atmospheric heat flux and the horizontal advection of heat transported both by the South Equatorial Current at the surface and the southward current at the bottom of the mixed layer. At the coast, the amplitude and duration of the upwellings are not constant all along the coast: in boreal summer, they are maximum east of the two capes (Palmas and Three Pointes) while in winter the strength of the cooling is maximum in the east of Cape Palmas and then decreases eastward. The meridional slope of the thermocline off the coast concerns a distance which is much larger than the internal Rossby radius of deformation and is coherent with the latitudinal extension and intensity of the Guinea current both in summer and winter. In addition to this geostrophic adjustment, the upward tilt of the thermocline in summer is enhanced by the intensification of the component of the wind parallel to the coast. The application of a simple linear model forced with an eastward wind at that time explains the surface maximum cooling at the coast and the current distribution in both vertical and meridional directions. The two maxima of the zonal component of the Guinea current, in summer (main one) and in winter are in phase with the intensity of the vertical component of the windcurl. The application of the Sverdrup equations shows that the current speeds computed, for both seasons, are of the same order of magnitude as the speeds observed.
Oceanologica Acta (0399-1784) (Gauthier-Villars), 1991 , Vol. 14 , N. 3 , P. 223-240

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00101/21252/18865.pdf
http://archimer.ifremer.fr/doc/00101/21252/