South Atlantic Basin Research Articles

Salt on the move: Multi stage evolution of salt diapirs in the Netherlands North Sea

After more than half a century of research, the dynamics of salt tectonics in the North Sea remains a contentious issue with unresolved questions regarding triggering and driving mechanisms. The North Sea is now covered by high-quality 3D seismic datasets which allow new insights into the interplay between salt tectonics and sedimentation. These insights are of both regional geological, societal and exploration interest as well as more widely applicable as analogues for salt tectonics in frontier regions such as the South Atlantic salt basins. A high quality 3D seismic volume from the Netherlands North Sea is used to show the thick and mobile Zechstein halite sequence of Late Permian age remobilized in several phases into complex salt structures. Early stage structural evolution was dominated by Triassic NNE–SSW and NW–SE trending salt ridges. Salt tectonic reorganization was triggered by regional shortening in the Campanian. Salt was transferred both laterally and vertically, creating salt anticlines and tall asymmetrical salt diapirs. Vertical growth of the tallest salt diapirs continued in pulses in the Plio-Pleistocene likely driven by increased sediment input from southern North Sea delta systems. The most recent deformation affects Late Pleistocene and Holocene sediment and could have impacted on the landscapes of Meso-Lithic human occupied sites. The recent activity of some salt domes should be considered when planning repositories for gas and nuclear waste.

A study of the air-sea interaction in the South Atlantic Convergence Zone through Granger causality

Air-sea interaction in the region of the South Atlantic Convergence Zone (SACZ) is studied using Granger causality (GC) as a measure of directional coupling. Calculation of the area weighted connectivity indicates that the SACZ region is the one with largest mutual air-sea connectivity in the south Atlantic basin during summertime. Focusing on the leading mode of daily coupled variability, GC allows distinguishing four regimes characterized by different coupling: there are years in which the forcing is mainly directed from the atmosphere to the ocean, years in which the ocean forces the atmosphere, years in which the influence is mutual and years in which the coupling is not significant. A composite analysis shows that ocean-driven events have atmospheric anomalies that develop first and are strongest over the ocean, while in events without coupling anomalies develop from the continent where they are strongest and have smaller oceanic extension.

Subtropical Cyclones over the Southwestern South Atlantic: Climatological Aspects and Case Study

Abstract Hurricane Catarina (2004) and subtropical storm Anita (2010) called attention to the development of subtropical cyclones (SCs) over the South Atlantic basin. Besides strong and organized storms, a large number of weaker, shallower cyclones with both extratropical and tropical characteristics form in the region, impacting the South American coast. The main focus of this study is to simulate a climatology of subtropical cyclones and their synoptic pattern over the South Atlantic, proposing a broader definition of these systems. In addition, a case study is presented to discuss the main characteristics of one weak SC. The Interim ECMWF Re-Analysis (ERA-Interim) and NCEP–NCAR reanalysis are used to construct the 33-yr (1979–2011) climatology, and a comparison between them is established. Both reanalyses show good agreement in the SCs’ intensity, geographical distribution, and seasonal variability, but the interannual variability is poorly correlated. Anomaly composites for austral summer show that subtropical cyclogenesis occurs under a dipole-blocking pattern in upper levels. Upward motion is enhanced by the vertical temperature gradient between a midtropospheric cold cutoff low/trough and the intense low-level warm air advection by the South Atlantic subtropical high. Turbulent fluxes in the cyclone region are not above average during cyclogenesis, but the subtropical high flow advects great amounts of moisture from distant regions to fuel the convective activity. Although most of the SCs develop during austral summer (December–February), it is in autumn (March–May) that the most “tropical” environment is found (stronger surface fluxes and weaker vertical wind shear), leading to the most intense episodes.

Southern Ocean velocity and geostrophic transport fields estimated by combining Jason altimetry and Argo data

AbstractZonal geostrophic velocity fields above 1975 dbar have been estimated for the Southern Ocean from 2004 into 2011 based on sea surface topography observed by Jason altimetry and temperature/salinity measured by Argo autonomous floats. The velocity at 1000 dbar estimated with the method has been compared to Argo drift trajectory at the same pressure level available from the Asia Pacific Data Research Center (APDRC). The inferred velocities agree with those from the Argo drift within the estimated sampling error of the latter, but have fewer gaps in space and time. The velocity has also been integrated from depth to surface to determine the mean and time‐variable zonal geostrophic transport in the Southern Ocean between 29.5°S and 58.5°S, primarily in the South Atlantic and South Indian Ocean basins, due to limitations in coverage of Argo. Analysis shows errors can be reduced by >70% by averaging gridded results over wide areas. Zonal transport averaged over the entire Indian Ocean basin shows a significant correlation with the Antarctic Oscillation (AAO) at low frequencies: transport is higher than normal during a positive phase of the AAO, and lower during the negative phase.

Intracontinental deformation in southern Africa during the Late Cretaceous

Intracontinental deformation accommodated along major lithospheric scale shear zone systems and within associated extensional basins has been well documented within West, Central and East Africa during the Late Cretaceous. The nature of this deformation has been established by studies of the tectonic architecture of sedimentary basins preserved in this part of Africa. In southern Africa, where the post break-up history has been dominated by major erosion, little evidence for post-break-up tectonics has been preserved in the onshore geology. Here we present the results of 38 new apatite fission track analyses from the Damara region of northern Namibia and integrate these new data with our previous results that were focused on specific regions or sections only to comprehensively document the thermo-tectonic history of this region since continental break-up in the Early Cretaceous. The apatite fission track ages range from 449±20Ma to 59±3Ma, with mean confined track lengths between 14.61±0.1μm (SD 0.95μm) to 10.83±0.33μm (SD 2.84μm). The youngest ages (c. 80–60Ma) yield the longest mean track lengths, and combined with their spatial distribution, indicate major cooling during the latest Cretaceous. A simple numerical thermal model is used to demonstrate that this cooling is consistent with the combined effects of heating caused by magmatic underplating, related to the Paraná-Etendeka continental flood volcanism associated with rifting and the opening of the South Atlantic, and enhanced erosion caused by major reactivation of major lithospheric structures within southern Africa during a key period of plate kinematic change that occurred in the South Atlantic and SW Indian ocean basins between 87 and 56Ma. This phase of intraplate tectonism in northern Namibia, focused in discrete structurally defined zones, is coeval with similar phases elsewhere in Africa and suggests some form of trans-continental linkage between these lithospheric zones.

New Insights into the Biogeographic and Karyotypic Evolution ofHoplias Malabaricus

Hoplias malabaricus is a species widely distributed throughout Brazil. Cytogenetic studies indicate the occurrence of extensive chromosomal rearrangements in population differentiation and speciation of the group that demonstrated an independent origin of sex chromosome systems. Seven karyomorphs were characterized for the species and are located in specific river basins, while others are distributed throughout several different basins. However, there are few studies linking the geographical distribution of H. malabaricus karyomorphs to the Brazilian hydrographic basins. This article provides new chromosomal information on five populations of H. malabaricus collected in a South Atlantic basin. The samples were analyzed by conventional and molecular cytogenetic techniques. Two karyomorphs, A (2n=42 m/sm) and C (2n=40 m/sm), were detected, and remarkable differences in the distribution of heterochromatin and GC-rich blocks were observed in the karyomorphs. A review of existing data is presented here, where we observe that dispersion is associated with the genesis of the South and Central America river basins. Coastal drainages represent an ancestral biogeographical component for many groups of fish, representing older basins, such as the basins of the Eastern Atlantic and San Francisco river, suggesting that existing karyomorphs found in these basins may represent a basal karyotype (karyomorph F) within H. malabaricus. The current cytogenetic data, including this article, for different karyomorphs of H. malabaricus in conjunction with the geological history of the continent allow us to determine that the ancestral group is most likely karyomorph F.

The evolution of Late Cretaceous deep‐ocean circulation in the Atlantic basins: Neodymium isotope evidence from South Atlantic drill sites for tectonic controls

The South Atlantic basins during the Late Cretaceous were characterized by overall expansion and the opening of deep‐water connections, eventually permitting deep‐ocean circulation with the Indian and North Atlantic basins. However, the evolving pattern of deep‐ocean circulation through the Late Cretaceous, particularly the timing of the connection between various basins, is not well constrained. Here we present new neodymium isotope data from five sites in the South Atlantic to reconstruct water mass chemistry and circulation patterns. The new data combined with previously published data indicate deep‐water formation occurred in the high‐latitude South Atlantic throughout the Late Cretaceous. Tectonic boundaries restricted the circulation of Southern Component Water until the opening of the Equatorial Atlantic Gateway and the subsidence of Rio Grande Rise between ∼79 and 75 Ma. Subsidence of these features permitted the northward flow of Southern Component Water, which then ventilated the deep North Atlantic. Nd isotope data indicate that the South Atlantic water column during the Cenomanian to Santonian was vertically stratified between intermediate and deep waters, with increased mixing evident during the Campanian and Maastrichtian.

Kinematics of the South Atlantic rift

Abstract. The South Atlantic rift basin evolved as a branch of a large Jurassic–Cretaceous intraplate rift zone between the African and South American plates during the final break-up of western Gondwana. While the relative motions between South America and Africa for post-break-up times are well resolved, many issues pertaining to the fit reconstruction and particularly the relation between kinematics and lithosphere dynamics during pre-break-up remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-break-up evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight-fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African Rift Zones, we have been able to indirectly construct the kinematic history of the pre-break-up evolution of the conjugate west African–Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic pre-salt sag basin and the São Paulo High. We model an initial E–W-directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities from 140 Ma until late Hauterivian times (≈126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ≈14 Myr-long stretching episode the pre-salt basin width on the conjugate Brazilian and west African margins is generated. An intermediate stage between ≈126 Ma and base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE–SW. From base Aptian onwards diachronous lithospheric break-up occurred along the central South Atlantic rift, first in the Sergipe–Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final break-up between South America and Africa occurred in the conjugate Santos–Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piauí-Ceará margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of these conjugate passive-margin systems and can explain the first-order tectonic structures along the South Atlantic and possibly other passive margins.

Major controlling factors on hydrocarbon generation and leakage in South Atlantic conjugate margins: A comparative study of Colorado, Orange, Campos and Lower Congo basins

Abstract We present a supra-regional comparative study of the major internal and external factors controlling source rock (SR) maturation and hydrocarbon (HC) generation and leakage in two pairs of conjugate margins across the South Atlantic: the Brazil (Campos Basin)–Angola (Lower Congo Basin) margins located in the “central segment”, and the Argentina (Colorado Basin)–South Africa (Orange Basin) in the “southern segment”. Our approach is based on the analysis and integration of borehole data, 1D numerical modeling, 2D seismic reflection data, and published reports. Coupling of modeling results, sedimentation rate calculation and seal-bypass system analysis reveal that: (1) oil window is reached by syn-rift SRs in the southern segment during the Early to Late Cretaceous when thermal subsidence is still active, while in the central segment they reach it in Late-Cretaceous–Neogene during a salt remobilization phase, and (2) early HC generation from post-rift SRs in the southern segment and from all SRs in the central segment appears to be controlled mainly by episodes of increased sedimentation rates. The latter seems to be associated with the Andes uplift history for the western South Atlantic basins (Campos and Colorado) and to a possibly climate-driven response for the eastern South Atlantic basins (Orange and Lower Congo). Additionally, we observe that the effect of volcanism on SR maturation in the southern segment is very local. The comparison of Cretaceous mass transport deposit (MTD) episodes with HC peak of generation and paleo-leakage indicators in the southern segment revealed the possible causal effect that HC generation and leakage have over MTD development. Interestingly, Paleogene leakage indicators, which were identified in the Argentina–South Africa conjugate margins, occur contemporaneously to low sedimentation rate periods. Nonetheless, present-day leakage indicators which were also identified in both pairs of conjugate margins might be related to seal failure events linked to eustatic sea-level drops.

Role of Equatorial Fracture Zones on Fluid Migration across the South Atlantic Margins

The continental margin basins of Brazil and West Africa share very similar tectono-stratigraphic megasequences that are recognizable in petroliferous basins, as a result of the Late Jurassic-Early Cretaceous rifting of the South Atlantic basins. A number of oil families present along the South Atlantic conjugated margins are composed of genetically related oils of mixed provenance. Motion of tectonic plates and their configurations which depend so much on the nature of the boundaries and their orientations strongly influence fault tectonics within both continents The tectonic evolution of the plates leads to the formation of fracture zones parallel to the direction of plate motion. The Middle Benue Trough of Nigeria and by extension, the whole Benue Trough, is bound by two offshore transform faults (the Chain and the Charcot Fracture Zones). These faults are asymmetric longitudinally with an oblique transverse fault bounding the basin, and have been outlined by the presence of magnetic lineation. Five E-W profiles across the Middle Benue Trough were selected for the application of Werner deconvolution and subjected to harmonic analysis. The magnetic dataset was used in concluding that the Equatorial Fracture Zones (EFZ) in the South Atlantic Ocean extending from South America into the Gulf of Guinea are mainly responsible for long distance migration of marine hydrocarbons from the West Africa margin to the offshore of Brazil.

Reconstructing deglacial North and South Atlantic deep water sourcing using foraminiferal Nd isotopes

During the last few decades, neodymium isotopes have been increasingly used as a paleoceanographic and paleoclimate proxy on million-year through millennial time-scales. The widespread use of the Nd isotope proxy depends on whether the Nd isotopic composition of past seawater can be reliably extracted from authigenic sediment phases. Here we show evidence that planktonic foraminifera with authigenic coatings preserve a deep water Nd isotopic signature and thus can be used to reconstruct past deep water Nd isotopes. Applying this method downcore on South Atlantic Deep Cape Basin core TNO57-21 (41.1°S, 7.9°E, 4981m), the Nd isotopic composition of the planktonic foraminiferal Fe–Mn oxide coatings is identical with and confirms that of the reductively leached bulk sediment fraction (Piotrowski et al., 2004, 2005, 2008) as reflecting changes in bottom water sourcing. In the case of northeastern North Atlantic core BOFS 8K (52.5°N 22.1°W, 4045m), the planktonic foraminiferal Fe–Mn oxide coatings have different Nd isotopes from reductive sediment leaches. Clearly acid-reductive leaching must be tested in more detail. We observe that the Nd isotopic composition of the deep northeastern Atlantic core shifted during the deglaciation, from glacial values similar to the South Atlantic Cape Basin towards Holocene values similar to the Bermuda Rise (Roberts et al., 2010). Taken as a whole, the Nd isotope record from foraminiferal authigenic coatings suggests large-scale changes in the proportion of northern and southern deep water sources occurred throughout the Atlantic Ocean, with substantial chemical gradients at times, during the last deglaciation.

Source rock characteristics and compositional kinetic models of Cretaceous organic rich black shales offshore southwestern Africa

The source rock potential of Cretaceous organic rich whole rock samples from deep sea drilling project (DSDP) wells offshore southwestern Africa was investigated using bulk and quantitative pyrolysis techniques. The sample material was taken from organic rich intervals of Aptian, Albian and Turonian aged core samples from DSDP site 364 offshore Angola, DSDP well 530A north of the Walvis Ridge offshore Namibia, and DSDP well 361 offshore South Africa. The analytical program included TOC, Rock-Eval, pyrolysis GC, bulk kinetics and micro-scale sealed vessel pyrolysis (MSSV) experiments. The results were used to determine differences in the source rock petroleum type organofacies, petroleum composition, gas/oil ratio (GOR) and pressure–volume–temperature (PVT) behavior of hydrocarbons generated from these black shales for petroleum system modeling purposes.The investigated Aptian and Albian organic rich shales proved to contain excellent quality marine kerogens. The highest source rock potential was identified in sapropelic shales in DSDP well 364, containing very homogeneous Type II and organic sulfur rich Type IIS kerogen. They generate P–N–A low wax oils and low GOR sulfur rich oils, whereas Type III kerogen rich silty sandstones of DSDP well 361 show a potential for gas/condensate generation. Bulk kinetic experiments on these samples indicate that the organic sulfur contents influence kerogen transformation rates, Type IIS kerogen being the least stable. South of the Walvis Ridge, the Turonian contains predominantly a Type III kerogen. North of the Walvis Ridge, the Turonian black shales contain Type II kerogen and have the potential to generate P–N–A low and high wax oils, the latter with a high GOR at high maturity.Our results provide the first compositional kinetic description of Cretaceous organic rich black shales, and demonstrate the excellent source rock potential, especially of the Aptian-aged source rock, that has been recognized in a number of the South Atlantic offshore basins.

Paleozoic crustal blocks of onshore and offshore central Argentina: New pieces of the southwestern Gondwana collage and their role in the accretion of Patagonia and the evolution of Mesozoic south Atlantic sedimentary basins

The southwestern Gondwana basement block configuration in the central Argentinean offshore area was analyzed using gravimetric, magnetic and seismic data and existing onshore tectonic models. The resultant maps, the distribution of the Mesozoic and Cenozoic basins and Paleozoic structural features were used to validate the interpretations and to produce a new regional tectonic model. Pre-Carboniferous southwestern Gondwana of South America was interpreted as an open margin formed from east to west by the Dom Feliciano Belt, the Río de la Plata Craton, the Pampean Belt and the Pampia and Cuyania terranes. The collision of the Patagonia allochthonous terrane during the Late Paleozoic resulted in the development of the Ventania-Cape Fold Belt, which was mapped for the first time off the Argentinean coast out to 600 km from the shore. A strong change in the orientation of the Fold Belt is referred to as the Colorado Syntaxis, a mirror image of the Cape Syntaxis in South Africa. This change reflects the buttressing effect of the cratonic blocks that hamper the northward propagation of syncollisional deformation and resulted in a 180-km shift of the orogenic front. The Mesozoic basins and the basement block distribution were analyzed. The Pampean Belt, a deformed area produced by the Pampia accretion to the cratonic area, is the locus to two episutural basins, the General Levalle and Macachín basins. The Salado Basin was interpreted as an episutural basin controlled by a 2.1–2.0 Ga suture within the Rio de la Plata Craton. The Colorado Basin is composed of four segmented depocenters that reflect different emplacement controls: the location of the western Colorado Basin was controlled by the Upper Paleozoic orogen; the distributions of the central and eastern Colorado depocenters, orthogonal to the continental boundary, were also strongly influenced by the Upper Paleozoic structures and were offset by lineaments that reflect Dom Feliciano fabric; the Colorado Basin external depocenter that parallels the continental margin was also controlled by these lineaments. We interpret a time gap of some 50 Ma between the beginning of the evolution of the margin-orthogonal depocenters and the Atlantic breakup.

New species of Terebellides (Polychaeta: Trichobranchidae) indicate long-distance dispersal between western South Atlantic deep-sea basins

During the expedition DIVA 3 in summer 2009, six new species of the genus Terebellides (Trichobranchidae) were discoveredfrom the deep Argentine and Brazil basins. Five of these (Terebellides gingko sp. n., Terebellides banalis sp. n., Terebellidesbulbosa sp. n., Terebellides concertina sp. n., Terebellides diva sp. n.) are formally described herein; the sixth species is onlybriefly described as it was represented by only a single specimen. While most species were represented by a few specimensfrom a single basin, T. gingko sp. n., was found in relative high abundances in both basins. Molecular analyses of 16S rDNAsequences confirmed the long-distance distribution of this species. Although branchiae are lost in the majority of specimensfound, all new species can clearly be separated from each other and Terebellides species formerly reported for shallow westernSouth Atlantic waters by the relationships between head structures and anterior segments, and the shape of thoracic and anterior uncini, presenting evidence for a diverse and previously undescribed diversity of Trichobranchidae in the world’s deep seas.

Advances in carbonate exploration and reservoir analysis

Abstract The development of innovative techniques and concepts, and the emergence of new plays in carbonate rocks are creating a resurgence of oil and gas discoveries worldwide. The maturity of a basin and the application of exploration concepts have a fundamental influence on exploration strategies. Exploration success often occurs in underexplored basins by applying existing established geological concepts. This approach is commonly undertaken when new basins ‘open up’ owing to previous political upheavals. The strategy of using new techniques in a proven mature area is particularly appropriate when dealing with unconventional resources (heavy oil, bitumen, stranded gas), while the application of new play concepts (such as lacustrine carbonates) to new areas (i.e. ultra-deep South Atlantic basins) epitomizes frontier exploration. Many low-matrix-porosity hydrocarbon reservoirs are productive because permeability is controlled by fractures and faults. Understanding basic fracture properties is critical in reducing geological risk and therefore reducing well costs and increasing well recovery. The advent of resource plays in carbonate rocks, and the long-standing recognition of naturally fractured carbonate reservoirs means that new fracture and fault analysis and prediction techniques and concepts are essential. A key area of progress has been integration of stratigraphic, structural, geomechanical and diagenetic analysis to populate reservoir models accurately. Dramatic increases in computing and digital imaging capabilities are being harnessed to improve spatial analysis and spatial statistics in reservoirs and ultimately improve 3D geocellular models.

An update of anthropogenic CO2 storage rates in the western South Atlantic basin and the role of Antarctic Bottom Water

The western basin of the South Atlantic from 10°N to 55°S and from the coast to the Mid-Atlantic Ridge is a region with large uncertainties as to the storage of anthropogenic CO2 (Cant). Our analysis of data of the last three decades provides a Cant storage rate of 0.92±0.13molm−2 y−1, i.e., 13%–35% higher than previous estimates in this area. The low but significant Cant concentrations ([Cant]) in the large volume of relatively well ventilated Antarctic Bottom Water (AABW) may well be the underlying cause of this higher storage rate. In fact, the significant contribution in terms of Cant of this ventilated AABW that enters the western South Atlantic Ocean was calculated to be 0.055±0.02 Pg C y−1 or 0.20molm−2 y−1. Instead of being based on the annual trend, the Cant specific inventory (in molm−2) evolution is more consistently computed as a function of the atmospheric xCO2 perturbation in ppm, (0.64molm−2ppm−1). This methodology allows improved projections of Cant storage rates over long periods.

Correlação entre as Temperaturas da Superfície do Mar e a quantidade da precipitação na estação chuvosa no Nordeste do estado de Pernambuco

O principal objetivo deste trabalho é avaliar o quanto a precipitação no nordeste do Estado de Pernambuco é devido à anomalias de Temperatura da Superfície do Mar (ATSM) do oceano Atlântico e do Pacífico. Para tanto, utilizou-se os totais mensais de precipitação registrados em 15 postos pluviométricos, distribuídos espacialmente ao longo da região estudada dos anos de 1963 a 1992, e ATSM extraídas da versão do Comprehensive Ocean-Atmospheric Data Set (COADS) de 1945 a 1993. Verificou-se que a região estudada possui uma forte correlação positiva com a área de estudo da Bacia do Atlântico Sul, e uma correlação negativa com a Bacia do Pacífico Equatorial. Entretanto, as anomalias na temperatura das águas superficiais da região do Atlântico, por estar adjacente a área analisada, apresentam uma correlação mais forte, ao mesmo nível de significância, que as anomalias do Pacífico. Os resultados mostram que um aquecimento anômalo nesta área do Atlântico pode causar eventos extremos de chuva na costa leste do Nordeste.

Diversity and distribution of Polychaeta in deep Antarctic and Subantarctic waters along the Greenwich meridian

In the course of the ANDEEP-SYSTCO project, during the ANT XXIV-2 expedition in austral summer 2007/2008, the diversity and composition of the Polychaeta of the Antarctic deep-sea and adjacent South Atlantic basins were analyzed. A total of 847 individuals of 31 families were found belonging to 86 different species. Calculation of diversity (Shannon-Wiener Index, Pielou's Evenness) and the general species composition of Polychaeta showed patterns typical for the deep sea, with high species richness and low abundances. Lowest diversity was found in the Agulhas Basin in over 4000 m water depth. Lowest Evenness was found on top of Maud Rise where one-third of all Polychaeta belonged to one species. Cluster analyses resulted in higher affinities of Maud Rise to the Agulhas Basin than to the Antarctic continental slope. Explanations are sought in similarities of environmental factors (e.g., sediment, food input).

Interannual to decadal changes in the western South Atlantic's surface circulation

[1] A combination of surface drifters and altimetry is used to analyze the seasonal to interannual variability of the surface velocity field in the Brazil-Malvinas confluence of the western South Atlantic Ocean. Longer-term changes are inferred from wind and sea surface temperature fields. During the period October 1992 to December 2007, a southward shift of −0.6 to −0.9° decade−1 is found in the confluence latitude of the Brazil and Malvinas currents. A comparable trend is found in the latitude of the maximum wind stress curl averaged across the South Atlantic basin, allowing a proxy for the confluence latitude to be calculated for the prealtimeter time period. This longer time series suggests that the recent trend may be part of a longer-term oscillation, which has returned to values last sustained in the early 1980s. This variation does not appear to be related to the multidecadal trend in the Southern Annular Mode, but instead is inversely related to long-term variations in the sea surface temperature anomaly in the Agulhas-Benguela pathway of the eastern South Atlantic subtropical basin.

Anthropogenic carbon dioxide in the South Atlantic western basin

The meridional WOCE line A17 was conducted during the austral summer of 1994 parallel to the eastern South American coast, from 55°S to 10°S, where one of the main limbs of the North Atlantic Deep Water (NADW), i.e., the southward-flowing Deep Western Boundary Current (DWBC) is found. Full-depth profiles of pH, total alkalinity and total inorganic carbon were measured and checked with analytical CO 2 certified reference materials (CRMs), providing a high-quality dataset with good internal consistency for the CO 2 system parameters that is well suited for anthropogenic CO 2 (C ANT) estimation. For the first time in the western Atlantic basin the C ANT has been calculated using four independent approaches and results are compared. The methods considered are the CFC-based TTD method and the φC T°, TrOCA and ∆C* carbon system-based back-calculation methods. All four methods have produced C ANT distribution patterns that are in general good agreement: maximum concentrations of C ANT (50–60 μmol kg − 1 ) are predicted for the upper warm South Atlantic central waters from the tropical gyres, while the minima (~ 5 μmol kg − 1 ) are located in the old northward-flowing branch of Circumpolar Deep Water. There are, however, some discrepancies detected. The TrOCA method yields the highest overall [C ANT] values, even over the theoretical limit of C ANT saturation for 1994 in the upper layers. The ∆C* approach consistently yielded negative estimates of C ANT below 2800 dbar, even after correcting a reported − 8 μmol kg − 1 bias in the alkalinity measurements of the WOCE A17 line. The main overall difference between the four methods corresponds to the relative C ANT maximum associated with the lower limb of NADW: this structure is well identified in the φC T° and TTD methods but seems to disappear in the case of TrOCA and ∆C*. In agreement with other intercomparison studies of C ANT, the specific inventories are significantly higher (~ 45%) than those reported in the GLODAP database obtained from the ΔC* method. This suggests that the South Atlantic stores more C ANT than initially expected, particularly towards the southernmost tip of the WOCE A17 line, close to the Southern Ocean. The φC T°, TrOCA and TTD methods confirm an increasing tendency of C ANT specific inventories south from the Equator, while the ΔC* method shows a decreasing trend south from 35°S.