Brazilian Continental Margin Research Articles

Plankton biochronology for the last 772,000 years from the western South Atlantic Ocean

Abstract Calcareous microfossils have potential in the biostratigraphy of Pleistocene sediments and a clear relationship with paleoceanographic studies. Conversely, there are few quantitative biostratigraphic studies of chronologically-tuned events or sections in the western South Atlantic. In order to improve the stratigraphical framework for South Atlantic paleoceanographic studies, the present work attempts to review the last 772,000 years by carrying out a quantitative analysis of the calcareous nannofossil and planktonic foraminiferal assemblages by comparing them with a high resolution marine isotopic record (δ 18 O). This work is based on the analysis of two piston cores obtained from the continental slope of the Santos Basin in the Santos Drift, southeastern Brazilian Continental Margin. Twelve Pleistocene calcareous nannofossil events and twenty planktonic foraminifera events calibrated with oxygen isotopes and correlated with literature stratigraphies are discussed. This is the first calcareous plankton biochronology study for the last 772 kyr in the western South Atlantic Ocean. More studies in this region will help to establish a more precise biochronology for these calcareous microfossils. This study also presents six new biostratigraphic events of isotopically-caibrated planktonic foraminifera which can be used as markers in the western South Atlantic.

Relations between Potiguar Rift structural framework and the adjacent basement: analysis of gravity and magnetic data

Os métodos gravimétricos e magnéticos classicamente apresentam boas respostas para o estudo do arcabouço tectônico de bacias sedimentares. Neste trabalho, foram aplicadas técnicas avançadas de processamento de dados geofísicos (redução ao polo, matched filter, gradiente horizontal total e inclinação do sinal analítico do gradiente horizontal total), com o objetivo de caracterizar a expressão geofísica das principais estruturas do Rifte Potiguar e do seu embasamento cristalino (NE do Brasil). Com o intuito de se estabelecer o comportamento dos principais blocos crustais da Província Borborema sob o pacote sedimentar da Bacia Potiguar, foram identificados oito domínios tectono-geofísicos através da correlação dos padrões anômalos das fontes causativas com dados geológicos disponíveis na literatura. Os extensos sistemas de falhas, que limitam o Rifte Potiguar, apresentam-se como expressivos lineamentos gravimétricos positivos. As principais zonas de cisalhamento, que limitam os blocos crustais da Província Borborema, foram também correlacionadas com expressivas anomalias alongadas na direção NE-SW. A integração dos diversos mapas geofísicos com a geologia de superfície permitiu correlacionar a expressão geofísica dessas estruturas e estabelecer a sua influência nos processos de rifteamento, pelos quais passou a bacia, inclusive as feições estruturais NE-SW, observadas nas regiões adjacentes a bacia. Foi possível definir a presença dessas feições abaixo da coluna sedimentar da Bacia Potiguar, as quais controlaram fortemente a estruturação interna do Rifte Potiguar. Tais resultados corroboram as hipóteses que sugerem a influência de estruturas pré-cambrianas na formação da bacia durante o rifteamento na margem continental leste do Brasil. É importante ressaltar que a inferência de deformações com trend NW-SE, reveladas principalmente pelos dados magnéticos, contribui para a hipótese de que tenha ocorrido a propagação dos esforços tectônicos E-W, que formaram a margem equatorial, para o interior do continente. Tais esforços também influenciaram o truncamento de estruturas do embasamento, deformações com direção NW-SE nos diques Rio Ceará-Mirim e causaram o arrasto lateral de direção E-W, que ocorre entre a Zona de Cisalhamento Portalegre e o Sistema de Falhas de Carnaubais.

Seismic characteristics and distribution of hydrothermal vent complexes in the Cretaceous offshore rift section of the Campos Basin, offshore Brazil

Hydrothermal vents are structures that occur close to volcanically active areas. Their occurrence can have considerable impacts on the evaluation of the oil reserves of a basin. In the Campos Basin, eastern Brazilian continental margin, no hydrothermal vents have been previously recognized and related with magmatic events on seismic sections. This work aims to map the hydrothemal vents, ducts and associated intrusive bodies observed in the rifte section. With the use of 2D seismic data and a well with lithologic information, the occurrence of ten hydrothermal vents and seven ducts were mapped and the interval with volcanic intrusive rocks was characterized. The identification of intrusive rocks in seismic data is given by reflectors with high amplitude and disk or half-moon geometry with abrupt endings. The vent geometries shows dome and eye-shaped types and the ducts are fault-related, downward-tapering cone type. This study highlights new geological elements that can influence the petroleum systems of the Campos Basin, such as the identification of hydrothermal vents in seismic sections and the correlation with regional tectono-magmatic events.

Interpretación geológica y geofísica de la Elevación de Río Grande, margen brasileño suroriental: tectónica extensional y rifting de las cortezas continental y oceánica

This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporate sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.

Bathymetric and regional changes in benthic macrofaunal assemblages on the deep Eastern Brazilian margin, SW Atlantic

Deep-sea continental slopes have valuable mineral and biological resources in close proximity to diverse, undersampled and fragile marine benthic ecosystems. The eastern Brazilian Continental Margin (19.01°S to 21.06°S, 37.88°W to 40.22°W) is an important economic region for both fishing and oil industries, but is poorly understood with respect to the structure of the soft-sediment benthic fauna, their regional distribution and their bathymetric patterns. To identify spatial and temporal patterns of benthic macrofaunal assemblages on the slope (400 to 3000m), the Espirito Santo Basin Assessment Project (AMBES, coordinated by Cenpes-Petrobras) sampled 42 stations across the Brazilian Eastern Slope during both Summer 2012 and Winter 2013. We found a significant decrease in macrofaunal abundance at the 400m isobath along the slope near the northern region of the Espirito Santo Basin, suggesting benthic responses to upwelling events towards the south in Campos Basin and southern Espirito Santo Basin. The taxonomic diversity and assemblage composition also changed significantly across depth zones with mid-slope peaks of diversity at 1000–1300m. In general, macrofaunal assemblages were strongly related to slope depth, suggesting a strong influence of productivity gradients and water mass distribution on this oligotrophic margin. Sediment grain size was marginally important to macrofaunal composition on the upper slope. In general, macrofaunal assemblages on the slope of Espirito Santo Basin are similar to other areas of the SE Brazilian margin, but regional changes in response to productivity and depth need to be considered for management strategies in the face of increasing economic activities off-shore.

A multiproxy approach to the climate and marine productivity variations along MIS 5 in SE Brazil: A comparison between major components of calcareous nannofossil assemblages and geochemical records

In this study, changes in terrigenous input, based on geochemical elementary ratios (e.g. Fe/Ca, Ti/Ca, Ba/Ca, Fe/K), and on marine primary productivity, based on calcareous nannofossil assemblages, from the southeastern Brazilian continental margin are analyzed for the time interval 140 to 55ka (MIS 6 to MIS 4). Variation in the terrigenous input follows the continental paleoclimate records, based on speleothems from SE Brazil, and the summer insolation curve from low latitudes for the southern hemisphere. In turn, marine productivity shows a high correlation with terrigenous input. Terrigenous input maxima correlate to increased marine productivity during sub-stages 5d and 5b. Both, terrigenous input and marine productivity would be strongly controlled by changes in the Intertropical Convergence Zone and winds associated with the South Atlantic Convergence Zone. Thus, NE winds would have prevailed promoting humidity and precipitation rates over SE Brazil, as well as, an intensified wind-driven upwelling and eddies associated with the Brazil Current, which favored the presence of a shallow nutricline. Our results are consistent with that of different proxies presented in other studies providing estimates of changes on the precipitation rate of SE Brazil and changes on marine productivity in the South Atlantic Ocean during MIS 5.

Megafaunal communities along a depth gradient on the tropical Brazilian continental margin

The species composition, abundance and diversity patterns of the epibenthic megafauna and fish community from the tropical Brazilian continental margin were analysed based on 42 bottom trawls from the Campos Basin continental shelf and slope. Trawls were collected aboard R/V Gyre during autumn 2008 from depths of 13 to 2030 m. Overall, 452 species belonging to five main taxa were identified: teleosts (Actinopterygii) were represented by 196 species, crustaceans by 113 species and echinoderms by 108 species. These three groups contributed 92% to the 452 taxa. Several species (289) were recorded only once or twice, and the species accumulation curves showed no signs of being close to reaching asymptotic values. A tendency of increasing diversity and richness with depth was observed. Analysis of the trawls showed the existence of megafaunal assemblages significantly associated with depth and water mass. The shelf group (13–100 m) had subgroups associated with Coastal Water (CW) and South Atlantic Central Water (SACW). The upper-slope group (376–501 m) was found exclusively under the influence of SACW. The mid- and lower-slope group (978–2030 m) had two subgroups associated with the presence of Antarctic Intermediate Water and North Atlantic Deep Water. The 39 typifying species contributed 90% to the global similarity. Teleosts and squids greatly contributed to the within-group similarity over the shelf, while decapods, echinoids and galatheids contributed to the upper-slope assemblage. Deep-sea isopods and decapods mostly contributed to the mid- and lower-slope assemblages.

Facies and depositional environments for the coquinas of the Morro do Chaves Formation, Sergipe-Alagoas Basin, defined by taphonomic and compositional criteria

Lacustrine carbonate rocks form important hydrocarbon accumulations along the Brazilian continental margin, some of which are contained in oil fields in which coquinas are one of the main reservoirs (viz. Campos Basin). The complexity and heterogeneity of these deposits make them a challenge in terms of reservoir description. For the necessary classification and paleoenvironmental interpretation of the coquinas, it is essential to evaluate many aspects including biological (such as carbonate productivity), sedimentological (energy regime in the depositional environment, transport of bioclasts, terrigenous supply), taphonomic (fragmentation of shells, abrasion) and diagenetic processes. The facies analysis applied in this study is considered a more appropriate classification approach to understand these coquinas, since it is more flexible and comprehensive than the existing classifications for carbonate rocks. The material investigated here consists of rock samples of the coquinas from the Atol Quarry of the Morro do Chaves Formation (Barremian/Aptian), Sergipe-Alagoas Basin. These rocks that crop out in the Atol quarry complex can be considered as a case study for similar coquinas reservoirs found in the Brazilian continental margin basins. Six sedimentary facies were described, using the main taphonomic (fragmentation of shells) and compositional (presence of micrite and siliciclastic matrix) features as a diagnostic criteria. Two carbonate facies, two mixed carbonate-siliciclastic facies and two siliciclastic facies (mudstones) were identified. From the facies succession, combined with a review of the literature on the subject, the following depositional paleoenvironments were defined: high-energy lake platform, lacustrine delta in a high-energy lake platform and lake-centre. In this paper, a new facies model for the studied coquinas succession is proposed.

Sea-level rise since 8.2ka recorded in the sediments of the Potengi–Jundiai Estuary, NE Brasil

Abstract We present the first data set on the evolution and Holocene infilling of the Potengi River Estuary at the NE Brazilian Continental Margin near the city of Natal. The study is based on integrated sedimentological, geochemical and palaeontological information from 6 boreholes drilled in the area of estuarine mangroves, complemented by radiocarbon dating of the in situ remains of mangrove vegetation. Four samples taken close to the pre-inundation substratum set the baseline of reference points for the sea-level reconstruction. This palaeoenvironmental reconstruction, coupled with a 14 C age model, complements previous sea-level studies along the Rio Grande do Norte coast, which have been based primarily on beach-rocks and other on-shore exposed sedimentary materials. A rapid sea-level rise, at an averaged rate of approximately 6.1 mm/yr − 1 , occurred between 8300 and 7000 cal. yr BP. Since then, the pace of relative sea-level rise slowed and non-eustatic factors, namely terrigenous sediment supply and coastal dynamics, became dominant in the evolution of the estuary. The post-transgression period witnessed the decline of mangroves, whose remnants were identified mostly in the initial 2 m of Holocene estuarine sediments. Accelerated sea-level rise, combined with geomorphological confinement of the studied intertidal zone, was probably responsible for the demise of halophyte vegetation, which thrived again in the late Holocene.

Sedimentation rates and age modeling of Late Quaternary marine sediments with unsupported 230Th

230Th is a radionuclide of the 238U decay series, characterized by natural abundance in sediments and a half-life of 75,380 years, suitable for direct evaluation of sedimentation rates and sediment ages in a time scale of 700,000 years before the present. 230Th cycle creates a contrast between supported and unsupported 230Th in the sedimentary matrix due to the different geochemical behaviors of 230Th and its parent, 234U. This study aimed at establishing and validating a mathematical model that evaluates sedimentation rates and age modeling based on 230Th data retrieved from a sediment core sampled in the southeastern Brazilian continental margin. The validation of the age modeling of this method was made through a statistical comparison between its results and those of benthic foraminifera δ 18O record technique, widely used for sediment dating and determination of sedimentation rates. It was observed that the model of unsupported 230Th was successful in determining Late Quaternary sedimentation rates and ages in a confidence level of 95 %. It was possible to determine several breaking points in unsupported 230Th vertical profile, and it was shown that those divides correspond to some major intervals between glacial and interglacial periods when compared with the benthic foraminifera record.

Avaliação do potencial gerador de petróleo da seção Aptiana-Albiana da Bacia de Sergipe, integrando análises palinofaciológicas e dados de geoquímica orgânica

Data presented in this investigation refer to studies integrating palynofacies and organic geochemistry analyses that were performed on 51 samples of PEC-41-SE-9 well, located in the Sergipe Basin, Eastern Brazilian continental margin. It was evaluated the potential for oil generation, identifying the type and degree of maturation of the organic matter. Optical analyzes show that the organic matter is composed in greater proportion by amorphous organic matter in good preservation, with high fluorescence. It also presents a 67% average concerning the total composition of the kerogen. The phytoclasts usually are well preserved in almost all the section. The palynomorphs are composed of an association, predominantly, of spores and pollen grains of genus Classopollis, featuring a predominantly arid to semi-arid climate environment at deposition time. Presence of the dinoflagellate in the upper section displays the first marine ingressions that happened in the end of Aptian. Based on the qualitative and quantitative results of kerogen, it was possible to characterize three distinct palynofacies, in which the palynofacies I had the best features for quality and preservation of organic matter. The results show total organic carbon values up to 6.39% and Rock-Eval pyrolysis data pointed to the type II kerogen rich in organic matter suitable for the formation of hydrocarbon accumulations. The hydrogen index is found between 200 and 550 mg HC/g of total organic carbon, which shows good potential for oil and gas generation. Spore color index data conclude that the organic matter is found predominantly on the immature maturation phase, with SCI values of 4.0 to 4.5, corroborating with data of maximum temperature, of up to 432°C.

Tectonic framework of the Barra de São João Graben, Campos Basin, Brazil: Insights from gravity data interpretation

The Barra de São João Graben (BSJG), shallow water Campos Basin, is part of the Cenozoic rift system that runs parallel to the Brazilian continental margin. This system was formed in an event that caused the reactivation of the main Precambrian shear zones of southeastern Brazil in the Paleocene. We proposed a new structural framework of BSJG based on gravity data interpretation. Magnetic data, one available 2D seismic line, and a density well-log of a nearby well were used as constraints to our interpretation. To estimate the top of the basement structure, we separated the gravity effects of deep sources from the shallow basement (residual anomaly). Then, we performed a 2D modeling exercise, in which we kept fixed the basement topography and the density of the sediments, to estimate the density of the basement rocks. Next, we inverted the residual anomaly to recover the depth to the top of the basement. This interpretation strategy allowed the identification of a complex structural framework with three main fault systems: a northeast–southwest-trending normal fault system, a northwest–southeast-trending transfer fault system, and an east–west-trending transfer fault system. These trends divided the graben into several internal highs and lows. Our interpretation was corroborated by the magnetic anomalies. The existence of ultradense and strongly magnetized elongated bodies in the basement was interpreted as ophiolite bodies that were probably obducted by the time of the shutdown of the Proterozoic Adamastor Ocean.

Quaternary fault control on the coastal sedimentation and morphology of the São Francisco coastal plain, Brazil

The role of neotectonic faulting in shaping topography and controlling sediment deposition has been increasingly documented along the stable continental margin of South America, but the level of the present-day tectonic stability of this margin is still a matter of debate. This paper presents an investigation of the effects of Quaternary fault reactivation on the São Francisco coastal plain (SFCP). This feature is a 56-km long progradational coastal plain located in the Sergipe–Alagoas Basin along the continental margin of Brazil. The SFCP is bounded on the west by a system of normal faults striking N–S to NE–SW. The horst that bounds this plain is capped by the Miocene Barreiras Formation and Quaternary debris-flow and fluvial deposits, whereas the graben is filled with braided stream deposits and sandy beach-ridge sediments. The faulting contributed to the generation of accommodation space for the deposition of coastal sediments on the plain. These processes started with the reactivation of Cretaceous faults during the Miocene. During the Quaternary, fault reactivation and associated soft-sediment deformation occurred between 82 and 8.3ka, with minor pulses between 1700 and 60calyrBP. Several faults along this margin are tectonically active and may represent the onshore continuation of gravitational collapses that have been documented offshore along the continental shelf and slope.

Denudation history and landscape evolution of the northern East-Brazilian continental margin from apatite fission-track thermochronology

We reconstruct the history of denudation and landscape evolution of the northern East- Brazilian continental margin using apatite fission-track thermochronology and thermal history modeling. This part of the Brazilian Atlantic margin is morphologically characterized by inland and coastal plateaus surrounding a wide low-lying inland region, the Sertaneja Depression. The apatite fission track ages and mean track lengths vary from 39 ± 4 to 350 ± 57 Ma and from 10.0 ± 0.3 to 14.2 ± 0.2 μm, respectively, implying a protracted history of spatially variable denudation since the Permian at relatively low rates (<50 m My−1). The Sertaneja Depression and inland plateaus record Permian-Early Jurassic (300–180 Ma) denudation that precedes rifting of the margin by > 60 Myrs. In contrast, the coastal regions record up to 2.5 km of Late Jurassic-Early Cretaceous (150–120 Ma) denudation, coeval with rifting of the margin. The samples from elevated coastal regions, the Borborema Plateau and the Mantiqueira Range, record cooling from temperatures above 120 °C since the Late Cretaceous extending to the Cenozoic. We interpret this denudation as related to post-rift uplift of these parts of the margin, possibly resulting from compressional stresses transmitted from the Andes and/or magmatism at that time. Several samples from these areas also record accelerated Neogene (<30 Ma) cooling, which may record landscape response to a change from a tropical to a more erosive semi-arid climate during this time. The inferred denudation history is consistent with the offshore sedimentary record, but not with evolutionary scenarios inferred from the recognition of “planation surfaces” on the margin. The denudation history of the northeastern Brazilian margin implies a control of pre-, syn- and post-rift tectonic and climatic events on landscape evolution.

The role of North Brazil Current transport in the paleoclimate of the Brazilian Nordeste margin and paleoceanography of the western tropical Atlantic during the late Quaternary

Reconstructions of surface paleoceanographic conditions of the western equatorial Atlantic and past climates of the adjacent Northeast Brazilian (the “Nordeste”) continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0° 20′ N on the equatorial Brazilian continental slope. The record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near-modern to approximately 110 Ka.High-resolution XRF analysis provides insight into the paleoclimate history of the Nordeste during the last glacial interval. Several large-amplitude and abrupt peaks are observed in the time series of Ti/Ca and are usually accompanied by peaks of Fe/K. Together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from Nordeste rivers into the Atlantic. Within the limits of dating accuracy, most Ti/Ca peaks correlate with Heinrich events in the North Atlantic. This record thus corroborates, and extends back in time, the previous record of Arz et al. (1998) determined on sediment cores from farther southeast along the Nordeste margin.Stable oxygen isotopic analysis and Mg/Ca paleothermometry on the near-surface-dwelling planktonic foraminiferal species Globierinoides ruber find that mean sea-surface temperature (SST) during glacial time (20–55 Ka, n=97) was 23.89±0.79°C and the mean SST during the late Holocene (0–5 Ka, n=14) was 26.89±0.33°C. SSTs were 0.5–2°C higher and inferred sea-surface salinities were lower during most of the periods of elevated Ti/Ca, thus, as observed in previous studies, the western equatorial Atlantic was warm (at least locally) and the adjacent southern tropical continent was wet at the same time that the high-latitude North Atlantic was cold.Using the SYNTRACE-CCSM3 fully coupled climate model with transient forcing for the period 22 Ka to present, we find that decreased transport of the North Brazil Current co-occurs with reduced Atlantic meridional overturning circulation, and colder-than-normal SSTs in the North Atlantic region. These simulated conditions are invariably associated with significantly increased precipitation in the Nordeste region.

Aptian/Albian (Early Cretaceous) paleogeography of the South Atlantic: a paleontological perspective

Paleontological data obtained in recent years reinforce the hypothesis that Aptian marine sedimentation in the sedimentary basins of the Brazilian continental margin - except the Pelotas basin, the southernmost Brazilian basin - took place under the domain of waters coming from the north through the Tethys Sea (Central Atlantic). Tethyan waters could reach the basins of the Brazilian continental margin via the seaway then existing in the present-day region of northeastern Brazil. Here there are records in several basins, notably in the Sao Luis (Codo Formation), Parnaiba (Codo Formation), Araripe (Santana Formation), Tucano (Marizal Formation), Sergipe (Riachuelo Formation) and Camamu (Algodoes Formation) basins. Despite irrefutable marine evidence - e.g., dinoflagellates, echinoids, foraminifera, molluscs and fishes, conspicuously present in the Araripe Basin - there are very few paleogeographic reconstructions that include the seaway which is totally ignored in the international literature. The skepticism is even greater in relation to the Tethyan affinity although the evidence has been well documented by molluscs and dinoflagellates, together with ammonoids in the Sergipe Basin. That skepticism may be due to the fact that, in tectonic and geodynamic terms, the opening of the South Atlantic indeed proceeded from south to north, at least in the part that extends from Argentina to the northeastern Brazilian state of Paraiba.

The geostatistics of the metal concentrations in sediments from the eastern Brazilian continental shelf in areas of gas and oil production

Geostatistical techniques were used to evaluate the differences in the geochemistry of metals in the marine sediments along the Eastern Brazilian continental margin along the states of Ceará and Rio Grande do Norte (Northeastern sector) and Espírito Santo (Southeastern sector). The concentrations of Al, Fe, Mn, Ba, Cd, Cu, Cr, Ni, Pb, V, Hg, and Zn were obtained from acid digestion and quantified using flame atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectrometry (ICP-AES). The metals showed a similar order of concentration: Al > Fe > Ba > Mn > V > Ni > Pb > Cr > Zn > Cu, in both the Ceará; and Rio Grande do Norte shelf regions but different in the Espírito Santo shelf (Fe > Al > Mn > Ba > Zn > V > Cr > Ni > Pb > Cu. The concentrations of Hg and Cd were below the detection limit in all areas. A multivariate analysis revealed that the metals of siliciclastic origin on the continental shelf of Ceará are carried by Al. In addition, a large portion of metal deposits is connected to the iron and manganese oxides on the continental margin of Rio Grande do Norte. The metals from the continental supply on the coast of Espírito Santo (Cu, Ni, Ba, and Mn) are associated with Al; whereas Cr, Pb, V, and Zn are associated with iron in this southern area. Geochemical evaluations are needed to distinguish the origin and mineralogical differences of marine sediments within the regions. Scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) applied to the sediments from the coast of Ceará showed the morphological diversity of sediment grains: biological fragments, multifaceted particles, aggregates, and crystals occurred in the three regions analyzed. Among these grains, calcite, Mg-calcite, and aragonite were predominant in the northeastern sector, whereas silicates and other minerals were predominant the southeastern sector. Mg, K, Ti, and Zr as well as the lanthanides La and Ce were identified using SEM/EDS and added to the geochemical analysis of the data.

Imaging proto-oceanic crust off the Brazilian Continental Margin

S U M M A R Y During the Sanba (Santos basin seismic transect) experiment in 2010–2011, a 380-km-long combined wide-angle and reflection seismic profile has been acquired using 30 ocean-bottom seismometers, a 4.5 km seismic streamer and a 8900 in. 3 airgun array. The Sanba 3 profile crosses the southern flank of the Sao Paulo Plateau, the Sao Paulo Ridge and the easternmost Santos Basin in an east–west direction. Its eastern end is located on undisturbed oceanic crust. Tomographic and forward modelling of the wide-angle seismic data reveals that the sedimentary thickness is variable with only 1–2 km on top of the ridge and thickening to 4–5 km in the basin. Crustal thickness at the ridge is about 18 km and the relative layer thickness and velocity gradients indicate a continental origin of this ridge. The eastern Santos Basin is underlain by crust of only 5 km thickness, characterized by high seismic velocities between 6.20 km s –1 in the upper crust and 7.40 km s –1 in the lower crust. Three hypotheses for the nature of the crust in this region are tested here: (i) thinned continental crust, (ii) serpentinized upper mantle material, (iii) thin oceanic crust. As seismic velocity gradients seem to rule out a continental origin of this region, and clear Moho reflections argue against serpentinized upper mantle, we propose that the crust underlying the easternmost Santos Basin is of oceanic origin. Deviations from normal oceanic crustal velocities in the lower crust (6.70–7.00 km s –1) can be explained by accretion at slow spreading rates leading to the inclusion of serpentinite into the lower crust at the onset of organized seafloor spreading.

Planktonic foraminifera in the sediment of a western boundary upwelling system off Cabo Frio, Brazil

Abstract The distribution of planktonic foraminifera in box-core tops under the influence of a western boundary upwelling system along the southeastern Brazilian continental margin was examined to evaluate the similarity percentage (SIMPER) and to create a biofacies model for paleoenvironmental applications. Species associated with warm and oligotrophic water were distributed in the Rio de Janeiro sector of the Campos Basin, while productive water species were most abundant in the Rio de Janeiro sector of the Santos Basin, and cold-water species were most abundant in the Cabo Frio Upwelling System (CFUS). Four major biofacies are associated with the oceanographic setting of the CFUS: one from the Campos Basin (A — Brazil Current front), one from the Santos Basin (B — mixture of coastal and oceanic waters), and two associated with the Cabo Frio High in the northern biofacies (C) and southern biofacies (D). The distribution of biofacies C and D was associated with temperature differences. Biofacies C represents a mixture of upwelling and tropical waters, while biofacies D represents a mixture of cold, nutrient-rich and Santos Basin waters. These biofacies were also defined in core CF10-01B, in which six main paleoceanographic phases were characterized during the last 9 cal kyr, predominantly showing alternating dominance between biofacies C and D. From 9.0 to 5.0 cal kyr before the present (BP), the system was dominated by biofacies C. Between 4.0 and 3.5 cal kyr BP, biofacies D was dominant. Between 3.5 and 2.5 cal kyr BP, a strong influence of coastal waters and weakened upwelling activity were indicated by the dominance of biofacies B + C + D. The last 2.5 cal kyr was dominated by biofacies D. The SST in this core was reconstructed using the Modern Analog Technique (MAT), which revealed 0.35 °C of variability, indicating no SST changes during the Holocene. The weak sensitivity of the MAT was due to the interplay among different oceanographic features.

Distribution of living planktonic foraminifera in relation to oceanic processes on the southeastern continental Brazilian margin (23°S–25°S and 40°W–44°W)

The vertical distribution (0 to 100m) of planktonic foraminifera was investigated based on 40 tow samples that were collected in eight stations, during the austral summer of 2002, in a geographically restricted area (23°S–25°S and 40°W–44°W) on the southeastern Brazilian continental margin. Species' abundances are low (less than 10 specimens/m3), which is typical of an oligotrophic area. The foraminifera assemblage is mainly composed of warm water species (Globigerinoides ruber white and pink forms), with a predominance of spinose and symbiont-bearing species. Temperature and inorganic nutrient enrichment of the surface are the main factors that control foraminiferal abundance and diversity; nevertheless salinity can also influence the ecological descriptors. The role of the deep chlorophyll maximum (DCM) in the distribution of foraminifera is not always clear, but the increase in the abundance of G. ruber (white and pink) seems to be related to a deeper DCM, and high salinities (S>36.5). The ecological habitat of these species is affected by the depth of the mixed layer, with a predominance of the white form in deeper layers. Increases in the foraminiferal diversity are related to the dynamics of the Brazil Current system, which displaces the area of high productivity in the euphotic zone off the coast. The abundances of Globigerina bulloides, Globigerina falconensis, Globigerinella calida and Globigerinella siphonifera follow the nutrient enrichment of the surface water mass, corroborating the usefulness of these species as paleoproductivity proxies in the study area. These data confirm the use of diversity measurements and assemblages composition for reconstructing past water column structures in subtropical oceans.