Coastal and shelf circulation in the vicinity of Camamu Bay (14 degrees S), Eastern Brazilian Shelf

The Camamu Bay (CMB) is located on the narrowest shelf along the South American coastline and close to the formation of two major Western Boundary Currents (WBC), the Brazil/North Brazil Current (BC/NBC). These WBC flow close to the shelf break/slope region and are expected to interact with the shelf currents due to the narrowness of the shelf. The shelf circulation is investigated in terms of current variability based on an original data set covering the 2002-2003 austral summer and the 2003 austral autumn. The Results show that the currents at the shelf are mainly wind driven, experiencing a complete reversal between seasons due to a similar change in the wind field. Currents at the inner-shelf have a polarized nature, with the alongshore velocity mostly driven by forcings at the sub-inertial frequency band and the cross-shore velocity mainly supra-inertially forced, with the tidal currents playing an important role at this direction. The contribution of the forcing mechanisms at the mid-shelf changes between seasons. During the summer, forcings in the two frequency bands are important to drive the currents with a similar contribution of the tidal currents. On the other hand, during the autumn season, the alongshore velocity is mostly driven by sub-inertial forcings and tidally driven currents still remain important in both directions. Moreover, during the autumn when the stratification is weaker, the response of the shelf currents to the wind forcing presents a barotropic signature. The meso-scale processes related to the WBC flowing at the shelf/slope region also affect the circulation within the shelf, which contribute to cause significant current reversals during the autumn season. Currents at the shelf-estuary connection are clearly supra-inertially forced with the tidal currents playing a key role in the generation of the along-channel velocities. The sub-inertial forcings at this location act mainly to drive the weak ebb currents which were highly correlated with both local and remote wind forcing during the summer season. (C) 2010 Elsevier Ltd. All rights reserved.

Daily morphological changes determined by high-energy events on an embayed beach: a qualitative model

This paper examines the daily morphological responses of Sununga Beach, an embayed beach located on the south-eastern Brazilian coast, to storms in the South Atlantic Ocean. The main mechanisms and timing of beach erosion and accretion, the relationship between wave height and direction, and beach volume changes are considered, to establish a qualitative model for short-term embayed beach morphological changes. The methodology consisted of daily topographic surveys during the month of May in 2001, 2002, and 2003, using an RTK-GPS (real-time kinematics global positioning system). Weather and wave model results were used to correlate hydrodynamics and beach morphology. The results indicate that the morphodynamics of Sununga Beach are characterized by a process of beach rotation, which occurred more or less clearly during all three surveys. Unlike what has been commonly described in the literature for longer time intervals and alternations of fair and stormy weather, the beach rotation processes on Sununga Beach occurred under conditions of moderate-to-high wave energy change (wave heights greater than 2 m). An integrated evaluation of the behaviour of the meteorological aspects, together with beach morphology, enabled us to recognize that extra-tropical cyclones were the most important agent in remobilizing the beach planform, whether in beach rotation or in cross-shore erosion. Copyright (C) 2010 John Wiley & Sons, Ltd.

Variability of the subtropical shelf front off eastern South America: Winter 2003 and summer 2004

Hydrographic data collected during surveys carried out in austral winter 2003 and summer 2004 are used to analyze the distributions of temperature (T) and salinity (S) over the continental shelf and slope of eastern South America between 27 degrees S and 39 degrees S. The water mass structure and the characteristics of the transition between subantarctic and subtropical shelf water (STSW), referred to as the subtropical shelf front (STSF), as revealed by the vertical structure of temperature and salinity are discussed. During both surveys, the front intensifies downward and extends southwestward from the near coastal zone at 33 degrees S to the shelf break at 36 degrees S. In austral winter subantarctic shelf water (SASW), derived from the northern Patagonia shelf, forms a vertically coherent cold wedge of low salinity waters that locally separate the outer shelf STSW from the fresher inner shelf Plata Plume Water (PPW) derived from the Rio de la Plata. Winter T-S diagrams and cross-shelf T and S distributions indicate that mixtures of PPW and tropical water only occur beyond the northernmost extent of pure SASW, and form STSW and an inverted thermocline characteristic of this region. In summer 2004, dilution of Tropical water (TW) occurs at two distinct levels: a warm near surface layer, associated to PPW-TW mixtures, similar to but significantly warmer than winter STSW, and a colder (T similar to 16 degrees C) salinity minimum layer at 40-50 m depth, created by SASW-STSW mixtures across the STSF. In winter, the salinity distribution controls the density structure creating a cross-shore density gradient, which prevents isopycnal mixing across the STSF. Temperature stratification in summer induces a sharp pycnocline providing cross-shelf isopycnal connections across the STSF. Cooling and freshening of the upper layer observed at stations collected along the western edge of the Brazil Current suggest offshore export of shelf waters. Low T and S filaments, evident along the shelf break in the winter data, suggest that submesoscale eddies may enhance the property exchange across the shelf break. These observations suggest that as the subsurface shelf waters converge at the STSF, they flow southward along the front and are expelled offshore, primarily along the front axis. (C) 2008 Elsevier Ltd. All rights reserved.

Records of sedimentary dynamics in the continental shelf and upper slope between Aveiro-Espinho (N Portugal)

The sedimentary unconsolidated cover of the Aveiro-Espinho continental shelf and upper slope (NW Portugal) records a complex interplay of processes including wave energy and currents, fluvial input, sediment transport alongshore and cross-shelf, geological and oceanographic processes and sediment sources and sinks. In order to study this record, a set of surface sediment samples was studied. Sediment grain size and composition, as well as the mineralogical composition (by XRD) of the fine (<63 mu m) and clay (<2 mu m) fractions and benthic microfaunal (foraminifera) data were analysed. Cluster analysis applied to the sedimentological data (grain size, sediment composition and mineralogy) allowed the establishment of three main zones corresponding to the: inner-, mid- and outer-shelf/upper slope. On the inner-shelf, the sedimentary coverture is composed of siliciclastic fine to very fine sand, essentially comprising modern (immature) terrigenous particles. The sediment grain size, as well as mineralogical and microfaunal composition, denote the high energetic conditions of this sector in which the alongshore transport of sand is predominantly southward and occurs mostly during the spring-summer oceanographic regime, when the main river providing sediments to this area, the River Douro, undergoes periods of drought. This effect may emphasize the erosive character of this coastal sector at present, since the Ria de Aveiro provides the shelf with few sediments. On the mid-shelf, an alongshore siliciclastic band of coarse sand and gravel can be found between the 40 m and 60 m isobaths. This gravelly deposit includes relic sediments deposited during lower sea-level stands. This structure stays on the surface due to the high bottom energy, which promotes the remobilization of the fine-grained sediments, and/or events of sediments bypassing. Benthic foraminifera density and "Benthic Foraminifera High Productivity" (BFHP) proxy values are in general low, which is consistent with the overall small supply of organic matter to the oceanic bottom in the inner- and mid-shelf. However, the Ria de Aveiro outflow, which delivers organic matter to the shelf, leaves its imprint mainly on the mid-shelf, identifiable by the increase in foraminifera density and BFHP values in front of the lagoon mouth. The higher values of BFHP along the 100 m isobath trace the present position of an oceanic thermal front whose situation may have changed in the last 3/5 ka BP. This zone marks a clear difference in the density, diversity and composition of benthic foraminifera assemblages. Here, in addition, sediment composition changes significantly, giving rise to carbonate-rich fine to medium sand in the deeper sector. The low bottom energy and the small sedimentation rate of the outer-shelf contributed to the preservation of a discontinuous carbonate-rich gravel band, between the 100 m and 140 m isobaths, also related to paleo-littorals, following the transgression that has occurred since the Last Glacial Maximum. The winter oceanographic regime favours the transport of fine grained sediments to the outer-shelf and upper slope. The inner- and mid-shelf, however, have low amounts of this kind of sediment and the Cretacic carbonated complexes Pontal da Galega and Pontal da Cartola, rocky outcrops located at the mid- and outer-shelf, act as morphological barriers to the cross-shelf transport of sediments. Thus a reduced sedimentation rate occurs in these deeper sectors, as indicated by the lower abundance of detrital minerals, which is compensated for the high sedimentary content of biogenic carbonates. The relatively high BFHP and Shannon Index values indicate water column stratification, high supply of organic matter and environmental stability, which provide favourable conditions for a diversified benthic fauna to flourish. These conditions also encourage authigenic chemical changes, favourable to glauconite formation, as well as illite and kaolinite degradation. Benthic foraminifera and clay mineral assemblages also reveal the effect of the internal waves pushing upward, and downslope losses of the sediments on the outer-shelf and upper slope.

Vertical distribution of benthic invertebrate larvae during an upwelling event along a transect off the tropical Brazilian continental margin

Abundance and composition of marine benthic communities have been relatively well studied in the SE Brazilian coast, but little is known on patterns controlling the distribution of their planktonic larval stages. A survey of larval abundance in the continental margin, using a Multi-Plankton Sampler, was conducted in a cross-shelf transect off Cabo Frio (23 degrees S and 42 degrees W) during a costal upwelling event. Hydrographic conditions were monitored through discrete CDT casts. Chlorophyll-a in the top 100 m of the water column was determined and changes in surface chlorophyll-a was estimated using SeaWiFS images. Based on the larval abundances and the meso-scale hydrodynamics scenario, our results suggest two different processes affecting larval distributions. High larval densities were found nearshore due to the upwelling event associated with high chlorophyll a and strong along shore current. On the continental slope, high larval abundance was associated with a clockwise rotating meander, which may have entrapped larvae from a region located further north (Cabo de Sao Tome, 22 degrees S and 41 degrees W). In mid-shelf areas, our data suggests that vertical migration may likely occur as a response to avoid offshore transport by upwelling plumes and/or cyclonic meanders. The hydrodynamic scenario observed in the study area has two distinct yet extremely important consequences: larval retention on food-rich upwelling areas and the broadening of the tropical domain to southernmost subtropical areas. (C) 2009 Elsevier B.V. All rights reserved.

Electronic stopping cross sections for protons in Al2O3: an experimental and theoretical study

The electronic stopping cross section (SCS) of Al2O3 for proton beams is studied both experimentally and theoretically. The measurements are made for proton energies from 40 keV up to 1 MeV, which cover the maximum stopping region, using two experimental methods, the transmission technique at low energies (similar to 40-175 keV) and the Rutherford backscattering at high energies (approximate to 190-1000 keV). These new data reveal an increment of 16% in the SCS around the maximum stopping with respect to older measurements. The theoretical study includes electronic stopping power calculations based on the dielectric formalism and on the transport cross section (TCS) model to describe the electron excitations of Al2O3. The non-linear TCS calculations of the SCS for valence electrons together with the generalized oscillator strengths (GOS) model for the core electrons compare well with the experimental data in the whole range of energies considered.

A multiproxy assessment of organic pollution in shore sediments from the Río de la Plata Estuary, SW Atlantic

The study of organic pollution in estuaries is very relevant as they are transitional zones, which control the fluxes of water, nutrients, particles and organisms from and to the continental margins, rivers and oceans. The aims of this study are:(1) to evaluate organic pollution in coastal sediments of Montevideo, Río de la Plata Estuary by a multi-biomarker approach, (2) to identify major sources of organic pollutants through qualitative analysis using molecular indices, (3) to assess the relative contribution of different sources of hydrocarbons through quantitative source apportionment employing (PCA/MLR) as chemometric technique. Sampling surveys were carried out in July 2009, January 2010 and March 2011 in 37 stations along the middle portion of the Río de la Plata Estuary across the coast of Montevideo. In each station surface (0–2 cm depth) sediment samples were taken with a 0.05 m2 van Veen grab. The Soxhlet extracted organic compounds included aliphatic hydrocarbons (AHs) and steroids, analysed by gas chromatograph with flame ionization detector (GC-FID), linear alkylbenzenes (LABs) and polycyclic aromatic hydrocarbons (PAHs) quantified by gas chromatograph with mass spectrometer (GC/MS). All biomarkers presented the highest concentrations in the stations of Montevideo Bay indicating high levels of organic pollution. The combination of molecular indices and the chemometric technique showed that major sources of AHs and PAHs are petroleum inputs and combustion, due to oil transport and refinement, harbour activities and vehicular emissions.Major sources of LABs and steroids are urban and domestic sewage. Identification, quantification and source assignment of those organic compounds are very important to assess pollution and to give tools to help minimize the inputs into the environment