Realistic modelling of shelf-estuary regions

A very high-resolution modelling configuration was created for the estuary of Baía de Todos os Santos – BTS, Brazil (300 to 400 m), and adjacent coastal waters (600 to 1200 m). The adoption of a multi-corner domain approach allowed the variable spatial resolution required to resolve the shelf, the bay and their interactions. Seven years were simulated using realistic oceanic, atmospheric and riverine forcing. Model validation was done against observations showing the model skill to reproduce the thermohaline field, the tidal currents, as well as the variability of the free surface at tidal and sub-tidal time scales. The results provide the first representation of the tidal wave propagation along the bay, in terms of maps of amplitudes, phases and ellipses of the barotropic currents for the main tidal constituents. By analysing the residual currents at different depths, in terms of averages over the simulation period, several prominent structures were identified and named: (i) Salvador eddy (up to 0.2 m s⁻¹); (ii) St Antonio current (up to 0.45 m s⁻¹); (iii) Salvador current (up to 0.5 m s⁻¹); (iv) Itaparica eddy (up to 0.2 m s⁻¹); (v) Ilha dos Frades southern eddy (up to 0.1 m s⁻¹); and (vi) Ilha dos Frades northern eddy (up to 0.2 m s⁻¹). The model set-up proved to be highly efficient and robust simulating the BTS shelf-estuary region and such an approach may be suitable to other estuarine systems.

     

Stationary underwater cameras assess more efficiently clear-water mangrove fish assemblages: A comparison of non-extractive techniques

Research in aquatic ecology requires adequate sampling designs if the underlying processes that govern population and community dynamics are to be understood. Adequate sampling design is particularly important for organisms inhabiting environments such as mangroves. Thus, collecting quantitative ecological data remains a major challenge. Here, we analysed the structural and temporal dynamics of a tidal cycle and mangrove fish assemblages at 15 sites along the Brazilian central coast. Specifically, we used two non-extractive sampling techniques, which included stationary underwater visual census (s-UVC) and stationary underwater wide-camera census (s-UWC). Our results showed that by reducing potential observer effects, the s-UWC protocol maximized fish counts, yielding more accurate and precise estimates. In contrast, although the s-UVC technique was a useful approach, this technique did not yield precise density and richness estimates. Furthermore, the negative effect associated with the presence of an observer (i.e., diver) was a significant constraint to the observer-based technique. Furthermore, the methods were more similar for the high tide samples and more divergent for the flood tide and ebb tide samples. These pronounced divergences among tidal periods allowed the application of a temporal mixed-sampling protocol (i.e., unbalanced temporal samples) based on s-UWC without the loss of statistical power. Methodological gains also were felt with a reduction in the costs associated with the presence of a trained observer in water and overcoming of constraints positively related to depth limitations. Overall, the s-UWC protocol adequately captures species using different stages of the tide to enter the mangroves.     

Coastal and shelf circulation in the vicinity of Camamu Bay (14°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.     

Pedo‐Geochemical Signatures of Archeological Sites in the Tapirapé‐Aquiri National Forest in Marabá, Amazonia,Brazil

The present study aims to interpret the occupation of terra firme (nonflooded uplands) archeological sites located at Tapirapé‐Aquiri National Forest in the Brazilian state of Pará, through an integrated analysis of pedological, archeological, and geochemical data. We focus on seven archaeological sites, selected among 22 identified in the region. Radiocarbon and thermoluminescence dating indicate distinct periods of occupation over the past ∼6000 years, and the pedo‐geochemical data identify intra‐ and inter‐site differences in soil. Archaeological, chronometric, and pedo‐geochemical data provide a basis for the functional classification of archeological sites found in the region and help to identify specific human activity areas. The results lead us to infer that many of the archeological sites were the result of multiple occupations that left a persistent pedological signature on the landscape.     

Reservoir-induced Seismicity in Brazil

—?A compilation of 16 cases of reservoir-induced seismicity in Brazil is presented with maximum magnitudes ranging from 1.6?M _L to 4.2?m _b . The compilation includes: location of the main epicentral area with respect to the reservoir (inside the lake, at the margin, or outside), predominant geology, and the temporal distribution of the main phase(s) of activity (initial or delayed in relation to impoundment). Data on the regional stress field for some reservoirs is also included. Four recent cases are discussed in more detail: Tucuruí, Nova Ponte, Miranda, and Serra da Mesa. A comparison with all other reservoirs deeper than 30?m and 50?m suggests that the hazard for induced-seismicity varies within Brazil: the NE part of the intracratonic Paraná basin has higher hazard as compared with the southern part of the same basin. No correlation of the induced hazard with variations in natural seismicity can be observed.     

How geological origin influences the quality of sand and the environmental impacts of its mining,Bahia state,Northeastern Brazil

The highly increasing demand for sand in Northeastern Brazil, due to governmental economic incentive focused to this region, has necessitated the opening of new extraction mines, generally without adequate environmental control. This study aimed to assess several sand extraction activities in the Recôncavo territory, Bahia state—area with high variability in sedimentary deposits—by evaluating the quality of the sand and the extraction impacts according to the geological basement. The methodology consisted of field research, using satellite images and maps for characterization, application of the environmental and economic impact checklists, and analysis of organic matter, particle size distribution, and salt content in 35 samples from different sand mines. The sands were classified as white sand (from Neossols/weathered aeolian sandstones), brown sand (from mature soils), and river sand. A strong relation between the geological origin and sand type and quality was established. Although the river sand appears to be the best option for extraction with regards to sand quality and low environmental impacts, the white sand derived from Jurassic sandstones is intensively extracted due to easy access, availability of wide deposits, and visual quality and homogeneity. However, the white sand has an inadequate particle distribution due to its poor gradation (aeolian selection of fine particle) and high environmental impacts due to its location in wet tropical ecosystem dominated by the Atlantic forest and the erosion of sandy soils. We demonstrated that the knowledge of lithology is fundamental for mapping new potential sand mines and minimizing the generated impacts.     

Observations of near-inertial oscillations along the Brazilian continental shelf break

Ocean Dynamics - Near-inertial oscillations (NIO) are intermittent motions with a frequency close to the inertial frequency and represent an important fraction of the energy to the currents in the...