Helenice Vital

Major and trace elements of stream sediments from the lowermost Amazon River

Geochemical results from river-bottom sediments taken from the lowermost Amazon River are presented. Bedload sediments were analyzed for 10 major and 38 minor elements. Mud bulk sediments exhibit a tendency to a slight enrichment in Cr, Mn, Rb, Sr, Zr, Cs, Ba and Hf, pointing to the importance of these elements in the silt fraction. Generally, Cu, Zn, Rb, Cs, Ba and Sn are clearly associated with clay minerals, while Y, Zr, Hf, Th and U are associated with heavy minerals. The location of this area in an intensive chemical weathering environment results in the strong modification of sediment composition. Chemical weathering processes strongly modify original sediment composition and can be recognized apart from provenance processes on the basis of their geochemical signature. Discriminant analysis makes the detection of less marked differences in sediment characteristics possible, thus, improving the accuracy of distinctions between groups. The Amazon Mouth and Xingu Mouth (XM) are discriminated principally by the textural maturity of their sediments. The Jari Mouth (JM) shows highest Co and Fe2O3 contents. Despite similarities between the Amazon North Branch (ANB) and Amazon South Branch (ASB), the ANB can be differentiated on the basis of its higher Sr, P2O5, and Na2O contents, the ASB on the basis of its higher Zn, MnO, and CaO contents, and the Estreitos de Breves (EST) based on its higher abundance of Ni and Cu. Higher contents in P2O5, Fe, and Mn are related to laterite covers.

Characterization of the Brazilian continental shelf adjacent to Rio Grande do Norte state, NE Brazil

B S T R A C T This study focuses on the analysis of high-resolution seismic profiles, integrated with sedimentological, echosounder, SRTM and satellite image datasets, of the Brazilian continental shelf adjacent to the Rio Grande do Norte State, NE Brazil. Located in the northeast of Brazil, the State of Rio Grande do Norte is bounded by two main coastal and shelf systems: the eastern coastal-shelf, from the Sagi River to the Touros High, and the northern coastal-shelf, extending from Touros High to Tibau. This shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system characterized by reduced width and shallow depths as compared with other parts of the Brazilian shelf. It has an average width of 40 km, the shelf-break lying at a depth of ~ 60 m. This shelf is subject to the full strength of the westerly South Equatorial current combined with high winds and moderate to high tides and waves. A sharply defined stratigraphic boundary, probably between the Pleistocene and Holocene deposits, is clearly to be observed in the seismic record. Incised-valleys extending from the main river mouths (e.g.the Potengi, Acu, and Apodi) to the shelf break dominate the area investigated and may indicate periods of lower sea level.

Mapeamento de recifes submersos na costa do Rio Grande do Norte, NE Brasil: Macau a Maracajau

Studies on the Brazilian continental shelf were developed between Touros and Macau-RN (NE Brazil) with the aim to mapping carbonate buildups, and especially, reef ecosystems. Remote sensing, submarine movies, bathymetric survey and diving were the methods used. This paper will focus on three main aspects of the Rio Grande do Norte reefs: 1) characterization of the coral fauna; 2) the classification and 3) the distribution of the main northriograndense carbonate buildups. Reefs environments are found predominantly on the inner shelf adjacent to Touros, at depths shallower than 10 m. These reefs generally show NW-SE orientation parallel to the coastline and constitute groupings of knolls and patch reefs. A prominent carbonate buildup, where the corals are almost absents and algae incrustation is thin, occurs in average depths of 25 m, along the northern part of Rio Grande do Norte State and is classified as sandstone bank. Others sandstone banks, with smaller lengths and heights were mapped around 10 m depth. They should be related to an ancient coastline; however the corresponding lithostratigraphic unit was not defined.This work is a contribution to the mapping of coral reef, in particular, and carbonate buildups, in general, on the Rio Grande do Norte coast.

Carta sedimentólogica da plataforma continental brasileira: área Guamaré a Macau (NE Brasil), utilizando integração de dados geológicos e sensoriamento remoto

Studies carried on the Brazilian continental shelf are little developed, especially on the northeastern Brazil, and particularly on Rio Grande do Norte State. This work intends to organize the existent information about the sediment cover on the NE Brazilian continental shelf, and to elaborate a sedimentological chart. The used methodology integrated the sedimentological data with remote sensing processing stored on a georeferenced bank. Ten main facies were mapped on the studied area. Siliciclastic sands are present on shallow waters, near the coast, while carbonate sands are present offshore. The muds are present in the mouth of the rivers and filling channels on the shelf. The sediment cover is one of the most important parameters to understand de shelf system as a whole, because they reflect geological and hydrodynamic processes, present and past, and define, together with others variables, the kind of biological community installed.

Lowermost Amazon River: morphology and shallow seismic characteristics

Abstract The riverbed morphology of the lowermost Amazon River was characterized by high-resolution acoustic profiling in 21 areas. This technique has been used for the first time in this area, and allowed the discovery of a variety of reflectors, which delineate areas either of sediment transport or of deposition on an older river bottom. Records obtained with the Pinger system made it possible to distinguish three main kinds of reflectors, both surface and sub-bottom: (1) flat, (2) wavy, and (3) irregular reflection patterns, which can be interpreted as strata deposited in a variable, mostly high energy setting, or reworked structures of initially flat or wavy strata that have been changed. Mapping of these different reflectors shows that they are not randomly distributed. The constant association of a given reflector with a particular morphological setting is indicative of its lithological significance (supported by riverbed sampling), and erosional/depositional processes. Depositional areas are characterized by transparent units consisting of soft silt overlying a highly reflective sub-bottom, related to old substrates (Alter do Chao or Barreiras Formation). This may result from inundation and incision caused by Quaternary sea-level changes. Previously eroded areas are covered by a wavy layer consisting of very fine to fine sand (unit FS), which can change locally to medium–coarse sand (unit MS). Reflectors are more abundant downstream. A minimum of 3 units were indicated by seismic records and could be related to sea level fluctuations and changes in the river discharge. Moreover, the detection of fault scarps, interpreted to result from neotectonic activity, shows that neotectonic features can be important locally and should not be ignored as a possible reason for modern river instability. The dynamic setting of the lowermost Amazon area results in patterns of deposition and erosion probably controlled by complex interactions of fluvial and oceanic processes. The effect of the Gurupa arch may result in a zone of sediment bypass in the central part and a zone of increased deposition in the distal downstream part.

Estruturas rasas de gás em sedimentos no estuário Potengi (nordeste do Brasil)

Areas with gas accumulations and gas seeps, where gas escapes from the river-bottom to the water column, have been mapped in the fill of a fluvial valley, the Potengi estuary. The various gas features have been classified into four types according to their specific seismic signatures: (1) acoustic blanket, (2) acoustic curtains, (3) acoustic columns, and (4) acoustic turbidity. At the same time, two types of gas escapes features have been distinguished: (1) acoustic plumes and (2) pockmarks. The neotectonic activity in the semigraben of the Potengi river may be one of the responsible factors for the migration of gas coming from the seismofacies located below the present river level where it is possible to recognize the connection with the level of the gas source through the acoustic column type seismic signature. It is concluded that sedimentary facies is the main factor determining whether gas accumulates or seeps, and also determines the specific type of accumulation or seep at each location.

The investigation of a mixed carbonate-siliciclastic shelf, NE Brazil: side-scan sonar imagery, underwater photography, and surface-sediment data

Side-scan sonar imagery and underwater photography, coupled with bathymetric and sedimentologic data, revealed noteworthy features and the interplay of multiple processes on the north-eastern Brazilian continental shelf. The high-resolution data cover a 500 km2 area of the shallow (~70 m) and narrow (~40 km) shelf, which comprises the submerged portion of the Potiguar Basin; the area includes the partially filled incised valley of the ancient Acu River.The seafloor imagery was integrated with 673 surface sediment samples to distinguish boundaries within the mixed siliciclastic-carbonate region. The sediment grain size increases from nearshore, muddy, siliciclastic sediments to biogenic, coarse sand and gravel on the outer shelf. Carbonate-rich sediment predominates, except on the eastern margin of the Acu Incised Valley (AIV), the nearshore zone and estuary mouths. The AIV is filled mainly with muddy sediments reworked from the latest transgressive deposits. Bedform configurations indicate bottom currents associated with tidal and wave-driven currents.Mixing processes are controlled by the interplay of high hydrodynamic energy regime and shelf physiography. Special attention was paid to the middle and outer shelves, where a large field of newly found reefs occurs between 20 and 50 m depths. The reef substrate is composed of limestones, probably belonging to the Guamare Formation.SCUBA dive inspections revealed benthic colonies (corals: Montastraea cavernosa, Porites astreoides, Siderastrea stellata; sponges: Scopalina ruetzleri, Callyspongia vaginalis, Ectyoplasia ferox, Ircinia sp., Spirastrella sp., Aiolochroia sp.) over fossil reefs. A preliminary examination of the foraminiferal species showed a Caribbean-type reef community, including Amphisorus hemprichii, Amphistegina gibbosa, Archaias angulatus, Heterostegina antillarum, Homotrema rubra, Peneroplis carinatus and P. proteus.

Evolution of the Parnaíba Delta (NE Brazil) during the late Holocene

Sedimentary processes and the evolution of the wave- and tide-dominated, asymmetric Parnaíba Delta during the late Holocene were investigated based on geochemical and sedimentological analyses of sediment cores collected in 2010, as well as satellite images and historical maps. This is a rare case of pristine deltas essentially unaffected by human activities worldwide. The lowermost part of the main Parnaíba River distributary exhibits several low-sinuosity bends and several anastomosing bifurcation patterns in the east, whereas three NW–SE-oriented tidal channels drain a large mangrove area in the west. Dating of various materials in sediment cores from the tidal flats, tidal channels and supratidal marshes revealed that the oldest sediment (4,853 to 4,228 cal. years BP) is paleo-mangrove soil from the main river distributary. Present-day mangroves and marshes up to 200 years old exhibit high sedimentation rates reaching 3.4 cm/year. The asymmetry of the delta is explained not only by the wind- and wave-induced westward-directed longshore drift but also by neotectonic processes, as revealed by satellite images. Faulting and eastward tilting may have triggered delta lobe switching from west to east. This would explain the erosional character and unusual updrift orientation of the main river-mouth channel. Consistent with existing knowledge on mangrove ecosystems worldwide, sediment carbon and nitrogen signatures lie in the range of freshwater or marine dissolved organic carbon and C3 terrestrial plants. In the western tidal channels, the low Corg/Ntot ratios (16–21) of young mangrove soil (deposited in the last 16 years) reflect a stronger influence of marine plants compared to older mangroves (1,390–1,525 cal. years BP; ratios of 20–37). Thus, there would have been a greater influence of the Parnaíba River on tidal-channel sedimentology 1,400 to 1,500 years ago, entailing a natural connection between the present-day tidal channels and the river in ancient times, which was abandoned later during delta lobe switching. This is substantiated by historical maps that indeed show this connection between the main distributary and the tidal-channel system.

Fluxo de processamento aplicado a dados de sísmica de alta resolução em ambiente de Plataforma Continental: exemplo: Macau-RN

This paper presents a proposal for a processing flow applied to high-resolution seismic data. It is organized in the implementation steps of operations and takes in account different factors in the flow organization. Usually used as raw data, the high resolution seismic processing is little discussed in literature. It differentiates in many aspects of processing used in conventional seismic data. The application of the processing flow to shallow seismic data was based on key criteria such as: behavior of a wave of high frequency, properties of the environmental, operation of the filters, gain, which allowed to obtaining a result with a better quality than the raw data. The flow defaults a logical sequence of procedures (header edition, filter, gain, deconvolution) for this kind of data, with basic steps, necessary for a general flow, and some additional processes, revealing potential problems and their solutions.

Beaches of Rio Grande do Norte

The Rio Grande do Norte coast extends 410 km, and consists of sandy beaches (72 %), active sea cliffs carved into Cenozoic sediments of the Barreiras and Tibau formations (26 %), and transgressive dune fields and beachrock. It comprises two different sectors: the northern (equatorial) coast trends east for 244 km, while the eastern (oriental) coast trends south for 166 km. The eastern sector is characterized by wave-dominated and some tide-modified beaches that are mainly reflective to intermediate states. In contrast, the northern sector has resulted in tide-modified and tide-dominated beaches that range from reflective (the dominant state) to intermediate. In general the Refletive + Low tide terrace (R + LTT) is present along the entire coast for most of the year, while wave-dominated Longshore bar and trough (LBT), Rhythmic bar and beach (RBB), Transverse bar and rip (TBR), Low tide terrace (LTT) and reflective (R) occur along the eastern sector, and tide-dominated Beach + tidal sand flats (B + TSF) occurs along parts of the northern sector. R + rock flats and coral reef flats are present in both sectors, where bedrock and beachrock reefs are prevalent. Beachrock reefs are very common along the Rio Grande do Norte shore, occurring in both the offshore and onshore zones. Beach morphodynamics is modified due the presence of the beachrocks. Sea level variability is dominated by tides (up 98 % of the energy spectra). The sea level subtidal component is well correlated with the winds, but demonstrate very low amplitudes. Longshore currents are wind modulated, while cross-shore currents are primarily modulated by tides and secondarily by winds. Erosional hotspots and, both natural and anthropogenic hazards, are present along the Rio Grande do Norte coast.