Search

Activities related to the marine exploration of oil and gas reservoirs tend to cause a concentration of natural radionuclides in related materials. As such, knowledge regarding the distribution of radionuclides in sedimentary basins with current oil operations is essential for modeling radiological hazards and possible risks of site contamination. This study investigated the distribution of 226Ra and 228Ra, radioisotopes from the 232U and 232Th decay chains in bottom sediment of the Santos basin. Sediment samples were collected from sites in a sampling grid based on depth contours and were analyzed through high-resolution gamma spectrometry. A distribution model of the spatial variation of these isotopes, the frst of its kind for the Basin, supported the interpretation of similar tendencies in their distribution. From studying the spatial trends of the mean levels of the isotopes with descriptive statistics and variance analysis, latitudinal and bathymetric diferences in the content of radionuclides emerged. These diferences are probably derived from the distinct sources of these elements, as 226Ra originates in the deep ocean from the decay of parent 230Th and from the patterns of sedimentation driven by open ocean circulation, while 228Ra is supplied by terrigenous materials transported by the northward-fowing Brazilian Coastal Current.

Supplementary Material of the article "Methane-related community of a carbonate-enriched pockmark, Brazilian Southeastern continental slope" -SF1. Sediment sampling in Box-corer, presenting the core positioning and sediment surface color -SM1. Organic compounds - Analytical procedures -SM2. Sediment dark carbon fixation (DCF) and Heterotrophic Microbial Production (HMP) - Analytical procedures -SF2. Rarefaction curve -SM3. Methodology details for the functional prediction database -ST1. ASVs alpha diversity indexes grouped by enrichment type and incubation time -SF3. Microbial community composition of deep-sea sediments of a pockmark of the Brazilian Southeastern Atlantic enriched with methane. Sequences were taxonomically classified using the Silva database v. 138 -SF4.Upset Plot composed by bacterial and archaeal ASVs detected on the harvesting times of the methane incubation of deep-sea intact pockmark sediment cores of Brazilian SE Atlantic. Blacklinesconnectingcirclesindicatesharedtaxonomies. Black points solely indicate unique taxonomy presence per set. Vertical bars indicate intersection size (number of complete taxonomies, up to Species from the assignment with Silva v.138) on each set. Datasets: Control_16h, cores without CH4 input harvested after 16h; Control_940h, core without CH4 input harvested on 40th day; Enriched_16h, core with CH4 input harvested after 16h; Enriched_120h, core with CH4 input harvested on the 5th day; Enriched_240h, core with CH4 input harvested on the 10th day; Enriched_720h, core with CH4 input harvested on the 30th day; Enriched_960h, core with CH4 input harvested on the 40th day. -SF5. Methane-enriched differentially abundant taxa recovered with ANCOM statistics

A combined analysis of sedimentological, molecular, and microfossil proxies was used to determine paleoclimatic and paleoceanographic variations and assess sediment transport pathways along the last c.a. 6300 years in a sedimentary record collected on the inner shelf of the southwestern Atlantic Ocean. The combined use of terrestrial and marine proxies enables a more complete environmental perspective of the La Plata River plume influence on the southwestern Atlantic shelf, regarding the relationship between climate, productivity, as well as hydrodynamic. Variations in marine productivity, lipid biomarkers of terrestrial plants, diagnostic geochemical indices, and sea-surface temperature point to three phases within the core. The first interval (6300–5200 cal yr BP) was characterized by low marine and terrigenous organic matter input, with pulses of phytodetritus indicated by benthic foraminifera. These features suggested dry climatic conditions over the southeastern portion of South America, with the influence of sediments from the adjacent continent and, on a smaller scale, transported from the Argentinian shelf due to the high relative sea level. The second interval (5200–2800 cal yr BP) was distinguished by a slight increase in marine productivity and terrigenous input, indicating a transition period with an increase of total rainfall influenced by an intensified South American Monsoon System (SAMS) and the modern El Niño-Southern Oscillation (ENSO) phenomenon. These features, associated with the changes in wind patterns and the lower relative sea level, suggest the onset of the influence of the La Plata River (LPR) plume over the southern regions of the Brazilian continental shelf for the last 6300 cal yr BP. The last interval (2800–890 cal yr BP) had the highest contents of marine and terrigenous proxies, suggesting less climatic variability, as the long-term intensification of the SAMS and a further increase in the amplitude of the ENSO resulted in the modern establishment and dominant influence of the LPR plume on the core site. Variations in sea-surface temperature up to 3.3°C were related to the influence of the LPR plume on the shelf off southern Brazil.

The present study determined total mercury (Hg) in four

The worldwide evidence of human activities on the environment led the scientific community to recognize a new geologic time unit known as the "Anthropocene." Since the twentieth century, urbanization and industrialization needs driven by population and economic growth have impacted several ecosystems including the estuaries. To assess the contamination, provenance, and fluxes of trace elements (As, Cr, Cu, Ni, Pb, Sc, V, and Zn) over the last century, a geochemical and chemometric technique was employed in sediment cores of an industrial and port region of international importance, the Santos and São Vicente Estuarine System (SSVES). The results indicated low contamination, with the highest enrichment factors (EFs) for Cu (EF = 3.1), Pb (EF = 2.7), Zn (EF = 2.4), and As (EF = 2.3) found next to the harbor area. The Pre-industrial records confirm the relatively high concentrations of As and its naturally enriched occurrence on the Brazilian shelf. Sediment accumulation rates and trace element fluxes showed a general increase over the years, since the early 1960s, associated with the "Great Acceleration" of the mid-twentieth century. These alterations are human-induced and include urbanization and industrialization. Nonetheless, as the contents and enrichment of trace elements indicate that the region is not severely polluted, we hypothesize that the contamination in the SSVES is likely related to the drainage and erosion of the urbanized adjacent area, rather than direct disposal of inorganic contaminants from the industrial activity.

Living (stained) benthic foraminifera from the continental shelf of Santos Basin, southeastern Brazilian continental margin, were investigated to understand their distribution patterns and their relationship with chlorophyll-a and particulate organic matter contents. Sediments were collected from 44 to 203 m of water depth. This study analysis benthic foraminifera and abiotic data, including grain size, total organic matter, distributions of ocean chlorophyll-a and particulate organic matter. Foraminiferal diversity and richness were positively correlated to the chlorophyll-a and particulate organic carbon contents. Stations located on the southern portion of the study area and sampled during austral winter are probably under the seasonal influence of the Brazilian Coastal Current intrusion, and Rio de la Plata and Lagoa dos Patos plumes. Ammonia rolshauseni, Textularia? sp. and Gyroidina umbonata are present within the foraminifera assemblages at these stations. On the other hand, stations in the north, are characterized by the presence of Uvigerina peregrina, Quinqueloculina atlantica, Hanzawaia concentrica and Bulimina marginata, and they are influence of Cabo Frio’s southward-moving plume.

The Great Amazon Reef (GARS) is an extensive mesophotic reef ecosystem between Brazil and the Caribbean. Despite being considered as one of the most important mesophotic reef ecosystems of the South Atlantic, recent criticism on the existence of a living reef in the Amazon River mouth was raised by some scientists and politicians. The region is coveted for large-scale projects for oil and gas exploration. Here, we add to the increasing knowledge about the GARS by exploring evolutionary aspects of the reef using primary and secondary information on radiocarbon dating from carbonate samples. The results obtained demonstrate that the reef is alive and growing, with living organisms inhabiting the GARS in its totality. Additional studies on net reef growth, habitat diversity, and associated biodiversity are urgently needed to help reconcile economic activities and biodiversity conservation.

In this paper, we used seismic stratigraphy for the study of deposition and sea-level evolution on the Sao Sebastiao Channel (SSC), southern Brazilian coast. About 60 km of high-resolution single-channel seismic lines were acquired to establish a 5th order seismic stratigraphy model for the area. After graphical processing for noise attenuation and enhancing of deep-reflections, we could interpret six seismic horizons and the acoustic basement which together defined six seismic units. Studying those units in a late Quaternary relative sea-level oscillations context, we propose a 4-phase deposition evolution model that took place since the Marine Isotopic Stage (MIS) 5e in the SSC area. The first phase comprises the deposition of seismic units U1 and U2 prior to MIS 4 with the deposition of one lowstand systems tract (LST) and one highstand systems tract (HST). Before the second phase, the sea-level dropped eroding the top layers of the previous units. During the second phase, after the Last Glacial Maximum (LGM), another LST (unit U3) was formed in one of the three stabilization periods of Upper Pleistocene/early Holocene sea-level rise on the Brazilian coast. Following an increase in sea-level rise velocity, we interpreted a transgressive system tract (unit U4) and another HST (unit U5) developed until the maximum sea-level of the Mid-Holocene, at 6 ky B.P during the third phase of our model. During the fourth phase, sea-level fall evolved to the present level, giving conditions to the deposition of the last HST (unit U6) interpreted in the region.

Current studies use indices and chemometric approaches to assess the health quality of estuarine systems to support estuarine management. This study aims to use a multivariate approach and a set of indices to evaluate sediment health quality. Levels of Al, As, Cd, Cr, Cu, Fe, Ni, Pb, Sc, V, and Zn were quantified by ICP-OES, and nine indices were applied and evaluated individually. Most of the indices presented high values of Cu, Pb, and Zn, and the aggregative indices differed from each other in magnitude; however, the spatial distribution demonstrated the same trends. From PCA results, it was possible to differentiate lithogenic from the anthropogenic contribution, especially by Cu, Pb, and Zn. Thus, the integration of geochemical approaches and chemometric tools supported the interpretation of elemental contribution in terms of sources and pathway of heavy metals, which was similar to the results of other studies conducted in the area.

The worldwide evidence of human activities on the environment led the scientific community to recognize a new geologic time unit known as the Anthropocene. Since the twentieth century, the estuaries have been largely impacted by urbanization and industrialization needs driven by population and economic growth. To assess the contamination, provenance, and fluxes of trace elements (As, Cr, Cu, Ni, Pb, Sc, V and Zn) over the last century, a geochemical and chemometric technique was employed in sediment cores of the Santos and São Vicente Estuarine System (SSVES), an industrial and port region of international importance. The results indicated low contamination, with the highest enrichment factors (EF) for Cu (EF = 3.1), Pb (EF = 2.7), Zn (EF = 2.4) and As (EF = 2.3) found next to the harbor area. The Pre-industrial records confirm the relatively high concentrations of As and its naturally enriched occurrence on the Brazilian shelf. Sediment accumulation rates and trace element fluxes showed a general increase over the years, since the early 1960s, associated with the “Great Acceleration” of the mid-twentieth century. These alterations are human-induced and include urbanization and industrialization. Nonetheless, as the contents and enrichment of trace elements indicate that the region is not severely polluted, we hypothesize that the contamination in the SSVES is likely related to the drainage and erosion of the urbanized adjacent area, rather than direct disposal of inorganic contaminants from the industrial activity.

A new mixed turbidite-contourite system is described in the northern Campos Basin, southeastern Brazilian margin. This system is developed in a middle slope setting and was formed through non-synchronous interaction between the turbidity current and a contour current in the same stratigraphic interval (Miocene). Different depositional cycles were accounted based on their diagnostic seismic features. Seismic attributes, seismic facies, and isochron maps were used to identify alternating cycles of downslope and alongslope processes in the study area, along with the intermediate stage with features from both processes (mixed system). Seismic units were then associated with the dominant type of current. Depositional processes resulted from alongslope current activity can be distinguished from the downslope current activity, based on the acoustic characteristics (root-mean-square (RMS) amplitude values), internal architecture, and external geometry pattern. While alongslope currents deposits consist of mainly low RMS amplitude values clinoforms with an alongslope trend; the downslope gravity deposits present high-amplitude or chaotic seismic facies, usually higher values of RMS amplitude, channel or channel-lobe features, erosive surfaces, and a basinward depositional trend. The first and oldest seismic unit (S1) was interpreted as a dominantly alongslope system, with aggrading sigmoidal clinoforms and high-frequency, low-amplitude reflections commonly associated with fine-grained sedimentary deposits, typical of a plastered drift. Basinward mass transport deposit derived from previous drift instability are often identified. Seismic unit S2 represents the intermediate stage where both gravity-driven and along-slope currents act asynchronously. It is referred to as a mixed turbidite-contourite sequence that shows high-amplitude sediment waves migrating upslope and a moat feature carved in its upslope front. The interfingering between high- and low-amplitude reflectors, distal chaotic facies, together with sediment waves and a channel moat, points to a sand-rich deposit reworked by northward-flowing contour currents. Seismic units S3 and S4 show downslope features with chaotic facies (S3) and paleochannels with coarse basal lag deposits interpreted after the high RMS amplitude values (S4). In S4, a series of long-lived submarine channels formed. The last seismic unit, S5, referred to as the second plastered drift sequence, is marked by low-amplitude clinoforms that thin basinward. Important information on the paleocurrents' direction was also made based on the final deposits display (e.g. terraces, sediment waves, paleochannels), where a northward-flowing bottom current was assumed. Research on alternating dominant processes and transitional stages or mixed depositional systems may provide a better understanding of deep-water depositional processes. Because these processes do not always fit previous depositional models that are mainly described for synchronous systems, new insights on cyclic non-synchronous mixed systems can improve our understanding of how mixed systems are organized through time and space. We can also determine which were the dominant processes that controlled the sedimentation by indicating periods where the margin was mostly submitted to sediment transfer from continent to the basin and periods where the oceanic currents prevailed by redistributing sediments along the isobaths and replacing the axis of downslope transfer conduits. Setting new models on cyclic deposits and intermediate stages can have a future economic impact on potential hydrocarbon reservoir architecture.

The Uruguayan continental margin (UCM), located in the Southwestern Atlantic margin’s subtropical region, is positioned in a critical transitional region regarding the global ocean circulation (Río de la Plata (RdlP) outflow and Brazil-Malvinas Confluence), as also reflected in seafloor features (northernmost distribution of a large depositional contourite system and RdlP paleovalley). This complex oceanographic scenario occurring in a relatively small area highlights the advantage of considering the UCM as a natural laboratory for oceanographic research. The present work provides the first conceptual “control” model of the physical drivers (i.e., climate, geomorphology) and main oceanographic processes (i.e., hydrodynamics, sediment, and carbon dynamics) occurring along the UCM, reviewing and synthesizing available relevant information based on a functional integrated approach. Despite the conspicuous knowledge gaps on critical processes, a general picture of the system’s functioning is emerging for this complex biophysical setting. This includes conceptualizations of the actual controls, main processes, feedbacks, and interactions responsible for system dynamics. The structure adopted for developing our conceptual models allows permanent improvement by empirical testing of the working hypothesis and incorporating new information as scientific knowledge advances. These models can be used as a baseline for developing quantitative models and, as representations of relatively “pristine” conditions, for stressors models by identifying sources of stress and ecological responses of key system attributes under a transboundary approach.

Polycyclic aromatic hydrocarbon (PAH) concentrations were determined in four dated sediment cores collected in mud depocenters of the southern Brazilian continental shelf. Core dating results covered the interval between 1925 and 2017. The total PAH concentrations (ΣPAHs) ranged from 44.69 ng g−1 to 305.43 ng g−1 and were similar between the analysed cores. Fine-grained sediments and total organic carbon (TOC) results did not correlate with the ΣPAHs, indicating that the variations in PAH concentrations are mostly related to variations in sources and emissions. PAH source appointment indicated a high input of a natural compound (perylene) and the predominance of anthropogenic PAHs from coal, biomass, and fuel combustion. Alkylated PAHs presented high contributions throughout all cores. The historical deposition of PAHs was associated with different periods of the socio-economic and industrial development of near coastal cities and reflected very well the history of coal production and consumption in the southern region of Brazil. The low levels of ΣPAHs before 1945 in all analysed cores may be related to the beginning of the industrialization process and the lower urbanization degree in the region. Between 1945 and 1965, the gradual ΣPAHs increase reflects the establishment and enlargement of the southern Brazilian industrial sector. The interval between 1965 and 1990 corresponded to the highest ΣPAHs in three of the four analysed cores. After 1990, a relative decrease in the ΣPAHs was observed in most cores and may be related the multiple cuts of incentives to the industrial usage of coal, as well as to Brazil's efforts in environmental regulation for coal extraction and consumption.

The coastal region of Brazil is historically impacted by anthropogenic activities, which release contaminants such as metals. In the estuaries of Paranagua, Cananeia, and Santos in southern Brazil, these activities are present at differing levels. In the present study, concentrations of aluminum, arsenic, cadmium, lead, copper, chromium, iron, nickel, zinc and mercury were investigated, and geochemical parameters in sediments. Bulk carbon and nitrogen isotopic analyses were performed by EA-IRMS, and metal analyses were performed by ICP-OES-VGA. The As, Cr, Ni, Zn and Hg had higher concentrations in Santos estuary sediments. The effects range-median quotient indicating the potential for adverse effects to biota was found in this estuary, coinciding with the site with the greatest intensity of anthropogenic activities and higher percentages of mud. The levels of metals in the Paranagua and Cananeia estuaries were similar, demonstrating that even with different degrees of anthropogenic pressures, the connection between these estuaries and the establishment of protected areas avoids contamination of these coastal regions that are World Heritage site and a biosphere reserve. Despite the decrease in the levels of metals in all the estuaries studied over time, the present study serves as a warning of the need for monitoring and coastal management actions. Mainly for Hg, which is available in sediments and above the sediment quality guidelines, to maintain the environmental quality of these important coastal regions of South America.

This study aims to investigate the spatial variability of surficial sedimentation processes on a large sector of the Brazilian shelf by using echo-character mapping. To reach this goal, almost 3200 km of high-resolution seismic lines collected with a 3.5 kHz Chirp source are compared with surficial sediment-related distribution maps. Based on seismic characteristics, such as sharpness, amplitude, and continuity of sub-bottom reflections, six echo types were identified in the study area. These echo types were grouped into two main classes according to the nature of seafloor morphology. Class 1 is characterized by flat-bottom echoes with a variable amplitude of reflections and acoustic penetration. This class is predominantly found on the inner and middle shelves. Class 2 consists of irregular seafloor morphologies and is classified based on the occurrence of internal reflections and relief variations. This class is mainly found on the outer shelf. The distribution of echo types and associated surficial sediment maps exhibit three main shelf domains: inner, middle, and outer. The echo types are mostly controlled by the influence of along-shelf currents, as well as by mid-shelf fronts associated with the entrance of cold-water tongues under changing wind conditions and upwelling phenomena. Whereas the inner shelf is mainly affected by variable wind-driven currents, the outer shelf is more persistently influenced by the southwestward movement of the Brazil Current. Between these areas, the middle shelf is affected by seasonal intrusions of the South Atlantic Central Water and northward extensions of the Brazilian Coastal Current. The studied area can be considered a case model for a shelf driven by hydrodynamic processes., This research was supported by grants (2010/06147–5, 2016/08029–6, and 2016/24251–0) from the São Paulo Research Foundation (FAPESP). We thank the NAP-GEOSEDEX Program for providing the data. We thank the crew and researchers who participated in the Alpha Crucis cruises. F.J. Lobo acknowledges the Brazilian program "Ciência sem Fronteiras" funded by CNPq, enabling him to conduct several research stages as "Pesquisador Visitante Especial" at the Instituto Oceanográfico, Universidade de São Paulo, under project number 401041/2014–0. Dr. Deniz Cukur and one anonymous reviewer are also greatly appreciated for their constructive comments on the original draft. The authors also acknowledge IHS Markit for providing the software Kingdom Suite under the Educational Grant Program.

Deep-sea carbonate mounds can harbor a wide variety of heterotrophic and chemosynthetic microbial communities, providing biodiversity hotspots among the deep-sea benthic ecosystems. This study examined the bacterial and archaeal diversity and community structure in the water column and sediments associated to a recently described giant carbonate mound named Alpha Crucis Carbonate Ridge (ACCR), Southwestern Atlantic. Due to the acoustic evidence of gas chimneys from a previous study, we further evaluated the chemosynthetic primary production through in situ-simulated dark carbon fixation rates. Pelagic microbial communities varied significantly with depth, showing a high abundance of photosynthetic groups in surface waters and taxa related to nitrification in intermediate and deep waters. The benthic communities from the top of the ACCR were very similar along with the sediment depth, while those from the base of ACCR showed a clear stratification pattern, with members in the deep strata mainly related to anoxic and chemosynthetic ecosystems. Dark carbon fixation rates were of the same order of magnitude as those of deep-sea cold seeps and hydrothermal vents. Our study provides the first description of the ACCR microbiome and adds new information to help formulate and implement future conservation and management strategies for vulnerable marine ecosystems.

The Southwestern Atlantic harbors unique reef environments with high proportions of endemic species. The most prominent reefs are located in the Abrolhos Bank, a 46,000 km2 extension of the South American continental shelf. However, just 100 km north of Abrolhos is the Royal Charlotte Bank (RCB), an area still poorly investigated. From a biological perspective, the only scientific information available is the historical record of lobster fisheries in the region. Hence, we performed an expedition to investigate what macrohabitats are found in the RCB and perform a preliminary biodiversity assessment. A dropcam was deployed in 67 sites and recorded images of macrohabitats and associated biodiversity. Four different macrohabitats were detected: rhodolith beds, macroalgal forests associated with rhodolith beds, calcareous sand deposits and coral reefs. The former two were the most frequent, and both rhodolith and macroalgae densities were higher on the outer shelf. A total of 83 species of fish, invertebrates and algae were identified, which comprises a comparatively high diversity for visual reef biodiversity assessments in the Southwestern Atlantic. Our findings show that the RCB is a large and diverse ecosystem complex mostly composed of rhodoliths, and also that it may be already undergoing anthropogenic impacts associated with climate change and fisheries. Therefore, further investigations are necessary to understand the extent of biodiversity and its main threats and evaluate the possible need of conservation and management measures.

The current study aims to do a comparative analysis of the results obtained with two End-Member (EM) grain-size modeling techniques (AnalySize and BasEMMA) and the Environmental Sensitive Grain-Size (ESGS) modeling for sediment samples collected in the mudbelts of the southern Brazilian shelf. The methods were compared using grain-size data from the south Brazilian shelf, and the results are discussed here. It is the first time that the concept of ESGS, in its present form, is utilized outside China. The results show that despite its relative ease of use, the ESGS is not fully comparable to the EM analyses, and only two grain-size ESGS classes were recognized as analogous to EMs. The comparison of the AnalySize and BasEMMA procedures also revealed significant differences between the techniques. A comparative analysis between the two EM techniques revealed advantages in the BasEMMA, especially in the better correlation of the end-members with the original grain size distribution. The analysis of the geographical distribution of the EM abundances allowed point sources of sandy populations to be recognized as well as the contribution of the Rio de la Plata as a source of silty sediments to be inferred.

The Uruguayan Continental Margin is located in a strategic location of great relevance to understand the dynamics of the regional and global climate and to study the sedimentary processes. The Río de la Plata represents the largest fluvial discharge of the Southwest Atlantic (23.000 m3/s of water and 57.000.000 m3/year of sediment for the South Atlantic Ocean). The sub-surface and deep circulation are characterized by the confluence of water masses with contrasting thermohaline characteristics that determine the formation of well-marked hydrological fronts. In the continental shelf, this characteristic is represented by the Subtropical Shelf Front, as a shallow extension of the Brazil-Malvinas Confluence that occurs in deep water off the shelf. Although the knowledge of the regional circulation is acceptable, there are significant gaps in the knowledge related to the influence of this complex hydrological system on sedimentation processes. The objective of this work is to synthesize the present knowledge about the morpho-sedimentary characteristics of the Uruguayan Continental Margin and the processes involved in its formation. It also aims to discuss some knowledge gaps that are the subject of current research in the region. The knowledge reflects the scientific-technological evolution at a global level that is translated in the increase of recent works on the continental slope. Nowadays, most of the available information is associated with the inner shelf and upper slope. Starting from the basis of the close relationship between bathymetry and sedimentological distribution it is clear that efforts should be concentrated on the better understanding of the interaction between the shelf and the slope. The scientific knowledge regarding sedimentology and geochemistry along the Uruguayan Continental Margin is still, scarce and heterogeneous. However, it indicates complexity in the sedimentary coverture present both in the continental shelf and slope. The Uruguayan Continental Margin can be considered as an in situ laboratory to improve the understanding of the influence of oceanic hydrodynamics on the margin configuration, a key point to achieve better paleoceanographic interpretations. In this sense, due to the complexity of both modern hydrology and morphological configuration of the margin, it would be interesting to understand the relative importance of the control of each environment on surface sedimentation. A better understanding of the processes that link the near bottom circulation and continental slope features is needed. At the same time, this will allow deepening a new field of research in Uruguay, considering the joint and interrelated vision between sedimentary and oceanographic processes. ResumoA Margem Continental do Uruguai está localizada num local estratégico de grande relevância para entender a dinâmica do clima regional e global e para estudar os processos sedimentares. O Río de la Plata representa a maior descarga fluvial do Atlântico Sudoeste (23.000 m3/s de água e 57.000.000 m3/ano de sedimento para o Oceano Atlântico Sul). A circulação de subsuperfície e profunda são caracterizadas pela confluência de massas de água com características termohalinas contrastantes que determinam a formação de frentes hidrográficas bem marcadas. Na plataforma continental, essa característica é representada pela Frente Subtropical da Plataforma, uma extensão rasa da Confluência Brasil-Malvinas que ocorre em águas profundas ao largo da plataforma. Embora seja considerável o conhecimento da circulação regional, existem lacunas significativas no conhecimento da influência deste complexo sistema hidrológico nos processos de sedimentação. O objetivo deste trabalho é sintetizar o conhecimento atual sobre as características morfo-sedimentares da Margem Continental Uruguaia e os processos envolvidos na sua formação. Também pretende discutir algumas lacunas de conhecimento que são objeto de pesquisas atuais na região. O conhecimento reflete a evolução científico-tecnológica a nível global, que se traduz no aumento de trabalhos recentes na vertente continental. Atualmente, a maior parte da informação disponível está associada à plataforma continental interna e ao talude continental superior. Partindo da base da estreita relação entre a batimetria e a distribuição sedimentológica, torna-se claro que os esforços devem ser concentrados na melhor compreensão da interação entre a plataforma e o talude continental. O conhecimento científico sobre sedimentologia e geoquímica ao longo da Margem Continental do Uruguai é ainda escasso e heterogêneo. No entanto, os dados existentes indicam que a cobertura sedimentar presente tanto na plataforma quanto no talude continental é complexa. A Margem Continental do Uruguai pode ser considerada um laboratório in situ para melhorar a compreensão da influência da hidrodinâmica oceânica na configuração da margem, ponto chave para se obterem melhores interpretações paleoceanográficas. Nesse sentido, devido à complexidade da hidrologia moderna e da configuração morfológica da margem, seria interessante entender a importância relativa do controle de cada ambiente sobre a sedimentação superficial. É necessário um melhor entendimento dos processos que ligam as características de circulação próxima do continente à que ocorre no talude continental. Este conhecimento permitirá aprofundar um novo campo de pesquisa no Uruguai, considerando a visão conjunta e inter-relacionada entre processos sedimentares e oceanográficos.

Continental margin sedimentation represents the terrestrial-marine geological transition. Coarser coastal sediments usually transition to fine-grained sediments across the shelf as the energy level decreases. As the distance from the continental source of terrigenous sediments increases, the mid-shelf mud belt transitions to a seaward organic/carbonatic sedimentation domain. However, this general pattern might change depending on the terrigenous sediment load. This is the case for the Amazon continental shelf, where the Amazon River empties and builds up the large Amazon River plume (ARP). The ARP promotes an inverse pattern with muddy nearshore prodelta sediments and siliciclastic sands offshore. This study investigates water and bottom characteristics to the east of the Amazon River mouth to understand the complex interplay that leads to simultaneous massive river plume spread and mesophotic reef development. Three cross-self profiles were performed, including physical-chemical water column measurements and bottom sample collection. The profiles were approximately 150 km apart, comprising a longitudinal gradient off of the ARP. The results reassure that there is enough light for mesophotic reefs to grow at the outer shelf and that at the mid-shelf, large-scale sand movement is likely to be the major limiting control on reef accretion. Also, mesophotic conditions (i.e., low light levels) occur in shallower waters, where a trend of reef expansion was identified, but high-energy levels and the lack of hard substrates hinder reef expansion.

This study describes the formation of two successive baymouth spits systems on the southeastern Brazilian coast and the degradation of the first system. The study area includes the Jureia Beach spit, the deflected Ribeira de Iguape River mouth, the central Iguape sandy headland, the Icapara Inlet of the Mar Pequeno Lagoon and the northern end of the Comprida Island barrier spit. The wave and river flow patterns were combined with the coastline evolution and the alongshore migration rates deduced from satellite images. Initially, both spits showed convergent alongshore migration rates equal to or less than 83 m/y. However, the extreme river flow due to high rainfall during a very strong El Nino event in 1983 eroded the inland side of the Jureia Beach spit, which finally retreated due to wave erosion. In 1989, a sand bank emerged in the river mouth, which attached to the central headland forming a recurved northeastward spit. In 1994, the high fluvial discharge associated with another very strong El Nino event caused the landward migration of the new spit and emersion of a second sand bank. This second sand bank merged with the Jureia Beach spit in 1997 at an alongshore migration rate of 1,795.6 m/y. Wave erosion of the central headland continued and the attached spit disappeared in 2000. In 2009, the headland erosion merged the river mouth and the Icapara Inlet, which resulted in flanking baymouth spits in a configuration that remains today. Therefore, two models for the formation of baymouth spits have been documented for wave-dominated microtidal coasts in humid tropical regions with intense fluvial discharge. The convergent longshore migration of the spits is controlled by both the bidirectional longshore drift and the fluvial discharge, the latter eroding the fronting spit, supplying sediments and acting as a hydraulic blockage for longshore drift.