Paleoproductivity changes during the Holocene in the inner shelf of Cabo Frio, southeastern Brazilian continental margin: Benthic foraminifera and sedimentological proxies

Sedimentological and benthic foraminifera analyses carried out on a core (length 4.15 in, collected at 22 degrees 56`31 `` S and 41 degrees 58`48 `` W, at a water depth of 43 in) sampled from the inner shelf of Cabo Frio, southeastern Brazilian continental margin, allowed identification of different hydrodynamic and productivity regimes related to sea-level fluctuations and/or climatic changes, during the last 9.4 ka cal BP. Prior to 7.0 ka cal BP, a less intense hydrodynamic and lower productivity regime occurred at lower sea levels and under drier climatic conditions. Between 7.0 and 5.0 ka cal BP, relatively stronger local oceanic circulation and relatively high productivity were observed, in a scenario of rising sea levels and more humid conditions. From 5.0 to 3.0 ka cal BP, bottom currents weakened and input of nutrients increased, with productivity levels similar to the previous phase at lower sea level and in a drier climate. From 3.0 ka cal BP up to the present, stronger hydrodynamic conditions and a higher productivity regime are linked to the establishment of the upwelling process in Cabo Frio. From 2.5 ka cal BP to the present, upwelling enhancement has been recognized, resulting from the combined action of NE winds and the intensification of the meandering pattern of the Brazil Current (BC). (C) 2008 Elsevier Ltd and INQUA. All rights reserved.

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.

PALEOPRODUCTIVITY CHANGES DURING THE LATE QUATERNARY IN THE SOUTHEASTERN BRAZILIAN UPPER CONTINENTAL MARGIN OF THE SOUTHWESTERN ATLANTIC

Changes in the Brazilian continental margin`s oceanic productivity and circulation over the last 27,000 years were reconstructed based on sedimentological and microfaunal analyses. Our results suggest that oceanic paleoproductivity and the supply of terrigenous sediments to the Brazilian continental margin were higher during the Last Glacial Maximum (LGM) than during the Holocene. These changes may have been primarily influenced by significant sea level fluctuations that have occurred since the late Pleistocene. During the LGM, the lower sea level, higher productivity and lower sea-surface paleotemperatures may have been the result of the offshore displacement of the main flow of the Brazil Current. However, during the Holocene, the warm waters of the Brazil Current were displaced toward the coast. This displacement contributed to the increase in water temperature and prevented an increase in oceanic productivity. The decrease in terrigenous supply since the LGM could be related to the increase of the extension of the continental shelf and/or drier climatic conditions.

Biological, physical and chemical properties at the Subtropical Shelf Front Zone in the SW Atlantic Continental Shelf

The physical aspects of the Subtropical Shelf Front (STSF) for the Southwest Atlantic Continental Shelf were previously described. However, only scarce data on the biology of the front is available in the literature. The main goal of this paper is to describe the physical, chemical and biological properties of the STSF found in winter 2003 and summer 2004. A cross-section was established at the historically determined location of the STSF. Nine stations were sampled in winter and seven in summer. Each section included a series of conductivity-temperature-depth (CTD) stations where water samples from selected depths were filtered for nutrient determination. Surface samples were taken for chlorophyll a (Chl-a) determination and plankton net tows carried out above and below the pycnocline. Results revealed that winter was marked by an inner-shelf salinity front and that the STSF was located on the mid-shelf The low salinity waters in the inner-shelf indicated a strong influence of freshwater, with high silicate (72 mu M), suspended matter (45 mg l(-1)), phosphate (2.70 mu M) and low nitrate (1.0 mu M) levels. Total dissolved nitrogen was relatively high (22.98 mu M), probably due to the elevated levels of organic compound contribution close to the continental margin. Surface Chl-a concentration decreased from coastal well-mixed waters, where values up to 8.0 mg m(-3) were registered, to offshore waters. Towards the open ocean, high subsurface nutrients values were observed, probably associated to South Atlantic Central Waters (SACW). Zooplankton and ichthyoplankton abundance followed the same trend; three different groups associated to the inner-, mid- and outer-shelf region were identified. During summer, diluted waters extended over the shelf to join the STSF in the upper layer; the concentration of inorganic nutrients decreased in shallow waters; however, high values were observed between 40 and 60 m and in deep offshore waters. Surface Chl-a ranged 0.07-1.5 mg m(-3); winter levels were higher. Three groups of zoo and ichthyoplankton, separated by the STSF, were also identified. Results of the study performed suggest that the influence of freshwater was stronger during winter and that abundance distribution of Chl-a, copepods and ichthyoplankton was related to the Plata Plume Waters (PPW), rather than to the presence of the STSF. During summer, when the presence of freshwater decreases, plankton interactions seem to take place in the STSF. (C) 2008 Elsevier Ltd. All rights reserved.

Biogeochemistry of a deep-sea whale fall: sulfate reduction, sulfide efflux and methanogenesis

Deep-sea whale falls create sulfidic habits Supporting chemoautotrophic communities, but microbial processes underlying the formation Of Such habitats remain poorly evaluated. Microbial degradation processes (sulfate reduction, methanogenesis) and biogeochemical gradients were studied in a whale-fall habitat created by a 30 t whale carcass deployed at 1675 m depth for 6 to 7 yr on the California margin. A variety of measurements were conducted including photomosaicking, microsensor measurements, radio-tracer incubations and geochemical analyses. Sediments were Studied at different distances (0 to 9 in) from the whale fall. Highest microbial activities and steepest vertical geochemical gradients were found within 0.5 m of the whale fall, revealing ex situ sulfate reduction and in vitro methanogenesis rates of up to 717 and 99 mmol m(-2) d(-1), respectively. In sediments containing whale biomass, methanogenesis was equivalent to 20 to 30%, of sulfate reduction. During in vitro sediment studies, sulfide and methane were produced within days to weeks after addition of whale biomass, indicating that chemosynthesis is promoted at early stages of the whale fall. Total sulfide production from sediments within 0.5 m of the whale fall was 2.1 +/- 3 and 1.5 +/- 2.1 mol d(-1) in Years 6 and 7, respectively, of which similar to 200 mmol d(-1) were available as free sulfide. Sulfate reduction in bones was much lower, accounting for a total availability of similar to 10 mmol sulfide d(-1). Over periods of at least 7 yr, whale falls can create sulfidic conditions similar to other chemosynthetic habitats Such as cold seeps and hydrothermal vents.

Neotectonics in the northern equatorial Brazilian margin

An increasing volume of publications has addressed the role of tectonics in inland areas of northern Brazil during the Neogene and Quaternary, despite its location in a passive margin. Hence, northern South America plate in this time interval might have not been as passive as usually regarded. This proposal needs further support, particularly including field data. In this work, we applied an integrated approach to reveal tectonic structures in Miocene and late Quaternary strata in a coastal area of the Amazonas lowland. The investigation, undertaken in Marajo Island, mouth of the Amazonas River, consisted of shallow sub-surface geophysical data including vertical electric sounding and ground penetrating radar. These methods were combined with morphostructural analysis and sedimentological/stratigraphic data from shallow cores and a few outcrops. The results revealed two stratigraphic units, a lower one with Miocene age, and an upper one of Late Pleistocene-Holocene age. An abundance of faults and folds were recorded in the Miocene deposits and, to a minor extent, in overlying Late Pleistocene-Holocene strata. In addition to characterize these structures, we discuss their origin, considering three potential mechanisms: Andean tectonics, gravity tectonics related to sediment loading in the Amazon Fan, and rifting at the continental margin. Amongst these hypotheses, the most likely is that the faults and folds recorded in Marajo Island reflect tectonics associated with the history of continental rifting that gave rise to the South Atlantic Ocean. This study supports sediment deposition influenced by transpression and transtension associated with strike-slip divergence along the northern Equatorial Brazilian margin in the Miocene and Late Pleistocene-Holocene. This work records tectonic evidence only for the uppermost few ten of meters of this sedimentary succession. However, available geological data indicate a thickness of up to 6 km, which is remarkably thick for an area regarded as a passive margin. (C) 2012 Elsevier Ltd. All rights reserved.

Distribution of major elements in Atlantic surface sediments (36 degrees N-49 degrees S): Imprint of terrigenous input and continental weathering

Numerous studies use major element concentrations measured on continental margin sediments to reconstruct terrestrial climate variations. The choice and interpretation of climate proxies however differ from site to site. Here we map the concentrations of major elements (Ca, Fe, Al, Si, Ti, K) in Atlantic surface sediments (36 degrees N-49 degrees S) to assess the factors influencing the geochemistry of Atlantic hemipelagic sediments and the potential of elemental ratios to reconstruct different terrestrial climate regimes. High concentrations of terrigenous elements and low Ca concentrations along the African and South American margins reflect the dominance of terrigenous input in these regions. Single element concentrations and elemental ratios including Ca (e. g., Fe/Ca) are too sensitive to dilution effects (enhanced biological productivity, carbonate dissolution) to allow reliable reconstructions of terrestrial climate. Other elemental ratios reflect the composition of terrigenous material and mirror the climatic conditions within the continental catchment areas. The Atlantic distribution of Ti/Al supports its use as a proxy for eolian versus fluvial input in regions of dust deposition that are not affected by the input of mafic rock material. The spatial distributions of Al/Si and Fe/K reflect the relative input of intensively weathered material from humid regions versus slightly weathered particles from drier areas. High biogenic opal input however influences the Al/Si ratio. Fe/K is sensitive to the input of mafic material and the topography of Andean river drainage basins. Both ratios are suitable to reconstruct African and South American climatic zones characterized by different intensities of chemical weathering in well-understood environmental settings.

Currents controlling sedimentation, deposits recording palaeo-hydrodynamics – lessons from the continental shelf-slope system off SE South America

The continental margin off SE South America hosts one of the world’s most energetic hydrodynamic regimes but also the second largest drainage system of the continent. Both, the ocean current system as well as the fluvial runoff are strongly controlled by the atmospheric circulation modes over the region. The distribution pattern of particular types of sediments on shelf and slope and the long-term built-up of depositional elements within the overall margin architecture are, thus, the product of both, seasonal to millennial variability as well as long-term environmental trends. This talk presents how the combination of different methodological approaches can be used to obtain a comprehensive picture of the variability of a shelf and upper-slope hydrodynamic system during Holocene times. The particular methods applied are: (a) Margin-wide stratigraphic information to elucidate the role of sea level for the oceanographic and sedimentary systems since the last glacial maximum; (b) Palaeoceanographic sediment proxies combined with palaeo-temperature indicating isotopes of bivalve shells to trace lateral shifts in the coastal oceanography (particularly of the shelf front) during the Holocene; (c) Neodymium isotopes to identify the shelf sediment transport routes resulting from the current regime; (d) Sedimentological/geochemical data to show the efficient mechanism of sand export from the shelf to the open ocean; (e) Diatom assemblages and sediment element distributions indicating palaeo-salinity and the changing marine influence to illustrate the Plata runoff history. Sea level has not only controlled the overall configuration of the shelf but also the position of the main sediment routes from the continent towards the ocean. The shelf front has shifted frequently since the last glacial times probably resulting from both, changes in the Westerly Winds intensity and in the shelf width itself. Remarkable is a southward shift of this front during the past two centuries possibly related to anthropogenic influences on the atmosphere. The oceanographic regime with its prominent hydrographic boundaries led to a clear separation of sedimentary provinces since shelf drowning. It is especially the shelf front which enhances shelf sediment export through a continuous high sand supply to the uppermost slope. Finally, the Plata River does not continuously provide sediment to the shelf but shows significant climate-related changes in discharge during the past centuries. Starting from these findings, three major fields of research should, in general, be further developed in future: (i) The immediate interaction of the hydrodynamic and sedimentary systems to close the gaps between deposit information and modern oceanographic dynamics; (ii) Material budget calculations for the marginal ocean system in terms of material fluxes, storage/retention capacities, and critical thresholds; (iii) The role of human activity on the atmospheric, oceanographic and solid material systems to unravel natural vs. anthropogenic effects and feedback mechanisms