An indirect way to measure the speed of bottom currents may be using granulometric parameters such as the proxy sortable silt (). This proxy is associated with the sedimentation process and can estimate paleocurrents. This study aims to evaluate the range of the near-bottom current flow speed through the and related to oceanographic processes in the southeastern Brazilian continental slope. The values ranged from 17.08 to 26.23 µm, presenting a mean value of 19.87 µm. The flow speed values were obtained through Ledbetter's (1986) and Miller's (1977) formulas, varying from a minimum of 0.07 and 0.21 m s-1 to a maximum of 0.26 and 0.24 m s-1. The changes in bottom velocities observed in the Campos Basin are influenced by geomorphologic and hydrodynamic parameters, namely the off-shelf export of the sediment; the action currents (Brazil Current, Intermediate Western Boundary Current, and the Deep Western Boundary Current); and the geomorphological characteristics (island, ravine, and halokinesis movement).
amazônica, com foco na variação do ciclo hidrológico no continente e dinâmica sedimentar durante os últimos 150 anos.Foram analisados quatro registros sedimentares distribuídos ao longo do estuário e um registro sedimentar proveniente de um canal de maré.A geocronologia foi estabelecida por 210 Pb e indicou taxas de sedimentação média que variaram entre 0,58 cm ano -1 a 1,04 cm ano -1 .Primeiramente compararam-se os resultados de metais obtidos por fluorescência de raios-X portátil (XRF-p) e por ICP/OES, e avaliou-se a aplicabilidade das razões Fe/Ca, Ti/Ca, Fe/Ti, Fe/K, Rb/Sr como proxies de proveniência de sedimento e de período úmido da região.As concentrações de metais obtidas por XRF-p e por ICP/OES (Cr, Ca, Fe, K, Mn, Pb, Sr, Ti, V e Zn) exibiram equivalência em magnitude e amplitude.Os resultados do método XRF-p mostraram-se excelentes parâmetros para estudos de reconstruções ambientais.Com os resultados do testemunho proveniente do canal de maré (TAI), reconheceram-se grupos de elementos indicadores de aporte continental, identificado como componente principal 1 (CP1) (Al, Cr, Cu, Fe, K, Mg, Pb, Sc e V), e marinho (CaCO 3 , Mg, Sr, Ba) através da análise de componentes principais e agrupamento.A variação de CP1 contraposta a dados de precipitação anual do período entre 1973 e 2015 apresentou covariações em que valores positivos de CP1 foram sincrônicos a períodos úmidos, ao passo que valores negativos de CP1, a períodos de seca.Assim, foi possível inferir períodos úmidos e secos entre 1777 e 2015.As variações da CP1 foram aplicadas como indicador do fluxo de transporte de sedimento entre os estuários Caeté e Taperaçu, onde valores positivos da CP1 foram associados a fluxo de sedimento proveniente do estuário Caeté.A análise da evolução do aporte de metais por meio da aplicação das razões Fe/Ca, Ti/Ca, Fe/Ti, Fe/K e Rb/Sr foi realizada a partir dos resultados de XRF-p dos quatro testemunhos distribuídos ao longo do estuário.Através de imagens de satélite reconheceram-se evoluções e modificações de feições geomorfológicas em intervalos inferiores a cinco anos.Mudanças destas feições são influenciadas pela hidrodinâmica local e clima, o que se reflete nos resultados de conteúdo de metais, taxa de sedimentação e acumulação de massa.O desenvolvimento urbano de Bragança, caracterizado pelo aumento dos teores de metais a partir de 1890, propiciou alteração da dinâmica sedimentar.As razões Fe/Ca e Ti/Ca indicaram influência continental nos testemunhos internos ao estuário (CAT-S, CAT-N), enquanto que os testemunhos da região de médio estuário e região externa (CAT1, CAT-F), apontaram influência tanto continental quanto marinha.A razão Fe/Ti revelou diferentes fontes de sedimentos a partir de 1890.A amostra de superfície do testemunho do médio estuário (CAT1) apresentou valores de Nd(0) igual a -18,65 e 87 Sr/ 86 Sr igual 0,742560, enquanto que a amostra de superfície da plataforma continental interna adjacente ao estuário (P4) apresentou valores de Nd(0) igual a -11,05 e 87 Sr/ 86 Sr igual a 0,721749 Esses resultados mostram que há diferentes fontes de sedimentos nestas duas áreas, sugerindo que o estuário não tem capacidade de exportar sedimentos para a plataforma continental adjacente.As razões metal/metal analisadas em testemunhos costeiros mostraram-se de complexa interpretação, pois um mesmo elemento pode ter mais de uma proveniência.Palavras-chave: Metais.Fluorescência de raios-X portátil.Fonte de sedimento
R/V METEOR expedition M125 (“SAMBA”) focused on the influence of paleoceanographic
changes off NE Brazil on the continental hydrological cycle. For this purpose, we obtained 202 m
of gravity (24 stations) and piston cores (9) at seven sections on the shelf and continental slope
close to river mouths from Cabo Frio in the south to the Rio Sao Francisco in the north. Coring
stations were determined after intensive echosounder surveys (total: 1221 NM). On-board
foraminiferal biostratigraphy, as well as color and XRF-scanning already provided first
stratigraphic constraints, indicating the preservation of different regional paleoclimatic signals at
the respective sections. Based on the preliminary stratigraphy, we retrieved high-resolution
archives, covering Holocene sediments on the shelf and late Pleistocene sediments on the slope.
These high-resolution archives are complemented by long-term records covering up to 900 ka of
continuous sedimentation at deeper sites at smaller rivers. For proxy-calibration and the study of
present-day sedimentation dynamics and biogeochemical processes, surface sediments were
sampled via multicorer (47), Van Veen Grab (6) and box corer (3). Water samples for
determination of the water chemistry (trace elements, stable and radiogenic isotopes) and nutrient
composition were retrieved by 55 CTD/Rosette casts. In addition, we run multinet-hauls at seven
stations to investigate the planktonic foraminiferal communities in the water column down to 700
m water depth, complemented by filtering water from the ship’s pump twice a day.
The distribution of the agglutinated benthic foraminifer Marsipella elongata is studied from 70 oceanographic stations, located between 400m and 3000m depth in the Campos Basin (southeastern Brazilian continental margin). Samples of the upper 0–2 cm of surficial sediment were collected during two sampling campaigns in 2008 and 2009. Grain size, total organic carbon, total nitrogen, calcium carbonate, phytopigment, lipids biomarkers (sterols, fatty acids and n-alcohols), total lipids, and bacterial biomass are analyzed as environmental parameters. These data as well as the particulate organic matter flux to the sea floor are compared with the distribution of living (rose Bengal stained) Marsipella elongata. Living specimens of M. elongata occur in the range of 400–3000mwater depth. The distribution of this species appears to be related to relatively high particulate organic matter vertical flux, phytopigment content, and bacterial biomass under the influence of Brazil Current, Intermediate Western Boundary Current and Deep Western Boundary Current conditions (0.3–0.8 m/s).
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Abstract Modern precipitation over northeastern (NE) South America is strongly controlled by the seasonal meridional migration of the Intertropical Convergence Zone (ITCZ). Ample evidence from the Northern Hemisphere suggests a mid‐ to late Holocene southward migration of the ITCZ. Such a shift would be expected to increase precipitation over semi‐arid northern NE Brazil (Southern Hemisphere). However, the most robust precipitation record from northern NE Brazil shows a drying trend throughout the Holocene. Here, we address this issue presenting a high‐temporal resolution reconstruction of precipitation over northern NE Brazil based on data from a marine sediment core, together with analyses of mid‐ and late Holocene simulations performed with the fully coupled climate model FGOALS‐s2. Both, our data and the climate model simulations show a decrease in precipitation over northern NE Brazil from the mid‐ to the late Holocene. The model outputs further indicate a latitudinal contraction of the seasonal migration range of the ITCZ that, together with an intensification of the regional Walker circulation, were responsible for the mid‐ to late Holocene changes in precipitation over NE South America. Our results reconcile apparently conflicting precipitation records and climate mechanisms used to explain changes in precipitation over NE South America.
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