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Identificación de procesos sedimentarios en la Cuenca de Bransfield (Antártida): estudio preliminar de las características sedimentológicas y composición elemental (C, Si) de los sedimentos recientes

1997; Royal Spanish Society of Natural History; Volume: 93; Issue: 1 Linguagem: Inglês

0583-7510

José Abel Flores Villarejo, Santiago Ledesma, Guillermo Francés Pedraz, Joan Francès, Antonio Calafat Frau, Miquel Canals Artigas, María Ángeles Bárcena Pernía,

Polar Research and Ecology

espanolEl estudio de dos testigos de gravedad recuperados en las subcuencas Central y Oriental de Bransfield ha permitido identificar la importancia y participacion de los diferentes procesos de relleno de la cuenca mediante la determinacion de las facies sedimentarias y de las propiedades granulometricas, fisicas (porcentaje de agua, porosidad y densidad seca) y quimicas (contenido en carbono organico y silice biogenica y composicion de las particulas de la fraccion arena) de los sedimentos mas recientes alli depositados. Ademas, a partir de las tasas de sedimentacion (300 cm/Ka aprox.) citadas por otros autores (Barcena et al., 1996), se han calculado los flujos para el carbono organico, la silice biogenica y la fraccion detritica. Los sedimentos recuperados son mayoritariamente limos de color oliva grisaceo con varios grados de laminacion y algunos niveles de limos arenosos con la base nitida y techo gradual. El analisis de la densidad seca revela la existencia en ambos testigos de varias secuencias decrecientes separadas en algunas ocasiones por secuencias crecientes o tramos con valores constantes. Los porcentajes de carbono organico y silice biogenica presentan una evolucion opuesta a la de la densidad seca presentando valores maximos donde los de la densidad son minimos. El analisis microscopico de la fraccion arena revela la mayor abundancia de cenizas volcanicas en la base dichas secuencias. Este tipo de secuencias decrecientes en densidad pueden ser generadas por procesos turbiditicos durante los cuales las particulas mas densas y redondeadas (como las cenizas) se depositan primero y las menos densas e inequidimensionales (diatomeas, espiculas de esponja y radiolarios) mas tarde. En contraposicion a esto, en los tramos con valores constantes o secuencias crecientes de la densidad seca, predominaria la sedimentacion pelagica. Los flujos de silice biogenica, de carbono organico y especialmente de la fraccion detritica son de un orden de magnitud superiores a las calculadas a partir del estudio de flujos de particulas (Wefer et al., 1988), lo que corrobora la relevancia de los procesos turbiditicos en el relleno de la Cuenca de Bransfield durante la ultima parte del Holoceno. EnglishThe Bransfield Basin is a narrow, extensional, back-arc basin located between the north-eastern tip of Antarctic Peninsula and the South Shetland Islands, around 60-63o S. Recent sediments accumulated on this basin may be determinant in finding the role played by Southern Ocean in carbon and silica cycles during late Holocene times. It is furthermore necessary to determine the processes responsible for the accumulation of these sediments before their use as paleoceanographic markers. The work done on two gravity cores from Bransfield Basin has allowed us to determine the sedimentary facies and grain size, physical (water percentage and dry density) and chemical (organic carbon and biogenic silica content and lithology of sand sized particles) properties of the younger sediments settled on Bransfield Basin. Moreover, based on linear sedimentation rates (300 cm/Ky aprox.) collected from literature (Barcena et al., 1996) we have calculated total, carbon, silica and terrigenous fluxes. The studied cores, GEBRA-1 and GEBRA-2, were obtained from the Bransfield Central Basin (1652 m depth) and the Bransfield Eastern Basin (1106 m depth). GEBRA-1 core is located on a flat area closer to the basin axis of Central Basin while GEBRA-2 core is located on a slope break of the Antarctic Peninsula continental slope (Fig.1). The recovered sediments are mainly olive grey silts with various degrees of lamination and some levels of sandy silts characterised by a sharp base and a gradational top. Grain size analysis corroborates the silty nature of GEBRA-1 and GEBRA-2 sediments. The mean percentages are 71.8% and 75.2% of silt, 25.6% and 19.8% of clay and 2.5% and 5.1% of sand, respectively (Fig. 2). The dry density analysis gives mean values of 0.48 g/cm3 for GEBRA-1 and 0.54 g/ cm3 for GEBRA-2 and reveals the existence in both cores of some decreasing sequences separated sometimes by increasing sequences or sections with constant values. Organic carbon percentages (1.18% and 1.15% as a mean for GEBRA-1 and GEBRA-2, respectively) and biogenic silica (mean of 15.48% and 20.42%) show a reverse profile pattern to dry density (Fig. 2). These kind of sequences are better represented in GEBRA-1 than in GEBRA-2 where they are shorter and concentrated in the basal part of the core (Fig. 2). There is a significant degree of correlation between organic carbon and biogenic silica in GEBRA-1, as shown in Figs. 2 and 3, not being so significant in GEBRA-2. The microscopic analysis of the sand fraction reveals the presence of siliceous debris (diatom frustules, sponge spicules and radiolarians), quartz and volcanic ashes, being the last ones more abundant at the base of the sequences identified in the density log. Decreasing density sequences identified in both cores may be generated by turbiditic processes during which denser and more spherical particles (as ashes) settle first and less dense and inequidimensional (organic matter and biosiliceous debris) later. Facing to this, in the sections with constant values or increasing sequences of dry density, pelagic processes should predominate. The significant correlation of organic carbon and biogenic silica is then the result of the similar behaviour of biogenic silica debris and organic matter in a turbiditic flow. The shifts in the correlation degree and the slope of the regression curve in core GEBRA-2 with respect to GEBRA-1 could be explained by the lower presence of turbiditic sequences in GEBRA-2 and by the disimilarities between source areas. The relevance of turbiditic processes in the Bransfield Basin and other areas with similar characteristics has been already underlined by some authors (Yoon et al., 1994; McCoy, 1991; Chough, 1984; Yoon & Chough, 1993). Beside that, the unusually high linear sedimentation rates above mentioned, points out to some kind of process, as turbidity currents, capable to supply large amounts of sediments to basin floor. Total, carbon, silica and terrigenous fluxes calculated for GEBRA-1 and GEBRA-2 sediments are one order of magnitude bigger than the fluxes calculated from sediment trap analysis (Wefer et al., 1988) (Table I). That shift could be attributed to advective processes such as turbiditic flows. The highest differences with sediment trap fluxes are those of the terrigenous components, which would indicate that this is the component more prone to enter the basin via turbiditic flows.