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

• Autores: José Abel Flores Villarejo, Santiago Ledesma, Guillermo Francés Pedraz, Joan Fabrés Francés, Antonio Calafat Frau, Miquel Canals Artigas, María Ángeles Bárcena Pernía
• Localización: Boletín de la Real Sociedad Española de Historia Natural. Sección geológica, ISSN 0583-7510, Tomo 93, Nº 1-4, 1997 (Ejemplar dedicado a: Ciencia antártica: el papel del SCAR), págs. 73-84
• Idioma: español
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• Resumen
  • español

    El estudio de dos testigos de gravedad recuperados en las subcuencas Central y Oriental de Bransfield ha permitido identificar la importancia y participación de los diferentes procesos de relleno de la cuenca mediante la determinación de las facies sedimentarias y de las propiedades granulométricas, físicas (porcentaje de agua, porosidad y densidad seca) y químicas (contenido en carbono orgánico y sílice biogénica y composición de las partículas de la fracción arena) de los sedimentos más recientes allí depositados. Además, a partir de las tasas de sedimentación (300 cm/Ka aprox.) citadas por otros autores (Bárcena et al., 1996), se han calculado los flujos para el carbono orgánico, la sílice biogénica y la fracción detrítica.

    Los sedimentos recuperados son mayoritariamente limos de color oliva grisáceo con varios grados de laminación y algunos niveles de limos arenosos con la base nítida y techo gradual. El análisis 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 orgánico y sílice biogénica presentan una evolución opuesta a la de la densidad seca presentando valores máximos donde los de la densidad son mínimos. El análisis microscópico de la fracción arena revela la mayor abundancia de cenizas volcánicas en la base dichas secuencias.

    Este tipo de secuencias decrecientes en densidad pueden ser generadas por procesos turbidíticos durante los cuales las partículas más densas y redondeadas (como las cenizas) se depositan primero y las menos densas e inequidimensionales (diatomeas, espículas de esponja y radiolarios) más tarde. En contraposición a esto, en los tramos con valores constantes o secuencias crecientes de la densidad seca, predominaría la sedimentación pelágica. Los flujos de sílice biogénica, de carbono orgánico y especialmente de la fracción detrítica son de un orden de magnitud superiores a las calculadas a partir del estudio de flujos de partículas (Wefer et al., 1988), lo que corrobora la relevancia de los procesos turbidíticos en el relleno de la Cuenca de Bransfield durante la última parte del Holoceno.

  • English

    The 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 (Bárcena 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.