Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh) (Articolo in rivista)

Type
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  • Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh) (Articolo in rivista) (literal)
Anno
  • 2011-01-01T00:00:00+01:00 (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#doi
  • 10.1016/j.epsl.2010.11.043 (literal)
Alternative label
  • Eduardo Garzanti a,?, Sergio Andó a,1, Christian France-Lanord b,2, Paolo Censi c,3, Pietro Vignola d,4, Valier Galy b,5, Maarten Lupker b,2 (2011)
    Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh)
    in Earth and planetary science letters
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Eduardo Garzanti a,?, Sergio Andó a,1, Christian France-Lanord b,2, Paolo Censi c,3, Pietro Vignola d,4, Valier Galy b,5, Maarten Lupker b,2 (literal)
Pagina inizio
  • 107 (literal)
Pagina fine
  • 120 (literal)
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  • 302 (literal)
Rivista
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  • 14 (literal)
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  • 1-2 (literal)
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  • a Laboratorio di Petrografia del Sedimentario, Dipartimento di Scienze Geologiche e Geotecnologie, Università di Milano-Bicocca, 20126 Milano, Italy b Centre de Recherches Pétrographiques et Géochimiques, BP 20, 54501 Vandoeuvre-lès-Nancy, France c Dipartimento CFTA, Università di Palermo, 90123 Palermo, Italy d Consiglio Nazionale delle Ricerche, Istituto per la Dinamica dei Processi Ambientali, 20133 Milano, Italy (literal)
Titolo
  • Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh) (literal)
Abstract
  • Sediments carried in suspension represent a fundamental part of fluvial transport. Nonetheless, largely because of technical problems, they have been hitherto widely neglected in provenance studies. In order to determine with maximumpossible precision themineralogy of suspended load collected in vertical profiles fromwater surface to channel bottom of Rivers Ganga and Brahmaputra, we combined Raman spectroscopy with traditional heavymineral and X-ray diffraction analyses, carried out separately on low-density and dense fractions of all significant size classes in each sample (multiple-window approach). Suspended load resulted to be a ternary mixture of dominant silt enriched in phyllosilicates, subordinate clay largely derived from weathered floodplains, and sand mainly produced by physical erosion and mechanical grinding during transport in Himalayan streams. Sediment concentration and grain size increase steadily with water depth. Whereas absolute concentration of clay associated with Fe-oxyhydroxides and organic matter is almost depth-invariant, regular mineralogical and consequently chemical changes fromshallowto deep load result frommarked increase of faster-settling, coarser, denser, or more spherical grains toward the bed. Such steady intersample compositional variability can be modeled as a mixture of clay, silt and sand modes with distinct mineralogical and chemical composition. With classical formulas describing sediment transport by turbulent diffusion, absolute and relative concentrations can be predicted at any depth for each textural mode and each detrital component. Based on assumptions on average chemistry of detritalminerals and empirical formulas to calculate their settling velocities, the suspension-sorting model successfully reproduces mineralogy and chemistry of suspended load at different depths. Principal outputs include assessment of contributions by each detrital mineral to the chemical budget, and calibration of dense minerals too rare to be precisely estimated by optical or Raman analysis but crucial in both detrital-geochronology and settling-equivalence studies. Hydrodynamic conditions duringmonsoonal discharge could also be evaluated. Understanding compositional variability of suspended load is a fundamental pre-requisite to correctly interpret mineralogical and geochemical data in provenance analysis of modern and ancient sedimentary deposits, to accurately assessweathering processes, sediment fluxes and erosion patterns, and to unambiguously evaluate the effects of anthropogenic modifications on the natural environment. (literal)
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