Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh)
Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga-Brahmaputra, Bangladesh)
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DOI：
10.1016/j.epsl.2010.11.043
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发表时间：
2011-02-01
影响因子：
5.3
通讯作者：
Lupker, Maarten
中科院分区：
文献类型：
--
作者：
Garzanti, Eduardo;Ando, Sergio;Lupker, Maarten
悬浮泥沙是河流搬运的基本组成部分。然而，主要是由于技术问题，它们一直被广泛忽视的起源研究。为了尽可能精确地确定恒河和雅鲁藏布江从水面到河道底部的垂直剖面中收集的悬浮物的矿物学，我们将拉曼光谱与传统的重矿物和X射线衍射分析相结合，分别对每个样品中所有重要尺寸类别的低密度和致密部分进行分析（多窗口方法）。悬浮负荷导致的主要粉砂富含层状硅酸盐，从属粘土主要来自风化foodplains，砂主要是由物理侵蚀和机械研磨在喜马拉雅河流运输过程中产生的三元混合物。泥沙浓度和粒径随水深的增加而增加。而与铁羟基氧化物和有机质的粘土的绝对浓度几乎是深度不变的，定期矿物学，因此从浅到深的负载的化学变化的结果从显着增加的快速沉降，粗，致密，或更多的球形颗粒朝向床。这种稳定的样品间组成的变化可以模拟为粘土，粉砂和砂模式与不同的矿物和化学成分的混合物。利用描述湍流扩散输沙的经典公式，可以预测任何深度的每种结构模式和每种碎屑成分的绝对和相对浓度。基于对碎屑矿物平均化学性质的假设和计算其沉降速度的经验公式，悬浮分选模型成功地再现了不同深度悬浮物的矿物学和化学性质。主要产出包括评估每种碎屑矿物对化学收支的贡献，以及校准太罕见而无法通过光学或拉曼分析精确估计的致密矿物，但这些矿物在碎屑地质年代学和沉降等效性研究中至关重要。还可以评估季风排放期间的水动力条件。了解悬移质的成分变异性是正确解释现代和古代沉积物来源分析中的矿物学和地球化学数据，准确评估风化过程，沉积物通量和侵蚀模式，以及明确评估人为修改对自然环境的影响的基本前提。(C)2010 Elsevier B.V.保留所有权利。
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 maximum possible precision the mineralogy of suspended load collected in vertical profiles from water surface to channel bottom of Rivers Ganga and Brahmaputra, we combined Raman spectroscopy with traditional heavy-mineral 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 fioodplains, 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 from shallow to deep load result from marked 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 detrital minerals 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 during monsoonal 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 modem and ancient sedimentary deposits, to accurately assess weathering processes, sediment fluxes and erosion patterns, and to unambiguously evaluate the effects of anthropogenic modifications on the natural environment. (C) 2010 Elsevier B.V. All rights reserved.