Where is basalt in river sediments, and why does it matter?
Where is basalt in river sediments, and why does it matter?
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DOI：
10.1016/j.epsl.2014.09.033
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发表时间：
2014-12-01
影响因子：
5.3
通讯作者：
Chauvel, Catherine
中科院分区：
文献类型：
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作者：
Garcon, Marion;Chauvel, Catherine
风化、侵蚀和矿物分选过程改变了沉积物相对于其源岩的化学和同位素组成。然而，这些过程影响沉积物地球化学的方式和程度尚未完全了解。在这里，我们报告了在恒河、亚穆纳河和昌巴尔河的不同水深采样的沉积物的微量元素数据以及 Nd、Hf 和 Pb 同位素组成，这些河流排出了德干地盾玄武岩以及来自喜马拉雅山脉和印度北部地盾的结晶岩和沉积岩。在同一地点采样的表层沉积物和河床沉积物之间的同位素差异达到 6 Nd-ε 和约 15 Hf-ε 单位，这表明悬浮负载和床载不携带类似的来源信息。这种差异可以通过河流输送过程中差异侵蚀和矿物分选过程的综合影响来解释。玄武岩侵蚀的物质优先在河流表面附近以悬浮液形式输送，而更多结晶前体侵蚀的物质则在河底附近输送。河道内这种与深度相关的来源导致细粒悬浮物中玄武岩物质的过多，这些悬浮物最终被输送到海洋中并成为海洋陆源粘土的一部分。相比之下，较粗沉积物（如床质或浊积岩）中玄武岩物质的比例被低估。我们的结果对使用沉积物的 Nd、Hf 和 Pb 同位素组成作为物源代表具有重要影响，因为它们表明必须考虑所有粒度级分才能正确追踪来源组成。他们还表明，如果流域地区存在玄武岩露头，则来自细粒沉积物（例如悬浮载荷）的上大陆地壳估计可能会偏向于玄武岩成分。 (C) 2014 Elsevier B.V. 保留所有权利。
Weathering, erosion and mineralogical sorting processes modify the chemical and isotopic compositions of sediments relative to those of their source rocks. The way and extent to which those processes affect the geochemistry of sediments is however not yet fully understood. Here, we report trace element data as well as Nd, Hf and Pb isotopic compositions of sediments sampled at different water depths in the Ganges, Yamuna and Chambal Rivers draining the Deccan Traps basalts and the crystalline and sedimentary rocks from the Himalayan mountain range and the northern Indian shield.Isotopic differences between surface and bed sediments sampled at the same location reach 6 Nd-epsilon and about 15 Hf-epsilon units, suggesting that suspended loads and bedloads do not carry similar provenance information. Such differences are explained by the combined effects of differential erosion and mineralogical sorting processes during fluvial transport. Materials eroded from basalts are preferentially transported in suspension near the river surface while materials eroded from more crystalline precursors are transported near the bottom of the river. This depth-dependent provenance within the river channel leads to an overrepresentation of basaltic materials in fine-grained suspended loads that are finally delivered into the ocean and become part of the oceanic terrigenous clays. By contrast, the proportion of basaltic materials in coarser sediments such as bedloads or turbidites is underestimated.Our results have important consequences on the use of Nd, Hf and Pb isotopic compositions of sediments as provenance proxies because they indicate that all grain-size fractions must be taken into account to properly trace source compositions. They also suggest that upper continental crust estimates derived from fine-grained sediments, such as suspended loads, may be biased towards basaltic compositions if basaltic outcrops are present in the drainage area. (C) 2014 Elsevier B.V. All rights reserved.