The geological significance of cosmogenic nuclides in large lowland river basins
The geological significance of cosmogenic nuclides in large lowland river basins
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DOI：
10.1016/j.earscirev.2016.06.001
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发表时间：
2016-08
影响因子：
12.1
通讯作者：
H. Wittmann;F. Blanckenburg
中科院分区：
文献类型：
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作者：
H. Wittmann;F. Blanckenburg
从大型河流中的一袋沙中测量，宇宙成因核素如原位产生的10 Be，26 Al和14 C的浓度可以用来限制源头的平均泥沙通量，并评估泥沙从源到汇的储存时间。我们重新审视这些原则，从亚马逊河和恒河流域的例子，我们确定了两个端成员的情况下，控制宇宙成因核素的浓度在低地河流沉积物：1）如果洪泛平原沉积物存储的时间尺度相比，半衰期的核素，在situcogenic核素浓度没有显着改变在低地流域。在这种情况下，例如，在现场产生的10 Be在沉积物中采取的低地盆地等于在大多数情况下，从沉积物源区输出，但在源头流之间的核素浓度的变化显着平均了。因此，为了将测得的河流沉积物中的核素浓度转换为全流域的剥蚀率，生产率按比例调整，仅包括产沙山区的生产率，而不是整个集水区。因此，不包括低地的核素生产，因为低地没有沉积物。这种校正被称为“洪泛区校正”。2)如果沉积物埋藏的时间与核素的半衰期相当，那么这些同位素的差异衰变会使成对的核素（例如26 Al/10 Be的比值）重新夹带到活跃的河流中，从而限制了洪泛区沉积物的储存时间和重新混合。由于原位生成核素测量的是较长时间尺度上的剥蚀率，通常在103年至105年之间进行积分，它们与河流负荷测量的沉积物通量的现代估计相结合，提供了破译大流域地表通量控制的丰富潜力。通过这种组合，我们可以评估河流系统如何对气候变化或人类干扰等外部扰动做出反应，以及这些信号如何通过低地流域的冲积河段传播。这两个流域最重要的结果是：i）低地亚马逊河和恒河的核素浓度确实分别广泛反映了安第斯和喜马拉雅山脉剥蚀的空间平均值，ii）由此计算的沉积物通量在现代沉积物测量通量的不确定性范围内，表明完全向海洋输送沉积物而没有净损失到流域，iii）只有亚马逊河流域，鉴于其规模，最后，我们提供了一个简短的介绍使用大气宇宙成因核素10 Be和它的比例稳定9 Be，岩石风化释放，在低地。在亚马逊河流域，我们表明，10 Be浓度在细粒悬浮负荷，深度集成超过水柱使用Al/Si比，提供侵蚀率，而10 Be/9 Be比被吸附到颗粒提供剥蚀率。两者与10 Be以来的剥蚀速率相似。我们发现，该系统响应更敏感的沉积物存储比thein situscycle，和积累和衰减的流星浓度，因此可以用来确定沉积物停留时间。
Measured from a bag of sand taken in large rivers, the concentration of cosmogenic nuclides such asin situ-produced10Be,26Al, and14C can be used to constrain the mean sediment flux of the headwaters and assess the duration of sediment storage from source to sink. We revisit these principles, with examples from the Amazon and Ganga basins.We identify two end member cases controlling the concentration of cosmogenic nuclides in lowland river sediment: 1) if the time scale of floodplain sediment storage is short compared to the half-life of the nuclide,in situcosmogenic nuclide concentrations are not significantly altered in lowland basins. In this case the concentration ofe.g. in situ-produced10Be in the sediment taken in the lowland basin equals in most cases that exported from the sediment source area, but the variability in nuclide concentrations between headwater streams is significantly averaged-out. Thus to convert the measured river sediments'in situcosmogenic nuclide concentration into a catchment-wide denudation rate, production rates are scaled to include those of the sediment-producing mountainous areas only, rather than the entire catchment area. Nuclide production in the lowlands, where no sediment is being produced, is hence excluded. This correction is termed “floodplain correction”. 2) If sediment buried for periods of the order of the nuclides´ half-life is episodically re-entrained into the active river, paired nuclides, for example the ratio of26Al/10Be, through the differential decay of these isotopes, constrain the the storage duration and re-mixing of floodplain sediment.Asin-situcosmogenic nuclides measure denudation rates over longer time scales, typically integrating between 103and 105years, their combination with modern estimates of sediment fluxes from river load gauging offers a rich potential of deciphering the controls of Earth surface fluxes across large basins. From this combination, we can assess how river systems react to external perturbations such as climate change or human interference, and how such signals are transmitted through the alluvial reaches of lowland basins. The most important results in both basins are that i) lowland Amazon and Ganga nuclide concentrations indeed broadly reflect the spatial average of Andean and Himalayan denudation, respectively, ii) the thus calculated sediment fluxes are within uncertainty of modern fluxes from sediment gauging, indicating complete sediment delivery to the sea without net loss into the basin, and iii) only the Amazon basin, given its size, contains a substantial fraction of sediment buried temporarily in the Quaternary.Finally, we provide a short introduction to the use of meteoric cosmogenic nuclide10Be and its ratio to stable9Be, released by rock weathering, in lowlands. In the Amazon basin we show that10Be concentrations in fine-grained suspended loads, depth-integrated over the water column using Al/Si ratios, provide erosion rates whereas the10Be/9Be ratio on Be adsorbed to particles provides the denudation rate. Both are similar to denudation rates fromin situ10Be. We show that this system responds more sensitively to sediment storage than thein situsystem, and both accumulation and decay of meteoric concentrations may thus be used to determine sediment residence time.