Abstract The chemical weathering rate of silicates is a key parameter of several geochemical processes; however, the long-term field measurement of these rates remains challenging. Currently, there are several methods for quantifying field weathering rates, but each presents it's own unique set of difficulties. In this study uranium-series nuclides were used to investigate long-term biotite weathering in a soil profile. For this purpose, pseudomorph grains of biotites (including weathering products) were hand-picked from four horizons of a soil profile and from underlying granitic saprolite from the experimental Breuil-Chenue site (France). Unexpected behaviors of U and Th nuclides were recorded from these samples. An unambiguous leaching of 232Th occurs during the weathering of biotite, while reduced U release relative to Th was observed, implying an efficient redistribution of U between the primary mineral and weathering products within the pseudomorphs. The measured U-series activity ratios reflect a regular and self-consistent pattern evolution. However, the “U-series-derived-weathered-stage” is not always coherent with the weathering features of major elements and/or with the location of the samples within the soil profile. For instance, biotite from the saprolite displays U-series activity ratios typical of significantly weathered samples, while major elements present rather limited signs of weathering. These results suggest an incongruent leaching of U and Th isotopes and suggest that U- and Th-series nuclides probe some water-mineral interactions that occur before macroscopic mineral tansformation. Using an open-system leaching model, coupled (234U/238U), (234U/230Th), and (226Ra/230Th) disequilibria measured from the samples allow us to calculate a weathering duration range of 6–52 ky for most weathered samples of this soil profile. The biotite weathering rate deduced from these data ranges from 7.8 × 10−17 mol·m−2·s−1 to 6.5 × 10−16 mol·m−2·s−1, which is consistent with the range of field rates previously reported. This rate was compared to the weathering rate of biotites induced by the substitution of vegetation occuring 35 years ago in a nearby soil profile, which was about 2–3 orders of magnitude higher than the long-term rate reported above. This feature is perfectly consistent with the decrease in the weathering rate with time that was previously widely documented but that still remains debated. Our results allude to a valuable use of U-series isotopes for mineral-weathering field rate determination and report a decrease in silicate weathering rates over long- (10,000s of years) and short-term (10s of years) weathering events recorded from the same site. The results show that short term laboratory experiments can be representative of field processes, and suggest that some forms of mineral weathering reactivation, similar to that observed during laboratory experiments, may naturally occur in soils in response to high frequency perturbation processes such as land cover change.

中文翻译：

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使用 U 系列不平衡法测定现场黑云母风化率

摘要 硅酸盐的化学风化速率是几个地球化学过程的关键参数；然而，这些速率的长期现场测量仍然具有挑战性。目前，有几种方法可以量化田间风化率，但每种方法都有其独特的困难。在这项研究中，铀系列核素用于研究土壤剖面中的长期黑云母风化。为此，从土壤剖面的四个层面和来自实验 Breuil-Chenue 站点（法国）的下伏花岗岩腐泥土中手工挑选了黑云母的假晶粒（包括风化产物）。从这些样品中记录了 U 和 Th 核素的意外行为。在黑云母的风化过程中，232Th 发生了明确的浸出，同时观察到相对于 Th 的 U 释放减少，这意味着 U 在伪晶内的原生矿物和风化产物之间有效地重新分布。测量的 U 系列活动比率反映了规则和自洽的模式演变。然而，“U 系列衍生风化阶段”并不总是与主要元素的风化特征和/或样品在土壤剖面内的位置相一致。例如，来自腐泥土的黑云母显示出明显风化样品的典型 U 系列活动率，而主要元素则表现出相当有限的风化迹象。这些结果表明 U 和 Th 同位素的浸出不一致，并表明 U 和 Th 系列核素探测了宏观矿物变形之前发生的一些水-矿物相互作用。使用开放系统浸出模型，耦合 (234U/238U)、(234U/230Th)、和 (226Ra/230Th) 从样品测量的不平衡使我们能够计算出该土壤剖面的大多数风化样品的风化持续时间范围为 6-52 ky。由这些数据推导出的黑云母风化速率范围为 7.8 × 10−17 mol·m−2·s−1 到 6.5 × 10−16 mol·m−2·s−1，这与先前的野外风化率范围一致报道。将该速率与 35 年前附近土壤剖面中发生的植被替代引起的黑云母风化速率进行比较，该速率比上述长期速率高出约 2-3 个数量级。这一特征与风化率随时间降低的现象完全一致，这一点之前已被广泛记录，但仍有争议。我们的结果暗示了 U 系列同位素在矿物风化场速率测定中的宝贵用途，并报告了硅酸盐风化速率在长期（10,000 年）和短期（10 年）风化事件中的下降。同一个网站。结果表明短期实验室实验可以代表现场过程，并表明某些形式的矿物风化再激活，类似于实验室实验中观察到的，可能会在土壤中自然发生，以响应高频扰动过程，例如土地覆盖变化.