Abstract To further develop boron isotopes as a tool for understanding shale weathering, we explored patterns of boron concentrations and isotopes across the forested Susquehanna Shale Hills Critical Zone Observatory (CZO). We present boron measurements for all watershed components that provided a foundation for examining water-rock interactions in a shale dominated watershed, including water compartments (e.g., precipitation, stream water, groundwater) and solid compartments (e.g., soil, bedrock, stream sediments, suspended load, and leaf litter). Results show boron isotopes (δ11B) in the bedrock (− 4.6‰) and soil (− 5.9 to - 4.2‰) were very similar. All waters were enriched in 11B by comparison: precipitation (7.2 to 22.6‰), stream (10.3 to 15.5‰), and groundwater (2.2 to 17.4‰). Modeling revealed that isotopic fractionation observed in the surface water and groundwater could mainly be explained by water-rock interactions including clay mineral dissolution (e.g., chlorite) and coprecipitation/adsorption processes (e.g., coatings on illite particles), likely in the near surface soils (~2 m deep). We found that leaching, the loss of boron from vegetation to stream water, plays a secondary role. Specifically, such leaching likely contributes the equivalent of 10 to 26% of the B fluxes from the watershed outlet. Boron mass balance between bedrock and precipitation inputs and the exported flux of dissolved and solid pools identified a “missing” isotopically light solid flux (δ11B of −12.2 ± 5.3‰ at ~4.4 ± 3.8 mol/ha/y of B; uncertainty reported as 2 SD). We did not sample any pool with this isotopic signature. Here our data suggest the composition of this pool is more likely related to precipitation of secondary clays rather than adsorption or (co)precipitation on Fe oxides. We propose two hypotheses to explain the missing light B pool: 1) a significant portion of the particles carrying the missing 10B are not sampled because they enter groundwater at depth and are transported out of the catchment under the stream; and/or 2) the inputs and outputs of boron are not operating at steady state in the catchment today, suggesting that the missing boron particles were lost in the past in proportions higher than today. When this B budget is paired with studies of δ26Mg and δ56Fe from Shale Hills, both of which also show missing isotopic pools, the pattern indicates a fundamental gap in understanding of shale weathering. We concluded that light B particles, presumably generated in the upper soils, are likely transported deep beneath the surface in the groundwater system or episodically in the past through riverine fluxes.

中文翻译：

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开发硼同位素以阐明临界区的页岩风化

摘要 为了进一步开发硼同位素作为了解页岩风化的工具，我们探索了森林覆盖的 Susquehanna 页岩山临界区观测站 (CZO) 的硼浓度和同位素模式。我们提供了所有流域组分的硼测量结果，为检查页岩主导的流域中的水-岩相互作用奠定了基础，包括水隔间（例如，降水、河流水、地下水）和固体隔间（例如土壤、基岩、河流沉积物、悬挂负载和落叶）。结果显示基岩 (- 4.6‰) 和土壤 (- 5.9 至 - 4.2‰) 中的硼同位素 (δ11B) 非常相似。相比之下，所有水域都富含 11B：降水（7.2 至 22.6 ‰）、溪流（10.3 至 15.5 ‰）和地下水（2.2 至 17.4 ‰）。建模表明，在地表水和地下水中观察到的同位素分馏主要可以通过水-岩石相互作用来解释，包括粘土矿物溶解（例如绿泥石）和共沉淀/吸附过程（例如伊利石颗粒上的涂层），可能在近地表土壤中（~2 m 深）。我们发现浸出，即硼从植被流失到溪流中，起着次要作用。具体来说，这种浸出可能相当于流域出口 B 通量的 10% 到 26%。基岩和降水输入之间的硼质量平衡以及溶解和固体池的输出通量确定了一个“缺失”的同位素轻固体通量（δ11B 为 -12.2 ± 5.3‰，~4.4 ± 3.8 mol/ha/y B；不确定性报告为2 SD）。我们没有对具有该同位素特征的任何池进行采样。在这里，我们的数据表明该池的组成更可能与次生粘土的沉淀有关，而不是与 Fe 氧化物的吸附或（共）沉淀有关。我们提出两个假设来解释缺失的光 B 池：1) 大部分携带缺失 10B 的粒子没有被采样，因为它们进入深层地下水并被运出溪流下的集水区；和/或 2）今天流域中硼的输入和输出并未以稳定状态运行，这表明过去丢失的硼颗粒的损失比例高于今天。当此 B 收支与来自页岩山的 δ26Mg 和 δ56Fe 的研究相结合时，两者都显示缺少同位素库，该模式表明对页岩风化的理解存在根本性差距。我们得出结论，轻 B 粒子，