Lithium isotope ratios (δ⁷Li) of rivers are increasingly serving as a diagnostic of the balance between chemical and physical weathering contributions to overall landscape denudation rates. Here, we show that intermediate weathering intensities and highly enriched stream δ⁷Li values typically associated with lowland floodplains can also describe small upland watersheds subject to cool, wet climates. This behavior is revealed by stream δ⁷Li between +22.4 and +23.5‰ within a Critical Zone observatory located in the Cévennes region of southern France, where dilute stream solute concentrations and significant atmospheric deposition otherwise mask evidence of incongruence. The water-rock reaction pathways underlying this behavior are quantified through a multicomponent, isotope-enabled reactive transport model. Using geochemical characterization of soil profiles, bedrock, and long-term stream samples as constraints, we evolve the simulation from an initially unweathered granite to a steady state weathering profile which reflects the balance between (a) fluid infiltration and drainage and (b) bedrock uplift and soil erosion. Enriched stream δ⁷Li occurs because Li is strongly incorporated into actively precipitating secondary clay phases beyond what prior laboratory experiments have suggested. Chemical weathering incongruence is maintained despite relatively slow reaction rates and moderate clay accumulation due to a combination of two factors. First, reactive primary mineral phases persist across the weathering profile and effectively “shield” the secondary clays from resolubilization due to their greater solubility. Second, the clays accumulating in the near-surface profile are relatively mature weathering byproducts. These factors promote characteristically low total dissolved solute export from the catchment despite significant input of exogenous dust.

中文翻译：

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溪流锂同位素比记录地中海山区气候的风化不一致

河流的锂同位素比 (δ ⁷ Li) 越来越多地用于诊断化学风化和物理风化对整体景观剥蚀率的影响之间的平衡。在这里，我们表明，通常与低地洪泛区相关的中等风化强度和高度富集的河流δ ⁷ Li值也可以描述受凉爽、潮湿气候影响的小型高地流域。位于法国南部塞文地区的一个关键区域天文台内的+22.4和+23.5‰之间的δ ⁷ Li流揭示了这种行为，该地区稀流溶质浓度和显着的大气沉积掩盖了不一致的证据。这种行为背后的水-岩反应途径通过多组分、同位素反应输运模型进行量化。使用土壤剖面、基岩和长期溪流样本的地球化学特征作为约束，我们将模拟从最初未风化的花岗岩发展为稳态风化剖面，反映了 (a) 流体渗透和排水与 (b) 基岩之间的平衡隆起和土壤侵蚀。富集流 δ ⁷ Li 的出现是因为 Li 强烈结合到主动沉淀的次生粘土相中，超出了先前实验室实验的建议。尽管由于两个因素的结合，反应速率相对较慢且粘土积累适度，但化学风化的不一致性仍然存在。首先，反应性原生矿物相在整个风化过程中持续存在，并且由于其更大的溶解度而有效地“保护”次生粘土免于再溶解。其次，近地表剖面中堆积的粘土是相对成熟的风化副产物。尽管外源性粉尘输入量很大，但这些因素导致流域内溶解溶质出口总量偏低。