Predicting sedimentary bedrock subsurface weathering fronts and weathering rates: Dataset

Although bedrock weathering strongly influences water quality and global carbon and nitrogen budgets, the weathering depths and rates within subsurface are not well understood nor predictable. Determination of both porewater chemistry and subsurface water flow are needed in order to develop more complete understanding and obtain weathering rates. In a long-term field study, we applied a multiphase approach along a mountainous watershed hillslope transect underlain by marine shale. Here we report three findings. First, the deepest extent of the water table determines the weathering front, and the range of annually water table oscillations determines the thickness of the weathering zone. Below the lowest water table, permanently water-saturated bedrock remains reducing, preventing deeper pyrite oxidation. Secondly, carbonate minerals and potentially rock organic matter share the same weathering front depth with pyrite, contrary to models where weathering fronts are stratified. Thirdly, the measurements-based weathering rates from subsurface shale are high, amounting to base cation exports of about 70 kmolc ha−1 y−1, yet consistent with weathering of marine shale. Finally, by integrating geochemical and hydrological data we present a new conceptual model that can be applied in other settings to predict weathering and water quality responses to climate change.

尽管基岩风化（bedrock weathering）对水体水质以及全球碳、氮收支具有显著调控作用，但当前学界对地下岩层内的风化深度与速率仍缺乏足够认知与可预测性。为构建更全面的认知框架并精准获取风化速率数据，需同时测定孔隙水化学（porewater chemistry）特征与地下水流（subsurface water flow）规律。在一项长期野外研究中，我们沿海相页岩（marine shale）作为下伏基底的山地流域山坡样带（mountainous watershed hillslope transect），采用多相研究方法开展了系统观测。本文报告三项核心发现：其一，地下水位（water table）的最深埋深决定了风化锋面（weathering front）的分布位置，而年度地下水位的波动范围则决定了风化带（weathering zone）的厚度；在最低地下水位以下，永久饱水基岩仍维持还原环境，可阻断更深层黄铁矿（pyrite）的氧化反应。其二，碳酸盐矿物（carbonate minerals）与潜在赋存的岩石有机质（rock organic matter）与黄铁矿共享同一风化锋面深度，这与认为风化锋面呈层状分布的传统模型相悖。其三，基于实测数据得到的地下页岩风化速率较高，碱基阳离子输出量约为70 kmolc·ha⁻¹·y⁻¹，且该结果与海相页岩的风化特征相符。最后，通过整合地球化学与水文数据，我们提出了一种全新的概念模型（conceptual model），该模型可推广至其他研究场景，用以预测气候变化背景下的风化过程与水质响应。