Abstract Understanding weathering processes in clay formations is an issue of primary importance for the preservation of our natural environment. Reactive-transport modeling used to simulate weathering of clay formations has indicated that reactive gases (CO2 and O2) are major parameters in controlling weathering processes. The Lower Cretaceous Tegulines marine-clay formation outcropping in the area of Brienne-le-Chateau (north-eastern France) has been investigated in the context of a sub-surface waste repository. We developed gas monitoring (CO2, O2, N2, alkanes) of core samples from two boreholes that entirely crosscut the Tegulines Clay formation, to define the consequences of weathering and oxidation processes on gases dissolved in pore waters. We discuss amounts of gas and the carbon isotopic composition of CO2 in terms of pore water chemistry including dissolved-inorganic carbon (DIC) and alkalinity, mineral reactivity, organic-matter degradation and oxygen diffusion. Degassing of samples conditioned under He atmosphere provided evidence of very high CO2 production in the soil (0–30 cm), and high CO2 degassing associated with a high oxygen level in the first 2–10 m of the clay. The CO2 degassing increase observed in weathered clay relative to preserved clay resulted from calcite dissolution due to pyrite oxidation and organic matter degradation. The δ13C of CO2 indicates that organic matter degradation was a major source of CO2 at shallow depths and down to 10–12 m, which is the maximum depth at which we observed fossil roots. Then the CO2 degassing decreased down to a constant value in preserved clay, where the carbonate system and the mineral assemblage control dissolved carbonates in pore waters. The profile of the δ13CCO2 also provides evidence of progressive CO2 diffusion of organic origin from the underlying Greensands aquifer in the lower part of Tegulines Clay up to ~40 m in the AUB230 borehole. As a first step toward understanding interactions between Tegulines Clay and near surface waters or water at the Greensands interface, we developed a reactive-transport model to simulate in one dimension weathering processes under ambient temperature, constrained by geochemical reactions in soil (organic matter degradation) and in the clay (pyrite oxidation and calcite dissolution), exchange, DIC and pore water chemistry. The simulation was carried out for 10 kyrs, assuming that weathering and soil formation began after the last glacial maximum. The DIC profile cannot be simulated without considering evaporation processes in agreement with the isotopic data. This type of approach combining a complete field dataset (reactive-gas concentrations, δ13C of CO2, major-ion concentrations, δ18O and δD of pore waters) and reactive-transport modeling is necessary for better understanding of chemical weathering processes in the critical zone.

中文翻译：

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临界区粘土地层孔隙水中溶解气体（CO2、O2、N2、烷烃）的来源（Tégulines Clay，法国）

摘要 了解粘土地层的风化过程对于保护我们的自然环境至关重要。用于模拟粘土地层风化的反应输运模型表明，反应性气体（CO2 和 O2）是控制风化过程的主要参数。Brienne-le-Chateau（法国东北部）地区出露的下白垩统 Tegulines 海相粘土地层已在地下废物处置库的背景下进行了调查。我们开发了对完全横切 Tegulines 粘土地层的两个钻孔的岩心样品进行气体监测（CO2、O2、N2、烷烃），以确定风化和氧化过程对溶解在孔隙水中的气体的影响。我们从孔隙水化学（包括溶解无机碳 (DIC) 和碱度、矿物反应性、有机物降解和氧扩散）的角度讨论了气体的数量和 CO2 的碳同位素组成。在 He 气氛下对样品进行脱气提供了土壤 (0-30 cm) 中非常高的 CO2 产量的证据，并且在粘土的前 2-10 m 中与高氧水平相关的高 CO2 脱气。在风化粘土中观察到的 CO2 脱气增加是由于黄铁矿氧化和有机物降解导致方解石溶解导致的。CO2 的 δ13C 表明有机物降解是浅层和低至 10-12 m 处 CO2 的主要来源，这是我们观察到化石根的最大深度。然后 CO2 脱气在保存粘土中下降到恒定值，其中碳酸盐系统和矿物组合控制孔隙水中溶解的碳酸盐。δ13​​CCO2 的剖面也提供了证据表明，在 AUB230 钻孔中，来自 Tegulines Clay 下部底层 Greensands 含水层的有机源的 CO2 逐渐扩散至约 40 m。作为了解 Tegulines Clay 与近地表水或 Greensands 界面水之间相互作用的第一步，我们开发了一个反应输运模型，以模拟环境温度下的一维风化过程，受土壤中地球化学反应（有机物质降解）的限制并在粘土中（黄铁矿氧化和方解石溶解）、交换、DIC 和孔隙水化学。对 10 个 kyrs 进行了模拟，假设风化和土壤形成是在最后一次冰川盛期之后开始的。如果不考虑与同位素数据一致的蒸发过程，就无法模拟 DIC 剖面。这种将完整的现场数据集（反应性气体浓度、CO2 的 δ13C、主要离子浓度、孔隙水的 δ18O 和 δD）和反应性输运建模相结合的方法对于更好地了解关键区域的化学风化过程是必要的。