Catherine Lerouge

Mathieu Debure

Jean-Charles Robinet

Francis Claret

Christine Flehoc

Catherine Guerrot

Henry Benoit

Philippe Landrein

Benoit Madé

Nicolas Maubec

Chemical weathering processes in the 0-10 m of the critical zone developed on

The first meters of sediments overlying Tégulines Clay (1-8 m) consist of soils developed on Quaternary clay silty loams, then carbonate-free and Fe-hydroxide-rich colluvium. At the loam-colluvium/Tégulines clay interface, claystone show often slip surfaces along hillslopes. They have rather yellowish-greenish color, and are swelling clay-rich, quartz-poor, and carbonate-poor. Gypsum and iron hydroxide entirely replaced pyrite. Claystone is marked by a high Cation Exchange Capacity (CEC) up to 37 meq/100 g (reduced Tégulines clay: 16-19 meq/100 g), and dominated by Ca cation (~80 %) in interlayer.

The first meters of Tégulines clay corresponds to the most reactive greenish clay. Clay fraction consists of a mixing between kaolinite (~35 %), illite-smectite mixed layers (~40 %) and illite-micas including glauconite (20-25 %). Clayrock shows CEC of 18-21 meq/100 g slightly higher than reduced clay, and Ca remains the dominant cation on the exchanger. The top of this zone is characterized by the highest dissolved inorganic carbon concentrations (DIC up to ~10 mmol/L compared to reduced clay: 1.5 ± 0.5 mmol/L) and sulfates (up to 30 mmol/L compared to reduced clay: 2 mmol/L) in pore water, associated with a peak of [O 2 ] and [N 2 ]. The presence of O 2 drives oxidative weathering of pyrite, organic matter and other Fe 2+ bearing minerals including glauconite. The oxidation of pyrite releases protons, which leads to the calcite dissolution. Both oxidation of pyrite and organic matter contribute to the DIC increase. Based on the δ 13 C of CO 2 , organic matter degradation is the major source of CO 2 (~50-60 %) at the top of the reactive zone, then decreases to ~20-30 % at the bottom. Iron-hydroxides and gypsum are secondary minerals formed in the entire zone, whereas secondary carbonate concretions only precipitated at the top. The isotopic dating of concretions with δ 13 C, δ 18 O, 87 Sr/ 86 Sr and 14 C indicate that they precipitated from recent surface-derived fluids (<50 000 years). Calcite dissolution, pyrite oxidation, organic matter and glauconite oxidation reactions, and secondary minerals precipitation represent "reaction fronts" associated with chemical weathering that can be used to infer the depth at which dissolved oxygen infiltrate the