Probing the exchange of CO₂ and O₂ in the shallow critical zone during weathering of marl and black shale

Abstract. Chemical weathering of sedimentary rocks can release carbon dioxide (CO₂) and consume oxygen (O₂) via the oxidation of petrogenic organic carbon and sulfide minerals. These pathways govern Earth’s surface system and climate over geological timescales, but the present-day weathering fluxes and their environmental controls are only partly constrained due to a lack of in situ measurements. Here, we investigate the gaseous exchange of CO₂ and O₂ during the oxidative weathering of black shales and marls exposed in the French southern Alps. On six fieldtrips over one year, we use drilled headspace chambers to measure the CO₂ concentrations in the shallow critical zone, and quantify CO₂ fluxes in real-time. Importantly, we develop a new approach to estimate the volume of rock that contributes CO₂ to a chamber, and assess effective diffusive gas exchange, by first quantifying the mass of CO₂ that is stored in a chamber and connected rock pores. Both rock types are characterized by similar contributing rock volumes and diffusive movement of CO₂. However, CO₂ emissions differed between the rock types, with yields over rock outcrop surfaces (inferred from the contributing rock volume and the local weathering depths) ranging between 166 tC km^-2 yr^-1 and 2,416 tC km^-2 yr^-1 for black shales and between 83 tC km^-2 yr^-1 and 1,558 tC km^-2 yr^-1 for marls over the study period. Having quantified diffusive processes, chamber-based O₂ concentration measurements are used to calculate O₂ fluxes. The rate of O₂ consumption increased with production of CO₂, and with increased temperature, with an average O₂ : CO₂ molar ratio of 10 : 1. If O₂ consumption occurs by both rock organic carbon oxidation and sulfide oxidation, either an additional O₂ sink needs to be identified, or significant export of dissolved inorganic carbon occurs from the weathering zone. Together, our findings refine the tools we have to probe CO₂ and O₂ exchange in rocks at Earth’s surface and shed new light on CO₂ and O₂ fluxes, their drivers and the fate of rock-derived carbon.