08 Jun 2023
 | 08 Jun 2023
Status: this preprint is currently under review for the journal ESurf.

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

Tobias Roylands, Robert G. Hilton, Erin L. McClymont, Mark H. Garnett, Guillaume Soulet, Sébastien Klotz, Mathis Degler, Felipe Napoleoni, and Caroline Le Bouteiller

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.

Tobias Roylands et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esurf-2023-15', Aaron Bufe, 29 Jun 2023
  • RC2: 'Comment on esurf-2023-15', Anonymous Referee #2, 01 Aug 2023

Tobias Roylands et al.

Tobias Roylands et al.


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Short summary
Chemical weathering of sedimentary rocks can release carbon dioxide and consume oxygen. We present a new field-based method to measure the exchange of these gases in real-time, which allows us to directly compare the amount of reactants and products. By studying two sites with different rock types, we show that the chemical composition is an important factor in driving the weathering reactions. Locally, carbon dioxide release changes alongside temperature and precipitation.