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

<p>Chemical weathering of sedimentary rocks can release carbon dioxide (<span class="inline-formula">CO₂</span>) and consume oxygen (<span class="inline-formula">O₂</span>) 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 <span class="inline-formula">CO₂</span> and <span class="inline-formula">O₂</span> during the oxidative weathering of black shales and marls exposed in the French southern Alps. On six field trips over 1 year, we use drilled headspace chambers to measure the <span class="inline-formula">CO₂</span> concentrations in the shallow critical zone and quantify <span class="inline-formula">CO₂</span> fluxes in real time. Importantly, we develop a new approach to estimate the volume of rock that contributes <span class="inline-formula">CO₂</span> to a chamber, and assess effective diffusive gas exchange, by first quantifying the mass of <span class="inline-formula">CO₂</span> that is stored in a chamber and connected rock pores. Both rock types are characterized by similar contributing rock volumes and diffusive movement of <span class="inline-formula">CO₂</span>. However, <span class="inline-formula">CO₂</span> emissions differed between the rock types, with yields over rock outcrop surfaces (inferred from the contributing rock volume and the local weathering depths) ranging on average between 73 and 1108 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">t</mi><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">C</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">km</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="864643c20a98a4114c442a17c1538a95"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="esurf-12-271-2024-ie00001.svg" width="61pt" height="15pt" src="esurf-12-271-2024-ie00001.png"/></svg:svg></span></span> for black shales and between 43 and 873 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">t</mi><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">C</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">km</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8634a84972cdf024154194bce9242012"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="esurf-12-271-2024-ie00002.svg" width="61pt" height="15pt" src="esurf-12-271-2024-ie00002.png"/></svg:svg></span></span> for marls over the study period. Having quantified diffusive processes, chamber-based <span class="inline-formula">O₂</span> concentration measurements are used to calculate <span class="inline-formula">O₂</span> fluxes. The rate of <span class="inline-formula">O₂</span> consumption increased with production of <span class="inline-formula">CO₂</span>, and with increased temperature, with an average <span class="inline-formula">O₂</span><span class="inline-formula">:</span><span class="inline-formula">CO₂</span> molar ratio of 10<span class="inline-formula">:</span>1. If <span class="inline-formula">O₂</span> consumption occurs by both rock organic carbon oxidation and carbonate dissolution coupled to sulfide oxidation, either an additional <span class="inline-formula">O₂</span> 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 <span class="inline-formula">CO₂</span> and <span class="inline-formula">O₂</span> exchange in rocks at Earth's surface and shed new light on <span class="inline-formula">CO₂</span> and <span class="inline-formula">O₂</span> fluxes, their drivers, and the fate of rock-derived carbon.</p>