Contributions of biotic and abiotic oxidative processes in CO2 emissions from sedimentary rocks

Abstract

Carbon is stored in sedimentary rocks as petrogenic organic carbon (OCpetro) and carbonates on timescales of millions of years. When sedimentary rocks are exposed to the atmosphere through orogeneses and erosion, a suite of physical, chemical and biotic processes occur during oxidative weathering, and result in carbon exchanges with the atmosphere. Previous studies have focused on quantifying the rates of geochemical reactions and net carbon dioxide (CO2) transfers, showing that oxidative weathering in sedimentary rocks likely acts as a positive feedback to rising temperatures on geological time scales.1 However, the role of microbiology in these processes remains largely unknown and unexplored, despite their ability to catalyse reactions via enzymes and their capacity to assimilate OCpetro.2 We incubated surface regolith and subsurface (>10 cm depth) shale from two marly catchments, namely Moulin and Brusquet, in the Terres Noire region of southeastern France where in situ CO2 fluxes have previously been quantified.1 The Moulin material consists mainly of Callovian to Oxfordian marly limestones with carbonate, OCpetro and total sulfur contents averaging ~45, ~0.48 and 0.47% wt., respectively. The Brusquet catchment presents sequences of Bajocian marly limestones, Aalenian grey shales and limestones to Toarcian black shales with ~33% wt. carbonate, ~0.78% wt. OCpetro and 0.40% wt. total sulfur. The material was collected in July 2023 and incubated at 40C for 3–4 weeks as intact (biotic) triplicates or as MgCl2-sterilised duplicates. Headspace was subsampled at weekly intervals and analysed by Cavity Ring-Down Spectroscopy to access pCO2 and d13CCO2. The incubation experiment demonstrated that the high CO2 fluxes from Brusquet regolith appear to be predominantly linked to biotically-mediated oxidation of OCpetro; whereas in the Moulin regolith, lower CO2 fluxes are more likely associated with abiotic carbonate dissolution, potentially by pyrite-derived sulfuric acid. In the subsurface material, it is the Moulin material that releases more CO2 relative to Brusquet, with little to no difference between biotic and abiotic samples for both rock types. This experiment brings us one step closer understanding the underlying mechanics and kinetics of CO2 fluxes from sedimentary rocks. 1. Soulet, G. et al. Temperature control on CO2 emissions from the weathering of sedimentary rocks. Nat. Geosci. 14, 665–671 (2021). 2. Petsch, S. T., Eglinton, T. I. & Edwards, K. J. 14 C-Dead Living Biomass: Evidence for Microbial Assimilation of Ancient Organic Carbon During Shale Weathering. Science 292, 1127–1131 (2001).