Temperature control of CO2 fluxes from fossil organic carbon oxidation in sedimentary rocks is mediated by microbial activity — insight from incubation experiments

Abstract

In the geological carbon cycle, carbon can be stored in sedimentary rocks as petrogenic organic carbon (OCpetro) on timescales of millions of years, before re-emerging to the surface via orogenesis and erosion. As the rock enters the critical zone of Earth’s surface, a suite of physical, chemical and biotic processes occur during oxidative weathering and can result in CO2 fluxes towards the atmosphere at a rate significant enough to influence Earth’s climate on millennia timescales1,2. Whereas the chemically-mature OCpetro contained in sedimentary rocks was classically perceived as a largely inert pool of carbon in the textbook view of the carbon cycle, the emerging theory is that the positive correlation between temperature and CO2 fluxes from oxidative weathering of OCpetro in sedimentary rocks constitutes a positive feedback to global warming1,2. To better understand the mechanisms involved in this CO2 release, we incubated marly limestone and shale material from the Terre Noire region of the Prealps, France—a badland landscape known to display temperature sensitive in situ CO2 emissions1,3. Surface regolith (ca. 0–5 cm depth) and subsurface rock (ca. 5-10 cm) was retrieved from two catchments with different OCpetro (0.45–0.78% wt.) and carbonate (30 -45% wt.) contents, transferred to airtight bottles with a saline media and CO2-free headspace, and incubated at 4, 10, 16, 30 and 40 degrees Celsius. Half of the bottles were sterilised with mercuric chloride (HgCl2) prior to incubation. The gas phase was monitored weekly for four weeks and analysed for CO2(g) concentrations and stable isotopic (13C) composition. Early terminations of triplicate bottles allowed us to monitor changes in microbial biomass and community composition during the incubation experiment, using phospholipid fatty acids, and amplicon sequencing, respectively. Our data reveal that microorganisms accelerated the oxidation of OCpetro at higher temperatures, particularly in the higher OCpetro-containing material. This work demonstrates that the temperature sensitivity of CO2 fluxes from sedimentary rocks likely operates predominately via temperature control on microorganisms, bringing us one step closer to understanding the mechanics behind oxidative weathering in, and 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. Zondervan, J. R. et al. Rock organic carbon oxidation CO2 release offsets silicate weathering sink. Nature 1–5 (2023) doi:10.1038/s41586-023-06581-9. 3. Roylands, T. et al. Probing the exchange of CO2 and O2 in the shallow critical zone during weathering of marl and black shale. Earth Surf. Dyn. 12, 271–299 (2024)