Capturing the short-term variability of carbon dioxide emissions from sedimentary rock weathering in a remote mountainous catchment, New Zealand

• Published in: Chemical geology 2022-10, Vol.608, p.121024, Article 121024
• Main Authors: Roylands, Tobias, Hilton, Robert G., Garnett, Mark H., Soulet, Guillaume, Newton, Josephine-Anne, Peterkin, Joanne L., Hancock, Peter
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carbon isotopes; Microbial respiration; Oxidative weathering; Petrogenic organic carbon; Sciences of the Universe; Silicate weathering
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2022.121024

Weathering of organic carbon contained in sedimentary rocks (petrogenic OC, OCpetro) is an important control on the concentrations of carbon dioxide (CO2) and oxygen in the atmosphere. Of particular significance are steep mountainous catchments, where high rates of physical erosion introduce OCpetro to the surface, where oxygen in air and water can help drive oxidative weathering reactions, yet measurements of CO2 emissions from OCpetro oxidation are still scarce. Here, we use in situ gas accumulation chambers and show that CO2 fluxes, and their environmental controls, can be determined during a stand-alone, short-term (8 days) field campaign, applied to a remote setting. In the rapidly eroding Waiapu River catchment, New Zealand, dominated by mudstones, we measured high rates of CO2 release (222–1590 mgC m−2 d−1) in five accumulation chambers in the near-surface of naturally fractured and bedded rock outcrops. The corresponding CO2 concentrations are very high (pCO2 ~4700–27,100 ppmv), and such values could influence acid-hydrolysis reactions during chemical weathering of co-occurring silicate minerals. The CO2 is radiocarbon depleted (fraction modern, F14C = 0.0122–0.0547), confirming it is petrogenic in origin. Stable carbon isotopes suggest a source from OCpetro, but δ13C values of the CO2 are lower by ~3.5–3.7 ± 0.1 ‰ from those of OCpetro (−25.9 ± 0.1 ‰), consistent with isotope fractionation associated with microbial respiration of OCpetro. Over 6 days of measurement, we find that CO2 fluxes respond quickly to changes in temperature and humidity, indicating an environmental regulation that is captured by our short-term installation. The approaches applied here mean that future research can now seek to constrain the climatic, lithological and biological controls on OCpetro oxidation across regional to global scales. •CO2 fluxes from oxidative weathering can be measured during short field campaigns.•Their size, regulated by temperature and humidity, reaches that of soil respiration.•Carbon isotopes reveal that measured CO2 is sourced from petrogenic organic matter.•Microbial respiration is potentially associated with carbon isotope fractionation.•Partial pressures of CO2 in rock can be high and induce silicate weathering.