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Cited by 39CO2 Sequestration and Geologic InteractionsGroundwater and Isotope GeochemistryAtmospheric and Environmental Gas Dynamics

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Technical note: In situ measurement of flux and isotopic composition of CO 2 released during oxidative weathering of sedimentary rocks

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Soulet, Guillaume, Hilton, Robert G., Garnett, Mark H., Dellinger, Mathieu, Croissant, Thomas, Ogrič, Mateja, Klotz, Sébastien·2018·Biogeosciences
DOI 10.5194/bg-15-4087-2018

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Abstract

Oxidative weathering of sedimentary rocks can release carbon dioxide (CO 2 ) to the atmosphere and is an important natural CO 2 emission. Two mechanisms operate – the oxidation of sedimentary organic matter and the dissolution of carbonate minerals by sulfuric acid. It has proved difficult to directly measure the rates at which CO 2 is emitted in response to these weathering processes in the field, with previous work generally using methods which track the dissolved products of these reactions in rivers. Here we design a chamber method to measure CO 2 production during the oxidative weathering of shale bedrock, which can be applied in erosive environments where rocks are exposed frequently to the atmosphere. The chamber is drilled directly into the rock face and has a high surface-area-to-volume ratio which benefits measurement of CO 2 fluxes. It is a relatively low-cost method and provides a long-lived chamber (several months or more). To partition the measured CO 2 fluxes and the source of CO 2 , we use zeolite molecular sieves to trap CO 2 “actively” (over several hours) or “passively” (over a period of months). The approaches produce comparable results, with the trapped CO 2 having a radiocarbon activity (fraction modern, Fm) ranging from Fm = 0.05 to Fm = 0.06 and demonstrating relatively little contamination from local atmospheric CO 2 (Fm = 1.01). We use stable carbon isotopes of the trapped CO 2 to partition between an organic and inorganic carbon source. The measured fluxes of rock-derived organic matter oxidation (171 ± 5 mgC m −2 day −1 ) and carbonate dissolution by sulfuric acid (534±16 mgC m −2 day −1 ) from a single chamber were high when compared to the annual flux estimates derived from using dissolved river chemistry in rivers around the world. The high oxidative weathering fluxes are consistent with the high erosion rate of the study region. We propose that our in situ method has the potential to be more widely deployed to directly measure CO 2 fluxes during the oxidative weathering of sedimentary rocks, allowing for the spatial and temporal variability in these fluxes to be determined.

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Figures

Figure 1 A: Schematic diagram of the closed-loop monitoring-sampling system connected to the natural weathering chamber. Gas flow pathways are controlled by opening and closing the clips. Clips removed from the bypass allow pCO2 in the chamber to be monitored (IRGA stands for Infra-Red Gas Analyzer), thus measuring CO2 flux and ensuring that enough CO2 accumulated in the 5 chamber for 14 C analysis. To remove CO2 from the line before connecting to the chamber, clips are moved from the CO2 scrub (soda lime). When connected to the chamber, the CO2 scrub can be used to lower the CO2 concentration before flux measurement. To collect CO2 in the chamber for isotope analyses, clips are removed from the zeolite molecular sieve cartridge. B: Pictures showing the chamber design. Top picture is chamber (H6), diameter 2.9cm, drilled in the rock on a cleared surface, with white PVC tubing inserted at the outlet. Bottom picture shows the rubber stopper fitted in the PVC tubing. Two glass tubes go through the rubber 10 stopper and are fitted with Tygon tubing, sealed with the red clips, and the exterior of the chamber is sealed with outdoor sealant. C: View of the field site showing two chambers (top chamber is H6 and lower chamber is H4). The lower chamber is connected to the closed-loop system and is being monitored for flux measurement. The two large grey PVC tubes attached to the rock on the right of the chambers are cases in which zeolite molecular sieves are installed and left for months when connected to the chamber for passive CO2 trapping. 15
Figure 1 A: Schematic diagram of the closed-loop monitoring-sampling system connected to the natural weathering chamber. Gas flow pathways are controlled by opening and closing the clips. Clips removed from the bypass allow pCO2 in the chamber to be monitored (IRGA stands for Infra-Red Gas Analyzer), thus measuring CO2 flux and ensuring that enough CO2 accumulated in the 5 chamber for 14 C analysis. To remove CO2 from the line before connecting to the chamber, clips are moved from the CO2 scrub (soda lime). When connected to the chamber, the CO2 scrub can be used to lower the CO2 concentration before flux measurement. To collect CO2 in the chamber for isotope analyses, clips are removed from the zeolite molecular sieve cartridge. B: Pictures showing the chamber design. Top picture is chamber (H6), diameter 2.9cm, drilled in the rock on a cleared surface, with white PVC tubing inserted at the outlet. Bottom picture shows the rubber stopper fitted in the PVC tubing. Two glass tubes go through the rubber 10 stopper and are fitted with Tygon tubing, sealed with the red clips, and the exterior of the chamber is sealed with outdoor sealant. C: View of the field site showing two chambers (top chamber is H6 and lower chamber is H4). The lower chamber is connected to the closed-loop system and is being monitored for flux measurement. The two large grey PVC tubes attached to the rock on the right of the chambers are cases in which zeolite molecular sieves are installed and left for months when connected to the chamber for passive CO2 trapping. 15CC-BY · From the PDF
Image 2
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Figure 2 An example of the monitoring of the CO2 accumulating in a chamber. The orange curve is the partial pressure of CO2 (pCO2, in parts per million volume) through time in chamber H6 on 27/03/2017. A: The CO2 sampling-monitoring system is not connected to the chamber. Atmospheric CO2 has been removed from the system (pCO2 = 0 ppm) using the CO2 scrub cartridge. B: The closed-loop monitoring system has been connected to the chamber. pCO2 increases to reach a maximum value of ~5100 ppm, 5 then drops and equilibrates to ~3500ppm. This pattern reflects the increase in the total volume (by the volume of the CO2 samplingmonitoring system) which decreases pCO2 and requires some time for the pCO2 to equilibrate. We determined that when connected to the chamber, the maximum value of pCO2 read is 0.94 the actual pCO2 in the chamber. C: The CO2 in the chamber is lowered (scrubbed with the CO2 scrub, or trapped with the zeolite molecular sieve) to near atmospheric pCO2. D: residual CO2 that in the chamber homogenized with the rest of the total volume 'artificially' increasing pCO2 quickly. E: pCO2 in chamber is monitored, 10 reflecting the flux of CO2 from the rock surface to the chamber.
Figure 2 An example of the monitoring of the CO2 accumulating in a chamber. The orange curve is the partial pressure of CO2 (pCO2, in parts per million volume) through time in chamber H6 on 27/03/2017. A: The CO2 sampling-monitoring system is not connected to the chamber. Atmospheric CO2 has been removed from the system (pCO2 = 0 ppm) using the CO2 scrub cartridge. B: The closed-loop monitoring system has been connected to the chamber. pCO2 increases to reach a maximum value of ~5100 ppm, 5 then drops and equilibrates to ~3500ppm. This pattern reflects the increase in the total volume (by the volume of the CO2 samplingmonitoring system) which decreases pCO2 and requires some time for the pCO2 to equilibrate. We determined that when connected to the chamber, the maximum value of pCO2 read is 0.94 the actual pCO2 in the chamber. C: The CO2 in the chamber is lowered (scrubbed with the CO2 scrub, or trapped with the zeolite molecular sieve) to near atmospheric pCO2. D: residual CO2 that in the chamber homogenized with the rest of the total volume 'artificially' increasing pCO2 quickly. E: pCO2 in chamber is monitored, 10 reflecting the flux of CO2 from the rock surface to the chamber.CC-BY · From the PDF
Image 4
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Image 5
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Figure 4 A: Zeolite molecular sieve connected to a chamber for passive CO2 trapping. The zeolite molecular sieve is encased in the grey PVC tubing and connected for months to the chamber using a white connector. B: Zeolite molecular sieve ready to be disconnected from chamber. The red clips are positioned so that they seal both the zeolite molecular sieve and the chamber.
Figure 4 A: Zeolite molecular sieve connected to a chamber for passive CO2 trapping. The zeolite molecular sieve is encased in the grey PVC tubing and connected for months to the chamber using a white connector. B: Zeolite molecular sieve ready to be disconnected from chamber. The red clips are positioned so that they seal both the zeolite molecular sieve and the chamber.CC-BY · From the PDF
Image 7
CC-BY · From the PDF

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Metadata

Authors
Soulet, G., Hilton, R. G., Garnett, M. H., Dellinger, M., Croissant, T., Ogrič, M., & Klotz, S.
Affiliations
  • Durham University (GB)
  • Institut National de Recherche en Sciences et Technologies pour l'Environnement et l'Agriculture (FR)
Corresponding author
Soulet, Guillaume
ORCID
Venue
Biogeosciences
Publisher
Copernicus GmbH
Published
2018-07-04
Volume
15
Issue
13
Pages
4087-4102
ISSN
  • 1726-4170 (ISSN-L)
  • 1726-4189
DOI
10.5194/bg-15-4087-2018
OpenAlex
W2768393184
Cited by
39
References
5
Citations by year
2026: 1 · 2025: 2 · 2024: 4 · 2023: 12 · 2022: 3 · 2021: 12 · 2020: 3 · 2019: 1 · 2018: 1
FWCI
2.13
Type
journal article
Language
en
Open access
Licence
Locations
  • Biogeosciences (publishedVersion, cc-by)
  • ENLIGHTEN (Jurnal Bimbingan dan Konseling Islam) (acceptedVersion, cc-by-sa)
  • Durham Research Online (Durham University) (acceptedVersion)
  • Durham Research Online (Durham University) (submittedVersion, cc-by)
  • HAL (submittedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • Cemadoc (Cemagref / IRSTEA) (submittedVersion)
  • Biogeosciences (European Geosciences Union) (submittedVersion, cc-by)
Funders
European Research Council
Topics
  • Physical Sciences › Environmental Science › Environmental Engineering › CO2 Sequestration and Geologic Interactions
  • Physical Sciences › Earth and Planetary Sciences › Geochemistry and Petrology › Groundwater and Isotope Geochemistry
  • Physical Sciences › Environmental Science › Global and Planetary Change › Atmospheric and Environmental Gas Dynamics
Keywords
oxidative weathering, CO2 flux, sedimentary rock, organic matter oxidation, carbonate dissolution, sulfuric acid, CO2 isotopic composition, carbon-14, zeolite molecular sieves, erosion, stable carbon isotopes

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