EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Dissolved gas (CO2, alkanes, O2, N2) in critical zone developed on claystone

Catherine Lerouge1, Michaela Blessing1, Mathieu Debure1, Frédérick Gal1, Wolfram Kloppmann1, and Jean-Charles Robinet2
Catherine Lerouge et al.
  • 1BRGM, Orléans, France (
  • 2ANDRA, Chatenay-Malabry, France

Weathering processes in clay environments are of major importance because they participate to regulate elemental cycling and mass transfer in the critical zone with major implications for carbon and nitrogen cycling.

In that aim, we measured 1) dissolved CO2, alkanes, O2 and N2 concentrations in clay pore waters by rock degassing, 2) soil gas flux and concentrations, and 3) δ13C of CO2 and alkanes in two contrasted tectonic contexts.

The first context is the marine Jurassic black marls in the French Alps, characterized by deep burial, high erosion rates and dominant physical weathering processes. These marls are well-known for their occurrences of natural methane gas seeps. In this area, we carried out rock degassing on outcropping weathered claystone in complement of soil flux measurements to constrain the implication of weathering processes on the natural gas releases. These measurements are also tested as a new component of environmental baseline assessment in the field of unconventional hydrocarbons.

The second context is the marine Cretaceous Tégulines Clay of the north-eastern part of the Paris Basin, characterized by low burial, low erosion rates, and dominant chemical weathering processes. In this area, we carried out rock degassing on soil and weathered claystone accessible by deep boreholes, in order to define the depth of the critical zone and major reactions controlling the weathering profile.

Oxygen and nitrogen concentrations are the record of the atmospheric diffusion through the formations. Some values are higher than the gas solubility, which could be attributed to rock desaturation and air bubbles, and clay sorption (only for nitrogen).

Weathering processes induce a significant CO2 increase and a large range of δ13CCO2, providing evidence of two major CO2 sources: CO2 internally controlled by carbonates and organic-derived CO2 of internal and external origins. In Alpine black marls, field observations suggest a low depth affected by weathering, due to intense erosion. In Tégulines Clay, the CO2 increase provides evidence of a ~ 20 m-thick critical zone. The lowest δ13CCO2 indicates that the highest reactive zone (organic matter degradation, calcite dissolution and pyrite oxidation) is ~10 m deep, in agreement with the depth of the root network.

Nature and amounts of alkanes are contrasted in the two contexts. In deep burial environment, alkanes are abundant, in particular, in the “ fontaines ardentes” gas seeps in the French Alps. Composition of hydrocarbon gas and δ13C of methane strongly suggest a thermogenic origin. Outcropping black marls contain methane, suggesting oxidation of higher alkanes. That assumption is supported by δ13CCO2 of soil close to δ13C of alkanes. In low burial environment, small amounts of methane are present that rapidly disappear with weathering. Some methane concentrations could be attributed to diffusion of external methane formed by degradation of organic matter under reducing conditions in soil.

Overall those results suggest that dissolved gas and their isotopic signature are good markers of weathering processes in the critical zone.

This research was funded by the EU H2020 Programme (grant 764531 - SECURe), ANDRA-BRGM projects.


How to cite: Lerouge, C., Blessing, M., Debure, M., Gal, F., Kloppmann, W., and Robinet, J.-C.: Dissolved gas (CO2, alkanes, O2, N2) in critical zone developed on claystone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11430,, 2020

This abstract will not be presented.