Quantifying Terrestrial Carbon Fluxes during Shale Weathering by Modeling Subsurface Processes.

Seasonal climate forcing affects the transfer of atmospheric CO₂ to the subsurface and the net carbon balance associated with shale weathering.

The Science   

Shale weathering occurs when rocks are exposed to water, atmospheric gases, and products of biological respiration. Shale weathering absorbs atmospheric CO₂ in the process through carbonate mineral dissolution, but also releases CO₂ through the oxidation of pyrite and petrogenic organic carbon present in shale. Understanding how shale weathering affects and contributes to the carbon cycle is important for estimating its net contribution to atmospheric CO₂ level. Here, a numerical model was developed to quantify the mechanisms consuming and producing CO₂ during the weathering of shale in a mountainous watershed. The authors found that shale weathering acts as a net CO₂ sink when large amounts of water infiltrate from the surface allowing the seasonal transfer of CO₂ from soil to bedrock.

 

 

The Impact

The results of this study showed that the net carbon flux associated with shale weathering depends on seasonal climatic changes. In particular, the study found that spring snowmelt in high elevation areas represents an important event for the transfer of CO₂ from soil to bedrock; the rewetting of soil upon snowmelt enhances microbial activity, while percolating water transports CO₂ deeper to bedrock, therefore increasing carbonate weathering. These results suggest that declining snowpacks under warmer conditions could lead to a decrease in the potential of shale weathering to act as a CO₂ sink. In contrast, drier conditions are expected to enhance the breakdown of minerals through oxidation reactions, which generate CO₂.

 

Summary

Shale weathering is an important component of the global carbon cycle. However, the controls on carbon weathering fluxes remain poorly understood. This study implemented a numerical model to simulate the effects of water infiltration, diffusion of atmospheric gases, and temperature variations on soil respiration and mineral weathering that affect shale carbon dynamics. The model allows us to understand the net effect of these factors and interpret the influence of seasonal climate forcing on shale CO₂ fluxes. The authors found that a small but substantial fraction of atmospheric CO₂ was transferred to the subsurface during large water infiltration events because of the temporal coupling between microbial respiration and carbonate weathering. The authors showed that the CO₂ drawdown in a high elevation catchment primarily occurs during spring snowmelt and governs the aqueous carbon exports. Moreover, the authors calculate that the consumption of CO₂ by carbonate minerals exceeds the CO₂ efflux generated by the oxidation of sulfide and organic carbon in rocks, demonstrating that weathering acts as a net carbon sink where large infiltration events allow an efficient transfer of CO₂ between the soil horizon and shale bedrock. Considering the sensitivity of carbon fluxes to seasonal climatic forcing, the findings indicated that warmer and drier conditions may reduce the potential of shale weathering to act as a CO₂ sink.

Contact

Lucien Stolze

Lawrence Berkeley National Laboratory

lstolze@lbl.gov

Eoin L. Brodie, Watershed Function SFA LRM

Lawrence Berkeley National Laboratory

elbrodie@lbl.gov

Funding

This work was supported by the Watershed Function Science Focus Area project at Lawrence Berkeley National Laboratory funded by the U.S. Department of Energy, Office of Science, Biological and Environmental Research.

Publications

Stolze, L., et al. Climate Forcing Controls on Carbon Terrestrial Fluxes during Shale Weathering, Proceedings of the National Academy of Sciences, In press (2024).

Stolze, L., et al. Aerobic Respiration Controls on Shale Weathering, Geochimica and Cosmochimica Acta, 340, 172-188 (2023).