Submitted to: Chemical Geology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2024
Publication Date: 10/22/2024
Citation: Schlegel, M., Souza, J., Warix, S., Murray, E., Godsey, S., Seyfried, M.S., Cram, Z.K., Lohse, K. 2024. Estimated in-situ carbon sequestration rates in a weathered silicate basin, southwestern Idaho, U.S.A. Chemical Geology. 670. Article 1224630. https://doi.org/10.1016/j.chemgeo.2024.122460.
DOI: https://doi.org/10.1016/j.chemgeo.2024.122460

Interpretive Summary: Soil inorganic carbon (SIC) constitutes approximately 40% of terrestrial soil carbon and is an integral part of the global carbon cycle. Processes associated with weathering of basalt rock tends to sequester SIC and is known to store significant amounts of atmospheric carbon dioxide (CO2) currently and on geologic time scales. However, the global estimate of basalt CO2 sequestration is highly uncertain. At the Reynolds Creek Experimental Watershed - Critical Zone Observatory (RCEW-CZO), southwestern Idaho, USA, we estimate in-situ carbon sequestration rates in a semi-arid weathered basalt aquifer from 6 springs and 10 wells. We conclude that progressive water-rock interactions in deep groundwater of our weathered basalt system resulted in continued carbon sequestration, which may help to explain a missing carbon sink in weathered basalt basins and is significant in understanding global carbon cycles.

Technical Abstract: Silicate weathering can induce calcite precipitation from groundwater, enabling carbon dioxide (CO2) sequestration in the critical zone (CZ), which acts as a net carbon sink with significant implications for the global carbon budget. In weathered silicates, secondary calcite dissolution accompanies precipitation-dissolution reactions, and it is unclear how calcite dissolution affects CO2 consumption in natural settings. At the Reynolds Creek Experimental Watershed - Critical Zone Observatory (RCEW-CZO), southwestern Idaho, USA, we estimate in-situ carbon sequestration rates in a semi-arid weathered silicate aquifer using hydrochemical compositions and age tracers from 6 springs and 10 wells. We delineate water-rock interactions by using observed groundwater chemistry to model open system carbon evolution, evapoconcentration in wells, silicate weathering, and formation of clays along groundwater flowpaths. We suggest carbonate precipitation under closed system conditons in deep groundwater, as calcite saturation is reached and CZ CO2 drops to just 41% of initial concentrations in older waters. In a closed system, we estimate approximately 9% of CZ CO2 would precipitate, indicating that on-going water-rock interactions in our weathered silicate system appear to drive continued carbon sequestration. Carbon sequestration rates via silicate weathering may help to explain a missing carbon sink observed at the RCEW-CZO and in other weathered silicate basins.