Abstract

Enhanced rock weathering (ERW) is a negative emissions technology with the potential to remove gigatonnes of atmospheric carbon dioxide (CO₂). Accurate carbon accounting is challenging due to the slow reaction of CO₂ with Mg- and Ca-silicates, storage of carbon as either a soluble or solid phase, and spatial and temporal variabilities in soil carbon content in large open systems. Our field study aims to develop a scalable methodology for measuring CO₂ sequestration rates through ERW of kimberlite mine residues from De Beers’ Gahcho Kué Diamond Mine (Canada). These mine wastes contained 30.2 wt.% lizardite [Mg₃Si₂O₅(OH)₄] and 9.4 wt.% forsterite (Mg₂SiO₄), nutrients including K (2.12%) and P (0.12%), and numerous trace elements, e.g., Ba, Ni, Cr, Sr, and Co (1171, 1151, 785, 471, 69 ppm, respectively). Field trials (1 m²) involved mixing 10 and 20 kg of kimberlite residues (denoted K10 and K20, respectively) into local soil, followed by monitoring porewater chemistry, CO₂ fluxes, CO₂ pore gas concentrations, and soil moisture for 6 months (June to November, 2021). Porewaters from the K20 plot were distinguishable from the control soil plot, characterized by higher concentrations of major (Mg, Si, K, Na) and trace (Ba, Ni, Sr) elements, as well as greater alkalinity (8.5 ± 1.5 vs. 6.6 ± 0.9 mmol of HCO₃^-/L). Plots only had negative CO₂ fluxes on the day of amendment, removing up to 5.6 kg CO₂/m²/yr. Thereafter, CO₂ fluxes became positive and indistinguishable from the control plot (0.3–13.5 kg CO₂/m²/yr). In May 2022, 30 kg of kimberlite was added to the K10 plot. This plot emitted CO₂ at a rate of 8.9 kg CO₂/m²/yr prior to kimberlite addition, 1.9 kg CO₂/m²/yr immediately after amendment, and remained positive for the rest of the experiment (2.4–10.1 kg CO₂/m²/yr). Although greater alkalinity demonstrates solubility trapping of CO₂, the amended soils continued to have positive fluxes. These results suggest that CO₂ stored in waters originate from soil respiration rather than atmospheric sources, particularly given that CO₂ concentrations in the soil pore space (0.4–1.9%) were up to ~40 times greater than atmospheric levels. This raises the question of whether ERW is a negative emissions or a CO₂ avoidance technology, though both are beneficial. Results from this ongoing research will aid in establishing geochemical proxies that allow for reliable carbon monitoring and accounting, which is crucial for the implementation of ERW.