Impact of wet-dry cycles on enhanced rock weathering of brucite, wollastonite, serpentinite and kimberlite: Implications for carbon verification

Abstract

Enhanced rock weathering is a proposed CO2 removal strategy for mitigating climate change; its implementation can be facilitated by improving carbon verification methods. In this study, weathering experiments exposed brucite, wollastonite skarn, serpentinite, and kimberlite residues (Venetia Diamond Mine, South Africa), to wetting and drying cycles (4/day) for 1 yr to elucidate reaction pathways and rates while evaluating carbon verification tools, including mineral quantification, total inorganic carbon (TIC), and stable and radiogenic carbon isotopes. Two primary reaction pathways were identified: A) silicate and hydroxide dissolution leading to carbonate precipitation (desirable), and B) dissolution and reprecipitation of carbonates (undesirable). The kimberlite residues, containing serpentine (19–33 wt%) and calcite (4–5 wt%), experienced variable and minor changes in total inorganic carbon. The δ13C and δ18O values of the carbonate minerals approached those expected for atmospheric-derived CO2 and are best explained by the exchange of carbonate CO2 with atmospheric CO2 (pathway A) as opposed to net CO2 sequestration. In contrast, the reaction of brucite (1.22% to 5.98%) and wollastonite skarn (0.22% to 1.01%) with atmospheric CO2 was substantial, as indicated by the increases in TIC (pathway B) yet decreases in δ13C values were inconsistent with the incorporation of atmospheric CO2 as a result of kinetic isotope fractionation due to carbonation being CO2 supply limited. The pulverized serpentinite experience insufficient carbonation to be detected by TIC. Unweathered (blue ground) and naturally weathered (yellow ground) kimberlite from the Voorspoed Diamond Mine, South Africa were compared to kimberlite residues used in experiments. Yellow ground contained twice the TIC content compared to blue ground whereas quantitative scanning electron microscopy revealed calcite veinlets that are characteristic of secondary mineralization. While the blue and yellow ground samples were essentially radiocarbon dead (0.03 and 0.05 F14C, respectively), substantial modern carbon was incorporated into the kimberlite (up to +0.27 F14C; CO2 exchange), brucite (+0.55 F14C), and wollastonite skarn (+0.53 F14C; CO2 sequestration) powders during weathering experiments. Although stable and radiogenic carbon isotopes have been used to verify the mineralization of atmospheric CO2, these methods are affected by CO2 exchange and kinetic fractionation effects during weathering that can be misleading and, thus, should not be relied on as sole methods for carbon verification.