Rapid CO2 mineralisation into calcite at the CarbFix storage site quantified using calcium isotopes.

The engineered removal of atmospheric CO2 is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, in the case of Iceland's CarbFix experiment, precipitates dissolved CO2 as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO2 injection waters to quantify the amount of carbonate precipitated, and hence CO2 stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO2 were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means.

High reactivity of deep biota under anthropogenic CO2 injection into basalt.

Basalts are recognized as one of the major habitats on Earth, harboring diverse and active microbial populations. Inconsistently, this living component is rarely considered in engineering operations carried out in these environments. This includes carbon capture and storage (CCS) technologies that seek to offset anthropogenic CO2 emissions into the atmosphere by burying this greenhouse gas in the subsurface. Here, we show that deep ecosystems respond quickly to field operations associated with CO2 injections based on a microbiological survey of a basaltic CCS site. Acidic CO2-charged groundwater results in a marked decrease (by ~ 2.5-4) in microbial richness despite observable blooms of lithoautotrophic iron-oxidizing Betaproteobacteria and degraders of aromatic compounds, which hence impact the aquifer redox state and the carbon fate. Host-basalt dissolution releases nutrients and energy sources, which sustain the growth of autotrophic and heterotrophic species whose activities may have consequences on mineral storage.

Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions.

Carbon capture and storage (CCS) provides a solution toward decarbonization of the global economy. The success of this solution depends on the ability to safely and permanently store CO2 This study demonstrates for the first time the permanent disposal of CO2 as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO2 injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. This result contrasts with the common view that the immobilization of CO2 as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years. Our results, therefore, demonstrate that the safe long-term storage of anthropogenic CO2 emissions through mineralization can be far faster than previously postulated.