Tracing carbon: natural mineral carbonation and the incorporation of atmospheric vs. recycled CO2

摘要

Mineral carbonation is a process whereby CO2 reacts with ultramafic rocks to form carbonate minerals such as calcite (CaCO3) and magnesite (MgCO3). This process can be induced artificially at high pressures and temperatures and therefore has potential to be adapted as a carbon capture and storage (CCS) technology. Large-scale surface and subsurface carbonate deposits of probable Quaternary age are associated with major faulting across the Oman-UAE ophiolite. Here, fractured rock forms a natural fluid pathway and increases the surface area available for carbonation. Modern springs along these faults typically discharge hyperalkaline (pH～11), Ca(OH)2-rich waters that precipitate carbonates on reaction with atmospheric CO2. Carbonates formed by absorption of atmospheric CO2 into Ca(OH)2 waters tend to display low δ~(13)G (-20‰ PDB) as a result of kinetic fractionation. However, ancient travertines show a large range in δ~(13)C of-10.5 to -21.8‰ PDB, produced by the mixing of the low δ~(13)C end member with other carbon sources such as limestones or organic-derived soil bicarbonate. Strontium isotope ratios of samples indicate fluids that formed calcite and magnesite veins may have interacted with limestones around and beneath the ophiolite. These are a carbon source which can easily be reworked and incorporated into carbonate deposits elsewhere. Carbonate deposits may not be created solely from atmospheric CO2, but instead represent a mixture of carbon sources. Failure to account for multiple carbon sources or recycled carbon may result in poor estimates of the rates and volumes of carbon that natural systems sequester. Further investigation is therefore necessary to determine how much of the carbon held within carbonate deposits has been incorporated from reworked sources.