Geochemical controls on ultramafic carbon sequestration and mineralization. A case study at three terrestrial ultramafic sites

Natural CO 2 sequestration processes, such as those occurring within obducted ultramafic rocks, represent an important part of the global carbon cycle. However, the amount of CO 2 consumed via the weathering and low temperature serpentinization of ultramafic rocks is not well quantified. This study measured the CO 2 gas flux at various ultra-basic groundwater springs and basic surface waters, portals into subsurface weathering processes, located in terrestrial ultramafic environments. Three terrestrial ultramafic bodies, the Tablelands (Newfoundland), The Cedars (California), and Aqua de Ney (California), were selected for this study as they represent ophiolites of different lithology and extents of serpentinization, actively contributing to natural CO 2 sequestration. At the three sites, the rate of CO 2 sequestration was measured from 5 groundwater discharge locations and 3 surface waters ( i.e., creeks and brooks). The rate dependence of CO 2 dissolution strongly correlated with hydroxide ion concentration ( i.e., pOH), such that the CO 2 flux ( F CO2 ) had a rate constant ( k ) of 1 × 10 −3 and a reaction order ( n ) of 0.44. Strong correlations also existed between CO 2 flux and Mg, Na, Cl, and total dissolved solids (TDS). Upscaling experimental results indicate that at the air-water interface relatively small ultra-basic pools ( i.e., 1 m 2 ) sequester CO 2 on the order of kilograms annually. This quantity of CO 2 is likely much greater in reality as the system extends into the subsurface. Although the quantity of CO 2 measured in this study is negligible on an atmospheric scale, these pools nonetheless act as valuable windows into CO 2 sequestration processes at the site-wide scale.