Sources and cycling of carbon in continental, serpentinite-hosted alkaline springs in the Voltri Massif, Italy

Abstract Active serpentinization systems are abundant on present-day Earth and are of increasing interest because water–rock reactions lead to alkaline, Ca–OH fluids that have the potential to sequester CO2 and form reduced chemical species (e.g. CH4 and H2) that can support chemolithoautotrophy. We present a study of alkaline (pH 10–12) springs in the Voltri Massif (Italy) that focuses on the sources and cycling of inorganic carbon in the basement rocks, the interacting fluids, and the resulting surface carbonate deposits. Most springs are located in mantle rocks that underwent varying degrees of ocean-floor serpentinization and incorporation of carbon (as carbonate and organic carbon) in the Jurassic, which is preserved during subsequent subduction and uplift onto the continent. The springs are fed by meteoric water that evolves into alkaline, Ca-rich spring waters that have 2–3 times higher Ca concentrations than the adjacent rivers. Our study is consistent with previous reaction path modeling and identifies the formation of clay minerals in serpentinites that have been altered by alkaline fluids. These strongly altered basement rocks contain up to 2 wt.% C in the shallow subsurface, also documented by the presence of late calcite veins. Concentrations of dissolved inorganic carbon (DIC) of the alkaline fluids are generally low (< 16 μmol/L) and δ13CDIC is between − 24.2‰ and + 1.3‰. We argue that the concentrations and isotopic composition of the DIC in the alkaline waters provide evidence for 1) the precipitation of calcium carbonate under closed-system conditions with respect to atmospheric CO2 and 2) removal of DIC by microbial activity in the subsurface. Late-stage uptake of atmospheric CO2 in the shallow subsurface or at the exit sites subsequent to water–rock–microbe interactions in the basement result in fluids with lower pH and Ca, and enrichment in DIC and 13C. At the surface, interaction of the high pH, Ca–OH fluids with atmospheric CO2 causes precipitation of carbonates as travertines or crusts on the basement rocks, storing CO2 as calcium carbonate. Carbonate precipitation at the exit sites is strongly dominated by kinetic processes leading to carbon and oxygen isotope compositions as low as − 27.2‰ and − 18.7‰, respectively. With increasing distance from the springs, the 13C and 18O content of the carbonate increases and the fluid pH changes towards neutral. Our study shows that surficial carbonate precipitation plays a subordinate role in carbon sequestration, but that the evolution from Mg–HCO3 spring waters to Ca–OH waters removes significant amounts of carbon, presumably in the subsurface. We calculate that the serpentinites have the capacity to sequester up to 0.50 to 2.05 × 109 mol carbon per year and conclude that they can take up significantly more CO2 than they currently contain.