Stable carbon isotope fractionation data between H(2)CO(3)(*) and CO(2)(g) extended to 120 °C

Literature data on experimentally derived equilibrium stable carbon isotope fractionation (10(3) lnα(13) C) between H2 CO3 (*) (H2 CO3  + CO2(aq) ) and gaseous CO2 (CO2(g) ) are so far only available up to 60 °C and were typically determined at or near atmospheric pressures. Here we experimentally expand this dataset to temperature and pressure conditions close to the supercritical state for CO2 . The objective is to improve the applicability of stable carbon isotopes as a tracer in environments where such conditions prevail.

Eighteen stable carbon isotope laboratory experiments were conducted in a steel vessel. Deionised water that was acidified with hydrochloric acid (HCl, 1 N) to a pH of 2.4 was equilibrated with CO2(g) at pressures (pCO2 ) of 55 bar for durations between 2 and 188 h. The experiments were conducted at 20, 60, 80, 100 and 120 °C. H2 CO3 (*) and CO2(g) were sampled separately and their carbon isotope ratios were determined by isotope ratio mass spectrometry.

At 20 °C, average 10(3) lnα(13) CH2CO3 * -CO2(g) values of -1.0 ± 0.1 ‰ were observed with a preference for (12) C in H2 CO3 (*) consistent with previous research. At elevated temperatures of 120 °C, 10(3) lnα(13) CH2CO3 * -CO2(g) values decreased to an average value of -0.7 ± 0.1 ‰. The resulting temperature dependence for carbon isotope fractionation between H2 CO3 (*) and CO2(g) was 10(3) lnα(13) CH2CO3 * -CO2(g)  = (0.0025 ± 0.0004) T(°C) - (1.0 ± 0.03) ‰. Carbon isotope equilibrium between H2 CO3 (*) and CO2(g) was reached within reaction times of 18 h and mostly within 5 h or less.

10(3) lnα(13) CH2CO3 * -CO2(g) data are now available for temperatures up to 120 °C and for pressures of up to 55 bar. The results suggest that higher pCO2 levels possibly shorten carbon isotope equilibration times. These data are critically important for using δ(13) C values as tracers, for instance at geological CO2 sequestration sites and corresponding natural analogues.