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

RATIONALE Literature data on experimentally derived equilibrium stable carbon isotope fractionation (10 3 lnα 13 C) between H 2 CO 3 * (H 2 CO 3  + CO 2(aq) ) and gaseous CO 2 (CO 2(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 CO 2 . The objective is to improve the applicability of stable carbon isotopes as a tracer in environments where such conditions prevail. METHODS 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 CO 2(g) at pressures ( p CO 2 ) of 55 bar for durations between 2 and 188 h. The experiments were conducted at 20, 60, 80, 100 and 120 °C. H 2 CO 3 * and CO 2(g) were sampled separately and their carbon isotope ratios were determined by isotope ratio mass spectrometry. RESULTS At 20 °C, average 10 3 lnα 13 C H2CO3 * –CO2(g) values of ‐1.0 ± 0.1 ‰ were observed with a preference for 12 C in H 2 CO 3 * consistent with previous research. At elevated temperatures of 120 °C, 10 3 lnα 13 C H2CO3 * –CO2(g) values decreased to an average value of −0.7 ± 0.1 ‰. The resulting temperature dependence for carbon isotope fractionation between H 2 CO 3 * and CO 2(g) was 10 3 lnα 13 C H2CO3 * –CO2(g)  = (0.0025 ± 0.0004) T (°C) – (1.0 ± 0.03) ‰. Carbon isotope equilibrium between H 2 CO 3 * and CO 2(g) was reached within reaction times of 18 h and mostly within 5 h or less. CONCLUSIONS 10 3 lnα 13 C H2CO3 * –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 p CO 2 levels possibly shorten carbon isotope equilibration times. These data are critically important for using δ 13 C values as tracers, for instance at geological CO 2 sequestration sites and corresponding natural analogues. Copyright © 2014 John Wiley & Sons, Ltd.