Stable carbon isotope fractionation data between H2CO3* and CO2(g) extended to 120 degrees C

作者：Myrttinen A^*; Becker V; Mayer B; Barth J A C

来源：Rapid Communications in Mass Spectrometry, 2014, 28(15): 1691-1696.

DOI：10.1002/rcm.6950

RATIONALE: Literature data on experimentally derived equilibrium stable carbon isotope fractionation (10(3)ln alpha C-13) between H2CO3* (H2CO3 + CO2(aq)) and gaseous CO2 (CO2(g)) are so far only available up to 60 degrees 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. %26lt;br%26gt;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 CO2(g) at pressures (pCO(2)) of 55 bar for durations between 2 and 188 h. The experiments were conducted at 20, 60, 80, 100 and 120 degrees C. H2CO3* and CO2(g) were sampled separately and their carbon isotope ratios were determined by isotope ratio mass spectrometry. %26lt;br%26gt;RESULTS: At 20 degrees C, average 10(3)ln alpha C-13(H2CO3*-CO2(g)) values of -1.0 +/- 0.1 parts per thousand were observed with a preference for C-12 in H2CO3* consistent with previous research. At elevated temperatures of 120 degrees C, 10(3)ln alpha C-13(H2CO3*-CO2(g)) values decreased to an average value of -0.7 +/- 0.1 parts per thousand. The resulting temperature dependence for carbon isotope fractionation between H2CO3* and CO2(g) was 10(3)ln alpha C-13(H2CO3*-CO2(g)) = (0.0025 +/- 0.0004) T(degrees C) - (1.0 +/- 0.03) parts per thousand. Carbon isotope equilibrium between H2CO3* and CO2(g) was reached within reaction times of 18 h and mostly within 5 h or less. %26lt;br%26gt;CONCLUSIONS: 10(3)ln alpha C-13(H2CO3*-CO2(g)) data are now available for temperatures up to 120 degrees C and for pressures of up to 55 bar. The results suggest that higher pCO(2) levels possibly shorten carbon isotope equilibration times. These data are critically important for using delta C-13 values as tracers, for instance at geological CO2 sequestration sites and corresponding natural analogues.

• 出版日期2014-8-15