Stable carbon isotope fractionation data between H2CO3* and CO2(g) extended to 120 °C. (18th June 2014)

• Record Type: Journal Article
• Authors: Myrttinen, A.; Becker, V.; Mayer, B.; Barth, J. A. C.
• Is Part Of: Rapid communications in mass spectrometry. Volume 28:Number 15(2014)
• Page Start: 1691
• Page End: 1696
• Publication Date: 2014-06-18
• Subjects: Mass spectrometry -- Periodicals; 543.65
• DOI: 10.1002/rcm.6950
• Languages: English
• ISSNs: 0951-4198

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <sec id="rcm6950-sec-0001" sec-type="section"> <title>RATIONALE</title> <p>Literature data on experimentally derived equilibrium stable carbon isotope fractionation (10<sup>3</sup>lnα<sup>13</sup>C) between H<sub>2</sub>CO<sub>3</sub><sup>*</sup> (H<sub>2</sub>CO<sub>3</sub> + CO<sub>2(aq)</sub>) and gaseous CO<sub>2</sub> (CO<sub>2(g)</sub>) 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<sub>2</sub>. The objective is to improve the applicability of stable carbon isotopes as a tracer in environments where such conditions prevail.</p> </sec> <sec id="rcm6950-sec-0002" sec-type="section"> <title>METHODS</title> <p>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<sub>2(g)</sub> at pressures (<italic>p</italic>CO<sub>2</sub>) of 55 bar for durations between 2 and 188 h. The experiments were conducted at 20, 60, 80, 100 and 120 °C. H<sub>2</sub>CO<sub>3</sub><sup>*</sup> and CO<sub>2(g)</sub> were sampled separately and their carbon isotope ratios were determined by isotope ratio mass spectrometry.</p> </sec> <sec id="rcm6950-sec-0003" sec-type="section"> <title>RESULTS</title> <p>At 20 °C, average 10<sup>3</sup>lnα<sup>13</sup>C<sub>H2CO3</sub><sub>*</sub><sub>–CO2(g)</sub> values of ‐1.0 ± 0.1 ‰ were observed with a preference for <sup>12</sup>C in H<sub>2</sub>CO<sub>3</sub><sup>*</sup> consistent with previous research. At elevated temperatures of 120 °C, 10<sup>3</sup>lnα<sup>13</sup>C<sub>H2CO3</sub><sub>*</sub><sub>–CO2(g)</sub> values decreased to an average value of −0.7 ± 0.1 ‰. The resulting temperature dependence for carbon isotope fractionation between H<sub>2</sub>CO<sub>3</sub><sup>*</sup> and CO<sub>2(g)</sub> was 10<sup>3</sup>lnα<sup>13</sup>C<sub>H2CO3</sub><sub>*</sub><sub>–CO2(g)</sub> = (0.0025 ± 0.0004) <italic>T</italic>(°C) – (1.0 ± 0.03) ‰. Carbon isotope equilibrium between H<sub>2</sub>CO<sub>3</sub><sup>*</sup> and CO<sub>2(g)</sub> was reached within reaction times of 18 h and mostly within 5 h or less.</p> </sec> <sec id="rcm6950-sec-0004" sec-type="section"> <title>CONCLUSIONS</title> <p>10<sup>3</sup>lnα<sup>13</sup>C<sub>H2CO3</sub><sub>*</sub><sub>–CO2(g)</sub> data are now available for temperatures up to 120 °C and for pressures of up to 55 bar. The results suggest that higher <italic>p</italic>CO<sub>2</sub> levels possibly shorten carbon isotope equilibration times. These data are critically important for using δ<sup>13</sup>C values as tracers, for instance at geological CO<sub>2</sub> sequestration sites and corresponding natural analogues. Copyright © 2014 John Wiley & Sons, Ltd.</p> </sec> </abstract>