Chlorofluorocarbon‐113 in the northeast Atlantic

Abstract

An automated gas Chromatographic technique to measure the concentrations of chlorofluorocarbon 113 (CFC‐113:CCl2FCClF2) dissolved in seawater has been developed. The method also quantifies chlorofluorocarbons 11 and 12 (CFC‐11:CCl3F and CFC‐12:CCl2F2). Seawater collected from Niskin bottles in ground‐glass syringes is stripped by a gas stream and concentrated on a cryogenic trap in the manner of Bullister and Weiss (1988) and Gammon et al. (1982). By isolating and heating the trap, the chlorofluorocarbon compounds are reliberated and injected onto a high‐resolution capillary gas Chromatographie column, followed by electron‐capture detection. The analysis time for each sample is less than 15 min. Surface seawater precisions are 2.9%, 2.4%, and 1.2% for CFC‐113, CFC‐11, and CFC‐12, with detection limits of 0.003–0.004, 0.02, and 0.03‐0.05 pmol L−1, respectively. Although these statistics do not compare favorably with other CFC‐11 and CFC‐12 techniques (precision ∼1%, detection limit ∼0.005 pmol L−1 (Bullister and Weiss, 1988)), the dynamic ranges of the CFC‐113:CFC‐11 and CFC‐113:CFC‐12 “ventilation ages” are 20:1, better than that of the best CFC‐11:CFC‐12 age, albeit with inferior precisions. Estimates of the solubility ratios of CFC‐113:CFC‐11 and CFC‐113:CFC‐12 are 0.303 and 1.22, disagreeing with the work of Wisegarver and Gammon (1988), whose CFC‐113 results are believed to be boosted by coelution with methyl bromide. The optimum tracer ventilation age resolution is ±0.9 years for both CFC‐113:CFC‐11 and CFC‐113:CFC‐12 from a cast considered in the northeastern Atlantic. A plot of CFC‐113:CFC‐12 ventilation age is presented on an outcropping isopycnal. A strong correlation with pressure and dissolved oxygen concentration is noted and an oxygen utilization rate between 4.2 and 5.5±0.4 μmol L−1 yr−1 is implied, depending on the choice of CFC‐113 atmospheric history.