Argon Isotopes in Seawater: Fractionation During Air-Water Exchange and The δ 40Ar of Seawater

We present a status report on a study of the argon 40/36 ratio in seawater. Results from laboratory experiments of the equilibrium and kinetic isotope fractionation factors during air-water exchange are used to interpret initial seawater measurements. Equilibration experiments in which fresh water was incubated with air at constant temperature and pressure resulted in δ ⁴⁰Ar values of 1.05 ± 0.01 ‰ (with respect to air at 25° C) and 1.21 ± 0.01 ‰ (2° C), which are about 0.4 ‰ greater than values for O₂ and N₂ but have the same temperature dependence. Kinetic fractionation factor measurements, in which the isotope ratio of pure argon was monitored in the head space of a reaction vessel containing initially gas-free, distilled water, indicate a fractionation factor of -5 ‰ during gas exchange - two to four times the values previously determined for O₂ and N₂. Initial measurements in the ocean at the Hawaii Ocean Time series indicate surface water values of 1.06 ± 0.02 ‰ (5 m and 23.5° C) and 1.12 ± 0.01 ‰ (4000 m and 2° C). Surface values are, within error, in equilibrium with the atmosphere, but deep values are 0.1 ‰ lighter than expected at equilibrium. It has been shown recently that Ar is undersaturated with respect to atmospheric equilibrium in the deep ocean by 1-2 %. This result is interpreted to be caused by a combination of processes that occur during deep water formation in high-latitude surface waters - rapid cooling leading to undersaturation and bubble formation causing supersaturation. One must know the importance of each mechanism to use the inert gas saturation state as a tracer for deep-water formation processes. Because of the large kinetic isotope fractionation factor, the argon isotope ratio is sensitive to diffusion across the air-water interface and may be used to separate these two mechanisms. We use a simple model to show that a deep water isotope ratio 0.1 ‰ lighter than the equilibrium value indicates that the thermally-driven degree of argon undersaturation must be about 50% greater than the measured value. To derive useful interpretation from these data one must be able to make the isotope ratio measurement to an accuracy of ± 0.01 ‰ because of the small differences between the deep water measurements and saturation values. Thus, this initial interpretation will have to be confirmed with more measurements and perhaps a better model.