Kinetic isotopic fractionation of argon and neon during air–water gas transfer

The use of the noble gas isotope ratios 22Ne/20Ne and 40Ar/36Ar as geochemical tracers of environmental conditions (i.e. continental paleotemperatures from groundwater) and processes (i.e. air–sea gas exchange) requires accurate values for kinetic and equilibrium isotopic fractionations (αk and αeq). Until now, these values have been approximated using different theoretical models. Here we evaluated both the kinetic and equilibrium fractionation factors experimentally by measuring the relative mass transfer rates of the major and secondary isotopes of argon and neon in laboratory experiments at 20 °C during exchange across the air–water interface as gas saturations evolved from 0 to 100%. The kinetic isotopic fractionation factors, αk, for argon (40/36) and neon (22/20) are determined to be 0.9961 ± 0.0001 (εk = − 3.9 ± 0.1‰) and 0.9931 ± 0.0004 (εk = − 6.9 ± 0.4‰), respectively. The kinetic isotopic fractionation factors are much closer in agreement to theoretical values determined by molecular dynamics simulations than to values calculated using the square root of molecular reduced masses. The equilibrium fractionation factor for neon (22/20) at 20 °C is determined to be 1.00104 ± 0.00005 (εeq = 1.04 ± 0.05‰). An argon equilibrium fractionation factor of 1.00107 ± 0.00002 (εeq = 1.07 ± 0.02‰) determined at 20 °C agrees with a linear interpolation of previous measured values at 2 °C and 25 °C (Nicholson et al., 2010).