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

The use of the noble gas isotope ratios Ne-22/Ne-20 and Ar-40/Ar-36 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 (alpha(k) and alpha(eq)). 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 degrees C during exchange across the air water interface as gas saturations evolved from 0 to 100%. The kinetic isotopic fractionation factors, alpha(k), for argon (40/36) and neon (22/20) are determined to be 0.9961 +/- 0.0001 (epsilon(k) = -3.9 +/- 0.1 parts per thousand) and 0.9931 +/- 0.0004 (epsilon(k) = -6.9 +/- 0.4 parts per thousand), 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 degrees C is determined to be 1.00104 +/- 0.00005 (epsilon(eq) = 1.04 +/- 0.05 parts per thousand). An argon equilibrium fractionation factor of 1.00107 +/- 0.00002 (epsilon(eq) = 1.07 +/- 0.02 parts per thousand) determined at 20 degrees C agrees with a linear interpolation of previous measured values at 2 degrees C and 25 degrees C (Nicholson et al., 2010). Published by Elsevier B.V.