A kinetic isotope effect during ice formation by water freezing

Abstract

A kinetic isotope effect is known to occur during ice formation from water vapour in a cloud ; it is due to the difference in molecular diffusivities in air of HDO and H218O molecules. A similar effect is likely during water freezing since diffusion coefficients of HDO and H218O are also different in liquid water. Their values are however less different from each other than those in air. Therefore, such a kinetic isotope effect during water freezing is less frequently observed in Nature. This paper describes a situation in Antarctica where this effect is conspicuous in icings (aufeis). In this type of ice indeed there is no relationship between δD (or δ18O) and deuterium excess whereas a clear inverse relationship between these parameters exists in ice formed by water freezing when equilibrium isotopic fractionation applies. This kinetic effect is potentially present in hail within clouds, in infiltration ice (formed during sea ice growth) and in some kinds of ground ice.