Preferential exchange rate effect of isotopic fractionation in a melting snowpack

Stable isotope exchange processes between solid and liquid phases of a natural melting snowpack are investigated in detail by separating the liquid water from snow grains at different depths of the snowpack and collecting the bottom discharge using a lysimeter. In the melting-freezing mass exchange process between the two phases, the theoretical slope of the delta D-delta(18)O line for newly refrozen ice is calculated to be nearly that of pore water. However, based on observations of the isotopic evolution and snow grain coarsening of the snowpack, it is demonstrated that the slope of the delta D-delta(18)O line for newly refrozen ice is equal to that of the original ice. This is proved to be due to preferential water flow in the snowpack, which leads to relatively more deuterium and less oxygen-18 in the mobile water than the immobile water because of the kinetic effect. Higher mass exchange rate in the mobile water region results in excess deuterium in the bulk refrozen ice, compared with the fractionation of uniform fractionation factors and exchange rate. This effect, which is termed the 'preferential exchange rate effect of isotopic fractionation'. is shown to be larger in the lower part than the upper part of the snowpack. Copyright (C) 2008 John Wiley & Sons, Ltd.