The origin of Arctic precipitation under present and glacial conditions

Abstract

At low altitude locations, the deuterium excess d = δD - 8δ¹⁸O in precipitation generally varies with the season in antiphase with the δ's. In the high-altitude regions of the Greenland ice sheet, however, d in the snow varies with only a few months time lag behind the δ's. A model for d values in Greenland precipitation is developed on the basis of Rayleigh condensation/sublimation with due account taken of kinetic effects during both evaporation of sea water and sublimation. The model predicts that the initial mixing ratio w_so in precipitating air determines the slope of the d versus δ relationship at late stages of the precipitation process, and that the sea surface temperature T_s in the source area of the moisture only influences the d level. The generally high d-values in ice sheet precipitation are compatible only with high values of Wso and T_s, which suggests the subtropical part of the North Atlantic Ocean as a dominating moisture source for ice sheet precipitation. This is supported experimentally: when the model is run with monthly W_so and T_s mean values oberved at Ship E (35° N, 48° W), it reproduces the high d level, the amplitude of the seasonal d variations, and the few months phase difference between d and δ on the ice sheet. None of these features can be reproduced with a local, high-latitude moisture source.

Detailed isotope analyses of ice core increments, spanning several abrupt climatic shifts under glacial conditions, show close to present d values during the cold phases, but lower d values during the mild phases. This feature is discussed in the light of the model experiments.