The origin of Arctic precipitation under present and glacial conditions

At low altitude locations, the deuterium excess d = delta D - 8 delta O-18 in precipitation generally varies with the season in antiphase with the delta'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 delta'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 delta 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 w(so) 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 degrees N, 48 degrees W), it reproduces the high d level, the amplitude of the seasonal d variations, and the few months phase difference between d and delta 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.