The altitude effect on the isotopic composition of tropical rains

Data on the isotopic composition of yearly and monthly precipitation samples collected at various altitudes on Mount Cameroon, Africa, and in two transects from the Amazon to the Altiplano in Bolivia, South America, are presented. In Bolivia, the H-2/H-1 and O-18/O-16 ratios show seasonal variations, with lower values in the summer rainy months with respects to the winter dry ones. The delta H-2 and delta O-18 values are linearly correlated with a slope of 7.5 in all seasons. but the intercept is higher in winter than in summer. The delta -gradient vs. altitude is larger in rainy periods. The isotopic data are fitted by using a numerical model based on Rayleigh adiabatic condensation process. Model simulations show that the delta -altitude relationship slightly deviates from linearity, because the slope increases with altitude due to the lowering of temperature and the consequent increase of the condensation rate of atmospheric vapour, The parameters which most affect the shape of delta -altitude relationships are the temperature vertical gradient (lapse rate) and the initial relative humidity of the ascending air masses, while a change of the initial isotopic composition of water vapour determines a shift of the curve along the delta -axis. In addition, the model explains the observed increase of the deuterium excess with altitude. (C) 2001 Elsevier Science B.V. All rights reserved.