The altitude effect on the isotopic composition of tropical rains

Data on the isotopic composition of yearly and monthly precipitation sample s collected at various altitudes on Mount Cameroon, Africa, and in two tran sects from the Amazon to the Altiplano in Bolivia, South America, are prese nted. 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 i n summer. The delta -gradient vs. altitude is larger in rainy periods. The isotopic d ata are fitted by using a numerical model based on Rayleigh adiabatic conde nsation process. Model simulations show that the delta -altitude relationsh ip slightly deviates from linearity, because the slope increases with altit ude due to the lowering of temperature and the consequent increase of the c ondensation rate of atmospheric vapour, The parameters which most affect th e shape of delta -altitude relationships are the temperature vertical gradi ent (lapse rate) and the initial relative humidity of the ascending air mas ses, while a change of the initial isotopic composition of water vapour det ermines a shift of the curve along the delta -axis. In addition, the model explains the observed increase of the deuterium excess with altitude. (C) 2 001 Elsevier Science B.V. All rights reserved.