Jean-Claude Olivry

The altitude effect on the isotopic composition of tropical rains

Abstract 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 2H/1H and 18O/16O ratios show seasonal variations, with lower values in the summer rainy months with respects to the winter dry ones. The δ2H and δ18O values are linearly correlated with a slope of 7.5 in all seasons, but the intercept is higher in winter than in summer. The δ-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 δ–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 δ–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 δ-axis. In addition, the model explains the observed increase of the deuterium excess with altitude.

Évolution spatio-temporelle des teneurs isotopiques (δ2H–δ18O) des eaux de la cuvette lacustre du fleuve Niger (Mali)

Abstract The Niger inland delta is a flooded zone of major economic importance. Evaporation is higher than precipitation and the flooded surfaces depend on Niger and Bani Rivers’ inflow. Rainwater data pointed to an isotopic variability due to various climatic conditions and rainfall origins. In the upper basin, values are influenced by groundwater, sub-surface storage and direct runoff in turn affected by high evaporation rates during the low water stage. Because of low hydraulic gradient, the isotopic variations are dominated by evaporation. Nevertheless, the spatial evolution is not as important as expected. A vapour re-equilibration with the river surface can reduce isotope fractionation.