Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

This paper discusses the factors controlling the variations of heavy isotope contents of 57 rain events collected throughout the rain forest regions of Cameroon, Central Africa, during the great rainy season (August–November) of 1994. Our data display a wide range of values, from +1.69 to −10.80‰ for 18O and from +23.2 to −85.5‰ for 2H. The data follow closely the global meteoric water line suggesting that rain formation processes occurred under isotopic equilibrium conditions between both the condensate and the corresponding vapour. This also indicates that the observed heavy isotope contents have not been altered by evaporation during the descent of raindrops to the ground. The low heavy-isotope contents observed for some precipitation events are assumed to be controlled by the amount of rainfall and/or by the low condensation temperature. The former effect is related to the northward passage of the intertropical convergence zone (ITCZ) over the region under study, whereas the second effect is linked to high altitudes of condensation. Moreover, the latitudinal variations of the weighted mean 18O contents indicates that in coastal areas (0–230 km inland from the Atlantic coast) the stable isotopic compositions of precipitation are controlled by a continuous extraction from the atmospheric reservoir of vapour as rains according to the Rayleigh distillation model, whereas at distances >230 km, recycling of continental moisture is thought to have influenced the isotopic compositions of rains.