Stable isotope composition of tropical high-altitude fresh-waters on Mt. Kenya, Equatorial East Africa

Abstract

The stable isotope composition of equatorial high-altitude precipitation and surface-water bodies has been studied on Mt. Kenya (0°10′S; 37°20′E) in East Africa during July 1997. Regarding the local hydrology, glacier meltwater was identified as the main source of replenishment for the lakes, while direct precipitation and runoff appear to have a negligible contribution during this period of the year. Two through-flow lakes exhibit nearly no evaporative isotopic enrichment relative to the inflow isotopic composition, while two longer residence-time lakes display an oxygen and hydrogen isotopic build-up of 4.0–7.0‰ and 18.0–34.0‰, respectively. The data indicate that stream waters are composed of a mixture of high-altitude glacier meltwater and rainfall. The isotopic composition of an ice-sample obtained directly from the margin of a glacier is enriched relative to the average composition of the glacier, suggesting mixing with local rain. Samples of ground-frost crystals have an isotopic composition similar to the rain or slightly enriched. In addition, the results show the lack of a significant “altitude effect” on the mountain. This study demonstrates the importance of the characterization of local hydrological settings when interpreting lacustrine equatorial isotopic records of past climate and confirms that for East Africa, the total range in oxygen and hydrogen isotopic composition of meteoric waters is smaller than in high-latitudes.