Spatial and temporal ²H and ¹⁸O isotope variation of contemporary precipitation in the Bale Mountains, Ethiopia^*

ABSTRACT

East Africa is an underrepresented region in respect of monitoring the stable isotopic composition of precipitation (δ¹⁸O_prec and δ²H_prec). In 2017, we collected precipitation samples from ten weather stations located along an altitudinal transect ranging from 1304 to 4375 m a.s.l. The δ¹⁸O_prec and δ²H_prec values varied from –8.7 to +3.7 ‰ and –38 to +29 ‰, respectively. The local meteoric water line is characterised by a lower slope, a higher intercept and more positive d-excess values (δ²H = 5.3 ± 0.2 * δ¹⁸O + 14.9 ± 0.9) compared to the global meteoric water line. Both altitude and amount of precipitation clearly correlate with our isotope data. However, the δ¹⁸O_prec and δ²H_prec values show at the same time a seasonal pattern reflecting rainy versus dry season. More enriched isotope values prevailed shortly after the end of the dry season; more depleted isotope values coincided with high precipitation amounts recorded in May, August and September. Moreover, HYSPLIT trajectories reveal that during the dry season water vapour originates primarily from the Arabian Sea, whereas during the wet season it originates primarily from the Southern Indian Ocean. These findings challenge the traditional amount effect interpretation of paleoclimate isotope records from East Africa and rather point to a previously underestimated source effect.

KEYWORDS:

• Ethopia
• hydrogen-2
• isotope hydrology
• meteoric water line
• natural isotope variations
• oxygen-18
• precipitation
• seasonal effect
• weather

Acknowledgments

We are grateful to the Ministry of Water, Irrigation and Electricity, Ethiopia; Ethiopian Wildlife Conservation Authority, Bale Mountains National Park and Addis Ababa University for providing access permit to the rain water, access to the Bale Mountains National Park and for the scientific collaboration, respectively. We also extend our gratitude to Prof. G. Gebauer and his team from the Isotope Biogeochemistry Group of Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, for water isotope analyses. We are also indebted to Terefe Endale, Geremew Mebratu, Mekbib Fekadu, Falk Hänsel and members of the central committee of the DFG research unit 2358 for their support with establishing the rain gauges, monitoring and collecting precipitation samples. We kindly thank two anonymous reviewers and the Editor-in-Chief Dr Gerhard Strauch for constructive comments and feedback.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

* Originally presented at the IAEA Symposium on Isotope Hydrology 2019, Vienna, Austria, 20–24 May 2019.

Additional information

Funding

This research work funded by the German Research Foundation within the DFG Research Unit ‘The Mountain Exile Hypothesis’ [DFG GL327/18–1, ZE844/10–1]. Mr. Bruk Lemma expresses his gratitude to the Catholic Academic Exchange Services (KAAD, Germany) for the PhD fellowship.