Abstract:
East Africa is an underrepresented region in respect of monitoring
the stable isotopic composition of precipitation (δ18Oprec and
δ2Hprec). 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 δ18Oprec and δ2Hprec 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 δ18Oprec and δ2Hprec 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.
