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Zusammenfassung:
During the last decade compound-specific deuterium (2H) analysis of plant leaf wax-derived n-alkanes
has become a promising and popular tool in paleoclimate research. This is based on the widely accepted
assumption that n-alkanes in soils and sediments generally reflect δ2H of precipitation (δ2Hprec). Recently,
several authors suggested that δ2H of n-alkanes (δ2Hn-alkanes) can also be used as a proxy in paleoaltimetry
studies. Here, we present results from a δ2H transect study (∼1500 to 4000 m above sea level [a.s.l.])
carried out on precipitation and soil samples taken from the humid southern slopes of Mt. Kilimanjaro.
Contrary to earlier suggestions, a distinct altitude effect in δ2Hprec is present above ∼2000 m a.s.l., that
is, δ2Hprec values become more negative with increasing altitude. The compound-specific δ2H values of
nC27 and nC29 do not confirm this altitudinal trend, but rather become more positive both in the O-layers
(organic layers) and the Ah-horizons (mineral topsoils). Although our δ2Hn-alkane results are in agreement
with previously published results from the southern slopes of Mt. Kilimanjaro [Peterse F, van der
Meer M, Schouten S, Jia G, Ossebaar J, Blokker J, Sinninghe Damsté J. Assessment of soil n-alkane
δD and branched tetraether membrane lipid distributions as tools for paleoelevation reconstruction. Biogeosciences.
2009;6:2799–2807], a re-interpretation is required given that the δ2Hn-alkane results do not
reflect the δ2Hprec results. The theoretical framework for this re-interpretation is based on the evaporative
isotopic enrichment of leaf water associated with the transpiration process. Modelling results show
that relative humidity, decreasing considerably along the southern slopes of Mt. Kilimanjaro (from 78 %
in ∼2000 m a.s.l. to 51 % in 4000 m a.s.l.), strongly controls δ2Hleaf water. The modelled 2H leaf water
enrichment along the altitudinal transect matches well the measured 2H leaf water enrichment as assessed
by using the δ2Hprec and δ2Hn-alkane results and biosynthetic fractionation during n-alkane biosynthesis
in leaves. Given that our results clearly demonstrate that n-alkanes in soils do not simply reflect δ2Hprec
but rather δ2Hleaf water, we conclude that care has to be taken not to over-interpret δ2Hn-alkane records from
soils and sediments when reconstructing δ2H of paleoprecipitation. Both in paleoaltimetry and in paleoclimate
studies changes in relative humidity and consequently in δ2Hn-alkane values can completely mask altitudinally or climatically controlled changes in δ2Hprec.
