Revisiting Mt. Kilimanjaro: Do n-alkane biomarkers in soils reflect the δ²H isotopic composition of precipitation?

Abstract. During the last decade compound-specific deuterium (δ²H) 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 (δ²H_prec). Recently, several authors suggested that δ²H of n-alkanes (δ²H,sub>n-alkanes) can also be used as proxy in paleoaltimetry studies.

Here we present results from a δ²H transect study (~1500 to 4000 m 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 δ²H_prec is present above ~2000 m a.s.l., i.e. δ²H_prec 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 A_h-horizons (mineral topsoils). Although our δ²H_n-alkane results are in agreement with previously published results from the southern slopes of Mt. Kilimanjaro (Peterse et al., 2009, BG, 6, 2799–2807), a major re-interpretation is required given that the δ²H_n-alkane results do not reflect the δ²H_prec results. The theoretical framework for this re-interpretation is based on the evaporative isotopic enrichment of leaf water associated with transpiration process. Modelling results show that relative humidity, decreasing considerably along the southern slopes of Mt. Kilimanjaro (from 78% at ~ 2000 m a.s.l. to 51% at 4000 m a.s.l.), strongly controls δ²H_{leaf water}. The modelled δ²H leaf water enrichment along the altitudinal transect matches well the measured ²H leaf water enrichment as assessed by using the δ²H_prec and δ²H_n-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 δ²H_prec but rather δ²H_{leaf water}, we conclude that care has to be taken not to over-interpret δ²H_n-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 δ²H_n-alkane values can completely mask altitudinally or climatically-controlled changes in δ²H_prec.