Do n-alkane biomarkers in soils/sediments reflect the δ2H isotopic composition of precipitation? A case study from Mt. Kilimanjaro and implications for paleoaltimetry and paleoclimate research

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 δ ²H of precipitation (δ ²H ₚᵣₑc). Recently, several authors suggested that δ ²H of n -alkanes (δ ²H ₙ₋ₐₗₖₐₙₑₛ) can also be used as a proxy in paleoaltimetry studies. Here, we present results from a δ ²H 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 δ ²H ₚᵣₑc is present above ∼2000 m a.s.l., that is, δ ²H ₚᵣₑc values become more negative with increasing altitude. The compound-specific δ ²H values of n C ₂₇ and n C ₂₉ 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 δ ²H ₙ₋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 δ D and branched tetraether membrane lipid distributions as tools for paleoelevation reconstruction. Biogeosciences. 2009;6:2799–2807], a re-interpretation is required given that the δ ²H ₙ₋ₐₗₖₐₙₑ results do not reflect the δ ²H ₚᵣₑc 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 δ ²H ₗₑₐf wₐₜₑᵣ. The modelled ²H leaf water enrichment along the altitudinal transect matches well the measured ²H leaf water enrichment as assessed by using the δ ²H ₚᵣₑc and δ ²H 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 δ ²H ₚᵣₑc but rather δ ²H ₗₑₐf wₐₜₑᵣ, we conclude that care has to be taken not to over-interpret δ ²H ₙ₋ₐₗₖₐₙₑ records from soils and sediments when reconstructing δ ²H of paleoprecipitation. Both in paleoaltimetry and in paleoclimate studies changes in relative humidity and consequently in δ ²H ₙ₋ₐₗₖₐₙₑ values can completely mask altitudinally or climatically controlled changes in δ ²H ₚᵣₑc.