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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bgd-11-7823-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.
https://doi.org/10.5194/bgd-11-7823-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

  03 Jun 2014

03 Jun 2014

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This preprint has been withdrawn by the authors.

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

M. Zech2,1, R. Zech3, K. Rozanski4, A. Hemp5, G. Gleixner6, and W. Zech1 M. Zech et al.
  • 1Department of Soil Physics, Chair of Geomorphology and Institute of Soil Science and Soil Geography, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth, Germany
  • 2Department of Terrestrial Biogeochemistry, Martin-Luther-University Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle, Germany
  • 3Geological Institute, ETH Zurich, Sonneggstr. 5, 8092 Zurich, Switzerland
  • 4Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland
  • 5Department of Plant Systematics, University of Bayreuth, 95440 Bayreuth, Germany
  • 6Max-Planck-Institute for Biogeochemistry, Hans-Knöll-Str. 10, 07745 Jena, Germany

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 (δ2Hprec). 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 δ2Hprec is present above ~2000 m a.s.l., i.e. 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 et al., 2009, BG, 6, 2799–2807), a major 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 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 δ2Hleaf water. The modelled δ2H 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.

This preprint has been withdrawn.

M. Zech et al.

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M. Zech et al.

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