Coupling δ²H and δ¹⁸O biomarker results yields information on relative humidity and isotopic composition of precipitation – a climate transect validation study

Abstract

The hydrogen isotopic composition (δ2H) of leaf waxes, especially of n-alkanes (δ2Hn-alkanes), is increasingly used for paleohydrological and paleoclimate reconstructions. However, it is challenging to disentangle past changes in the isotopic composition of precipitation and changes in evapotranspirative enrichment of leaf water, which are both recorded in leaf wax δ2H values. In order to overcome this limitation, Zech M. et al. (2013) proposed a coupled δ2Hn-alkanes–δ18Osugar biomarker approach. This coupled approach allows for calculating (i) biomarker-based "reconstructed" δ2Hδ18O values of leaf water (δ2Hδ18Oleaf water), (ii) biomarker-based reconstructed deuterium excess (d-excess) of leaf water, which mainly reflects evapotranspirative enrichment and which can be used to reconstruct relative air humidity (RH) and (iii) biomarker-based reconstructed δ2Hδ18Oprecipitation values. Here we present a climate transect validation study by coupling new results from δ2H analyses of n-alkanes and fatty acids in topsoils along a climate transect in Argentina with previously measured δ18O results obtained for plant-derived sugars. Accordingly, both the reconstructed RH and δ2Hδ18Oprecipitation values correlate highly significantly with actual RH and δ2Hδ18Oprecipitation values. We conclude that compared to single δ2Hn-alkane or δ18Osugar records, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach will allow more robust δ2Hδ18Oprecipitation reconstructions in future paleoclimate research. Additionally, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach allows for the establishment of a "paleohygrometer", more specifically, the reconstruction of mean summer daytime RH changes/history.