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 (δ²H_n-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 δ²H_n-alkanes–δ¹⁸O_sugar biomarker approach. This coupled approach allows for calculating (i) biomarker-based "reconstructed" δ²Hδ¹⁸O values of leaf water (δ²Hδ¹⁸O_{leaf 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 δ²Hδ¹⁸O_{precipitation} 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 δ²Hδ¹⁸O_{precipitation} values correlate highly significantly with actual RH and δ²Hδ¹⁸O_{precipitation} values. We conclude that compared to single δ²H_n-alkane or δ¹⁸O_sugar records, the proposed coupled δ²H_n-alkane–δ¹⁸O_sugar biomarker approach will allow more robust δ²Hδ¹⁸O_{precipitation} reconstructions in future paleoclimate research. Additionally, the proposed coupled δ²H_n-alkane–δ¹⁸O_sugar biomarker approach allows for the establishment of a "paleohygrometer", more specifically, the reconstruction of mean summer daytime RH changes/history.