Should alkane biomarker results be corrected for degradation effects when reconstructing vegetation changes?
2 Abteilung Bodenphysik, Universität Bayreuth
3 Lehrstuhl für Bodenökologie, Universität Bayreuth
P 1.12 in Funktion von Ökosystemen
N-alkanes with 25 to 33 carbon atoms are important constituents of plant leaf waxes and thereby typically reveal a strong odd over even predominance (OEP). The homologues C27 and C29 are characteristically dominant in most trees and shrubs, whereas C31 and C33 are dominant in grasses and herbs. With the litter-fall the cuticular alkanes are also deposited in sediments and soils, where they are regarded to be relatively resistant to degradation and therefore may serve as plant-derived biomarkers. Hence, alkane ratios like C31/C27 have been used in Quaternary studies to reconstruct vegetation changes. But in fact, a change of alkane patterns has been reported when comparing fresh litter and the soil organic matter developing from it. On the one hand, the alkane patterns are getting more balanced, i.e. dominances of C27 in trees and C31 in grasses, respectively, are reduced and the ratios C31/C27 are approximating the value 1. On the other hand, it has been shown for several litter types that the OEP values, being high in fresh plant material, are becoming lower during organic matter degradation. We therefore recently proposed to use the OEP as proxy for degradation and plotted the alkane ratios in percent against the OEP in order to obtain ‘degradation lines’ for grassland- versus forest- derived organic matter (Zech et al., 2009, Eiszeitalter und Gegenwart – Quaternary Science Journal, submitted). Accordingly, for instance for a given loess or palaeosol sample, an improved estimation of the percentage contribution of tree- versus grass-derived alkanes can be determined by calculating firstly degradation corrected end members and secondly using a two component mixing equation.