Effect of leaf litter degradation and seasonality on D/H isotope ratios of n-alkane biomarkers

Abstract

Abstract During the last decade, compound-specific hydrogen isotope analysis of plant leaf-wax and sedimentary n -alkyl lipids has become a promising tool for paleohydrological reconstructions. However, with the exception of several previous studies, there is a lack of knowledge regarding possible effects of early diagenesis on the δD values of n -alkanes. We therefore investigated the n -alkane patterns and δD values of long-chain n -alkanes from three different C3 higher plant species ( Acer pseudoplatanus L., Fagus sylvatica L. and Sorbus aucuparia L.) that have been degraded in a field leaf litterbag experiment for 27 months. We found that after an initial increase of long-chain n -alkane masses (up to ∼50%), decomposition took place with mean turnover times of 11.7 months. Intermittently, the masses of mid-chain n -alkanes increased significantly during periods of highest total mass losses. Furthermore, initially high odd-over-even predominances (OEP) declined and long-chain n -alkane ratios like n -C 31 /C 27 and n -C 31 /C 29 started to converge to the value of 1. While bulk leaf litter became systematically D-enriched especially during summer seasons (by ∼8‰ on average over 27 months), the δD values of long-chain n -alkanes reveal no systematic overall shifts, but seasonal variations of up to 25‰ ( Fagus , n -C 27 , average ∼13‰). Although a partly contribution by leaf-wax n -alkanes by throughfall cannot be excluded, these findings suggest that a microbial n- alkane pool sensitive to seasonal variations of soil water δD rapidly builds up. We propose a conceptual model based on an isotope mass balance calculation that accounts for the decomposition of plant-derived n -alkanes and the build-up of microbial n -alkanes. Model results are in good agreement with measured n -alkane δD results. Since microbial ‘contamination’ is not necessarily discernible from n -alkane concentration patterns alone, care may have to be taken not to over-interpret δD values of sedimentary n -alkanes. Furthermore, since leaf-water is generally D-enriched compared to soil and lake waters, soil and water microbial n -alkane pools may help explain why soil and sediment n -alkanes are D-depleted compared to leaves.