δ D values of individual n-alkanes from terrestrial plants along a climatic gradient - Implications for the sedimentary biomarker record

The compound-specific hydrogen isotope composition (delta D) of sedimentary n-alkanes is increasingly used as a palaeo-climate proxy. To explore the origin of the hydrogen isotope signal of n-alkanes from higher terrestrial plant biomass in sedimentary organic matter of small lakes we have analysed the n-alkane composition and delta D values of 31 plant biomass samples (Birch (Betula), Beech (Fagus), Oak (Quercus), Alder (Alnus), Hombeam (Carpinus) and Myrtle (Myrtus) as well as non-stomata containing Sphagnum, Cladonia and moss) from 14 sites along a climatic gradient from Northern Finland to Southern Italy. Eleven of the sites were previously investigated for the n-alkane delta D values in adjacent lake sediments and are compared to the leaf wax n-alkane delta D values. The distribution of terrestrial n-alkanes in lake sediments was not found to be characteristic of the dominant vegetation around the lake. Evidently, n-alkane distributions within the same species at different sites were found to be variable. These findings complicate the use of certain n-alkane distributions in sediments as a palaeoenvironmental indicator. delta D values of plant biomass n-alkanes show a significant positive correlation with the precipitation delta D values. Biomass n-alkane delta D values are in general less negative than the associated sedimentary n-alkanes of terrestrial origin, possibly due to the stronger evapotranspiration during the summer, when sampling took place. This suggests, that n-alkanes in leaves show seasonal variations and the sedimentary organic matter receives the lighter isotope signal of the autumn, when most leaves are incorporated into the sediment. The variability of the delta D values from non-stomata containing plants is much higher than for broadleaf trees, since stomata can regulate water loss through evapotranspiration. The average chain length (ACL) of n-alkanes from deciduous tree leaves increases from North to South. With increasing ACL the hydrogen isotopic fractionation (epsilon) between source water and n-alkanes also increases. Our results prove that terrestrial n-alkane delta D values record the delta D value of precipitation, modified by the amount of isotope enrichment in the leaf water due to plant anatomy (stomata vs. no stomata) and site specific meteorological conditions, such as evapotranspiration, relative humidity and soil moisture. Therefore, terrestrial n-alkanes in lake sediments can deliver information on changes in these parameters over geological timescales. In lake sediments where aquatic n-alkanes, such as n-C-17 record the lake water delta D value, the isotopic difference between terrestrial and aquatic n-alkanes could therefore be used to reconstruct changes in evapotranspiration. (c) 2005 Elsevier Ltd. All rights reserved. [References: 32