Environmental and biochemical controls on the molecular distribution and stable isotope composition of leaf wax biomarkers
Leaf wax n-alkyl lipids are increasingly used as proxies in palaeoclimate studies. Palaeovegetation assemblages are reconstructed from their molecular distribution patterns, while their δ13C and δ2H signals are thought to reflect plant-environment interactions and palaeohydrological shifts, respectively. Such applications depend, however, upon these compounds faithfully recording environmental conditions. To explore the influence of environmental, physical and biochemical controls on n-alkane composition, leaf waxes from seven UK saltmarsh plants were analysed over two growing seasons. Linked analysis of sedimentary n-alkanes enabled further investigation of leaf wax biomarker integration into saltmarsh sediments. The molecular distribution and concentration of n-alkanes from the saltmarsh plants varied significantly. Bulk and n-alkane δ13C recorded different seasonal shifts, with a range of up to 13‰ in the offset between bulk and n-alkane 13C/12C values. This indicated that post-photosynthetic 13C/12C fractionation may be an important additional control on n-alkane δ13C signals. n-Alkane δ2H also varied among the sampled species by >100‰, and could not be explained by physical processes controlling the movement of water inside/outside and within leaves. Comparison with the 2H/1H of chloroplast-synthesised compounds (fatty acids, phytol) suggested these differences instead reflected the varied biochemical mechanisms operating in the chloroplast and cytosol. Sedimentary biomarker analysis further highlighted that small/moderate vegetation change could drive shifts of ~40‰ in sedimentary nalkane 2H/1H, while using globally averaged “typical” values to correct for fractionation between source water and n-alkane 2H/1H may not be representative of a specific geographical location. Results demonstrate: (i) the importance of biochemical mechanisms in controlling the molecular and isotopic composition of n-alkyl lipids; and (ii) the need to further constrain the influence of vegetation change on the isotope composition of sedimentary n-alkanes. Future research should address these areas in other biomes and depositional environments, to ensure accurate interpretation of modern and ancient leaf wax lipid data.
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