Understanding 2H/1H systematics of leaf wax n-alkanes in coastal plants at Stiffkey saltmarsh, Norfolk, UK

Interpretation of sedimentary n-alkyl lipid delta H-2 data is complicated by a limited understanding of factors controlling inter-species variation in biomarker H-2/H-1 composition. To distinguish between the effects of interrelated environmental, physical and biochemical controls on the hydrogen isotope composition of n-alkyl lipids, we conducted linked delta H-2 analyses of soil water, xylem water, leaf water and n-alkanes from a range of C-3 and C-4 plants growing at a UK saltmarsh (i) across multiple sampling sites, (ii) throughout the 2012 growing season, and (iii) at different times of the day. Soil waters varied isotopically by up to 35 parts per thousand depending on marsh sub-environment, and exhibited site-specific seasonal shifts in delta H-2 up to a maximum of 31 parts per thousand. Maximum interspecies variation in xylem water was 38 parts per thousand, while leaf waters differed seasonally by a maximum of 29 parts per thousand. Leaf wax n-alkane H-2/H-1, however, consistently varied by over 100 parts per thousand throughout the 2012 growing season, resulting in an interspecies range in the epsilon(wax/leaf water) values of -79 parts per thousand to -227 parts per thousand. From the discrepancy in the magnitude of these isotopic differences, we conclude that mechanisms driving variation in the H-2/H-1 composition of leaf water, including (i) spatial changes in soil water H-2/H-1, (ii) temporal changes in soil water H-2/H-1, (iii) differences in xylem water H-2/H-1, and (iv) differences in leaf water evaporative H-2-enrichment due to varied plant life forms, cannot explain the range of n-alkane delta H-2 values we observed. Results from this study suggests that accurate reconstructions of palaeoclimate regimes from sedimentary n-alkane delta H-2 require further research to constrain those biological mechanisms influencing species-specific differences in H-2/H-1 fractionation during lipid biosynthesis, in particular where plants have developed biochemical adaptations to water-stressed conditions. Understanding how these mechanisms interact with environmental conditions will be crucial to ensure accurate interpretation of hydrogen isotope signals from the geological record. (C) 2013 Elsevier Ltd. All rights reserved.