N-Alkane biosynthetic hydrogen isotope fractionation is not constant throughout the growing season in the riparian tree Salix viminalis. (15th September 2015)

• Record Type: Journal Article
• Authors: Newberry, Sarah L.; Kahmen, Ansgar; Dennis, Paul; Grant, Alastair
• Is Part Of: Geochimica et cosmochimica acta. Volume 165(2015:Sep. 15)
• Page Start: 75
• Page End: 85
• Publication Date: 2015-09-15
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.05.001
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Compound-specific δ 2 H values of leaf wax n -alkanes have emerged as a potentially powerful paleohydrological proxy. Research suggests terrestrial plant n -alkane δ 2 H values are strongly correlated with meteoric water δ 2 H values, and may provide information on temperature, relative humidity, evaporation, and precipitation. This is based upon several assumptions, including that biosynthetic fractionation of n -alkanes during synthesis is constant within a single species. Here we present a multi-isotope study of the n -alkanes of riparian Salix viminalis growing in Norwich, UK. We measured n -alkane δ 2 H, leaf water δ 2 H, xylem water δ 2 H, and bulk foliar δ 13 C and evaluated the variability of n -alkane δ 2 H values and net biosynthetic fractionation ( ε lw-wax ) over a whole growing season. S. viminalis n -alkane δ 2 H values decreased by 40‰ between the start of the growing season in April and the time when they stabilized in July. Variation in leaf and xylem water δ 2 H did not explain this variability. ε lw-wax varied from −116‰ during leaf expansion in April to −156‰ during the stable phase. This suggests that differential biosynthetic fractionation was responsible for the strong seasonal trends in S. viminalis n -alkane δ 2 H values. We suggest that variability in ε lw-wax is driven by seasonal differences in the carbohydrate source and thus the NADPH used in n -alkane biosynthesis, with stored carbohydrates utilized during spring and recent occurring growing season assimilates used later in the season. This is further supported by bulk foliar δ 13 C values, which are 13 C-enriched during the period of leaf flush, relative to the end of the growing season. Our results challenge the assumption that biosynthetic fractionation is constant for a given species, and suggest that 2 H-enriched stored assimilates are an important source for n -alkane biosynthesis early in the growing season. These findings have implications for the interpretation of sedimentary n -alkanes and call for a careful design of calibration studies using contemporary samples.