Controls on leaf wax fractionation and delta H-2 values in tundra vascular plants from western Greenland

摘要

Hydrogen isotope ratios of leaf waxes are used to reconstruct past hydroclimate because they are a reflection of meteoric water, but the interpretation of these signatures from ancient sedimentary archives relies on a thorough understanding of the drivers of modern isotope variability and controls on fractionation. These studies are particularly valuable in the high latitudes, regions especially vulnerable to rapid climate change and increasingly used for plant-based proxy reconstructions of past hydroclimate, but also where modern vegetation is understudied compared to the lower latitudes. Here we investigate delta H-2 values from leaf wax n-alkanes of vascular tundra plants in the Kangerlussuaq area of western Greenland. We collected a variety of common tundra species to study possible interspecies variability in delta H-2 values including dwarf shrubs (Betula nana, Empetrum hermaphroditum, Salix glauca), forbs and graminoids (Vaccinium uliginosum, Rhododendron tomentosum, and Calamagrostis lapponica), a horsetail species (Equisetum arvense), and a submerged aquatic macrophyte from a local lake (Stuckenia filiformis). Using previously measured leaf and stem waters to help constrain potential drivers of leaf wax n-alkane delta H-2 values, we find that the overall net fractionation (epsilon(app)) from the studied species is -75 +/- 20%. The eapp at Kangerlussuaq is consistent with other studies of Arctic vegetation that find smaller eapp than from the majority of lower latitude sites. The fractionation of leaf water and xylem water (epsilon(lw/xw)) and the fractionation of xylem water and precipitation (epsilon(xw/p)) are both relatively constant, suggesting stable leaf and soil related fractionations across species. Estimates of biosynthetic fractionation (epsilon(bio)), as evidenced from the fractionation of the delta H-2 values of n-alkanes and leaf water (epsilon(wax/ lw)), are not constant across species as sometimes assumed, and are small (average of ebio is -120 +/- 27%) compared to many published estimates. This supports a significant role in ebio shaping the eapp in this high latitude setting, where lipid biosynthesis may be driving differences in n-alkane delta H-2 values. This finding suggests that lipids in the Kangerlussuaq plants studied rely on the use of some proportion of different hydrogen sources during lipid synthesis, such as stored NADPH. The cumulative results of this survey of Kangerlussuaq area n-alkane delta H-2 values and water-wax fractionations suggest that fractionation in the high latitudes during the short summer growing season may play an important role in governing the small eapp compared to many low latitude sites. Better understanding of appropriate eapp and the importance of ebio in controlling plant wax fractionation from the high latitudes is necessary for future reconstructions of hydroclimate using leaf wax delta H-2 values in these regions. (C) 2018 Elsevier Ltd. All rights reserved.