Distribution of n-alkanes and their δ2H and δ13C values in typical plants along a terrestrial-coastal-oceanic gradient. (15th July 2020)

• Record Type: Journal Article
• Authors: He, Ding; Nemiah Ladd, S.; Saunders, Colin J.; Mead, Ralph N.; Jaffé, Rudolf
• Is Part Of: Geochimica et cosmochimica acta. Volume 281(2020)
• Page Start: 31
• Page End: 52
• Publication Date: 2020-07-15
• Subjects: n-Alkanes -- Hydrogen isotopes -- Carbon isotopes -- Leaf -- Root -- Wetland -- Paleo-reconstruction; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.05.003
• Languages: English
• ISSNs: 0016-7037

Abstract

Highlights: Abundances, δ 2 H and δ 13 C values of n -alkanes were measured from plants across a coastal wetland. Most of the geochemical proxies differed among plant habitats and between leaves and roots. Differences in the isotopic values are larger among plants than between the leaves and roots. PCA is a valuable tool differentiating organic matter source in coastal wetland sediments. Abstract: Reconstructing past responses of coastal wetlands to climate change contextualizes ongoing and future developments in these globally important ecosystems. The molecular distributions and stable isotope ratios (δ 2 H and δ 13 C) of sedimentary plant wax n -alkanes are frequently used to infer past vegetation and hydroclimate changes in wetland systems. However, there is limited modern information available about these compounds in subtropical wetlands. Here we analyzed mature leaves from 30 typical plant species and roots from 6 plant species collected in the Florida Everglades, including tree island plants, freshwater wetland plants, mangroves, and seagrass. The n -alkane abundance (2 to 884 µg/g dry weight), percent of aquatic plants ratio (P aq, 0 to 1), average chain length (ACL23-33, 24.0–30.7), concentration weighted average (CWA) δ 2 H (−231 to −78‰) and δ 13 C values (−38.9 to −14.4‰) spanned wide ranges with plant growth habit. Significant differences in n -alkane abundances, P aq, ACL23-33, CWA δ 2 H and δ 13 C values were found to exist between the leaves and roots of some emergent aquatic plants. Simple mass balance calculations of wetland aquatic plants suggest that long chain n -alkanes (e.g., C29 n -alkanes) are predominantly derived from leaves rather than roots in wetland surface sediments/soils. However, the contribution from mid-chain n -alkanes (e.g., C23 n -alkane) from roots may be equal to or greater than those from leaves. This implies that the differences in the isotopic compositions between root and leaf derived material need to be taken into account when interpreting down core changes in mid-chain n -alkane δ 2 H and δ 13 C values, which may be derived from variable contributions from leaves and roots rather than a change in hydroclimate or vegetation. Considering the large variation in both n -alkane distribution proxies and isotopic composition, no single molecular index or stable isotope ratio can capture multivariate changes of wetland ecosystems in the past. Nevertheless, principal component analysis shows promising potential to resolve different plant functional types. Paleo-reconstruction of subtropical aquatic ecosystems using n -alkanes will be most useful if the full molecular and isotopic distribution information of plant waxes are used.