Isotopic and molecular distribution of leaf-wax in plant-soil system of the Gangetic floodplain and its implication for paleorecords

Abstract

The stable isotopic composition of bulk organic matter (OM), as well as the molecular and isotopic composition of n-alkanes and n-alkanoic acids, are compared in various plant growth-forms (trees, shrubs and grasses) and soils from the Gangetic floodplain. Abundance of higher molecular weight (HMW) n-alkanes is found to be highest in grasses and shrubs and lowest in trees, whereas n-alkanoic acids show an opposite trend. Total lipid production is highest in trees and lowest and comparable in shrubs and grasses, reflecting differences in metabolic conditions and water use efficiency during synthesis of these biomolecules in trees and overground species. In all plant growth-forms, higher (5‰) carbon isotopic composition (δ13C) are observed in bulk OM compared to 13C/12C ratio of n-alkyl compounds, while a small difference (0.2‰) is seen in between n-alkane and n-alkanoic acids. The trees and shrubs show lower average hydrogen isotopic composition (δD) in n-alkanes (δDalk) and higher values in n-alkanoic acids (δDacid) compared to the grass species. Interestingly, the grass and shrub species register higher variation (ca. 110‰) in δD values of n-alkyl compounds, reflecting the ability of groundcover species to proliferate under varying environmental conditions. The lower D/H (ca. 74‰) variation in the tree species possibly suggests consistent dependency on the perennial water sources. Although the study site is equally dominated by C3 and C4 vegetation, mean δ13C values of bulk OM and n-alkanoic acid in soil show proximity to the C4 end-member, while n-alkanes show proximity to C3 trees. Mean δDalk values in the soil reflect the cumulative hydrological condition of the overground vegetation, while average δDacid values mimic the ambient conditions specific to the C3 species. The variation in OM mixing from different plant types in soil was triggered by soil-forming processes, which possibly resulted in variation in responses between the n-alkyl compounds. Compared to the global record, the C3-C4 vegetation of the Gangetic floodplain is characterised by a lower (ca. 0.5–2‰) difference between bulk leaf tissues and n-alkyl compounds (Δ13CBulk–alk/acid). H-isotopic fractionation with respect to the ambient water (εapp) shows higher (ca. +30‰) values for n-alkanoic acids and lower (ca. −60‰) values for n-alkanes compared to the global datasets, implying a role of regional environmental conditions on the isotopic fractionation in n-alkyl compounds in plants. As molecular and isotopic compositions of plant-derived proxies are related to environmental conditions, emphasis must be given to establish the response of proxies to regional conditions before applying them in paleoenvironmental reconstructions.