Sedimentary n-alkanes and n-alkanoic acids in a temperate bog are biased toward woody plants

Abstract

Abstract Sedimentary plant waxes and their hydrogen and carbon (δ2H and δ13C) isotopes are important proxies for past hydrologic and vegetation change. However sedimentary waxes accumulate from diverse sources, integrating uncertainties from: (i) variable isotope fractionation among plant species, and (ii) unresolved processes controlling the transport of waxes from plants to sediments. We address these uncertainties by comparing the molecular and isotopic composition of n-alkanes and n-alkanoic acids in recent bog sediments with all major plant species growing in the catchment of Browns Lake Bog (BLB) in Ohio, USA. There are two distinct plant assemblages at BLB, including a forest dominated by trees and a bog shoreline composed of shrubs, woody groundcover, herbs and graminoids. n-Alkane concentrations in trees were 10–300 times higher than in shoreline plants, while n-alkanoic acid concentrations were generally lower and comparable across all species. The overall range of wax δ2H values among individual plants (77‰ for n-C29 alkane and 84‰ for n-C28 alkanoic acid) was likely driven by interspecies differences in biosynthetic δ2H fractionation as well as source water differences between forest and shoreline plants. A considerably smaller range of δ2Hwax values in the bog sediments (9‰ for n-C29 alkane and 11‰ for n-C28 alkanoic acid) suggests that sediments are either biased toward specific plants, or that signal averaging processes during or after deposition are constant. The combined δ2H and δ13C signatures of plant sources and sediments indicate a sediment bias mainly toward trees, with contributions from woody shrubs and groundcover growing in the bog shoreline. Within trees and woody shrubs, we observed δ2Hwax–δ13Cwax relationships of opposite sign for n-C29 alkane and n-C28 alkanoic acid, which we speculate may reflect contrasting seasonal timing of synthesis and plant metabolic status between compound classes. The net apparent δ2H fractionation between precipitation and wax (eapp) was approximately 30‰ larger for n-alkanes (−133‰) than for n-alkanoic acids (−103‰), both at the plant level and in sediments. These results demonstrate the sensitivity of sediments in a hydrologically closed basin to woody plants growing in the associated catchment and can guide eapp estimates for sedimentary records from similar depositional settings.