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

Sedimentary plant waxes and their hydrogen and carbon (delta H-2 and delta C-13) 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 delta H-2 values among individual plants (77 parts per thousand for n-C-29 alkane and 84 parts per thousand for n-C-28 alkanoic acid) was likely driven by interspecies differences in biosynthetic delta H-2 fractionation as well as source water differences between forest and shoreline plants. A considerably smaller range of delta H-2(wax) values in the bog sediments (9 parts per thousand for n-C-29 alkane and 11 parts per thousand for n-C-28 alkanoic acid) suggests that sediments are either biased toward specific plants, or that signal averaging processes during or after deposition are constant. The combined delta H-2 and delta C-13 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 delta H-2(wax)-delta C-13(wax) relationships of opposite sign for n-C-29 alkane and n-C-28 alkanoic acid, which we speculate may reflect contrasting seasonal timing of synthesis and plant metabolic status between compound classes. The net apparent delta H-2 fractionation between precipitation and wax (epsilon(app)) was approximately 30 parts per thousand larger for n-alkanes (-133 parts per thousand) than for n-alkanoic acids (-103 parts per thousand), 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 epsilon(app) estimates for sedimentary records from similar depositional settings. (C) 2019 Elsevier Ltd. All rights reserved.