Multiple plant-wax compounds record differential sources and ecosystem structure in large river catchments

Abstract

The concentrations, distributions, and stable carbon isotopes (δ13C) of plant waxes carried by fluvial suspended sediments contain valuable information about terrestrial ecosystem characteristics. To properly interpret past changes recorded in sedimentary archives it is crucial to understand the sources and variability of exported plant waxes in modern systems on seasonal to inter-annual timescales. To determine such variability, we present concentrations and δ13C compositions of three compound classes (n-alkanes, n-alcohols, n-alkanoic acids) in a 34-month time series of suspended sediments from the outflow of the Congo River. We show that exported plant-dominated n-alkanes (C25 – C35) represent a mixture of C3 and C4 end members, each with distinct molecular distributions, as evidenced by an 8.1 ± 0.7‰ (±1σ standard deviation) spread in δ13C values across chain-lengths, and weak correlations between individual homologue concentrations (r = 0.52 – 0.94). In contrast, plant-dominated n-alcohols (C26 – C36) and n-alkanoic acids (C26 – C36) exhibit stronger positive correlations (r = 0.70 – 0.99) between homologue concentrations and depleted δ13C values (individual homologues average ≤ -31.3‰ and -30.8‰, respectively), with lower δ13C variability across chain-lengths (2.6 ± 0.6‰ and 2.0 ± 1.1‰, respectively). All individual plant-wax lipids show little temporal δ13C variability throughout the time-series (1σ ≤ 0.9‰), indicating that their stable carbon isotopes are not a sensitive tracer for temporal changes in plant-wax source in the Congo basin on seasonal to inter-annual timescales. Carbon-normalized concentrations and relative abundances of n-alcohols (19 – 58% of total plant-wax lipids) and n-alkanoic acids (26 – 76%) respond rapidly to seasonal changes in runoff, indicating that they are mostly derived from a recently entrained local source. In contrast, a lack of correlation with discharge and low, stable relative abundances (5 – 16%) indicate that n-alkanes better represent a catchment-integrated signal with minimal response to discharge seasonality. Comparison to published data on other large watersheds indicates that this phenomenon is not limited to the Congo River, and that analysis of multiple plant-wax lipid classes and chain lengths can be used to better resolve local vs. distal ecosystem structure in river catchments.