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

The concentrations, distributions, and stable carbon isotopes (delta C-13) 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 delta C-13 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 (C-25-C-35) represent a mixture of C-3 and C-4 end members, each with distinct molecular distributions, as evidenced by an 8.1 +/- 0.7 parts per thousand (+/- 1 sigma standard deviation) spread in delta C-13 values across chain-lengths, and weak correlations between individual homologue concentrations (r = 0.52-0.94). In contrast, plant-dominated n-alcohols (C-26-C-36) and n-alkanoic acids (C-26-C-36) exhibit stronger positive correlations (r = 0.70-0.99) between homologue concentrations and depleted delta C-13 values (individual homologues average <=-31.3% and -30.8 parts per thousand, respectively), with lower delta C-13 variability across chain-lengths (2.6 +/- 0.6 parts per thousand and 2.0 +/- 1.1 parts per thousand, respectively). All individual plant-wax lipids show little temporal delta C-13 variability throughout the time-series (1 sigma <= 0.9 parts per thousand), 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. (C) 2016 Elsevier Ltd. All rights reserved.