Publication Date:
2022-05-26
Description:
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 184 (2016): 20-40, doi:10.1016/j.gca.2016.04.003.
Description:
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.
Description:
J.D.H. was supported by the National Science Foundation
Graduate Research Fellowship under Grant No. 2012126152. V.V.G. was partly supported by
the US National Science Foundation, grants OCE-0851015 and OCE-0928582. Parts of this work
were supported by the DFG Research Center/Cluster of Excellence “The Ocean in the Earth
System” at MARUM - Center for Marine Environmental Science, University of Bremen.
Description:
2017-04-08
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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