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  • 1
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    Multiple plant-wax compounds record differential sources and ecosystem structure in large river catchments (2016)
    Details
    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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  • 2
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    Hydrologic controls on seasonal and inter-annual variability of Congo River particulate organic matter source and reservoir age (2017)
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    Publication Date: 2022-05-26
    Description: © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Chemical Geology 466 (2017): 454-465, doi:10.1016/j.chemgeo.2017.06.034.
    Description: We present dissolved organic carbon (DOC) concentrations, particulate organic matter (POM) composition (δ13C, δ15N, ∆14C, N/C), and particulate glycerol dialkyl glycerol tetraether (GDGT) distributions from a 34-month time-series near the mouth of the Congo River. An end-member mixing model using δ13C and N/C indicates that exported POM is consistently dominated by C3 rainforest soil sources, with increasing contribution from C3 vegetation and decreasing contribution from phytoplankton at high discharge. Large C4 inputs are never observed despite covering ≈ 13 % of the catchment. Low and variable ∆14C values during 2011 [annual mean = (-148 ± 82) ‰], when discharge from left-bank tributaries located in the southern hemisphere reached record lows, likely reflect a bias toward pre-aged POM derived from the Cuvette Congolaise swamp forest. In contrast, ∆14C values were stable near -50 ‰ between January and June 2013, when left-bank discharge was highest. We suggest that headwater POM is replaced and/or diluted by C3 vegetation and pre-aged soils during transit through the Cuvette Congolaise, whereas left-bank tributaries export significantly less pre-aged material. GDGT distributions provide further evidence for seasonal and inter-annual variability in soil provenance. The cyclization of branched tetraethers and the GDGT-0 to crenarchaeol ratio are positively correlated with discharge (r ≥ 0.70; p-value ≤ 4.3×10-5) due to the incorporation of swamp-forest soils when discharge from right-bank tributaries located in the northern hemisphere is high. Both metrics reach record lows during 2013, supporting our interpretation of increased left-bank contribution at this time. We conclude that hydrologic variability is a major control of POM provenance in the Congo River Basin and that tropical wetlands can be a significant POM source despite their small geographic coverage.
    Description: J.D.H. was supported by the NSF Graduate Research Fellowship Program under grant number 2012126152; E.S. was supported by the DFG Research Center/Cluster of Excellence “The Ocean in the Earth System” at MARUM – Center for Environmental Sciences; V.V.G. was partly supported by the US National Science Foundation, grants OCE-0851015 and OCE-0928582; R.G.M.S. was partly supported by the US National Science Foundation, grants OCE-0851101, OCE-1333157, and OCE-1464396; and T.I.E. was partly supported by the Swiss National Science Foundation (SNF Grant No. 200021_140850).
    Keywords: Biomarkers ; Congo River ; GDGTs ; Particulate Organic Matter ; Radiocarbon
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
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    The biogeochemistry of carbon across a gradient of streams and rivers within the Congo Basin (2014)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 119 (2014): 687–702, doi:10.1002/2013JG002442.
    Description: Dissolved organic carbon (DOC) and inorganic carbon (DIC, pCO2), lignin biomarkers, and theoptical properties of dissolved organic matter (DOM) were measured in a gradient of streams and rivers within the Congo Basin, with the aim of examining how vegetation cover and hydrology influences the composition and concentration of fluvial carbon (C). Three sampling campaigns (February 2010, November 2010, and August 2011) spanning 56 sites are compared by subbasin watershed land cover type (savannah, tropical forest, and swamp) and hydrologic regime (high, intermediate, and low). Land cover properties predominately controlled the amount and quality of DOC, chromophoric DOM (CDOM) and lignin phenol concentrations (∑8) exported in streams and rivers throughout the Congo Basin. Higher DIC concentrations and changing DOM composition (lower molecular weight, less aromatic C) during periods of low hydrologic flow indicated shifting rapid overland supply pathways in wet conditions to deeper groundwater inputs during drier periods. Lower DOC concentrations in forest and swamp subbasins were apparent with increasing catchment area, indicating enhanced DOC loss with extended water residence time. Surface water pCO2 in savannah and tropical forest catchments ranged between 2,600 and 11,922 µatm, with swamp regions exhibiting extremely high pCO2 (10,598–15,802 µatm), highlighting their potential as significant pathways for water-air efflux. Our data suggest that the quantity and quality of DOM exported to streams and rivers are largely driven by terrestrial ecosystem structure and that anthropogenic land use or climate change may impact fluvial C composition and reactivity, with ramifications for regional C budgets and future climate scenarios.
    Description: This work was supported by the National Science Foundation as part of the ETBC Collaborative Research: Controls on the Flux, Age, and Composition of Terrestrial Organic Carbon Exported by Rivers to the Ocean (0851101 and 0851015).
    Description: 2014-10-30
    Keywords: Dissolved organic matter ; Lignin ; CDOM ; pCO2 ; Aquatic ; Hydrology
    Repository Name: Woods Hole Open Access Server
    Type: Article
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    Format: application/vnd.ms-excel
    Format: application/pdf
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