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Hydrologic controls on seasonal and inter-annual variability of Congo River particulate organic matter source and reservoir age (2017)

Hemingway, Jordon D. ; Schefuß, Enno ; Spencer, Robert G. M. ; [et al.]

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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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Isotopic evidence for sources of dissolved carbon and the role of organic matter respiration in the Fraser River basin, Canada (2022)

Voss, Britta M. ; Eglinton, Timothy I. ; Peucker-Ehrenbrink, Bernhard ; [et al.]

Springer

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Publication Date: 2022-11-04

Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Voss, B., Eglinton, T., Peucker-Ehrenbrink, B., Galy, V., Lang, S., McIntyre, C., Spencer, R., Bulygina, E., Wang, Z., & Guay, K. Isotopic evidence for sources of dissolved carbon and the role of organic matter respiration in the Fraser River basin, Canada. Biogeochemistry. (2022), https://doi.org/10.1007/s10533-022-00945-5.

Description: Sources of dissolved and particulate carbon to the Fraser River system vary significantly in space and time. Tributaries in the northern interior of the basin consistently deliver higher concentrations of dissolved organic carbon (DOC) to the main stem than other tributaries. Based on samples collected near the Fraser River mouth throughout 2013, the radiocarbon age of DOC exported from the Fraser River does not change significantly across seasons despite a spike in DOC concentration during the freshet, suggesting modulation of heterogeneous upstream chemical and isotopic signals during transit through the river basin. Dissolved inorganic carbon (DIC) concentrations are highest in the Rocky Mountain headwater region where carbonate weathering is evident, but also in tributaries with high DOC concentrations, suggesting that DOC respiration may be responsible for a significant portion of DIC in this basin. Using an isotope and major ion mass balance approach to constrain the contributions of carbonate and silicate weathering and DOC respiration, we estimate that up to 33 ± 11% of DIC is derived from DOC respiration in some parts of the Fraser River basin. Overall, these results indicate close coupling between the cycling of DOC and DIC, and that carbon is actively processed and transformed during transport through the river network.

Description: Open Access funding provided by the MIT Libraries. This work was supported by the WHOI Academic Programs Office, the MIT EAPS Department Student Assistance Fund, and the PAOC Houghton Fund to BMV; NSF-ETBC grants OCE-0851015 to BPE, VG, and TIE and OCE-0851101 to RGMS; NSF grant EAR-1226818 to BPE; NSF grant OCE-0928582 to TIE and VG; and a WHOI Arctic Research Initiative grant to ZAW.

Keywords: River ; Carbon isotopes ; Radiocarbon ; Weathering ; Carbon cycle