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

© 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.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.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)