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

摘要

We present dissolved organic carbon (DOC) concentrations, particulate organic matter (POM) composition (delta C-13, delta N-15, Delta C-14, 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 delta C-13 and N/C indicates that exported POM is consistently dominated by C-3 rainforest soil sources, with increasing contribution from C-3 vegetation and decreasing contribution from phytoplankton at high discharge. Large C-4 inputs are never observed despite covering approximate to 13% of the catchment. Low and variable Delta C-14 values during 2011 [annual mean= (-148 +/- 82) parts per thousand], 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, Delta C-14 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 C-3 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 x10(-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.