Organic matter characterization and distribution in sediments of the terminal lobes of the Congo deep-sea fan: Evidence for the direct influence of the Congo River

• Lobe complex and Congo River sediments have similar geochemical characteristics.
• Concentration of soil-derived organic carbon is high in lobe sediments.
• Turbiditic deposition explains the geochemical homogeneity of lobe sediments.
• 137Cs reveals recent and substantial turbiditic deposition in the lobe complex.
• The lobe complex represents a huge sink for terrestrial organic carbon.

The terminal lobe complex of the Congo River submarine fan sits on the abyssal Atlantic plain, at 5000 m water depth, 760 km offshore from the river mouth estuarine area. While most rivers deliver particulate material to the continental shelf, particulate matter from the Congo River largely bypasses the shelf and is transported by turbidity currents through the Congo submarine canyon system. We determined the quantity and quality of the organic matter reaching the terminal lobe complex at five sites with marked morphological differences that may influence the distribution of organic matter. A suite of bulk geochemical (% OC, δ13Corg, δ15N, C: N), 137Cs and palynofacies analyses were done on cores collected from the terminal lobe area. These results were also compared to the composition of sediments collected upstream at the Malebo Pool (Congo River).Distal lobe complex sediments contain high amounts of terrestrial organic carbon (3–5 wt.%) that is homogeneously distributed in surficial (22 cm) and deeper sediments (580 cm) silty–clay facies. Strongly altered soil-derived organic matter with well-preserved land plant detritus from the Congo River predominates. A terrestrial soil origin for the particulate load was confirmed by the elevated 137Cs activity in lobe sediments. The vertical distribution of the 137Cs signal suggests that there has been a massive arrival of terrestrial sediments since 1963, consistent with a turbiditic origin. From the locations surveyed, we estimate a maximum accumulation of terrestrial organic carbon of ca. 1 kg OC m− 2 y− 1 for the distal lobe. However, transport modifies the organic matter both in terms of quantity and quality. Observed differences were attributed to preferential degradation of nitrogenous matter during diagenesis and to the addition of highly remineralized marine organic matter.Results from our temporal reference site (E) suggest that organic matter may be preserved in turbidite facies for thousands of years. The good preservation state of the accumulated organic matter shows that turbiditic lobe complexes should be considered as a sink for terrestrial organic carbon in the deep ocean.

Relationship between stable isotopes of nitrogen and carbon measured on OM in all the cores analyzed (n = 94) in the overall recent and abandoned lobe complexes. The majority of points are tightly clustered and associated with the deep sea fan environment; these have isotopic signatures typical of C3 plants. Each box is constructed with data from different sources. Congo River data are from Mariotti et al. (1991) and Spencer et al. (2012). “Active zone” illustrates the recent lobe complex where turbiditic inputs dominate (data from this study and Baudin et al., 2010). “Abandoned zone” illustrates the northern lobe complex and other parts of the Congo deep sea fan where pelagic inputs dominate (data from this study and Holmes et al., 1996; Müller et al., 1994; Schefuss et al., 2004). Marine POM data are from Tyson (1995), Meyers (1997), Fischer et al. (1998) and Schefuss et al. (2004).Figure optionsDownload as PowerPoint slide