The Congo Canyon and its deep-sea fan: a unique example of intensified remineralisation and burial of organic matter in the deep-sea

Canyons are natural conduits of particulate matter between the coastal and open oceans. Most world major rivers are disconnected from their original canyons and discharge their particulate material on the continental shelf. On the contrary, the lobes of the Congo deep-sea fan are still connected to the river mouth by the Congo canyon and channel-levee system and fed by recurrent turbidites. These gravity flows contain a large proportion of labile terrigenous organic matter (OM) which is deposited in the lobe zone, thereby creating high sedimentation rates of organic-rich sediments. In some localized spots, these organic-rich inputs support the development of ecosystems containing rich assemblages of Vesicomydae bivalves and bacterial mats. Six stations localized in different zones of the lobes and 3 different types of habitats were studied using in situ and ex situ measurement of oxygen microprofiles coupled to ¹³C and ¹⁴C measurements of particulate organic matter and dissolved inorganic carbon (DIC) in porewaters. Oxygen profiles provide a quantification of the organic carbon remineralization rate while the isotopic signature of carbon offers a mean to study the origin of the organic material and its mineralization in the sediments.

The results clearly indicate that the distinct biological habitats which are mostly found in the channel show a very active remineralization activity which largely exceeds that in the background sediment, in the levees and around the biological habitats. The origin of the particulate material is largely terrestrial with a ¹³C and ¹⁴C signature which correlates with the Congo River particulate matter, and a substantially older organic matter in the biological habitats. These results confirm the hypothesis of the Congo lobe region to be a hot spot for terrestrial organic carbon remineralization and burial in the Southern Atlantic Ocean.