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WP4 Diagenesis

 
 

DSMP-InsituContinental margins are the main location of carbon transfer from land to sea and have been recognized as an important pathway in the global carbon and silica cycles.

The Congo River is a unique river system in several matters: in addition to being the second world largest river and carrying a relatively low particle load, it is the only large river in the world to be still connected to its submarine canyon. Indeed, most of the present large rivers (Amazon, Mississippi, Yangtze) discharge on their continental shelf far from their former canyon. This last characteristic has interesting consequences on the fate of particulate organic matter originating from the Congo River and the associated ecosystems fuelled by these inputs. A fraction of the river discharge is rapidly transferred mixed with margin sediments, after a short retention near the canyon head, through large turbidity currents to the deep-sea fan with an average return time of 2-3 years, thus creating a unique “deep-ocean coastal system” where recent terrestrial particles are deposited in a marine environment and chemosynthetic macrofauna is observed.

MultitubeThe aim of the CONGOLOBE project is to establish the functional relationship between the organic matter input from the Congo canyon, its recycling in the sediment and the structure and functioning of the biological assemblages.

Within this project, the objective of WP4 is the study of organic matter and silica input, recycling and burial, i.e. determine quantitatively and qualitatively the fate of organic particles (carbon and silica) transferred to the terminal lobes. Budgets of organic carbon and biogenic silica will be calculated including inputs, recycling and burial. An emphasis will be put on organic matter recycling and reduced fluid generation which could be linked to chemosynthesis over different time scales from years to thousand years.

Large inputs of organic matter and biogenic silica from the Congo canyon are deposited in the terminal lobes. They mix with pelagic inputs from the marine production and induce a series of microbial reactions leading to decomposition of organic matter (OM). The mineralisation of OM creates geochemical and microbial gradients in the sediment in which oxidants are consumed in the following order: O2àNO3àMn(IV)à Fe(III)àSO42-àCO2. Sulfate plays a major role in reducing sediments and 25-50% of OM can be mineralized using this metabolic pathway and producing HS. Below these layers, methane is produced by methanotrophs. The reduced compounds (CH4 and HS) are transported by diffusion or in microfaults between turbidites. The production of HS can be intensified by Anaerobic Oxydation of Methane (AOM). Sulfide can then be used by sulphide-oxydizing bacteria which leave free or in symbiotic association with chemosynthetic fauna.

The specific question raised  are the following

–      How much organic carbon and particulate silica is transferred to the Lobe zone and what share of the Congo particulate input does it represent?

–      How is this input flux partitioned between burial and recycling in the sediment?

–      What are the pathways of organic matter mineralization and what is the quantity of reduced compounds (methane, sulphide) produced during anoxic diagenesis?

–      What is the importance of reverse weathering reactions (RWR) in the Lobe sediments?

–      Are the Si and C recycling coupled in the sediments of the Lobe regions?

 

Team:

Coordinator : C. Rabouille (LSCE)

Participants:   LSCE : JL. Reyss, B. Bombled, C. Rabouille, C. Cathalot, F. Toussaint

                        IFREMER: A. Khripounoff, JC. Caprais, L. Ruffine, J. Le Bruchec

                        IUEM: O. Ragueneau, R. Corvaisier

Georgia Institute of Technology (USA): M. Taillefert, J. Beckler

HCMR (Greece): P. Michalopoulos

LOCEAN : D. cardinal