Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption

C. Rabouille (1, 2) , J.-C. Caprais (3) , B. Lansard (1, 2) , P. Crassous (4) , K. Dedieu (5) , J.L. Reyss (1, 2) , A. Khripounoff (4)

1 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
2 OCEANIS - Océan et Interfaces
3 IFREMER - Institut Français de Recherche pour l'Exploitation de la Mer
4 LEP - Laboratoire Environnement Profond
5 LEFE - Laboratoire Ecologie Fonctionnelle et Environnement

A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO3, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds. The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4±0.6, 3.6±0.5 mmol m−2 d−1 at 1300 and 3100 m depth, respectively, and between 1.9±0.3 and 2.4±0.2 mmol m−2 d−1 at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8±1.1, 2.3±0.8, 0.8±0.3 and 1.2±0.1 mmol m−2 d−1, respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel-levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5-8 times larger than the vertical flux recorded in traps. Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y−1) found on this site. The Lobe region could receive as high as 19 mol C m−2 y−1, 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.

Mots clés: In situ oxygen demand; Organic carbon recycling in sediments; Congo Submarine Canyon; Organic carbon burial; Sediment carbon budget

Domaines: Océanographie; Géochimie

C. Rabouille, J.-C. Caprais, B. Lansard, P. Crassous, K. Dedieu, et al.. Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption. Deep Sea Research Part II: Topical Studies in Oceanography, 2009, 56 (23), pp.2223-2238. ⟨10.1016/j.dsr2.2009.04.005⟩. ⟨hal-02881056⟩