Isotopic analysis of methane by Cavity Ringdown Spectroscopy (CRDS) Application to the deep-sea Congolobe fan

Channeling all the continental material exported from the Congo River to the terminal lobes, the Congo deep-sea fan constitutes an unrecognized hotspot for biology and biogeochemistry in the Atlantic Ocean. Assemblages of benthic ecosystems in this peculiar environment mimic the ones observed only in active cold-seep regions. Massive organic matter inputs from the Congo canyon likely induce a sedimentary production of reduced fluids bearing sulphide and methane. These reduced compounds may support the development of bacterial mats based on chemo-autotrophy and the presence of biological communities feeding on these mats, as already observed in sediment from the lobe zone. Yet, the processes and driving forces controlling the structure of benthic communities in the lobe of the Congo submarine canyon are still poorly understood. Isotopic fractionations occurring during methanogenesis (depletion), thermic alteration of organic matter (enrichment), and microbial anaerobic oxidation (enrichment) lead to distinct δ13CH4 signatures 1,2. Hence, stable methane isotopes are increasingly being used to determine methane source in the surrounding sediments and infer the gas provenance 3. In the frame of the Congolobe project, this study investigates the functioning of benthic communities in relation with the main environmental conditions. Specifically, it focuses on the applicability of the stable methane isotopes (δ13CH4) in understanding the sediment processes involved and the metabolism of the benthic ecosystems (chemo-autotrophy vs heterotrophy). A total of 5 sites (A, B, C, E, F) were investigated, at a water depth of approximately 5000 m. Three sites (A,F,C) were located along the main axis of the currently active lobe. Site B was located on a lobe which has been disconnected from the active canyon for several decades. Site E corresponds to a fossil lobe, and is taken as a reference station for hemipelagic deposition. At site C, sediment cores of ~20 cm length were sampled from different biological habitats (microbial mats, reduced sediments, and vesicomyides) using the ROV Victor (IFREMER). Distribution of dissolved methane in pore-waters and its isotopic composition (δ13CH4) were measured at these stations using a new versatile tool based on Cavity RingDown Spectroscopy (CRDS) from Picarro The preliminary results indicate active diagenetic and chemo-synthetic processes within the surface sediment of the Congo lobes (e.g. methanogenesis and anaerobic oxidation of methane). The δ13CH4 ratio differs from one site to another and seems to follow a path related to the distribution of the living communities, suggesting that the biogeochemical processes within the sediment directly structure the benthic communities at the seafloor. References: 1 Alperin, M. J. et al. Glob. Biogeochem. Cycle 2, 279-288 (1988). 2 Barker, J. F. & Fritz, P. Nature 293, 289-291 (1981). 3 Schmale, O. et al. Biogeosciences 9, 4969-4977 (2012).