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The project

 
 

The terminal lobes of the Congo deep-sea fan are a unique area in the world ocean. These lobes are fuelled quasi-continuously by turbidites containing a large proportion of labile organic matter delivered by the Congo River (the second largest river in the world by its freshwater discharge) which is linked to its submarine canyon by an incision in the shelf. This connection between the river and the canyon is unique for large rivers on a global scale. The lobe zone is the receptacle of the organic inputs channelized by the canyon and covers an area of 3000 km2. The sedimentation of labile organic matter in the lobe zone located at 750 km from shore and at 5000 m depth which displays the same features as a river delta (high burial rate, fresh organic matter) allows the development of an exuberant ecosystems which is visualized by large bivalves, bacterial mats, mucus blankets of polychetes, an assemblage which has never been observed out of peculiar regions of active cold seeps.
Despite this specificity, the ecosystem from the terminal lobes of the Congo fan have been only poorly observed (preliminary pictures during ROV dives and rare multiple coring). This prevented the investigation of their exact composition, spatial extension and, furthermore, their functioning.

The goal of the CongoLobe project is first 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 ecosystem based on chemosynthesis in the area by performing a multi-disciplinary study of the lobe zone from the Congo deep-sea fan. The terminal lobes of the Congo deep-sea fan constitute also a hot spot for biology and biogeochemistry in the region. In addition to the biological description of the ecosystem in terms of biodiversity, we will test a hypothesis concerning the functioning of this ecosystem which composition seems to be close to chemosynthesis based ecosystems. Our hypothesis aims at establishing a link between organic inputs from the Congo canyon/channel, their diagenesis in the first meters of the sediment (or deeper), the possible production of reduced fluids bearing sulphide and methane, and the presence of chemo-autotrophic fauna and bacterial mats in the lobe zone.
Precisely, we aim to:

  • Determine quantitatively and qualitatively the origin and 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.
  • Study habitat heterogeneity in this deposition lobe, biological community structure from bacteria to megafauna, and ecosystem functioning. The link with the direct input of organic matter from the Congo channel and with the reduced fluids generated by the recycling of formerly buried organic matter will be sought. The interactions between the size classes of the living organisms including microbes with consideration of meiofauna and macrofauna will be particularly studied. Functional biodiversity of this peculiar site in the deep-sea will be compared to other known cold seep sites in this region (active “pockmarks”) and phylogenetic links will be looked after.

In the heterogeneous context of the lobes, a multidiciplinary approach is needed, gathering geologists specialists of this deep-sea fan, organic geochemists capable of characterizing the origin and reactivity of organic matter, marine geochemists estimating the recycling and burial of biogenic compounds, microbiologists assessing the nature and activity of bacteria and archea of the sediment, and biologists studying biodiversity and functioning of the fauna of all size.

The working approach chosen is based on sea expeditions, such as those that IFREMER masters. One preliminary field campaign is already programmed in February 2011 with 5 days in the lobe zone (WACS leg 2) and will be occupied by a biogeochemical and biological survey of the zone. The second expedition in december2011-january 2012 with the ROV Victor 6000 of IFREMER has allowed the visualization and precise sampling of the biological and geological structures; in situ technologies were fully employed with 3 landers (movable by the ROV) carrying either standard or polarographic micro-electrodes and benthic chambers.