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The Cruise

 
 

The Congolobe project was built upon two sampling efforts during two oceanographic cruises on board the R.V.” Pourquoi Pas?” equipped with the ROV Victor 6000:

Pp aerien3Victora

 

 

 

 

 

N/O Pourquoi Pas? and the submersible Victor 6000

 

Challenges

There are many challenges faced by the accomplishment of such a programme. In abyssal zones as deep as the lobe zone where the pressure is around 500 atm, only a few ROV and landers can be used, namely those who are strickly working down to 6000 meters. Furthermore, the use of a ROV is essential to fulfil the objectives of the programme because of the heterogeneity of the ecosystem and geological structures of the terminal lobes. At this depth in the equatorial ocean, the use of in situ measurements is also a pre-requisite, since decompression and warming introduces major biases in the measurement. Only a few groups throughout the world can organize such sea expeditions with a ROV 6000m, among which IFREMER in Brest. Furthermore, this project gathers state of the art in situ technologies:  benthic chambers, microelectrodes, sediment traps, current meters which we will be deployed from the surface or manipulated with the ROV.

calmar+profileurTwo autonomous equipments Calmar (left) and micro-profiler (right) using to measure the water-sediment exchanges

 

Another challenge is the multiple disciplines which need to be involved in such a programme. Geologists, organic geochemists, marine geochemists, microbiologists and biologists needed to work in synergy. The strength of the present group is the already existing collaboration which has run for a decade between the biologists and geologists of IFREMER and the marine geochemists of LSCE and IUEM. The inclusion of a third partner (UPMC) provides new collaboration to carry out organic geochemistry and go one step further in the understanding of this ecosystem.

multitube3 Multicorer to sample the sediment

 

Cruises

A short preliminary cruise (WACS-leg2, February 2011) using ROV and ship sampling was occupied by a biogeochemical, sedimentological and biological survey of the zone. During this cruise, 3 sites were investigated (A, C, and D) by means of sea-surface sampling (coring by multicorer and Calypso piston corer), autonomous landers (Respirometer, microprofiler) and exploratory ROV dives to perform photographic surveys and precise sampling. Special biological assemblages were detected on site C especially, and sampling was performed on dense microbal mats and vesichomidae habitats.

 

carte generalestations lobe

Maps showing the Congolobe sampling stations and the route of the ship during the cruise

A long research cruise Congolobe was conducted from December 2011 to January 2012 for 30 days. The scientific team surveyed and sampled 5 different zones and performed a wealth of different measurements. The sampling zone is located at the outlet of the Congo submarine canyon at around 750 km off the coast and 5000m depth. On this zone of 3000 km2, 5 sub-sites will be chosen which will allow us to study the spatial variability and the temporal evolution of the lobe deposits:

Spatial variability: Four sites (A, C, D, and F) are located in the main deposition zone of the Congo canyon along the track of the main channel and characterized by different deposition intensities. Site C is obviously the main deposition centre whereas A and F located “upstream” receive deposits during main turbidites events. Site D is located beyond the main deposition and receives limited amount of turbidite material.

Temporal evolution: Two disconnected sites (B and E) were chosen: site B is close to the canyon and has seemingly been disconnected for a decade or more and site E in the Northern zone was disconnected for several millennia. This allowed a look into time to the persistence of these ecosystems through time.

A high resolution bathymetry of the sampling areas was acquired by the ROV Victor 6000 and treated in real-time in order to orientate the survey of biological habitats. Over the five zones surveyed, five different biological habitats were studied in situ using microelectrodes, sediment incubations, and core collections (box or tube cores). Overlying water and faunal collection by nets was also performed.

bouche

View of the oasis of life composed by large Bivalves

On board, sediment and porewaters were separated and analysed for the whole suite of electron acceptor on ROV cores, multicores or Calypso cores. Some cores were sectioned and kept frozen for microbiological analysis, some were investigated using on board micro-electrodes in the cold room. The in situ box cores were sectioned and fauna was sorted in different size classes, in and out of the habitats in order to contrast the faunal composition of the two media.