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WP2 Sediment

 
 

sediment11The Congo River delivers a huge amount of sediment into the Atlantic Ocean, up to 55 Million Tons per year. But, the river mouth is not the end of the story for the sediment. As it enters into the submarine world it starts another journey towards the abyss. Because the sediment is constantly supplied ad into the adjacent submarine canyon, sometimes (probably several times every year) this accumulation becomes instable (just like the snow on a mountains) and triggers avalanches of sediment that we call turbidity currents.

 The turbidity currents move downwards, inside the canyon, driven by gravity. They can reach important heights, in order of several hundreds of meters and can reach important speeds, in order of several meters per seconds. They tend to sort the sediment by keeping the coarse particles (sand) close to the bottom and rejecting the finest particle (clay) to the top. This mechanism generates more deposition of sediment on the sides and gradually creates a channel flanked by two aggrading lateral levees that we call a “Turbiditic Channel-Levées”.

sediment3Turbiditic Channel-Levées can be described as “undersea river beds”, but the way the sediment is transported and deposited by turbidity currents is drastically different of rivers. However, they have very similar meandering patterns that are remarkably visible on the seabed morphology at the outlet of the Congo submarine canyon. At a given time only ONE channel-levées is working while others are abandoned.

During years, millennia and million years this sediment has accumulated on the continental margin to create a several kilometres thick and several hundred of kilometres wide gigantic pile. This pile of sediment is actually very well structured because it is a stack of thousands of Turbiditic Channel-Levées. It is called a Turbidite System or Submarine Fan.

 The presently working channel-levées (Fig. 2) runs along 800 km from the submarine canyon and down to 5000 m water depth. The overall length is 1100 km when “unfolding” the meanders and this is the distance travelled by a sediment avalanche triggered in the submarine canyon before it dies naturally by lack of energy (i.-e. lack of slope). The termination of the channel-levées has a complex and intricate morphology that is characterized by a diverging pattern of small-size channel-levées that gradually vanish. Because of its shape it is called a “lobe”.

sediment2 Because we want to understand where, why and how benthic organisms can live on the organic carbon supplied by the turbidity current it is necessary to understand the role that play the morphology and the composition of the lobes in the occurrence of these biological oasis. Our main problem is that the occurrence of the biologic “spots” and the characteristics of the seabed (morphology and composition) in the lobes vary a lot. For instance the size of a “spot” of biologic community ranges from few meters to no more than few tens of meters while the pixel size on the images of Figures 3 and 4 is only about 50 m. This means that such variability cannot be resolved with tools operated from the sea surface. Therefore we have used a tool operated close to the seabed, the IFREMER Remotely Operated Vehicle (ROV) called VICTOR that allowed us to acquire very high resolution bathymetry (pixel size 1 m) and to directly describe the sedimentary structures on the seabed.

The pattern of distributaries channels network in the lobes permits to outline several distinct sedimentary areas, such as the channels floors, levees, areas adjacent to channel-levées, and numerous landslides whose characteristics may vary. Similarly, the benthic biologic communities are not homogeneously distributed on the lobe and we can suppose that these geological and biological heterogeneities are likely correlated.

Bearing that in mind and using the high degree of organization of the area as a guide, the objectives of task 5 are:

(1) to refine our knowledge of the morphology and the internal structure of the lobe complex

(2) to determine accurately the sedimentary facies and their distribution

(3) to guide further field investigations during cruises

(4) to determine the geological constraints on the distribution and composition of the benthic communities.

Team:

Coordinator: Bernard DENNIELOU