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The identification of different sources of organic matter


elsaElsa Stetten, doctorante UPMC (ED398)


The terminal lobe complex of the Congo deep-sea fan receives a mixture of terrestrial organic matter inputs from turbidity currents which are initiated in the Congo River submarine canyon and is also fed with degraded marine organic matter. Performing biogeochemical analyses to characterize these distinct sources brings us essential information concerning the origin and fate of the organic matter in this particular abyssal region and is also required to understand why so many surprising habitats are present:

Given that continental inputs are mainly composed of fine and altered soil-derived organic matter, we used the recent BIT index proxy (based on glycerol dialkyl glycerol tetraethers (GDGTs) composition) to study this pool of organic matter. This proxy varies between 0 and 1, where 1 corresponds to organic matter deriving from soils only. In lobe sediments BIT index values averaging 0.8 were found and indicate that about 80% of the organic matter comes from soils eroded from the Congo River watershed. This abundant organic matter pool is known to be refractory compared to other more reactive classes of organic matter and should persist in sediments over geological time scales.

Other lipidic molecules such as fatty acids are useful class of biomarkers because they have biosynthetic pathways that are specific to a group of organisms and can thus discriminate precisely between the different sources of sediment organic matter even for minor fractions that cannot be detected by bulk approaches. We analyzed sediments from a terrestrial reference (Malebo Pool) in order to compare the results with what we obtain in lobe sediments (Fig. 1). The sediment collected in the lobe area (site A) presents similar concentrations and compositions in Fatty Acids (FAs) compared to those measured in samples collected at the Malebo Pool. A major input from land vegetation was highlighted by the abundance of very long chain FAs (VLCFAs on Fig. 1) deriving from cuticular waxy leaf coatings of higher plants and by the presence of large higher plant debris (Fig. 2).

Figure 2. Microphotographs of palynofacies after filtration (10 ┬Ám) on sediments collected at the site A. This higher plant ligno-cellulosic debris presents discernible vegetal fiber and is well-preserved.

The identification of different sources of organic matter by biomarkers (soils, plant debris) will lead us to investigate their potential for recycling and burial in the context of the Lobe region. A collaboration with Ming-Yi Sun (University of Georgia, USA), who makes measurements of the carbon isotopic signature of the fatty acids will help clarifying their provenance and the degradation processes occurring during the turbiditic transfer and after deposition.