Stable isotopes (C-13/C-12 and N-15/N-14) in settling organic matter of the northwestern Mediterranean Sea: biogeochemical implications

Within the framework of the High Frequency Flux (HFF) experiment (MATER programme), time-series sediment traps have been deployed for two months on the continental slope off Marseilles to measure downward particle fluxes at a high frequency sampling rate (two and six days). Combined isotopic analyses of carbon and nitrogen have been performed on selected samples. Both isotopic tracers have been used for the first time on organic material of the Mediterranean Sea (Gulf of Lions) to determine the main biological sources and to address the biogeochemical processes that affected this material. Settling particles were characterised by very low values of delta N-15 (near 0 parts per thousand) and delta C-13 (near -24.5 parts per thousand) that indicate the existence of a mixed material with two sources that differ according to the considered element (C or N). The terrestrial source slightly dominates the carbon pool because of its higher C:N ratio, whereas the nitrogen pool may mostly originate from N-2-fixing cyanobacteria (delta N-15 = 0 parts per thousand, low C:N ratio). These preliminary data suggest that dissolved atmospheric N, may act as a significant new nitrogen source in the Mediterranean Sea.