The biochemical composition of the sediments in a Mediterranean submarine cave (Marseille, France) was studied, and a budget for the biogeochemical cycle of organic carbon was calculated. Chloropigment and lipid levels were markedly lower in sediments from the dark inner section of the cave compared to the twilight outer section. These decreased levels were related to the decrease in the vertical inputs of particulate organic matter. Lower decreases were recorded in the sediment content of other organic constituents (organic carbon, organic nitrogen, carbohydrates, proteins). The analysis of carbohydrate and protein extracted from sediments (NaOH 1 N, 24 h, 4-degrees-C) yielded no significant information, demonstrating the ambiguous significance of such a chemical approach in sediments low in organic matter. A biogeochemical budget for the cycling of organic compounds in the sediment was achieved using previous studies on suspended particle composition and vertical flux. Benthic degradation processes were highly efficient, approximately 90 % of the sedimenting organic carbon was degraded in the top 15 cm sediment layer, and only approximately 10 % of this input was buried. Lipids, with approximately 100 % used in the top layer, proved to be highly degradable compounds. Despite a high degradation rate of proteins and carbohydrates, respectively approximately 3 and approximately 8 % of their initial inputs were still present at 15 cm depth showing that some of these compounds are stable and may resist diagenetic decomposition. Complex organic matter was a significant source of organic carbon in these sediments, and, despite the high energetic investment required, must be considered as a potential resource, especially in oligotrophic environments.