Organic sedimentation in the water column in the Arabian Sea: Relationship between the lipid composition of small and large-size,surface and deep particles

Small and large-size particulates (>50 μm) have been collected during the ORGON IV cruise in the Arabian Sea (North-western Indian Ocean), using conventional metallic bottles and vertical near-surface (200-0 m) and deep (below 1000 m) plankton nets. This area presents remarkable conditions for studying organic sedimentation: a high superficial productivity, low oxygen content of seawater and low terrigenous inputs.Lipids have been quantified and analysed by gas-liquid chromatography and gas-liquid chromatography/mass spectrometry for fatty acids, saturated hydrocarbons and sterols. The distribution of biological species, size range and biological nature have been determined through microscopic examination.Principal results are presented in a geochemical context: small and large-size particles belong to two different reservoirs both for concentrations and chemical compositions.The comparison between the size range of particles and chemical characteristics observed through analyses of net and bottle samples emphasizes the importance of large-size particles in the geochemical process of material transfer between surface and sediment. For various lipids, this transfer concerns freshly biosynthesized molecules such as long-chain polyunsaturated fatty acids, pristane and sterols, compounds which are of high metabolic value for abyssal and pelagic organisms.The lipid fluxes, determined from in situ concentrations, and settling velocity estimated from the size range of deep particulate material, are compared to those reported by different authors using other sampling procedures for deep settling matter.