Ascending and descending fluxes of lipid compounds in North Atlantic and North Pacific abyssal waters

TWO classes of particles are generally recognized in oceanic waters, suspended and sinking. Previously, we demonstrated that there is also an upward flux 1 . This represents a small proportion of the descending material but it is mostly composed of organic-rich particulate matter. A subsequent study 2 confirmed these results providing quantitative data on the ascending organic carbon and nitrogen. Based on sediment-trap experiments, we show here that these upward fluxes determine the biogeochemical cycling of some lipids in abyssal waters. Up-to-down flux ratios ranging between 0.19–190% and 15–13,000% have been found for the sterols and fatty acids, respectively. The ascending organic material corresponds essentially to zooplankton and crustacean debris, whereas algal patterns dominate in the settling particles. Our results indicate that formation of buoyant matter in abyssal waters may act as a selection mechanism by which some lipid components are recycled back to the upper water levels whereas others keep descending towards the bottom. This previously unreported process has implications for the interpretation of the lipid record in oceanic sediments, especially when molecular stratigraphy is used for palaeoenvironmental assessment 3 .