Particle export from the euphotic zone: Estimates using a novel drifting sediment trap, 234Th and new production

We describe the first results obtained using a novel free-drifting neutrally buoyant sediment trap called PELAGRA (Particle Export measurement using a LAGRAngian trap). The trap uses an APEX float (Autonomous Profiling EXplorer) to maintain its location at a predetermined depth or density horizon and is designed to be deployed in the depth range 100–600 m for periods of up to a week. PELAGRA was deployed into a well characterised region of the Northeast (NE) Atlantic over the Porcupine Abyssal Plain (PAP) during the summers of 2003, 2004, 2005 and 2006 in post-bloom conditions. In parallel, measurements were made of total and new primary production and estimates of particle export based on 234Th deficits in the upper water column. Samples of sinking material from PELAGRA were recovered almost uncontaminated by “swimmers”. The material collected differed from that obtained using water bottles and an in situ filtration system and this indicated selective export of centric diatoms. Fluxes of 234Th into the traps were less than that calculated from the deficit of this isotope in the upper water column. It is concluded that this reflects export events that had occurred prior to these deployments. Fluxes of organic carbon into the PELAGRA traps were similar to those expected from measurements of new production in the upper mixed layer. During 2006, simultaneous PELAGRA deployments at depths of 150 and 250 m provided a direct measurement of the decrease in flux with depth. This decrease was substantially more than that predicted by the often-used “Martin equation” and yielded a “b value” of 1.7. Using this value in the NE Atlantic about 14% of the total production reaches a depth of 150 m during the post-bloom period under nutrient limitation.