Direct Measurements of Particulate Organic Carbon Flux Through the Twilight Zone During the North Atlantic Bloom Using Neutrally Buoyant Sediment Traps

The spring phytoplankton bloom in the North Atlantic is thought to contribute a significant proportion of the global export of particulate organic carbon (POC) out of the euphotic zone and through the mesopelagic zone. Primary production in this region has been previously estimated at 200 g C m-2 year-1 and higher. POC fluxes were measured between 150 m and 700 m from four deployments of neutrally buoyant sediment traps (PELAGRA) made during the initial development of the North Atlantic spring bloom in May 2008. In addition, fluxes of particulate organic nitrogen, inorganic carbon, and biogenic silica were measured. These data show that the sinking of the bloom is characterised by sedimentation of discrete pulses of material such that at certain times deeper traps collect several-fold more material than shallower traps, reflecting the passage of particle pulses through the water column. During the time period of maximal flux, increasing concentrations of suspended particles at depth were observed in CTD and Seaglider profiles of chlorophyll fluorescence and optical backscatter. We have applied a range of depth-normalisations, from published b-values of the 'Martin-curve', to estimate the export flux at 100 m depth. Daily fluxes during these pulsed events contributed between 1 and 8 % of the annual export reported in the literature using very different approaches (36 g C m-2 year-1). Data from concomitant ship-board measurements of total 234Th disequilibrium during the cruise provide a further constraint on the magnitude of export production.