Trajectories of sinking particles in the Sargasso Sea: modeling of statistical funnels above deep-ocean sediment traps

Characteristics of statistical funnels above moored deep-ocean sediment traps at the Oceanic Flux Program (OFP) site in the Sargasso Sea were determined by Lagrangian analysis of particles sinking through a realistic horizontal velocity field. Stochastic simulations support previous assertions that the trajectories of sinking particles are, for the most part, far from vertical, and that traps sample particles from rather large “catchment” areas when evaluated over long time scales (much greater than 1 year). The dimensions and geographic centers of these catchment areas are determined by the characteristics of the ocean flow field, particle sinking speeds and depth of the traps. The predicted extent and center of the 3200 m OFP trap is nearly identical to that inferred from a previous analysis of OFP trap fluxes and Coastal Zone Color Scanner (CZCS) imagery. Traps moored at different depths may collect particles originating in widely separated areas at the sea surface. This is an important issue when the catchment area is assessed over short time scales (less than 60 days), which typify the collection times of most moored sediment traps. Given the typically patchy distribution of particles and particle producers, this can result in short-period flux measurements that show little or no coherence between collections made at the same location for adjacent times or those made simultaneously but at different depths. The effects of eddy dispersion will be greater for most other oceanic regions, as the Sargasso Sea is characterized by relatively low levels of both eddy and mean kinetic energy. The results of this study demonstrate that an understanding of the temporal and spatial characteristics of the flow field above deep-ocean sediment traps is just as important to the interpretation of flux measurements as is the analysis of the material collected by the traps.