A bottom-up view of the biological pump : modeling source funnels above ocean sediment traps

Abstract

The sinking of particles that make up the biological pump is not vertical, but nearly horizontal. This means that the locations where the particles are formed may be distant from their collection in a sediment trap. This has led to the development of the concept of the statistical funnel to describe the spatial-temporal sampling characteristics of a sediment trap. Statistical funnels can be used to quantify the source region in the upper ocean where collected particles were created (source funnels) or the location of the collected particles during that deployment (collection funnels). Here, we characterize statistical funnels for neutrally-buoyant, surface-tethered and deep-ocean moored trap deployments conducted just north of Hawaii in the Pacific Ocean. Three-dimensional realizations of the synoptic velocity field, created using satellite altimeter and shipboard acoustic Doppler current profiler data, are used to advect sinking particles back to their source for sinking velocities of 50 to 200 m per day. Estimated source and collection funnel characteristics for the 5-day collections made by neutrally-buoyant and surfacetethered traps are similar with typical scales of several km to several 10’s of km. Deep moored traps have daily source funnel locations that can be many 100’s of km distant from the trap and have long-term containment radii that range from 140 to 340 km depending upon sinking rate. We assess the importance of particle source regions using satellite estimates of chlorophyll concentration as a surrogate for the spatial distribution of particle export. Our analysis points to the need to diagnose water-parcel trajectories and particle sinking rates in the interpretation of sinking particle fluxes from moored or freely-drifting sediment traps, especially for regions where there are significant horizontal gradients in the export flux.