Functional separation of colloids and gravitoids in surface waters based on differential settling velocity: Coupled cross‐flow filtration—split flow thin‐cell fractionation (CFF‐SPLITT)

Given recognized challenges with shallow sediment trap and 234 Th‐based particle flux studies, it behooves us to test the picture of surface ocean fluxes provided by such investigations with alternative and new analytical techniques. To this end, cross‐flow filtration (CFF) has been coupled with split flow thin‐cell fractionation (SPLITT) to afford active but mild separation of aquatic particles through a dynamic settling‐velocity discrimination. Operation parameters have been developed to yield accurate settling‐velocity cutoffs in the range >0.1–>10 m d −1 , element recoveries in the range 85–110%, and minimal influence of CFF preconcentration up to at least a factor 110. Chemical fingerprints of truly settling matter (i.e., gravitoids) in coastal waters provided by the CFF‐SPLITT technique demonstrated that gravitoidal particles settling out of surface waters exhibit distinctly different compo‐sition than the bulk filterable particles. For instance, both gravitoidal POC and P belonged in a coastal Baltic regime largely to a slowly settling (1–2 m d −1 ) particle pool (presumably amorphous organic aggregates), which was decoupled from a more rapidly settling (>4 m d −1 ) Si‐containing particle pool (presumably diatom dominated). Both of these biogenic gravitoid pools were in turn distinct from an Fe‐containing gravitoid pool settling >4 m d −1 , which, in contrast to POC, P, and Si gravitoids, had a geochemical composition similar to that of bulk particles.