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 Th-234-based par ticle flux studies, it behooves us to test the picture of surface ocean flu xes provided by such investigations with alternative and new analytical tec hniques. To this end, cross-flow filtration (CFF) has been coupled with spl it flow thin-cell fractionation (SPLITT) to afford active but mild separati on of aquatic particles through a dynamic settling-velocity discrimination. Operation parameters have been developed to yield accurate settling-veloci ty cutoffs in the range >0.1->10 m d(-1), element recoveries in the range 8 5-110%, and minimal influence of CFF preconcentration up to at least a fact or 110. Chemical fingerprints of truly settling matter (i.e., gravitoids) i n coastal waters provided by the CFF-SPLITT technique demonstrated that gra vitoidal particles settling out of surface waters exhibit distinctly differ ent composition than the bulk filterable particles. For instance, both grav itoidal POC and P belonged in a coastal Baltic regime largely to a slowly s ettling (1-2 m d(-1)) particle pool (presumably amorphous organic aggregate s), which was decoupled from a more rapidly settling (>4 m d(-1)) Si-contai ning particle pool (presumably diatom dominated). Both of these biogenic gr avitoid pools were in turn distinct from an Fe-containing gravitoid pool se ttling >4 m d(-1), which, in contrast to POC, P, and Si gravitoids, had a g eochemical composition similar to that of bulk particles.