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)

Abstract

Given recognized challenges with shallow sediment trap and 234Th-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 composition 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.