Phosphorus Phase Associations in the Deep Ocean Particle Flux in the Sargasso Sea

The carrier phases of phosphorus influence its flux and remineralization in the water column and burial in ocean sediments. Sequential extraction methods (SEDEX, Ruttenberg et al. 1992) have been developed to measure sedimentary phosphorus associated with specific mineral analogs, however these may not be representative of key carrier phases in the water column. To better understand P carrier phases in the deep particle flux, we applied a modified SEDEX method to partition P in sinking particles collected by the Oceanic Flux Program sediment traps (500m, 1500m and 3200m depths) into seven fractions, which were then analyzed by ICPMS for elemental composition. The samples spanned a time period from Nov 2008 (seasonal flux minimum) to Mar 2009 (spring bloom). At 500m depth, approximately 75% of the total P flux (~200 ugP m^-2 d^-1) was loosely sorbed and released into the supernatant during sample collection. This flux of loosely-sorbed P decreased by an order of magnitude between 500 and 1500m depths, and by 3200m depth constituted only 25% of the total P flux (~40 ugP m^-2 d^-1). In contrast, the P flux in non-soluble phases decreased by only 50% between 500m (~55 ugP m^-2 d^-1) and 3200m (~34 ugP m^-2 d^-1) depths. SEDEX/ICPMS results indicate that the distribution of P among non-soluble carrier phases varies little with depth: approximately 30-35% is associated with Mn-Fe hydroxides, 20-30% with opal, and 15-30% with carbonate/authigenic P/biogenic apatite phases. Smaller fractions are associated with ion exchangeable phases (12-15%), detrital apatite (4-8%), and refractory organics and inorganic minerals released by a final ashing/acid extraction step (1-4%). There is a small trend toward more recalcitrant Mn-Fe hydroxides and opal carrier phases with depth. The P flux associated with Mn/Fe hydroxides and opal (and at 500m, ion exchangeable material) showed strong seasonality and covaried over the time-series with peak fluxes during the spring bloom, while P fluxes associated with other carriers were less variable.