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doi:10.2204/iodp.proc.336.205.2015

Materials and methods

Solid-phase P concentrations in different reservoirs are determined following the SEDEX sequential extraction procedure. This method separates sedimentary P into five operationally defined categories: (1) loosely sorbed P, (2) ferric iron-bound P (abbreviated Fe-bound), (3) authigenic carbonate fluorapatite + biogenic apatite + CaCO₃-associated P (abbreviated auth/bio apatite + CaCO₃-bound P), (4) detrital apatite, and (5) refractory organic P (Ruttenberg et al., 2009). The separation of sedimentary P pools is based on the reactivity of each given pool to a particular extractant solution. Briefly, frozen squeezed sediment cakes are lyophilized, ground, and sieved to <125 µm. Weighed samples (0.08 g) are extracted along with an internal standard for reproducibility. The procedure begins with a 1 M magnesium chloride (MgCl₂) extraction that targets phosphate loosely bound to mineral phases. The extraction is followed by a reductive step during which sodium dithionite reduces iron oxyhydroxides and citrate complexes with iron, thus releasing P associated with iron oxides. The residue is then extracted in 1 M MgCl₂ to remove any residual oxide-bound P. Next, authigenic/biogenic apatite and P bound to calcium carbonate are extracted in 1 M sodium acetate, and any residual P from that pool is extracted in two consecutive 1 M MgCl₂ extractions. Following this step, detrital apatite is extracted in 1 M hydrochloric acid (HCl). Lastly, refractory organic P and residual P are extracted in 1 M HCl following ashing of sample residues at 550°C. Total P concentrations in sediment extracts are measured using inductively coupled plasma–optical emission spectroscopy (ICP-OES) on axial mode (Perkin-Elmer Optima 4300 DV Inductively Coupled Plasma Optical Emission Spectrometer operated by the University of California, Santa Cruz). Standards are prepared with the same solutions as those used for the SEDEX procedure in order to minimize matrix effects on P measurements. Sediment extracts and standards (0, 0.1, 1, and 10 ppm) are diluted to lower salt content to prevent salt buildup on the nebulizer. Concentration data from both wavelengths (213 and 214 nm) are averaged to obtain extract concentrations for each sample. The detection limit for P on this instrument for both wavelengths is 11 ppb. Low-P blanks associated with polycarbonate filters in the manifold reaction vessels were determined for each extraction step and subtracted from sample P measurements. We determined the precision for each SEDEX extraction step using repeat extraction of an internal sediment standard and analyses by inductively coupled plasma optical emission spectroscopy to be 28.4%, 8.7%, 11.4%, 17.0%, and 15.4% for steps 1–5, respectively. The higher error associated with loosely sorbed P measurements is due to the low P concentrations in those sediment extracts, which introduces uncertainty when measuring P concentrations on the ICP-OES.

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