Locations:

Arctic Ocean: P-1-94-AR P21, 84°5' N, 174°58' W
California margin: W-2-98-NC TF1, 41°5' N, 125°1' W
Equatorial Pacific: TT013-06MC, 12°00' S, 134°56' W

Methodology:

We used the ignition method to determine total P and molybdate-reactive P concentrations (MRP, which includes primarily free orthophosphate) for each sediment sample used for this study. Samples for total P analyses were ashed in crucibles at 550oC for 2 h and then extracted in 25 mL of 0.5 M sulfuric acid for 16 h. Samples for MRP analyses were extracted in the same manner, without the ashing step (Olsen and Sommers 1982; Cade-Menun and Lavkulich 1997). We derived molybdate-unreactive P concentrations (MUP, which includes primarily organic P and polyphosphates) in supernatants by subtracting MRP from total P concentrations. For ashed and unashed extracts, MRP was determined as described below.

Total P concentrations in sediment extracts were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES). Standards were prepared with the same solutions as those used for the extraction procedure in order to minimize matrix effects on P measurements. Sediment extracts and standards (0 μM, 3.2 μM, 32 μM and 320 μM) were diluted to lower salt content to prevent salt buildup on the nebulizer. Concentration data from both wavelengths (213 nm and 214 nm) were averaged to obtain extract concentrations for each sample. The detection limit for P on this instrument for both wavelengths is 0.4 μM. The MRP concentrations were measured on a QuikChem 8000 automated ion analyzer. Standards were prepared with the same solutions used for the extraction step to minimize matrix effects on P measurements. Sediment extracts and standards (0 – 30 μM PO4) were diluted ten-fold to prevent matrix interference with color development. The detection limit for P on this instrument is 0.2 μM. We derived MUP concentrations by subtracting MRP from total P concentrations.