We measured P concentrations in sediment samples using a sequential extraction technique as modified from Ruttenberg (1992) and as described in Delaney and Anderson (1997). The original sequential extraction technique isolates five sedimentary P components: adsorbed, oxide associated (sorbed to oxides or oxide coatings), authigenic (carbonate-fluorapatite), detrital (terrestrial silicates and apatite), and organic P. We applied this method to some of the samples, with results listed for all five fractions. For most of the samples reported, we modified this to a four-step procedure by eliminating an initial exchange step and instead beginning with a reduction step, assuming that most of the adsorbed fraction is associated with oxides. This has the effect of combining the adsorbed and oxide-associated fractions of the five-step procedure into a single fraction, with results listed as the oxide-associated component.
Splits of 10-cm3 samples were freeze dried and lightly ground. Weighed samples (typically 0.1 g each) were processed in replicate through the P sequential extraction procedure. P concentrations in known volumes of extractants (and thus in the solid samples) were determined using an automated spectrophotometric flow injection analysis system (Lachat QuickChem 8000). Results are reported as the means ± 1s (sample standard deviation) for each component. Total reactive P and total P are calculated as the sums of the relevant components, with the errors calculated from the propagation of absolute errors of the individual components. Analytical detection limits in typical size samples, assessed as three times the standard deviation of replicate measures of a low concentration solution standard, were typically 0.06 µmol P/g sediment for adsorbed P, 0.3 µmol/g for oxide-associated P, 0.1 µmol/g for detrital P, and 0.1 µmol/g for organic P. With the exception of oxide-associated P concentrations in some samples, P concentrations were generally well above these detection limits. We assessed long-term analytical variability by analyzing three composite samples here termed "consistency standards" (the first, a high calcium carbonate, low detrital P, and low organic carbon composite; the other two, homogenized from splits from a number of Leg 167 samples, with one composite higher in detrital P). The relative errors (1s) on the long-term means of these consistency standards from the four-step procedure were relatively high for the components that constitute small fractions of total P, ranging from 20% to 80% for oxide associated, 2% to 10% for higher detrital P consistency standards to as high as 74% for the low detrital P standard, and from 20% to 44% for organic P. The relative errors on the long-term means for authigenic P (the dominant sedimentary component) were 8%-17%, and for total P the errors were 6%-10%.
Using subsamples of the freeze-dried splits, we measured the weight percent calcium carbonate using a UIC, Inc., Coulometrics Model 5012 CO2 coulometer and the weight percent biogenic opal using the technique of Mortlock and Froelich (1989). Relative standard deviations on the means for (1) multiple determinations of a pure calcium carbonate standard, (2) samples run in duplicate within a given analytical run, and (3) replicate analyses of the consistency standards were always <1%. The effective detection limit for weight percent CaCO3 depended on the sample size; for typical sample sizes of 5-10 mg, the detection limit is 0.5-1.0 wt%. We adjusted sample sizes as needed to increase the detection limit, which decreases proportionally to the increase in sample size. The detection limit for biogenic opal, defined from three times the standard deviation of replicate measurements of a blank, was equivalent to 0.9 wt% in a typical size sample. The relative errors on long-term means of the two Leg 167 consistency standards were 6%-10%, with higher relative errors for the consistency standard with lower biogenic opal content. Analytical reproducibility for replicate samples had relative errors typically <5%, and up to 10% or greater for samples with low biogenic opal contents.