Sampling and Analytical Methodology

Water-column samples for trace metal analysis were collected in modified 5-L teflon-lined, external-closure Niskin-X samplers (General Oceanics Inc.) suspended from a non-metal line. In addition, a limited number of deeper water-column samples were collected by Mak Saito in 10-L teflon-lined GoFlo samplers (General Oceanics Inc.). All samples were filtered as soon as possible after collection using a 0.2 µm Supor Acropak filter cartridge (Pall Corp.), except for the Go-Flo samples which were filtered by Mak Saito through 0.4 µm polycarbonate membranes. The filtered seawater samples were acidified to pH 1.7 with Seastar Baseline ultrapure hydrochloric acid, stored for at least 24 hours, then dissolved iron (dFe) was determined by flow injection analysis modified after the method of Measures et al. [1995] with analytical figures of merit as detailed by Sedwick et al. [2005, 2008].

The efficacy of our sample collection, sample processing and analytical methods for dissolved iron in seawater have been verified in the SAFe intercomparison exercise [Johnson et al., 2007]. Our laboratory values for SAFe seawater reference materials are as follows:
SAFe surface seawater S1: dFe = 0.11 ± 0.01 nM (n = 15) vs consensus value of 0.097 ± 0.043 nM
SAFE deep seawater D2: dFe = 0.97 ± 0.06 nM (n = 14) vs consensus value of 0.91 ± 0.17 nM.

In addition, unfiltered splits of all samples were acidified to pH 1.7 with Seastar Baseline ultrapure hydrochloric acid, stored for at least 6 months, and then total-dissolvable iron (TDFe) was determined by flow injection analysis using the same method as used for dFe. Total-dissolvable iron is therefore equal to the concentration of dissolved iron plus acid-labile particulate iron [see Sedwick et al., 2005]. Dissolved macronutrients were measured at sea in 0.2-µm filtered using JGOFS-standard autoanalyzer methods.

References

Johnson, K. S., et al. (2007), The SAFe iron intercomparison cruise: An international collaboration, Eos, Trans. Am. Geophys. Un., 88, 131-132.

Sedwick, P. N., T. M. Church, A. R. Bowie, C. M. Marsay, S. J. Ussher, K. M. Achilles, P. J. Lethaby, R. J. Johnson, M. M. Sarin, and D. J. McGillicuddy (2005), Iron in the Sargasso Sea (Bermuda Atlantic Time-series Study region) during summer: Eolian imprint, spatiotemporal variability, and ecological implications, Global Biogeochemical Cycles, 19, doi:10.1029/2004GB002445.

Sedwick, P. N., A. R. Bowie, and T. W. Trull (2008), Dissolved iron in the Australian sector of the Southern Ocean (CLIVAR-SR3 section): meridional and seasonal trends, Deep-Sea Research I, doi:10.1016/j.dsr.2008.03.011.

Fe_profile.csv