
Sampling and Analytical Methodology
Near-surface seawater samples were collected from ~3 m depth while
underway at ~5 knots, using a towed, trace-metal clean pumping
system developed by Ken Bruland and Geoffrey Smit (University
of California, Santa Cruise).  Briefly, seawater was collected
through the intake of a 'towfish' deployed at ca. 3 m water depth
and ca. 5 m outboard of the ship, and then pumped though acid-
cleaned low-density polyethylene tubing into a shipboard clean-air
laboratory container, where it was filtered in-line using a 0.2 µm
Supor Acropak filter cartridge (Pall Corp.). 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 analytical method for dissolved iron in seawater has 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.
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.