Dataset: GT10-11 - dFe and dFe-II
Deployment: KN199-04

Sample collection and processing:
Water-column samples for trace metal analysis were collected in modified 12 L Teflon-lined GO-FLO samplers (General Oceanics Inc.), deployed on the U.S. GEOTRACES clean CTD rosette system with a total of 24 samplers. All samples were filtered as soon as possible after recovery by GEOTRACES sampling personnel using 0.2 µm Supor Acropak filter cartridge (Pall Corp.) inside the U.S. GEOTRACES clean-air laboratory van. Near-surface samples (from ~2 m depth) were collected while underway on approach to each station using the Bruland-Smith trace-metal towfish, which was towed outside of the ship’s wake. Seawater from the towfish was pumped directly into a clean-air laboratory van, where it was filtered through 0.2 µm Supor Acropak filter cartridges.

Dissolved iron (dFe) analysis:
The filtered seawater samples were collected in 125 mL acid-cleaned Nalgene wide-mouth low-density polyethylene bottles, acidified to pH ~1.7 with Fisher Optima grade ultrapure hydrochloric acid, and then stored at room temperature until post-cruise analysis at Old Dominion University by Bettina Sohst. Dissolved iron was determined by flow injection analysis with colorimetric detection after in-line preconcentration on resin-immobilized 8-hydroxyquinoline [Sedwick et al., 2005, 2008], using a method modified from Measures et al. [1995]. The efficacy of our analytical method for dFe was verified through the analysis of the SAFe seawater reference materials. Over the period during which these samples were analyzed (2010-2012), we determined mean dFe values of 0.122 ± 0.021 nM (n = 7) for SAFe seawater reference material S and 1.13 ± 0.19 nM (n = 2) for SAFe seawater reference material D2, versus community consensus values of 0.095 ± 0.008 nM and 0.955 ± 0.024 nM, respectively (as of May 2013). The analytical limit of detection is estimated as the dFe concentration equivalent to a peak area that is three times the standard deviation on the "zero-loading blank" (or "manifold blank"), from which we estimate a detection limit of less than 0.04 nM [Bowie et al., 2004; Sedwick et al., 2005]. Blank contributions from the ammonium acetate sample buffer solution (added on-line during analysis) and hydrochloric acid (added after collection) are typically negligible (i.e., too low to quantify). Robust estimates of our analytical precision are derived from multiple (separate-day) determinations of the SAFe seawater reference materials, which yield analytical uncertainties (expressed as ± one relative sample standard deviation on the mean) of ±14.7% (n = 33) at the concentration level of SAFe S seawater (0.095 nM), and ±9.2% (n = 16) at the concentration level of SAFe D2 seawater (0.955 nM).

Dissolved iron(II) (dFe(II)) analysis:
The filtered seawater samples were transported from the sampling station to a shipboard clean-air laboratory van in a cooler, and were analyzed at sea by Bettina Sohst as soon as possible after sampling. Dissolved iron(II) was determined by flow injection analysis with in-line preconcentration modified after the method of Bowie et al. [2005]. The analytical limit of detection was estimated daily as the dFe(II) concentration corresponding to a signal three times the standard deviation on triplicate analyses of the blank [Bowie et al., 2004; Sarthou et al., 2011]. For the blank solution, we used filtered, aged, low-iron seawater that was stored in the dark ("low Fe(II) seawater"), for which the chemiluminescence signal was analytically indistinguishable from reagents injected without seawater [see Bowie et al., 2005]. KN199-04: The limit of detection averaged <0.06 nM for all of the daily shipboard analyses (n = 16). KN204-01: The limit of detection averaged <0.01 nM for all of the daily shipboard analyses (n = 31).

There is no standard reference material for the determination of dissolved iron(II) in seawater, thus we are unable to provide rigorous estimates of the accuracy and precision of the dFe(II) determinations. However, some indication of our analytical uncertainty is provided by repeat measurements of standards prepared in low-Fe(II) seawater, which yielded average relative standard deviations of <17% (KN199-04) or <13% (KN204-01) and <11% for triplicate injections of 0.2 nM and 0.4 nM dFe(II) standards, respectively.

References:
Bowie, A. R., P. N. Sedwick, and P. J. Worsfold (2004), Analytical intercomparison between flow injection-chemiluminescence and flow injection-spectrophotometry for the determination of picomolar concentrations of iron in seawater, Limnology & Oceanography Methods, 2, 42-54. doi: 10.4319/lom.2004.2.42.

Bowie, A. R., E. P. Achterberg, S. Ussher, and P. J. Worsfold (2005), Design of an automated flow injection-chemiluminescense instrument incorporating a miniature photomultiplier tube for monitoring picomolar concentrations of iron in seawater, Journal of Automated Methods & Management in Chemistry, 2, 37-43. doi: 10.1155/JAMMC.2005.37.

Measures, C. I., J. Yuan, and J. A. Resing (1995), Determination of iron in seawater by flow injection analysis using in-line preconcentration and spectrophotometric detection, Marine Chemistry, 50, 3-12. doi: 10.1016/0304-4203(95)00022-J.

Sarthou, G., E. Bucciarelli, F. Chever, S. P. Hansard, M. González-Dávila, J. M. Santana-Casiano, F. Planchon, and S. Speich (2011), Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre, Biogeosciences, 8, 2461-2479, doi: 10.5194/bg-8-2461-2011.

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, GB4006, 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, 55, 911-925, doi: 10.1016/j.dsr.2008.03.011.