Dataset: GP16 Dissolved Iron - ODU

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 a 0.2 um Supor Acropak filter cartridge (Pall Corp.) inside the U.S. GEOTRACES clean-air laboratory van. Near-surface samples (~3 m depth) were collected whilst underway near each station using the Bruland-Smith "Geofish" sampling system towed outside of the ship’s wake; seawater from the Geofish was pumped into a clean-air laboratory van, where it was filtered through 0.2 um 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 at-sea to pH ~1.8 with Fisher Optima grade ultrapure hydrochloric acid, and then stored at room temperature until post-cruise analysis by Bettina Sohst at Old Dominion University. Dissolved iron was determined by flow injection analysis (FIA) with colorimetric detection after in-line preconcentration on resin-immobilized 8-hydroxyquinoline (Sedwick et al., 2015), using a method modified from Measures et al. (1995). Analyses were performed on a volumetric basis, so concentrations are reported in units of nanomole liter-1 (nM), which may be converted to units of nmol kg-1 by assuming a mean solution density of 1.023 kg L-1 at laboratory temperature. Analytical precision is estimated from multiple (separate-day) determinations of the SAFe seawater reference materials, which yield uncertainties (expressed as one relative standard deviation on the mean, or one sigma) of ~15% at the concentration level of SAFe S seawater (0.090 nM), and ~10% at the concentration level of SAFe D2 seawater (0.90 nM). 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"), which yield a detection limit below 0.04 nM (Bowie et al., 2004). 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).

Intercalibration information:
A preliminary intercomparison of DFe data from the U.S. GEOTRACES EPZT cruise carried out as part of a cruise data workshop in November 2015 indicated that our shipboard DFe analyses were in some cases as much as 10-20% higher than corresponding post-cruise determinations by the other analysts, which is an issue that has been previously noted for shipboard versus post-cruise DFe measurements using FIA. Consequently, we undertook a re-analysis of all cruise samples at Old Dominion University, which yielded mostly lower values that agree more closely with the other EPZT DFe data sets. A subsequent intercalibration of EPZT DFe data for samples collected from stations 1, 11, 17, 18, 26 and 36 has been carried out by PIs Ken Bruland, Seth John, Peter Sedwick and Jingfeng Wu. At this writing (November 2016), the results of this intercalibration exercise are being evaluated by the International GEOTRACES Standards and Intercalbration Committee. 

Related 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.

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.

Sedwick, P. N., B. M. Sohst, S. J. Ussher, and A. R. Bowie, A. R. (2015), A zonal picture of the water column distribution of dissolved iron(II) during the US GEOTRACES North Atlantic transect cruise (GEOTRACES GA03). Deep Sea Research Part II: Topical Studies in Oceanography, 116, 166-175.