Dataset: GT11 - Fe Soluble

Trace metal-clean seawater samples were collected using the U.S. GEOTRACES sampling system consisting of 24 Teflon-coated GO-FLO bottles that had been pre-rinsed with a 24+ hour treatment of filtered surface seawater at the beginning of the cruise (see Cutter & Bruland, 2012 for more information on the sampling system). At each station, the bottles were deployed open and tripped on ascent at 3 m/min. On deck, the bottles were kept in a trace metal clean sampling van over-pressurized with HEPA-filtered air, except immediately prior to and following deployments, in which cases they were covered on both ends with shower caps to avoid deck contamination.

During sampling in the clean van, the GO-FLO bottles were pressurized to ~0.4 atm with HEPA-filtered air, and their spigots were fitted with an acid-cleaned piece of Bev-a-Line tubing that fed into an Acropak-200 Supor capsule filter (0.2 µm pore size made of polyethersulfone). Before use, this filter had been filled with filtered surface seawater that had been acidifed to pH 2 with 4x Vycor-distilled hydrochloric acid ('trace metal clean HCl') and left overnight to rinse. Before collecting any subsamples, at least 500mL of seawater was passed through the filter (while eliminating air bubbles in the capsule reservoir). Subsamples were taken into acid-cleaned HDPE bottles after a single rinse. Acropak filters were used for at most 3 casts before a new filter was used, and they were stored empty in the refrigerator while not in use. GEOFish surface samples were taken using an all-plastic 'towed fish' pumping system as described in Bruland et al. 2005 at approximately 2 m depth and were subsampled identically.

Subsamples were immediately taken into a HEPA-filtered flow bench constructed in a 'plastic bubble' over-pressurized with HEPA-filtered air. They were then further filtered through an offline filter rig fitted with a 0.02 µm pore-sized alumina Anodisc filter (47mm) and pressurized to ~0.4atm. Un-rinsed filters were loaded straight into the filter rig and were first rinsed with >50mL of pH 2 trace metal clean HCl, and then they were rinsed with >50mL of Milli-Q water. Before each sample was processed, >50mL of sample was passed through the filter. Subsamples were taken into acid cleaned HDPE bottles after a single bottle rinse. Filters were shared for at maximum 2 sampling depths to avoid filter clogging. All samples were filtered within 3-6 hours of being collected to avoid Fe loss to the bottle walls (Fitzsimmons & Boyle, 2012), and samples were acidified at sea to pH 2 with trace metal clean HCl.

Samples were analyzed at least 5 months after acidification over 18 analytical sessions by the isotope-dilution ICP-MS methods described in Lee et al. 2011, which includes pre-concentration on nitrilotriacetate (NTA) resin and analysis on a Micromass IsoProbe MC-ICP-MS. On each day of sample analyses, procedure blanks were determined using 12 replicates of 300 µL of an in-house standard reference material seawater (SAFe surface underway), where the known amount of Fe in the 300 µL was subtracted out. The procedure blank over the relevant sessions ranged from 0.006-0.108 nmol/kg, averaging 0.051 nmol/kg. Procedure blanks were very reproducible with an average standard deviation of 0.010 nmol/kg, resulting in a detection limit (3x this standard deviation) of 0.030 nmol/kg. Replicate analyses of the SAFe surface underway sample (not the official SAFe surface SRM but an in-house, large volume sample from the same cruise/site) averaged 0.079±0.050 nmol/kg (±1SD, n=73), which is within error of the consensus value of 0.090 nmol/kg. Replicate analyses of the SAFe D2 sample averaged 0.916±0.023 nmol/kg (±1SD, n=37, Bottle 446), which is within error of the consensus value of 0.90 nmol/kg.

References:
Cutter, G.A and Bruland, K.W. 2012. Rapid and noncontaminating sampling system for trace elements in global ocean surveys. Limnology & Oceanography: Methods 10: 425-436. doi: 10.4319/lom.2012.10.425
Bruland, K.W., Rue, E.L., Smith, G.J., and DiTullio, G.R. 2005. Iron, macronutrients and diatom blooms in the Peru upwelling regime: brown and blue waters of Peru. Marine Chemistry 93:81-103. doi: 10.1016/j.marchem.2004.06.011
Fitzsimmons, J.N. and Boyle, E.A. 2012. An intercalibration between the GEOTRACES GO-FLO and the MITESS/Vanes sampling systems for dissolved iron concentration analyses (and a closer look at adsorption effects). Limnology & Oceanography: Methods 10: 437-450.
Lee, J.-M., Boyle, E.A., Echegoyen-Sanz, Y., Fitzsimmons, J.N., Zhang, R., and Kayser, R.A. 2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry. Analytica Chimica Acta 686: 93-101. doi: 10.1016/j.aca.2010.11.052