Dataset: GP16 Iron Zinc and Cadmium isotopes dissolved
Deployment: TN303

Samples were provided from the GEOTRACES trace-metal clean sampling rosette and were sampled into acid-cleaned 1L or 4L LDPE bottles by the onboard GEOTRACES clean sampling team. Upon receipt of samples on shore, they were acidified to pH 2 using clean hydrochloric acid and left at this pH for at least 3 months before processing.

A preliminary analysis of Fe, Zn, and Cd concentration was performed by extraction of 10 mL seawater onto Nobias PA-1 resin at pH 5-7 and analysis on an Element ICPMS by isotope dilution. These data are generally not reported, except in rare cases where the concentration data based on 1L samples was not available. Data generated by this method are flagged (see below).

Every sample was simultaneously extracted and purified for the analysis of delta56Fe, delta66Zn, delta114Cd, [Fe], [Zn], and [Cd] using published methods (Conway et al., 2013). Briefly, Fe, Zn, and Cd double-spikes were added to the acidified samples and allowed to equilibrate for > 1 hour. Metals were then extracted from seawater onto an ethylenediaminetriacetic acid resin (Nobias PA-1; Hitachi) for 2 hours at pH 2 and 2 hours at pH between 6-6.3. Metals were eluted off this resin using 3 N HNO3, and were then purified by anion exchange chromatography before being reconstituted in 0.1 N HNO3 for analysis.

Concentrations and stable isotope ratios were determined simultaneously by analysis on a Neptune multi-collector ICP-MS. Fe and Zn were analyzed in high resolution mode, while Cd was analyzed in low resolution mode. Every sample was analyzed twice, and the mean isotope ratios are presented here, expressed relative to international isotope standards. Instrumental and procedural isotopic fractionation was corrected for using a double-spike data reduction scheme.

Concentrations were calculated using isotope-dilution to calculate ng of metal per sample, and then converted to nmol kg-1 or pmol kg-1 using the weight of each sample processed. Concentration numbers represent the mean of two ICP-MS analyses of each sample. The largest sources of error for concentration analyses are determining the weight of the sample and determining the volume of double-spike added to each sample. We approximate 2% error for each sample based on the combination of these two factors. Accuracy was checked by analysis of S, D1 and D2 SAFe reference standards (for which we determined values identical to the consensus values; see Conway et al, 2013) and intercomparison with two other groups measuring trace metal concentrations on GP16 samples.

Analytical accuracy of stable isotope ratios is equal to the analytical precision (i.e. the analysis is free from systematic errors; John, 2012) and is calculated separately for each sample. Uncertainty for isotope ratios is expressed as the 2 sigma uncertainty on the average isotope ratio, calculated from a combination of the internal standard error of each of two measurements and the bracketing isotope standards used, as described in Conway et al., (2013). 

Data are flagged as follows:

1 (good): data appear to be of high quality

4 (no data): samples either were not analyzed, or there was a complete failure during analysis such as failure to recover the metals onto Nobias resin or failure of the insturment during analysis of this sample

7 (questionable): data based on “oceanographic consistency” these data do not appear to be correct, for example they fall very far off the trends apparent in other data above and below the point suggesting that they are “flyers”

8 (low-volume): concentration data from the 1L isotope sample was not available, so concentrations from the 10 mL samples have been used instead