Dataset: GP15 Fe Zn Cd Dissolved and Stable Isotope Ratios - Leg 2
Deployment: RR1815

Seawater samples were collected using the GEOTRACES trace-metal clean sampling system (rosette with 24 12-liter GO-FLO bottles) or a towfish for surface samples (Cutter and Bruland, 2012). Samples were filtered through acid-cleaned AcroPak capsule filters (0.2 micrometers (μm)) into acid-cleaned LDPE bottles. For concentration and isotope analysis at the University of South Florida (USF), 2 liters of filtered seawater were collected, acidified ashore at USF by addition of 2.4 milliliters (mL) 10 molar (M) Teflon-distilled HCl, and stored for at least 12 months at pH ∼2 before processing.

Seawater samples were processed for isotope analysis following Sieber et al. (2019). Briefly, a double-spike for cadmium (Cd), iron (Fe), and zinc (Zn) was added prior to batch extraction using Nobias PA-1 chelating resin, followed by purification by anion-exchange chromatography using AG-MP1 resin. Isotope analyses were performed on a Thermo Neptune Plus MC-ICPMS in the Tampa Bay Plasma Facility at the University of South Florida using an Apex-Q (Cd) or Apex Ω (Fe, Zn) introduction system.

We express Cd stable isotope ratios in delta notation (δ¹¹⁴Cd) relative to the NIST SRM-3108 Cd standard. A secondary standard, BAM-I012, was analyzed over 8 sessions on the same timescale as the samples to provide an estimate of long-term instrumental precision. We obtain a value of −1.32 ± 0.06‰ (2SD, n = 172), in agreement with consensus values (Abouchami et al., 2013). Using the 2SD of offsets from the mean of full replicate measurements based on 26 pairs of replicate analysis on separate seawater samples collected at the same depth (the GP15 sampling strategy collected overlapping samples between casts), we obtain a second estimate of external precision (0.05‰), which is similar to analytical precision. Therefore, we consider a 2SD uncertainty of 0.06‰ as a conservative estimate of analytical precision, and have applied it to all samples, except for low-concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected ¹¹⁴Cd/¹¹¹Cd ratios measured simultaneously with isotope analyses (Sieber et al., 2019). We express uncertainty (1SD) on Cd concentrations as 2%, based on replicate analysis on separate seawater samples collected at the same depth (n = 26).

We express Fe stable isotope ratios in delta notation (δ⁵⁶Fe) relative to the IRMM-014 standard. A secondary Fe standard, NIST-3126, was analyzed over 44 sessions to provide an estimate of long-term instrumental precision. We obtain a value of +0.36 ± 0.05‰ (2SD, n = 604), in agreement with consensus values (Conway et al., 2013). As a second estimate of external precision, we use the 2SD of offsets from the mean of full replicate measurements based on 26 pairs of replicate analysis using separate seawater samples collected at the same depth (0.08‰), which is similar to the analytical precision. Therefore, we consider a 2SD uncertainty of 0.05‰ as an estimate of analytical precision, and have applied it to all samples, except for low-concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected ⁵⁷Fe/⁵⁶Fe ratios measured simultaneously with isotope analysis. We express uncertainty (1SD) on Fe concentrations as 2%, based on replicate analysis on separate seawater samples collected at the same depth (n = 26).

We express Zn stable isotope ratios in delta notation (δ⁶⁶Zn) relative to the JMC-Lyon standard. A secondary Zn standard, AA-ETH, was analyzed on the same timescale as the samples (over 10 sessions) to provide an estimate of long-term instrumental precision. We obtain a value of +0.28 ± 0.03‰ (2SD, n = 147), in agreement with consensus values (Archer et al., 2017). As a second estimate of external precision, we use the 2SD of offsets from the mean of full replicate measurements based on 26 pairs of replicate analysis using separate seawater samples collected at the same depth (0.03‰), which is similar to the analytical precision. Therefore, we consider a 2SD uncertainty of 0.03‰ as an estimate of analytical precision, and have applied it to all samples, except for low-concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected ⁶⁷Zn/⁶⁶Zn ratios measured simultaneously with isotope analysis. We express uncertainty (1SD) on Zn concentrations as 5%, based on replicate analysis on separate seawater samples collected at the same depth (n = 26).