Sampling Methods
The samples (with the exception of surface samples) were collected for trace metal determinations at 11 stations from the RV Knorr using a custom-built US GEOTRACES trace metal clean rosette consisting of an epoxy painted Al rosette frame containing 24x12 L GO-FLO bottles (Cutter and Bruland, 2012). Immediately after the package was recovered, the tops of the GO-FLO bottles were covered with plastic bags and the bottles were removed from the frame and carried into the US GEOTRACES clean van for sub-sampling. The GO-FLO bottles were pressurized to 10 psi using 0.2 µm-filtered compressed air and samples were filtered through 0.2 µm Acropak filters. All sub-sampling was undertaken in the clean van using rigorous trace metal protocols. Surface samples are from the towed fish surface sampling system.

Analytical Methods
FIA Sample Preparation: Samples were drawn into pre-numbered 125 ml PMP bottles after three rinses and were stored in plastic bags in the dark at room temperature before determination which was usually within 12 -36 hours of collection. Prior to determination samples were acidified by the addition of 125 ul sub-boiling distilled 6N HCl (hereinafter 6N HCl) and were microwaved in groups of 4 for 3 minutes in a 900 W microwave oven to achieve a temperature of 60+10 °C. Samples were allowed to cool for at least 1 hour prior to determination. Samples were determined in groups of 8.

FIA Standards: Shipboard mixed standards (Al and Fe, Mn) were prepared in the shore-based laboratory by serial dilution of commercial Al, Fe, and Mn standards (BDH Aristar) into distilled water which was acidified with the equivalent of 4 ml sub-boiled 6N HCl. Standards for instrument calibration were prepared daily from filtered seawater by acidifying 1 L of low Fe seawater from a previous cast with 1 ml of 6N HCl and microwaving for 5 minutes to reach a temperature of 60 + 10°C. After 1 hour, 200 + 2 ml of the cooled seawater was added to each of three 250 ml PMP bottles each of which had been rinsed three times with the microwaved seawater and shaken dry. Working standards were prepared by adding 0, +100µL, +200µL spikes of the shipboard mixed standard to these bottles, to yield a standard curve of +28.4nM and +56.81nM for Al, +1.26nM and +2.53nM for Fe, +5.11nM and +10.22nM for Mn. The system blank from the addition of the acid and buffer to samples was determined by double spiking a replicate sample i.e. by adding 2 x 125 ul 6N HCl and 5 ml of sample buffer to the replicate bottle and comparing the resulting signal to the original sample.

Dissolved Al Concentrations: Dissolved Al was determined using a Flow Injection Analysis scheme with fluorometric detection. Major components were a Rabbit peristaltic pump, a Dynamax FL-1 fluorometer, a Rainin A/D board and a Macintosh G3 computer running Rainin MacIntegrator v 1.4.3 to log and reduce data. The analytical scheme produces a complex between lumogallion and dissolved Al which when excited at 484 nm produces fluorescence at 552 nm. Detailed description of the methodology is published in Resing and Measures (1994).

A 3-minute pre-concentration of sample (~9 ml) onto an 8-hydroxyquinoline (8-HQ) resin column yielded a detection limit of 1.02 and a precision of 1.94% at 17.58 nM.

Dissolved Fe: Dissolved Fe was determined using a Flow Injection Analysis scheme with spectrophotometric detection (Rainin Dynamax UV-C). Major components were a Rabbit peristaltic pump, a Dynamax FL-1 fluorometer, a Rainin A/D board and a Macintosh G3 computer running Rainin MacIntegrator v 1.4.3 to log and reduce data. The spectrophotometric detection of the iron eluted from the column is achieved through its catalytic effect on the oxidation of N,N-dimethylp-phenylenediamine dihydrochloride (DPD) the oxidised product is measured at 514 nm. Detailed description of the methodology is published in Measures et al (1995).

A 3-minute pre-concentration of sample (~9 ml) onto an 8-hydroxyquinoline (8-HQ) resin column yielded a detection limit of 0.064 nM and a precision of 3.1%.

Calculation of FIA data: Calculation of sample concentrations was by dividing the peak height derived from sample using the A/D software by the calculated slope of the standard curve. Variations in the slope of the standard curve during a day's run were corrected by the following procedure.

The change in the value of the slope of the standard curve between each run of standards was divided by the number of samples run between those standards to provide a calculated value for the slope of the standard curve at the point each sample was run. The value of the peak height for each sample was then recalculated by the estimated ratio of the standard curve slope at the point that sample was run. The estimate of the slope at each sample run is calculated by: (Initial slope + (incremental change per sample X # of samples run since initial standard was run)). The sample concentration is then calculated from the initial standard curve slope.

Data correction with shore-based ICPMS data
Shipboard determined dissolved Fe and Mn have been corrected using the shore-based ICPMS data that was measured by co-PI Dr. Jingfeng Wu. That data will be submitted separately. The corrected FIA data was calculated using the slope and the intercept of the correlation plot between the ICPMS data and FIA data on each of the days the FIA was run.

Shipboard dissolved Fe: Shipboard calculation:
The Fe concentration of samples was determined by dividing the sample peak height by the slope of the Fe standard solution. The standard curve was produced by adding +0 µL (+0 nM), +100 µL (+1.01 nM), +200 µL (+2.02 nM) of the shipboard standard solution into 200mL aliquots of seawater. This seawater was obtained from a 1L sample of seawater (usually from near the chlorophyll max) that had been filtered through a 0.2 µm pore-size Acropack cartridge, acidified with 1 ml of 6N HCl (sub boiling distilled) and microwaved to achieve a temperature of 60 + 10°C. A typical standard curve is shown in Figure 1 below.

Figure 1. Fe standard curve from the standard addition method. (click on the image to view a larger version)

The shipboard standard solution was 2.020 µM and had been prepared by gravimmetric dilution from a high purity standard solution (1000 µg/mL) at the shore-based laboratory of University of Hawaii. Overall precision is 3.1%, and the average detection limit is 0.064 nM.