Sampling methods:
The water column samples were collected from The U.S. GEOTRACES trace-metal clean CTD carousel (GTC). This sampler has a plastic-coated aluminum frame, titanium pressure housings for electronics and sensors, no sacrificial zinc anodes, and 24 ×12 liter General Oceanics GO-FLO bottles modified for trace metal sampling (Cutter and Bruland, 2012). Before deployment and immediately upon recovery, the tops and bottoms of the GO-FLO bottles were covered with polyethylene shower-caps, and the bottles were removed from the frame and carried into the U.S. GEOTRACES clean container laboratory for sub-sampling. The GO-FLO bottles were pressurized to 6 psi using 0.2 μm HEPA-filtered compressed air, and samples were passed through 0.2 μm Pall ACROPAK SUPOR filter capsules that had been pre-cleaned by soaking overnight and flushed with 5 L of unacidified seawater.

For the Loihi Seamount stations (Station 18.3 and 18.6), the seawater samples were also collected from a 36 place, ∼10 liter Niskin bottle rosette (ODF) in order to avoid questions of colocation due to short term time and space scale variability between hydrocasts. The samples were filtered through 0.8/0.45 μm Pall ACROPAK 500 capsules that had been pre-cleaned by soaking three days with 10% hydrochloric acid (1.2 M HCl) and flushed with 3-5 L of Milli-Q water, and stored refrigerated between casts. The capsule filter was pre-flushed with 0.5-1 L of unacidified seawater. The filtered seawater samples were acidified to pH 2 using sub-boiling distilled hydrochloric acid. Although the samples were drawn from a non trace-metal clean Niskin rosette, the Fe and Mn values are much larger than measured contamination levels for this system.

In order to evaluate the potential contamination for dAl from the 36 place Niskin rosette, the seawater samples were collected at Station 35 from GTC and ODF casts.

Surface seawater sampling was carried out using a towfish deployed from the starboard side of the vessel at a depth of ~3 m (Bruland et al., 2005), with seawater pumped through Teflon tubing from the fish into a clean “bubble” set up onboard. Filtered samples (0.2 μm Acropak) were taken while underway just before arrival at each hydro-station and occasionally during transit approximately halfway between stations. Sampling into acid-cleaned plastic bottles was conducted following GEOTRACES protocols (Cutter et al., 2017).

Samples of the 0.2 μm-filtered seawater were collected acid pre-washed 125 mL polymethylpentene bottles (PMP) 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. Samples were analyzed shipboard for dissolved Al, Fe & Mn using flow injection analysis methods (Resing and Measures, 1994; Measures et al., 1995, Resing and Mottl, 1992 respectively). Prior to determination samples were acidified to 0.006 M hydrochloric acid (HCl) using sub-boiling distilled 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.

For preparation for standard solutions:
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 +4.94nM and +9.88nM for Al, +0.60nM and +1.20 nM for Fe, +0.62 nM and +1.23 nM 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 µl 6N HCl and 5 ml of sample buffer to the replicate bottle and comparing the resulting signal to the original sample.

For dissolved Al analysis:
Dissolved Al was determined using a Flow Injection Analysis scheme with fluorometric detection. Major components were a Rabbit peristaltic pump, a Dynamax FL-1 flourometer, 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 flourescence 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 0.14 and a precision of 1.8% at 2.51 nM.

For dissolved Fe analysis:
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 Rainin Dynamax UV-C, 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 oxidized 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 0.071 nM and a precision of 1.9% at 1.4 nM.

For dissolved Mn analysis:
Dissolved Mn was determined using a Flow Injection Analysis scheme with spectrophotometric detection (Rainin Dynamax UV-C). Major components were a Rabbit peristaltic pump, a Rainin Dynamax UV-C, 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 manganese eluted from the column is achieved through its catalytic effect on the formation of malachite green which is measured at 620 nm. Detailed description of the methodology is published in Resing and Mottl (1992). A 3-minute pre-concentration of sample (~9 ml) onto an 8-hydroxyquinoline (8-HQ) resin column yielded a detection limit of 0.15 nM and a precision of 9.7% at 0.51 nM.

Calculation of each dissolved trace element concentration:
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.

The analytical blanks of dissolved Fe and Mn were determined by the shore-based ICPMS data that was measured by co-PI Dr. J. Fitzsimmons (Texas A&M University). The ICPMS data and its metadata will be submitted separately. The blank corrected FIA data was obtained from the intercept of the correlation plot between the ICPMS data and FIA data on each of the days the FIA was run.

Inter-calibration for dissolved Al:
GEOTRACES standard seawater and internal standard seawater were analyzed periodically. A large batch of seawater was acidified prior to the cruise and used as a primary standard during measurements. This standard seawater was run along with each sample analysis. Our laboratory has participated the GEOTRACES intercalibration effort using this flow injection method.

We report our laboratory values for the GEOTRACES GS standard analyses using this flow injection method to be: GEOTRACES GS = 29.63±1.15nM (n=2), D1=0.78, D2=0.88, GSP=1.10±0.03nM (n=2), GSC=0.54±0.08nM (n=2).

These results are in good agreement with this from the GEOTRACES intercalibration effort for Al and demonstrate that the methodologies employed to produce this dataset detect concentrations within the standard deviation of current consensus values.

The Al data at Station 35 from the GEOTRACES PMT cruise (GP15) were compared with the value at Station 36 from the GP16 EPZT cruise obtained by Joe Resing which all show very similar results throughout the water column.