Seawater samples were collected from the surface ocean to depths of 750\u20131000 m, then sub-sampled for suspended particulate matter under trace-element-clean conditions (see Measures et al., 2008 for details). Briefly, suspended particulate matter samples were collected by pressurizing GO-FLO bottles with <10 psi filtered, compressed air. Samples were filtered through acid-cleaned 0.4 um polycarbonate filters (Nuclepore) in polypropylene holders. Acid-cleaned backing filters of mixed cellulose esters were used to ensure even loading on sample filters.\u00a0 Samples were rinsed while on the filter holders with 15\u201320 mL deionized (DI) water adjusted to pH 8 with ammonium hydroxide from the edges to the center, with a low vacuum applied to avoid loss or re-distribution of particles. Filtration was started approximately 30 to 60 minutes after water samples were collected and was generally completed within 60 minutes.
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Particulate trace element concentrations were determined by energy-dispersive X-ray fluorescence (ED-XRF) using a thin film technique described by Feely et al. (1991). An optical subsample of each filter was analyzed on a Thermo Fisher Quant\u2019X equipped with a Rhodium Target X-Ray tube and an electronically cooled, lithium-drifted solid state detector. X-rays for primary sample excitation were filtered for optimum control of peak-to-background ratios. Standards for calibration consisted of commercial thin film standards (MicroMatter), geochemical reference sample material MAG-1 (Gladney and Roelandts, 1988) finely ground and loaded onto polycarbonate filters (Feely et al., 1991), and a series of standards prepared using a modification of the method reported by Holynska and Bisiniek (1976) using sodium diethyldithiocarbamate (NaDDTC) to quantitatively precipitate trace metals from a solution of known concentration. These standards were prepared using a 1% NaDDTC solution that was added to trace metal solutions at pH 4 with a ligand-to-metal ratio of 4:1. The resulting precipitate was filtered over an acid-cleaned, 0.4 um polycarbonate Nuclepore filter. Procedural blanks were made by addition of a NaDDTC solution to a sample of acidified DI water followed by filtration. MicroMatter and MAG-1 standards were used to create individual standard curves for all elements; standard curves for Fe and Mn also included NaDDTC standards.\u00a0
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Four different excitation conditions, all conducted under a vacuum atmosphere, were used for sample analysis and are detailed along with minimum determination limits (MDL) in Table 1. MDLs are defined as 3 times the square root of the background intensity measured from a standard of known concentration:
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MDL = ( 3 * \u221aIb)/(Ip/conc)\u00a0\u00a0 \u00a0
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where\u00a0Ib\u00a0is the background intensity,\u00a0Ip\u00a0is the peak intensity, and\u00a0conc\u00a0is the concentration of the standard. MicroMatter standards were used to calculate MDLs.\u00a0
ED-XRF output is in ng cm-2\u00a0and is reported here as seawater concentrations (nM) using the following formula for each trace element of interest.
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Seawater concentration (nmol L-1) = filter concentration (ng cm-2) * filter sample area (cm2) / filtered seawater volume (L) / atomic weight (g/mol)
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The filter sample collection area is 12.41 cm2. The volume of seawater filtered per sample ranged from 1 to 11 L, average 9 L.
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For values below the analytical detection limit, values are indicated as \u201cBDL\u201d\u00a0
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Quality flags are indicated as:
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1 = sample drawn from bottom but measurement not received
\n2 = acceptable
\n3 = questionable
\n9 = no sample drawn