Sampling and Analytical Methodology:

Seawater samples were collected from the surface ocean to depths of 750-1000 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 μm polycarbonate filters (Nuclepore) in polypropylene holders. Acid-cleaned backing filters of mixed cellulose esters were used to ensure even loading on sample filters. Samples were rinsed while on the filter holders with 15-20 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.

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'X 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 μm 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.

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:

MDL = ( 3 * √I_b)/(I_p/conc)

where I_b is the background intensity, I_p is the peak intensity, and conc is the concentration of the standard. MicroMatter standards were used to calculate MDLs.

Related files and references:
Barrett, Pamela M., Resing, Joseph A., Buck, Nathaniel J., Buck, Clifton S., Landing, William M., Measures, Christopher I., The trace element composition of suspended particulate matter in the upper 1000 m of the eastern North Atlantic Ocean: A16N, Marine Chemistry (2012), doi: 10.1016/j.marchem.2012.07.006.

Feely, R.A., Massoth, G.J., Lebon, G.T., 1991. Sampling of marine particulate matter and analysis by x-ray fluorescence spectrometry. In: Hurd, D.C. and Spencer, D.W. (eds.), Marine Particles: Analysis and Characterization, AGU Geophysical Monograph Series, p. 251-257.

Gladney, E.S., Roelandts, I., 1988. 1987 Compilation of elemental concentration data for USGS BHVO-1, MAG-1, QLO-1, RGM-1, SCo-1, SDC-1, SGR-1 and STM-1. Geostandards Newsl. 12 (2), 253–362.

Holynska, B., Bisiniek, K., 1976. Determination of trace amounts of metals in saline water by energy dispersive XRF using the NaDDTC preconcentration. J. Radioanal. Nucl. Chem. 31, 159–166.

Measures, C.I., Landing, W.M., Brown, M.T., Buck, C.S., 2008. A commercially available rosette system for trace metal-clean sampling. Limnology and Oceanography Methods 6, 384-394.