Total, refractory, and labile particulate element concentrations via ICPMS:
Labile and total suspended particulate trace elements concentrations are reported for: Al, Ba, Cd, Co, Cu, Fe, La, Mn, Ni, P, Pb, Sc, Th, Ti, V, Y, Zn. Concentrations of the labile fraction of these particulate elements are indicated as element names followed by the suffix ‘-Labile’, refractory portions are indicated with the suffix ‘-Refractory’, and concentrations of total particulate elements (the sum of labile and refractory) are followed by ‘-Total’. Concentrations are reported in units of picomoles per liter (pmol/L).

Sampling Methodology:
Trace metal-clean seawater samples were collected using a sampling system consisting of Teflon-coated GO-Flo bottles and following methods described in Bruland et al (1979). Additional samples were collected from surface waters (~2 meters) using a towed ‘fish’ deployed by Ken Bruland's lab. Water collected with GO-Flo bottles and the ‘fish’ was transferred into acid-washed 4-liter (L) LDPE carboys for off-line filtration.

All filtration was conducted in a HEPA-filtered 'bubble' (temporary clean room). A filter holder containing a 25-millimeter (mm) diameter Pall Supor 0.4-micrometer (um) polyethersulfone membrane was attached to the opening at the carboy top. Carboys were pressurized with 0.2‐um filtered air and inverted during filtration to ensure that all particles were captured on the membrane. Filtrate was collected to determine the volume of seawater filtered: an average of 2.1L was filtered through each membrane. After filtration, membranes were folded into quarters, placed in 1.7-milliliter (mL) polypropylene vials, and stored at -20 degrees Celsius until analysis.

Analytical Methodology:
All digestion steps were performed in a Class-100 clean room using standard clean techniques. Filters were sequentially digested, first following the protocol of Berger et al. (2008) to obtain labile particulate concentrations and then digested using a 4M HCl, 4M HNO3, and 4M HF mixture as described in Ohnemus et al. (2014) to obtain refractory particulate element concentrations.

For the labile particulate leach, a 1-milliliter solution of 25% Optima-grade acetic acid and 0.02 M hydroxylamine was added to the filter stored in a 1.7 mL polypropylene vial. Following the recommendation of Berger et al. (2008), the solution was heated to 95 degrees C in a water bath for 10 minutes and then allowed to cool to room temperature. The filter was in contact with the acetic acid leach for a total of two hours, after which the filter was removed from the polypropylene vial and placed in an acid-cleaned 22-mL PFA vial. The acetic acid/hydroxylamine leachate was centrifuged at 14,000 rpm for 10 minutes to sediment all particles. Without disturbing particles on the bottom of the tube, approximately 0.8 mL of leachate was transferred into an acid-cleaned 7 mL PFA digestion vial. Optima-grade HNO3 was added (100 uL) to the digestion vial, which was subsequently heated uncapped at 110 degrees C to near dryness. Vial contents were redissolved with 2% HNO3 (Optima grade).

Refractory particulate metals were determined by subsequent digestion of the filter. Two milliliters of a solution containing 4M HCl, 4M HNO3, and 4M HF (all Optima grade) was added to the filter which was placed in a cleaned 22-mL PFA vial. The vial was tightly capped and heated to 110°C for 4 hours. This procedure has been determined to be adequate for digestion of all particulate material, while allowing the Supor filter to remain intact (Ohnemus et al. 2014). Following heating, the acid solution in the bomb was poured into a second PFA vial, leaving the filter piece behind. To ensure complete transfer of acid, the bombs were thoroughly rinsed with 3 × 0.5 mL aliquots of ultrapure water which were poured into the secondary vial. The secondary vial was then heated to dryness and the contents redissolved with 1 mL of a 50% Optima-grade HNO3 + 15% Optima-grade H2O2 (v/v of concentrated reagents) solution. This solution was again dried down and the contents redissolved with 2% HNO3.

All digests were analyzed using a Finnigan-MAT Element2 HR-ICP-MS at the University of Maine following the protocols outlined in Twining et al. (2011). The instrument is equipped with a cyclonic nebulizer, an autosampler contained under a HEPA filter, and nickel cones. Ba, Cd, La, Th, and Y were analyzed in low-resolution mode, while the remaining isotopes were analyzed in medium-resolution mode.

Quantification was performed by external calibration, and In-115 was used as an internal standard to correct for variations in instrumental sensitivity during analyses. Cs-133, spiked during the initial sample leaches, was used as a process recovery monitor, but no samples were discarded or corrected using the Cs recoveries, as typical Cs recoveries were 90-110%. Certified reference materials were digested alongside refractory sample digests. Average recoveries for each element are given in the attached supplemental file "CRMs.png".