Dataset: GI04 Dissolved Lead and Lead Isotope Ratios

A CTD carousel multi-sampling system (CTD-CMS) was used for collection of water samples during the KH-09-5 cruise of R/V Hakuho-Maru. The system was composed of a carousel (Sea-Bird, SBE-32), a CTD fish (Sea-Bird, Model SBE-9-plus) with a DO sensor, an Epoxy-coated Al frame, and 24 Teflon-coated Niskin-X bottles (General Oceanics, 12-liter type). A pinger (Benthos, Model BFP-312) and/or altimeter (Benthos, Model PSA-916) were installed on the frame to monitor the distance above the sea bottom. A turbidity meter (SeaPoint) was used during Legs 1 and 2. The fluorometer used was Aquatracka Mk III (Chelsea). Zn anodes except those on the CTD housing had been removed in order to avoid Zn contamination and replaced by Al anodes before the cruise.

All the Niskin-X bottles were filled with 1.5% aqueous solution of Extran MA01 for 1 day, 0.1 M HCl for 1day, and ultra-high purity water for more than 2 days, successively. Teflon spigots of the bottles were soaked in 1% Extran solution and 1M HCl for 1 day. They were then soaked in a mixed solution of HClO4, H2SO4 and HNO3 (1:1:1 mixture) at 120 ˚C for 3 hrs, 6 M HCl at 120 ˚C for 3 hrs, and ultra-high purity water at 100 ˚C for 3 hrs, successively. Viton O-rings were soaked in 1% Extran (1 day), 0.1 M HCl (1 day), 0.1 M HCl at 60 ˚C (12hrs), and ultra-high purity water at 68 ˚C (12 hrs).

The CTD-CMS system was attached at the end of a Ti-armored cable (8 mm o.d.) from the No. 2 winch of the vessel and controlled by a CTD deck unit (Sea-Bird, Model 11plus) connected with a computer. During the hydrocasts, the system was lowered down to a depth of ~10 m above the bottom, and water samples were collected at appropriate depths while the system was coming up to the surface. The vessel steered so that surface water came from starboard during the sampling in order to minimize contamination from the ship. Upon retrieved, Niskin-X bottles were transferred into a clean space called "Bubble" and hold vertically on a wooden frame in the Bubble. The Bubble space has a volume of about 10 m3 (2.5 m x 2 m x 2 m), to which filtered clean air was always introduced through two HEPA filter units. A 0.2 um nominal pore size capsule type cartridge filter (Pall Scientific, AcroPak 200) was connected to the spigot of the Niskin-X bottle and water inside the bottle was pressurized with compressed clean air supplied through the top air vent of each bottle. Filtered water samples were obtained after flushing for 30 sec. Transfer of the bottle and sub-sampling inside the clean space follow the GEOTRACES Cookbook recommendations.

Pb concentration analysis: Total Pb concentration was analyzed by the NTA ID-ICPMS method described by Lee et al. (2011). Acid-leached 1.5 mL vials were used. The acidified seawater sample was poured into the vial and 1.3 mL of the sample was pipetted out using a clean pipette tip. The 1.3 mL sample in the pipette tip was pipetted back into the same vial after quickly emptying the vial. Then, the sample was spiked with 25 uL of stable ²⁰⁴Pb isotope spike and left for a few minutes to establish equilibrium between added isotope spikes with their natural isotopes in the samples. Then, the pH of the sample was adjusted to pH=5.3 using ammonium acetate buffer solution (pH= 7.9-8.0). Finally, 150 uL of an NTA resin suspension (2400 beads) were added to the vial and it was shaken on an orbital table at 2000 rpm for 4 days to allow the Pb and resin to bind. After 4 days, the sample vial was centrifuged (for 45 seconds at 14,500 rpm) and the overlying seawater was siphoned off carefully, leaving the resin beads at the bottom. The resin beads were then rinsed three times with high purity H₂O to remove seasalt. In each rinse, the vial was filled with high purity water, centrifuged down, and the supernatant was siphoned off. After rinsing, 150 uL of 0.5 M HNO₃ were added to the vial, and then two days allowed for Pb to be released into solution. The final solution was then brought to the ICPMS for measurement of the isotope ratio (²⁰⁸Pb/²⁰⁴Pb) of this solution (after on-peak zero acid blank correction) which is used to calculate the Pb concentration.

Pb isotopes were analyzed in the same laboratory from another aliquot of the same seawater samples. First, samples were concentrated and purified for Pb isotope analysis by the double Mg(OH)₂ precipitation method described by Reuer et al. (2003) as modified by Boyle et al. (2012). Depending on the Pb concentration in each sample, 270-550 ml of sample was poured into a clean 1-L polyethylene separatory funnel. A small amount of vapor-distilled ammonia solution was added to form Mg(OH)₂ precipitate, and the next day, the settled precipitate was drawn off into clean 50 mL polypropylene centrifuge tubes, centrifuged, and the supernatant siphoned off. The precipitate was then re-dissolved in high-purity 6M HCl, and a few drops of ammonia solution added again to repeat the Mg(OH)₂ precipitation. The amount of ammonia added to the samples for the first and second precipitation was determined empirically to produce Mg(OH)₂ precipitate that scavenges Pb nearly quantitatively (Wu and Boyle, 1997b), but minimizes the amount of precipitate so as to allow for a single final anion purification step. The final wet Mg(OH)₂ precipitate was usually less than 100 ul in volume. It was dissolved in high-purity 1.1M HBr for the HCl-HBr anion exchange column separation.

As described in Boyle et al. (2012), Si(OH)₄ scavenged during Mg(OH)₂ precipitation tends to precipitate as a thick gel if the amount of HBr added to the final precipitate is insufficient, and this gel clogs the anion columns during the purification procedure. This problem occurred for high-Si samples, e.g. deep samples and the samples from station 14. For most of the samples, this problem was resolved by adjusting the volume of 1.1M HBr (0.8 to 2.8 ml) added to the samples to avoid precipitation. However, for some samples, the silica gel was not dissolved completely even after increasing the volume of HBr. In that case, we centrifuged the vials to settle the silica gel to the bottom and carefully loaded the supernatant HBr solution onto the columns. This may lower the recovery efficiency of the procedure as some Pb is trapped in the silica gel. However, the loss of Pb does not seem to affect the resulting Pb isotope ratios (i.e., little fractionation occurs by Pb entrapment in the silica gel) given that 1) duplicates of a few samples processed with different amount of HBr (different amount of silica gel) gave identical results, and 2) the Pb isotope ratios of the samples processed with silica gel were consistent with samples from immediately above and below in the water column that were processed without silica gel formation.

Samples purified by passing through HCl-HBr anion exchange columns were dried on a hotplate in a clean recirculating fume hood, and then re-dissolved in 0.2M HNO₃ on the day of ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analysis. The samples were spiked with Tl for exponential "beta" mass fractionation correction based on ²⁰⁵Tl/²⁰³Tl normalization. Pb isotopes (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb) of the samples were measured using a GV/Micromass Isoprobe multiple collector ICP-MS, with ²⁰⁴Pb detection on an ion-counting Daly-style detector with WARP (Wide Aperture Retarding Potential) filter. Data processing and corrections were performed as in Boyle et al. (2012), which is the method modified from Reuer et al. (2003).