Dataset: GP16 Pb Dissolved

Sample storage bottle lids and threads were soaked overnight in 2N reagent grade HCl, then filled with 1N reagent grade HCl to be heated in an oven at 60 degrees C overnight, inverted, heated for a second day, and rinsed 5X with pure distilled water. The bottles were then filled with trace metal clean dilute HCl (~0.01N HCl) and again heated in the oven for one day on either end. Clean sample bottles were emptied, and double-bagged prior to rinsing and filling with sample.

Trace metal-clean seawater samples were collected using the U.S. GEOTRACES sampling system consisting of 24 Teflon-coated GO-FLO bottles that had been pre-rinsed with a 24+ hour treatment of filtered surface seawater at the beginning of the cruise (see Cutter & Bruland, 2012 for more information on the sampling system). At each station, the bottles were deployed open and tripped on ascent at 3 m/min. On deck, the bottles were kept in a trace metal clean sampling van over-pressurized with HEPA-filtered air, except immediately prior to and following deployments, in which cases they were covered on both ends with shower caps to avoid deck contamination.

During sampling in the clean van, the GO-FLO bottles were pressurized to ~0.4 atm with HEPA-filtered air, and their spigots were fitted with an acid-cleaned piece of Bev-a-Line tubing that fed into an Acropak-200 Supor capsule filter (0.2 um pore size made of polyethersulfone). Before use, this filter had been filled with filtered surface seawater that had been acidified to pH 2 with trace metal clean HCl and left overnight to rinse. Before collecting any subsamples, at least 500 mL of seawater was passed through the filter (while eliminating air bubbles in the capsule reservoir). Subsamples were taken into acid-cleaned (see above) Nalgene HDPE bottles after a triple rinse with the sample. Acropak filters were used for at most 3 casts before a new filter was used, and they were stored empty in a refrigerator while not in use. GEOFish surface samples were taken using an all-plastic "towed fish" pumping system as described in Bruland et al. 2005 at approximately 3 m depth and were subsampled identically.

All samples were acidified back in the Boyle laboratory at 2 mL per liter seawater (pH ~2) with trace metal clean 6N HCl.

Samples were analyzed at least 1 month after acidification over 52 analytical sessions by one of two methods: a resin pre-concentration or magnesium hydroxide co-precipitation for very low level Pb concentration analyses. Details of the two methods are as follows:

(1) Resin pre-concentration: This method utilized the isotope-dilution ICP-MS method described in Lee et al. 2011, which includes pre-concentration on nitrilotriacetate (NTA) resin and analysis on a Fisons PQ2+ using a 400 uL/min nebulizer. Briefly, samples were poured into 30 mL subsample bottles. Then, triplicate 1.5 mL polyethylene vials (Eppendorf AG) were rinsed three times with the 30 mL subsample. Each sample was pipetted (1.3 mL) from the 30 mL subsample to the 1.5 mL vial. Pipettes were calibrated daily to the desired volume. 25uL of a 204Pb spike were added to each sample, and the pH was raised to 5.3 using a trace metal clean ammonium acetate buffer, prepared at a pH of between 7.9 and 7.98.  ~2400 beads of NTA Superflow resin (Qiagen Inc., Valencia, CA) were added to the mixture, and the vials were set to shake on a shaker for 4 days to allow the sample to equilibrate with the resin. After 4 days, the beads were centrifuged and washed 3 times with pure distilled water, using a trace metal clean syphon tip to remove the water wash from the sample vial following centrifugation. After the last wash, 320 uL of a 0.1N solution of trace metal clean HNO3 was added to the resin to elute the metals, and the samples were set to shake on a shaker for 2 days prior to analysis by ICP-MS. NTA Superflow resin was cleaned by batch rinsing with 0.1N trace metal clean HCl for a few hours, followed by multiple washes until the pH of the solution was above 4. Resin was stored at 4 degrees C in the dark until use, though it was allowed to equilibrate to room temperature prior to the addition to the sample. Eppendorf polyethylene vials were cleaned by heated submersion for 2 days at 60 degrees C in 1N reagent grade HCl, followed by a bulk rinse and 4X individual rinse of each vial with pure distilled water. Each vial was then filled with trace metal clean dilute HCl (~0.01N HCl) and heated in the oven at 60 degrees C for one day on either end. Vials were kept filled until just before usage.

(2) Magnesium hydroxide co-precipitation (a.k.a “Low level method”): This method works for all seawater samples, but was employed especially for lower level Pb concentration samples. The method is a lower volume seawater adaptation of the isotope ratio method of Reuer et al., 2003, which includes pre-concentration by co-precipitation and analysis on a Fisons PQ2+ using a 400 uL/min nebulizer. Briefly, 50 mL polypropylene conical centrifuge tubes (Corning) were weighed and recorded before rinsing three times with seawater sample or blank (low trace metal seawater) and filling with ~40 mL of sample (10 mL of blank). Samples were processed in duplicate. Each sample vial was then weighed and recorded for an accurate measurement of sample volume by mass. Each sample and blank was then spiked with 100 uL of a 204Pb spike; pipettes were calibrated daily to the desired volume. After a 10 – 60 minute equilibration period, trace metal clean ammonium hydroxide, prepared at a pH of between 8.5 and 9.5 was added such that co-precipitation would occur within 8 hours. Samples were left to precipitate for 12 – 24 hours prior to centrifuging, siphoning off of seawater using a trace metal clean siphon tip, and dissolution in 50 – 100 uL trace metal clean HCl (6M), determined visually for each sample. Samples were then re-precipitated with 20 – 50 uL of ammonium hydroxide (instantaneous) and left to sit for 12 – 24 hours prior to centrifuging, siphoning off of seawater using a trace metal clean siphon tip, and dissolution in 200 – 500 uL trace metal clean HBr (1.1M). Excess salts in the samples were then removed by processing through an anion exchange column chemistry based on Krogh, 1973 and Manhesa et al., 1978. In brief, trace metal cleaned Eichrom AG-1x8 resin was used with in-house made mini-Teflon columns; resin was cleaned with 2mL HCl (6M), equilibrated with 1mL HBr (1.1M); samples were loaded (in 1.1M HBr), washed with 1.5 mL HBr (1.1M), washed with 0.8 mL HCl (2M) and eluted for collection with 2 mL HCl (6M). Concentrated samples were evaporated to dryness until analysis. Prior to analysis, samples were dissolved in 500 uL of 0.1M trace metal clean HNO3. For each set of samples, three low trace metal seawater blanks were concurrently extracted; for each column separation, 1 chemistry blank was run. Eichrom AG-1x8 resin was cleaned by three batch rinses with 6N trace metal clean HCl for a ~12 hours on a shaker table, followed by multiple washes with distilled water until the pH of the solution was above 4.5. Resin was stored at room temperature in the dark until use. Corning polypropylene conical centrifuge tubes (50mL) were cleaned by heated submersion for 2 days at 60 degrees C in 1N reagent grade HCl, followed by a bulk rinse and 4X individual rinse of each vial with pure distilled water. Each vial was then filled with trace metal clean dilute HCl (~0.01N HCl) and heated in the oven at 60 degrees C for one day on either end. Vials were kept filled until just before usage.

On each day of sample analysis, procedure blanks were determined. For the resin preconcentration method, 12 replicates of 300 uL of an in-house standard reference material seawater (low Pb surface water) were used, where the amount of Pb in the 300 uL was verified as negligible. For the "low level" magnesium hydroxide co-precipitation method, 3 replicates of ~10 mL of the same in-house standard reference material seawater were used. The procedural blank over the relevant sessions for resin preconcentration method ranged from 2.2-7.3 pmol/kg, averaging 4.9 ± 1.4 pmol/kg; for the magnesium hydroxide co-precipitation method, blanks ranged from 0.2 – 3.9 pmol/kg, averaging 1.3 ± 0.9 pmol/kg. Within a day, procedure blanks were very reproducible with an average standard deviation of 0.8 pmol/kg (resin preconcentration method) and 0.3 pmol/kg (magnesium hydroxide co-precipitation method), resulting in detection limits (3x this standard deviation) of 2.4 pmol/kg and 0.9 pmol/kg. Replicate analyses of three different large-volume seawater samples (one with ~13 pmol/kg, another with ~24 pmol/kg, and a third with ~35 pmol/kg) indicated that the precision of the analysis is similar for both methods: 4% or 1.2 pmol/kg, whichever is larger, for the resin preconcentration method; 10% or 0.9 pmol/kg, whichever is larger for the "low level" method.

Triplicate analyses of an international reference standard gave SAFe D2: 27.2 ± 1.7 pmol/kg. However, this standard run was linked into our own long-term quality control standards that are run on every analytical day to maintain long-term consistency.  

The Pb concentrations of these samples were quite low, so in general the errors are determined more by uncertainty in the blank corrections rather than as a fixed percentage of the concentration. Profile samples were intercalibrated with Dr. Ken Bruland and his student Claire Parker and Dr. Russ Flegal and his technician Ralph Till, who analyzed separate samples from USGT-EPZT stations, and each used different analytical schemes. The standard deviation of the concentration differences for 213 GoFlo samples (excluding 5 samples with lab to lab differences of >5 pmol/kg) analyzed by MIT and the UCSC Bruland lab was 1.5 pmol/kg. The standard deviation of the concentration differences for 77 GoFlo samples (excluding 5 samples with lab to lab differences of  >5 pmol/kg) analyzed by the UCSC-Flegal lab and the UCSC Bruland lab was 1.6 pmol/kg. We therefore believe that data from all three labs for this cruise are comparable to about 1.6 pmol/kg (one sigma). Analyses from an individual lab relative to other data from the same lab for this cruise are comparable to that lab’s standard deviation on standards or samples near the detection limits. For the MIT lab, the pooled standard deviation of 17 samples near the detection limit (analyzed by the "low level" method) for station 30 was 0.4 pmol/kg.

One third of the MIT samples were analyzed by Jong-Mi Lee using the resin preconcentration method, and the remaining two-thirds of the cruise samples were analyzed by Cheryl Zurbrick using both the resin preconcentration and "low level" methods. There was no significant difference between them for the lowest concentration large-volume seawater reference sample (JML averaged 13.2 ± 0.2 pmol/kg; CMZ averaged 13.3 ± 1.3 pmol/kg).