Dataset: GT10-11 - Particulate trace elements

Samples were collected from sub-surface waters using the GEOTRACES GO-Flo rosette. The filtration was performed directly from pressured GO-Flo bottles onto membranes (25-mm diameter Supor 0.45-µm polyethersulfone) mounted in Swinnex polypropylene filter holders.  Filters were cleaned following the protocol outlined in the GEOTRACES Sampling Document (Cutter et al. 2010). Pressurization to <8 psi was achieved with 0.2-µm filtered air.  Prior to filtration each GO-Flo bottle was gently mixed by manually inverting the bottle several times (after removal of unfiltered salt samples to provide some headspace).  Filtration was continued until the entire bottle was empty or 2 hours had elapsed.  An average of 6.5-L of water was filtered through each membrane.  Filter holders were removed from the GO-Flo bottles and a vacuum applied to remove any residual water.  Filters were then folded, stored in acid-washed centrifuge tubes, and frozen at -20ºC until digestion and analysis.

Filters were digested in rigorously cleaned 22-mL PFA digestions vials (Savillex).  All digestion steps were performed in a Class-100 clean room using standard clean techniques.  A subset of filters collected from the shallowest GO-Flo bottle and the deep chlorophyll maximum bottle were first digested in a solution of 25% Optima-grade acetic acid and 0.02 M hydroxylamine hydrochloride following the protocol of Berger et al. (2008).  One milliliter of this solution 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°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. The filter was removed to an acid-cleaned PFA bomb and was later digested using the mixture of concentrated acids described below to recover the refractory elements. 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 a 7-mL PFA digestion vial.  Optima-grade HNO3 was added (100 µL) to the 7-mL digestion vial, which was subsequently heated uncapped at 110ºC to near dryness.  Vial contents were redissolved with 2% HNO3 (Optima grade).

The refractory fraction of particulate metals was then measured on the sample filters.  The filter (and any remaining leachate) was transferred to a 22-mL PFA vial, 2-mL of a mixture of 4M HCl, 4M HNO3, and 4M HF (all Optima grade) was added, and 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 (Twining, P. Lam, P. Morton et al., unpublished).  Following heating, the acid solution in the bomb was poured into a second PFA vial. To ensure complete transfer of acid, the bombs were thoroughly rinsed with 3 × 0.5-mL aliquots of ultrapure water. This water was also poured into the secondary vial.  The secondary vial was then heated to dryness and the contents redissolved with 2-mL of a 50% Optima-grade HNO3 + 15% Optima-grade H2O2 solution.  This solution was again dried down and the contents redissolved with 2% HNO3. 

Filter samples from depths outside of the shallowest bottle and deep chlorophyll maximum bottle were digested only in the HCl/HNO3/HF solution to provide just a ‘total’ particulate metal fraction.  

All digests were analyzed using a Finnegan-MAT Element2 HR-ICP-MS at the University of Maine following the protocols outlined in Twining et al. (2011).  The instrument is equipped with an ESI Apex desolvation nebulizer, an autosampler contained under a HEPA filter, and nickel cones.  Cd-111 and Mo-98 were analyzed in low-resolution mode, and the remaining isotopes were analyzed in medium-resolution mode.  Multiple isotopes were analyzed for some elements (Fe-56, Fe-57, Zn-66, Zn-68).  Concentrations were within 4%, on average, for both Fe isotopes and within 10%, on average, for both Zn isotopes.   Concentrations of Fe-56 and Zn-66 are reported here.  Quantification was performed by standard addition, and In-115 was used as an internal standard to correct for variations in instrumental sensitivity during analyses.

Digestions of the certified reference materials BCR-414 (plankton, Community Bureau of Reference, Commission of the European Communities) and PACS-2 (marine sediment, National Research Council of Canada) were done alongside sample digestions in order to assess accuracy.  Recoveries were typically within 10% of the certified values (and within the error of the data, taken from replicate measurements).

Precision was determined through replicate analyses of digests of three different particle-laden filters collected by Dr. Phoebe Lam (WHOI).  These filters were distributed to US GEOTRACES investigators working on particulate trace element analyses.  Each filter was digested, and the digestion solutions were separately diluted and analyzed during analytical runs in February, November and December 2012 and July 2013.  Coefficients of variation (CV) were calculated from the four analyses for each element, and the results are shown in the ‘precision’ sheet of the attached Excel workbook.  Mean precision was generally 10-15% for most elements.  

SXRF samples were prepared from unfiltered water taken from GEOTRACES GO-Flo bottles at the shallowest depth and deep chlorophyll maximum.  Cells were preserved with 0.25% trace-metal clean buffered glutaraldehyde and centrifuged onto C/formvar-coated Au TEM grids.  Grids were briefly rinsed with a drop of ultrapure water and dried in a Class-100 cabinet.  SXRF analysis was performed using the 2-ID-E beamline at the Advanced Photon source (Argonne National Laboratory) following the protocols of Twining et al. (2011).