Netherlands Polar Data Center

Sampling and Analytical Methodology: Samples were taken using 24 internally Teflon-coated PVC 12 litre GO-FLO Samplers (General Oceanics Inc.) mounted on an all-titanium frame (De Baar et al., 2008). This frame was connected to a 17.7 mm diameter Kevlar hydrowire with seven independent internal signal/conductor cables (Cousin Trestec S.A.) and controlled from onboard. Each GO-FLO sampler had a special ultraclean all-teflon PTFE valve (Cole Parmer; PN A-06392-31) installed. Samples for trace metal analysis were collected from the GO-FLO bottles in a class 100 clean room environment. The water was filtered over a 0.2 μm filter cartridge (Sartrobran-300, Sartorius) under pressure (1.5 atm) of (in-line prefiltered) nitrogen gas exerted via a special connector instead of the regular air bleeding valve at the top of each GO-FLO sampler. Sub-samples for Mn were taken in cleaned LDPE sample bottles (125 mL) from each GO-FLO bottle. All sample bottles were rinsed five times with the sample seawater. Analyses of dissolved Manganese (Mn) were performed on shipboard with the method developed by Doi et al. (2004) with some slight modifications in the preparation and brands of the chemicals used. Furthermore, samples were buffered in-line with an ammonium borate sample buffer (see text below). Samples were acidified with 12 M ultraclean HCl (Baseline® Hydrochloric Acid, Seastar Chemicals Inc.) to a pH of 1.8 and stored inside double plastic bags in plastic crates. At the home institute, in a flow injection system, the samples were buffered in-line to a pH of 8.5 ± 0.2 with ammonium borate sample buffer. This buffer was produced by dissolving 30.9 grams of boric acid (Suprapure, Merck) in 1 L MQ water and adjusting the pH to 9.4 with ammonium hydroxide (Suprapure, Merck). Dissolved Mn in the buffered sample was preconcentrated for 200 seconds on a Toyopearl AFChelate 650M (TesoHaas, Germany) column. Hereafter the column was rinsed for 60 seconds with MQ water to remove interfering salts. The Mn was subsequently eluted from the column for 200 seconds with a solution of 0.1M three times quartz distilled formic acid (reagent grade, Merck) containing 0.1 M hydrogen peroxide (Suprapure, Merck) and 12 mM ammonium hydroxide (Suprapure, Merck). The pH of this carrier solution was adjusted to 2.9 ± 0.05. The eluate with the dissolved Mn passed a second column of immobilized 8-hydroxyquinoline (Landing et al., 1986) to remove interfering iron ions in the carrier solution (Doi et al., 2004). Hereafter the carrier mixed subsequently with 0.7 M ammonium hydroxide (Suprapure Merck) and a luminol solution. The latter luminol solution was made by diluting 600 μl luminol stock solution and 10 μl TETA (triethylenetetramine, Merck) in 1 L MQ. The luminol stock solution was made by diluting 270 mg luminol (3-aminophtalhydrazide, Aldrich) and 500 mg potassium carbonate in 15 ml MQ. The resulting mixture of carrier solution, ammonium hydroxide and luminol solution had a pH of 10.2 ± 0.05 and entered a 3 meter length mixing coil placed in a water bath of 25°C. Hereafter the chemiluminescence was detected with a Hamamatsu HC135 Photon counter. Concentrations of dissolved Mn were calculated in nanomol•L-1 (nM) from the photon emission peak height (see below). The system was calibrated using standard additions from a 5000 nM Mn stock solution (Fluka) to filtered acidified seawater with low concentrations of Mn that was collected in the region. A five-points calibration line (0, 0.36, 0.73, 1.46 and 2.18 nM standard additions) and blank determination were made daily. The 3 lowest points (0, 0.36 and 0.73 nM) of the calibration line were measured in triplicate and the 2 highest points in duplicate to add more weight to the lower part of the calibration line. The blank was determined by measuring column cleaned seawater. Latter was obtained by passing seawater over a Toyopearl AFChelate 650M and an 8-hydroxyquinoline column which retained the dissolved Mn. The average blank value was 20 pM. The limit of detection defined as three times the standard deviation of the blank was < 0.01 nM. The flow injection system was rinsed every day with a 0.5 M HCl solution. Data Processing: A standard was measured in triplicate every day. This standard was a sub-sample of a 25 L volume of filtered seawater that was taken at the beginning of the cruise in the Barents Sea. The relative standard deviation of the replicate analysis seawater sample that was analysed 22 times on different days in triplicate was 2.19%. The relative standard deviation on the separate days was on average 1.21 %. The average concentration of Mn of this standard was 1.0 nM and the deviation from this average for a given measuring day was used as a correction factor. To verify whether this correction was decreasing the inter-daily variability in the dataset, every day a sample which was collected and measured the previous measuring day, was analysed once again. The deviation between the concentrations measured on the different days decreased from 3.28% to 2.68%, indicating the data correction procedure is beneficial. As an independent comparison, the certification samples collected on the American SAFe cruise (Johnson et al., 2007) were analysed in triplicate for Mn. The resulting concentrations of Mn for both SAFe Surface (S) and SAFe deep (D2) from 1000 m, agreed very nicely the community consensus value that is starting to emerge on the absolute concentrations of Mn in the SAFe samples. The dataset was scanned for obvious outliers and these have been quality flagged with the number 4. Some samples gave anomalous nutrient results for the intended depth and were assumed to have been closed at the wrong depth and quality flagged with the number 8. In the Laptev Sea some very high concentrations of Mn were observed and these were out of range for the method used and flagged with the number 5. All other values are assumed to be correct and flagged with the number 0.