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CTD Bottle hydrography, nutrients, iron and manganese concentrations from GEOTRACES GAc01 cruise KN192-5

Responsible investigator

Mak A Saito ¹

¹ Woods Hole Oceanographic Institution Department of Marine Chemistry and Geochemistry

Acquisition description:

Sampling Strategy

Three transects were sampled using the Trace Metal Rosette (TMR) on the CoFeMUG cruise aboard R/V Knorr in 2007. Refer to Noble et al. (2012) for detailed information on acquisition and processing.

In brief acid-cleaned, Teflon-coated X-Niskin sampling bottles (OceanTest Equipment) of 8-liter capacity were deployed on an epoxy-coated rosette (Sea-Bird Electronics) attached to a nonmetallic line and were programmed to trip at predetermined depths and record sampling depths. All bottles used to store seawater prior to analysis were soaked overnight in the acidic detergent Citranox, rinsed thoroughly with Milli-Q water (Millipore), filled with 10% HCl to soak for 10 d, rinsed thoroughly with Milli-Q water adjusted to pH 2, and double-bagged. Low-density polyethylene bottles were used for trace metal sample storage and high-density polyethylene bottles were used for nutrient sample storage. Immediately following collection, the X-Niskins were pressurized with 99.999% N2, and seawater was filtered through Teflon tubing and a 142-mm, 0.4-mm polycarbonate plastic sandwich filter (Geotech Environmental Equipment) into sample storage bottles in a positive pressure class-100 clean environment. All tubing and filters were acid-washed prior to use. Samples intended for dissolved iron and manganese analyses were acidified to pH 1.7 with high-purity HCl (Seastar) within 6 months of sampling, and stored acidified at room temperature for at least 8 months prior to analysis.

Analytical methodology

Nutrient Analyses: Nutrient samples were filtered and frozen in acid-washed 60-mL high-density polyethylene bottles until analysis. The frozen samples were thawed in a warm water bath and stored in the dark for 20-24 h prior to analyses. Immediately before analysis, aliquots of the samples were transferred to 15-mL polypropylene cups and an Alpkem autosampler. Technicon AutoAnalyzer II components were used to measure phosphate and ammonium; and Alpkem rapid flow analyzer (RFA) 300 components were used for silicic acid, nitrate + nitrite, and nitrite.

Iron and Manganese Analyses: Total dissolved iron and manganese were measured using ICP-MS, as described by Saito and Schneider (2006). Briefly, 13.0-mL aliquots of acidified seawater were weighed into cleaned polypropylene centrifuge tubes and 57Fe was added for isotope dilution analysis and equilibrated overnight. Concentrated ammonium hydroxide (Seastar) was added to induce Mg(OH) ₂ and trace metal coprecipitation for 3 min, followed by centrifugation for 3 min at 3000 rpm (1460 x g) using an Eppendorf Centrifuge 5810R. The supernatant was decanted, the sample was centrifuged and decanted again, and the pellet was dissolved in 5% nitric acid (Seastar) containing 1 µg/L indium. The 5% nitric acid resuspension solution was used to estimate the ⁵⁶ Fe, ⁵⁷ Fe, and ⁵⁵ Mn reagent and instrument blank, and signal suppression due to matrix effects was accounted for by using a ratio of the indium in the blank to the indium in the resuspended sample solution, as described in Saito and Schneider (2006). The Sampling and Analysis of Fe (SAFe) seawater intercalibration standards were analyzed at the beginning of each analysis day with iron concentrations of 0.96 ± 0.095 nmol L ^-1 for the D2 seawater standard and 0.125 ± 0.046 nmol L ^-1 for the S1 seawater standard (n=10), which are within the reported ranges of 0.91 + 0.17 nmol L ^-1 and 0.097 + 0.043 nmol L ^-1 (Johnson et al. 2007).

References

Noble, A.E, C. H. Lamborg, D. C. Ohnemus, P. J. Lam, T. J. Goepfert, C. I. Measures, C. H. Frame, K. L. Casciotti, G. R. DiTullio, J. Jennings, M. A. Saito. 2012. Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean. Limnology and Oceanography, 57(4) 989-1010. doi:10.4319/lo.2012.57.4.0989

Johnson, K.S. et al. 2007. Developing iron standards for seawater. EOS Trans. 88: 11, 131-132. doi: 10.1029/2007EO110003

Saito, D.L. and Schneider. 2006. Examination of precipitation chemistry and improvements in precision using the Mg(OH)2 preconcentration ICP-MS method for high-throughput analysis of open-ocean Fe and Mn in seawater. Anal. Chim. Acta 565: 222-233, doi:10.1016/j.aca.2006.02.028

BODC Data Processing

Data Processing

Data were received and were loaded using established BODC data banking procedures. Data were supplied in per l and converted into per kg using standard density conversion of /1.025.

This document covers data submitted using the following GEOTRACES parameters: