A method for determining the isotopic composition of dissolved zinc in seawater by MC-ICP-MS with a 67Zn-68Zn double spike

The biogeochemical cycling of dissolved zinc in seawater is dominated by biological uptake in the surface ocean and its regeneration at depth leading to a large concentration gradient between the surface and deep ocean. The advent of multi-collector inductively coupled mass spectrometry (MC-ICP-MS) provides the marine community with a new way of probing the biogeochemical zinc cycle through changes in its isotope composition. In this paper we document the mathematical and analytical procedures involved in determining the dissolved zinc isotopic composition of seawater using the double spike technique. Seawater samples were spiked with a Zn-67-Zn-68 double spike, before processing. Measurement of zinc by the MC-ICP-MS requires the sample to be free of the seawater matrix and other interfering elements; for this purpose Toyopearl AF-Chelate 650 M and AG1X8 resins were used to pre-concentrate zinc from seawater and separate it from major cations and anions. The optimum pH range for loading zinc for affinity chromatography was between 5 and 8. After pre-concentration, zinc was purified from other trace element by anion exchange chromatography. The reproducibility of the procedure was excellent for a deep water sample [6.55 +/- 0.34 nmol L-1 and 0.59 +/- 0.02 parts per thousand (n = 9) w.r.t. JMC-Lyon] and a shallow water sample [0.05 +/- 0.01 nmol L-1 and 0.08 +/- 0.02 parts per thousand (n = 3). w.r.t. JMC-Lyon] analyzed over a period of 1 year. Analysis of GEOTRACES IC1 BATS and SAFe inter-calibration samples produced data in good agreement with data published by other groups. As a further test of the methodology, we determined the isotopic composition of dissolved zinc for samples collected at various depths from station 40 degrees S, 162 degrees E located in the Tasman Sea, SW Pacific Ocean. The delta Zn-66 vs. depth profile showed isotope variability in the upper water column (0-200 m) with values ranging between -0.03 parts per thousand and 0.53 parts per thousand. These variations were attributed to biological activity. In deep waters the isotope composition of dissolved zinc (0.50 +/- 0.10 parts per thousand) is comparable to other data published. (C) 2016 Elsevier B.V. All rights reserved.