Determination of the tungsten isotope composition in seawater: The first vertical profile from the western North Pacific Ocean

Abstract

The stable isotope ratio of W is a new tracer in oceanographic studies and a new proxy in paleoceanographic studies; however, precise data for modern seawater have not been reported to date. Because the concentration of W in seawater is as low as 49 pmol kg−1, an ~3000-fold preconcentration is necessary prior to measurement by multicollector inductivity coupled plasma mass spectrometry (MC-ICP-MS). For the preconcentration, we investigated solid-phase extraction using chelating resins, namely, NOBIAS Chelate-PA1 with ethylenediaminetriacetic acid groups and TSK-8HQ with 8-hydroxyquinolie groups. We report that TSK-8HQ is useful because the effects of the seawater matrix are minor thermodynamically and kinetically. We present a novel method for analysis of the concentrations and isotope ratios of W and Mo in seawater, consisting of solid phase extraction, chromatographic separation using anion exchange resin AG1 X8, and measurement by MC-ICP-MS. Both W and Mo are quantitatively recovered by this method, which was applied to seawater samples collected from the North Pacific Ocean. The measured concentration of W and the concentration and isotope ratio of Mo are consistent with those in the literature. The isotope ratio of W is found to be uniform throughout the water column in the western North Pacific Ocean; δ186/184W is 0.55 ± 0.12‰ (ave ± 2sd, n = 7) using NIST SRM 3163 as a reference for W. On the basis of this data, we determined that the isotopic difference in δ186/184W is ~0.49‰ between seawater and oxic sediments in the modern ocean. This value accords with the reported experimental data for the isotope fractionation of W during adsorption on manganese and iron (oxyhydr)oxides, suggesting the validity of our data.