Distributions of zirconium, niobium, hafnium, and tantalum in the subarctic North Pacific Ocean revisited with a refined analytical method

Abstract

Although zirconium (Zr), niobium (Nb), hafnium (Hf), and tantalum (Ta) in seawater are potential tracers for water masses, their determination is still a challenge in analytical chemistry. We have refined our preconcentration method using 8-hydroxyquinoline chelating resin (TSK-8HQ) and reinvestigated concentration profiles of the four elements in dissolved (d) and total dissolvable (td) fractions at five different stations from 47 °N, 160 °E to 51 °N, 160 °W in the subarctic North Pacific Ocean. The new method has saved analytical time and reduced systematic errors compared with previous methods. The concentration ranges were 30–276 pmol/kg for dZr, 1.0–2.6 pmol/kg for dNb, 0.09–0.78 pmol/kg for dHf, and 0.006–0.026 pmol/kg for dTa in the subarctic North Pacific Ocean. The concentrations of Zr and Hf increased from surface water to deep water, whereas those of Nb and Ta were nearly constant over the water depth. The profiles of dZr, dNb, and dHf were consistent with those in previous studies. However, we found that dTa is uniformly distributed at 0.015 ± 0.005 pmol/kg (mean ± sd, n = 75), which is approximately one-fifth of that in a previous study. It is likely that the previous dTa data were affected by a systematic error. Negligible differences between td and d fractions suggest that the particulate concentrations of these elements are lower than those reported in a previous study.