247 Journal of Oceanography, Vol. 64, pp. 247 to 257, 2008 Keywords: ⋅ Zirconium, ⋅ hafnium, ⋅ niobium, ⋅ tantalum, ⋅ molybdenum, ⋅ tungsten, ⋅ seawater, ⋅ North Pacific Ocean, ⋅ speciation. * Corresponding author. E-mail: lutfi@inter3.kuicr.kyoto- u.ac.jp Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer Dissolved and Labile Particulate Zr, Hf, Nb, Ta, Mo and W in the Western North Pacific Ocean M. LUTFI FIRDAUS*, KAZUHIRO NORISUYE, YUSUKE NAKAGAWA, SEIJI NAKATSUKA and YOSHIKI SOHRIN Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan (Received 30 April 2007; in revised form 29 October 2007; accepted 30 October 2007) Dissolved and labile particulate Zr, Hf, Nb, Ta, Mo and W were determined at sta- tions K1 (51°N, 165°E), K2 (47°N, 160°E), KNOT (44°N, 155°E) and 35N (35°N, 160°E) in the western North Pacific Ocean. A portion of seawater for dissolved species (D) was passed through a 0.2 μ μ μm Nuclepore filter and acidified to pH 2.2 with HCl and HF. A portion of seawater for acid-dissolvable species (AD) was acidified without fil- tration. Labile particulate (LP) species is defined as AD minus D, which represents a chemically labile fraction of particulate species. D-Zr, Hf and Ta increase with depth, Nb shows a slight depletion in surface water, whereas Mo and W have a conservative vertical profile. The concentration range of D-Zr, Hf, Nb, Ta and W is 31–275, 0.14– 0.95, 4.0–7.2, 0.08–0.29 and 40–51 pmol kg –1 , respectively, whereas that of Mo is 97– 105 nmol kg –1 . LP-species of Zr, Hf and Ta account for 10–14% of AD in average and increase up to 25% below 4000 m, whereas those for Mo and W are negligible. In contrast, LP-Nb shows maxima (up to 27%) in surface water. We also found that D- Zr/Hf, Nb/Ta and Mo/W mole ratios generally increase in the order continental crust < river water < coastal sea < open ocean. surfaces of sinking particles, a process referred to as “scavenging”. Both of these factors result in very low concentrations in seawater. Thus, these metals are thought to have a potential use as an oceanographic tracer (McKelvey and Orians, 1998). Mo and W are dominated by oxyacid species, and are more soluble than hydroxide species (Turner et al., 1981). Some papers on their oce- anic distributions have been published. Boswell and Elderfield (1988) were the first to report the D-Zr and Hf concentrations in open ocean seawater, in addition to that in river and coastal water. They found that D-Zr in sur- face and deep water of the Indian Ocean was 80 and 185 pmol kg –1 , respectively, whereas it was 200 pmol kg –1 in the Atlantic deep water. They reported that D-Hf was 19– 22 pmol kg –1 . McKelvey and Orians (1993) reported the first vertical profile of D-Zr in the ocean from the central North Pacific. They found that D-Zr ranged from 12–95 pmol kg –1 in surface water to a maximum of 300 pmol kg –1 in deep water. Godfrey et al. (1996) reported that the concentration of D-Zr and Hf in the northeastern Atlantic Ocean was in the range 70–180 pmol kg –1 and 0.4–1.1 pmol kg –1 , respectively. They stated that the dis- tributions with depth indicate a nutrient-like scavenging- regeneration behavior, although the dissimilarity with sili- cate and nitrate in samples below the nutricline suggest 1. Introduction Zr, Hf, Nb, Ta, Mo and W are adjacent metals in the periodic table, known as “high field strength elements” (HFSE) (Rudnick and Gao, 2005), and are of great inter- est in geochemistry. During the fractional crystallization of magma or magma generation by the partial melting of mantle, HFSE cannot easily enter cation sites of the min- erals and concentrate in the magma (liquid phase). Be- cause of their close chemical similarities (e.g. ionic charge and ionic radius), the ratios of Zr to Hf, Nb to Ta and Mo to W show limited variations in crustal materials (Jochum et al., 1986; Li, 2000). The average mole ratio in the bulk continental crust is estimated to be 69 for Zr/Hf, 22 for Nb/Ta and 1.6 for Mo/W (Rudnick and Gao, 2005). In seawater, Zr, Hf, Nb and Ta are classified as “re- fractory metals” (Orians and Merrin, 2001) and are domi- nated by hydroxide species (Turner et al., 1981). They are not readily dissolved in seawater. Their supply to the ocean is low relative to their abundance in the crust. They are rapidly removed from solution by interaction with the