Understanding the circulation of water masses in the world's oceans is critical to our knowledge of the Earth's climate system. Trace elements and their isotopes have been explored as tracers for the movement of water masses 1 . One type of candidate elements 2 are the high-field-strength elements zirconium (Zr), hafnium (Hf), niobium (Nb) and tantalum (Ta). Here we measure the distributions of dissolved Zr, Hf, Nb and Ta along two meridional sections in the Pacific Ocean that extend from 65 • to 10 • S and from 10 • to 50 • N. We find that all four elements tend to be depleted in surface water. In the deep oceans, their concentrations rise along our transects from the Southern Ocean to the North Pacific Ocean, and show strong correlations with the concentration of silicate. These results indicate that terrigenous sources are important to the budget of Zr, Hf, Nb and Ta in sea water, compared with hydrothermal input. Unexpectedly, the weight ratios for Zr/Hf fall between 45 and 350 and those for Nb/Ta between 14 and 85 in Pacific sea water, higher than the ratios observed in fresh water, in the silicate Earth or in chondritic meteorites. We conclude that the fractionation of Zr/Hf and Nb/Ta ratios will be useful for tracing water masses in the ocean.In the modern ocean, deep water is formed in the northern North Atlantic and the Southern oceans and flows to the Indian and the Pacific oceans. This thermohaline circulation results in an oceanic mixing time in the range 500 to 1,000 years. Silicate (Si(OH) 4 ), a major nutrient, is taken up by diatoms from surface sea water to form siliceous tests, and remineralized from the sinking tests in deep water 3 . The silicate concentration increases with the age of the deep water. Such a biogeochemical cycle is important in controlling the biological productivity in the ocean. An international research collaboration program, GEOTRACES, has been launched recently to determine the global distribution of trace elements and their isotopes (TEIs) and to characterize more completely their biogeochemical cycles 1 . Refractory elements have a low supply to the oceans relative to their abundance in the Earth's crust 2 . They are also rapidly removed from sea water by adsorption on sinking particles, a process referred to as scavenging. These factors result in large variations in the oceanic distributions that typically reflect their sources, making them potential tracers of water masses. For this purpose, rare earth elements (REEs), which occur as carbonate complexes of a trivalent (+3) been measured in iron-manganese (Fe-Mn) crusts and nodules, sediments, and rocks 5-9 . The first dissolved seawater Hf isotope data were recently published 10,11 . Together with neodymium isotope ratios ( 143 Nd/ 144 Nd, expressed as ε Nd ), the Hf-Nd isotope system is used as an excellent proxy to elucidate the past change in continental weathering and/or hydrothermal sources to the ocean. It is now well known that for a given ε Nd , sea water and precipitates, such as ironmagnanese (Fe-Mn) crust...