Distribution of stable isotopes of Mo and W from a river to the ocean: signatures of anthropogenic pollution

Molybdenum and tungsten are redox-sensitive elements, and their stable isotope ratios have attracted attention as paleoceanographic proxies. However, our knowledge of the distribution of stable Mo and W isotopes in the modern hydrosphere remains limited. In this study, we provided the concentrations and isotope ratios of dissolved Mo and W in the oceans (the North Pacific and Indian Oceans), marginal seas (the East China Sea and Sea of Japan), and a river-estuary system in Japan (from the Uji-Yodo rivers to Osaka Bay). In the North Pacific and Indian Oceans, the W concentration was 48.2 +/- 6.2 pmol/kg (ave +/- 2sd, n = 109), delta W-186/184 was 0.52 +/- 0.06 parts per thousand, the Mo concentration was 105.1 +/- 8.0 nmol/kg, and delta Mo-98/95 was 2.40 +/- 0.06 parts per thousand. The results indicate that W has the constant concentration and isotopic composition in the modern ocean as well as Mo. In the East China Sea and the Sea of Japan, the W concentration and delta W-186/184 in the upper water (< 1000 m depth) were different from those in the ocean (W = 56 +/- 18 pmol/kg, delta W-186/184 = 0.45 +/- 0.06 parts per thousand, n = 24). However, the concentrations in deeper water were congruent with those in the oceans (W = 49.9 +/- 7.6 pmol/kg, delta W-186/184 = 0.50 +/- 0.02 parts per thousand, n = 7). The Mo concentration was 105.4 +/- 3.1 nmol/kg and delta Mo-98/95 was 2.36 +/- 0.03 parts per thousand (n = 31) throughout the water column, congruent with those in the ocean. In the Uji River-Yodo River-Osaka Bay system, the W concentration reached 1074 pmol/kg and delta W-186/184 reached 0.20 parts per thousand. We propose that the enrichment of W with a low delta W-186/184 in the river-estuary system and marginal seas is caused by anthropogenic pollution. Anthropogenic Mo pollution was not detected in marginal seas. However, the Mo concentration and delta Mo-98/95 showed high anomalies above the mixing line of river water and seawater in the lower Yodo River and Osaka Bay, implying possible anthropogenic pollution of Mo in the metropolitan area.