AUTHOR=Matsuoka Kohei , Tatsuyama Tomomichi , Takano Shotaro , Sohrin Yoshiki 

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

JOURNAL=Frontiers in Marine Science

VOLUME=10

YEAR=2023

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1182668

DOI=10.3389/fmars.2023.1182668

ISSN=2296-7745

ABSTRACT=<p>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, <italic>n</italic> = 109), <italic>δ</italic><sup>186/184</sup>W was 0.52 ± 0.06 ‰, the Mo concentration was 105.1 ± 8.0 nmol/kg, and <italic>δ</italic><sup>98/95</sup>Mo was 2.40 ± 0.06 ‰. 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 <italic>δ</italic><sup>186/184</sup>W in the upper water (&lt; 1000 m depth) were different from those in the ocean (W = 56 ± 18 pmol/kg, <italic>δ</italic><sup>186/184</sup>W = 0.45 ± 0.06 ‰, <italic>n</italic> = 24). However, the concentrations in deeper water were congruent with those in the oceans (W = 49.9 ± 7.6 pmol/kg, <italic>δ</italic><sup>186/184</sup>W = 0.50 ± 0.02 ‰, <italic>n</italic> = 7). The Mo concentration was 105.4 ± 3.1 nmol/kg and <italic>δ</italic><sup>98/95</sup>Mo was 2.36 ± 0.03 ‰ (<italic>n</italic> = 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 <italic>δ</italic><sup>186/184</sup>W reached 0.20 ‰. We propose that the enrichment of W with a low <italic>δ</italic><sup>186/184</sup>W 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 <italic>δ</italic><sup>98/95</sup>Mo 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.</p>