TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 64 (No. 3), pp. 429-441, 2008

Temporal and Spatial Variations of Radiocarbon in Japan Sea Bottom Water

Yuichiro Kumamoto1*, Takafumi Aramaki2, Shuichi Watanabe1, Minoru Yoneda2, Yasuyuki Shibata2, Orihiko Togawa3, Masatoshi Morita2 and Kiminori Shitashima4

1Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan
2Environmental Chemistry Division, National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki 305-8506, Japan
3Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Shirakata Shirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
4The Environmental Science Research Laboratory, Central Research Institute of ElectricPower Industry, Abiko, Abiko, Chiba 270-1194, Japan

(Received 14 May 2007; in revised form 24 November 2007; accepted 14 December 2007)

Abstract: In 1995 and 2000, the radiocarbon ratio (Δ14C) of total dissolved inorganic carbon was measured in the Japan Sea where deep and bottom waters are formed within the sea itself. We found that (1) since 1979, the Δ14C in bottom water below about 2000-m depth in the western Japan Basin (WJB) had increased by about 30‰ by 1995, and (2) the bottom Δ14C in the WJB did not change between 1995 and 2000. The former finding was due to penetration of surface bomb-produced radiocarbon into the bottom water owing to bottom ventilation, whereas the latter was caused by stagnation of the bottom ventilation there. In the eastern Japan Basin (EJB), the bottom Δ14C also increased by about 30‰ between 1979 and 2002. Recent stagnation of the bottom ventilation in the EJB is also suggested from analyses of constant bomb-produced tritium between 1984 and 1999. The temporal variations of Δ14C, tritium, and dissolved oxygen in the bottom waters indicate that: (1) new bottom water is formed south of Vladivostok in the WJB only in severe winters; and (2) the new bottom water then follows the path of a cyclonic abyssal circulation of the Japan Sea, which results in the increases in dissolved oxygen and the transient tracers in the bottom waters in the EJB and Yamato Basin with an approximate 3- to 6-year time lag. This process is consistent with the spatial variations of Δ14C, bomb-produced 137Cs, and chlorofluorocarbon-11 in the bottom waters of the Japan Sea.

*Corresponding author E-mail: kumamoto@jamstec.go.jp

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