[MAG44-P13] Re-circulation of 137Cs concentrations in surface seawater in the western North Pacific Ocean
Keywords:radiocaesium, North Pacific Ocean , large scale nuclear weapons test, global fallout, Fukushima Dai-ichi Nuclear Power Plant accident, Apparent half-residence time
In this study, we focus on temporal variation of 137Cs concentrations in the western North Pacific Ocean (NPO) and its marginal sea. In the NPO where there was high fallout from atmospheric nuclear weapons tests, the rates of decrease in the 137Cs concentrations changed over the five decades: the rate of decrease from the 1950s to the 1970s was much faster than that after the 1970s, and the 137Cs concentrations were almost constant after the 1990s. The 137Cs released into the western North Pacific Ocean by global fallout were transported eastward with the Kuroshio and North Pacific Currents and accumulated in the eastern NPO. Following then, global fallout-137Cs were southward transport in the NPO and subducted into the subsurface layer (1). It was found that global fallout-137Cs were westward transported into subtropical mode water and Central Mode Water in the NPO, suggesting the re-circulation of global fallout-137Cs (2). The apparent half residence time (Tap) of 137Cs in the surface sea water in the western NPO, Sea of Japan (SOJ), East China Sea (ECS) between 2000 and 2010 were 17.3yr, 23.5yr, and 18.0 yr, respectively. The longer Tap in SOJ and ECS suggests that 137Cs derived from global fallout may be recirculated into the SOJ through ECS from wNPO. The recirculation of 137Cs is also found by the intrusion of Fukushima Daiichi Nuclear Plant derived 137Cs (FNPP1-137Cs) after 2012 (3,4). Considering that the increase of FNPP1-137Cs occurred within several years after the FNPP1 accident, the seawater transport route into the SOJ from the NPO with shorter time scale would be existed.
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