The Kuroshio radiocesium stream

The Fukushima accident released short-lived Cs-134 and longer-lived Cs-137 to the ocean. The amount, although substantial, is much less than that produced during the atomic bomb tests 60 years ago. But, the latter has received little attention. Here we found only Cs-137 in waters near the origin of the Kuroshio. The speed of the Kuroshio current generally decreases with water depth, yet, the Cs-137 activity increases with depth to reach a subsurface maximum of 2.4 Bq m−3. As a result, a core of high Cs-137 flux, or a radiocesium stream, exists at approximately 200–400 m in depth. In total, the Kuroshio transports about 1 PBq year−1 Cs-137 northward between 121 and 123°E, 1000 times more than the 0.73–1.0 TBq year−1 discharged to the ocean at Fukushima in 2016–2018.