Abstract pnas.1120794109 www.pnas.org/cgi/doi/10.1073/pnas.1120794109

The Tohoku earthquake and tsunami of March 11, 2011, resulted in

unprecedented radioactivity releases from the Fukushima Dai-ichi

nuclear power plants to the Northwest Pacifc Ocean. Results are

presented here from an international study of radionuclide con-

taminants in surface and subsurface waters, as well as in zooplank-

ton and fish, off Japan in June 2011. A major finding is detection of

Fukushima-derived 134Cs and 137Cs throughout waters 30–600 km

offshore, with the highest activities associated with near-shore

eddies and the Kuroshio Current acting as a southern boundary

for transport. Fukushima-derived Cs isotopes were also detected

in zooplankton and mesopelagic fish, and unique to this study we

also find 110mAg in zooplankton. Vertical profiles are used to cal-

culate a total inventory of ~2 PBq 137Cs in an ocean area of 150,000 km2.
Our results can only be understood in the context of our

drifter data and an oceanographic model that shows rapid advec-

tion of contaminants further out in the Pacific. Importantly, our

data are consistent with higher estimates of the magnitude of

Fukushima fallout and direct releases [Stohl et al. (2011) Atmos

Chem Phys Discuss 11:28319–28394; Bailly du Bois et al. (2011) 

J Environ Radioact, 10.1016/j.jenvrad.2011.11.015]. We address risks

to public health and marine biota by showing that though Cs iso-

topes are elevated 10–1,000× over prior levels in waters off Japan,

radiation risks due to these radionuclides are below those gener-

ally considered harmful to marine animals and human consumers,

and even below those from naturally occurring radionuclides.