The 129-iodine content of subtropical Pacific waters: impact of Fukushima and other anthropogenic 129-iodine sources

Abstract. Results obtained from a dedicated radiochemistry cruise approximately 100 days after the 11 March 2011 Tohoku earthquake and subsequent disaster at the Fukushima Daiichi Nuclear Power Plant show that Fukushima derived radionuclides in the nearby ocean environment had penetrated, on average, to ≤250 m depth (1026.5 kg m³ potential density surface). The excess inventory of Fukushima-derived ¹²⁹I in the region (∼150 000 km²) sampled during the cruise is estimated to have been between 0.89 and 1.173 billion Bq (∼136 to ∼179 grams) of ¹²⁹I. Based on a tight tracer–tracer relation with ¹³⁴Cs (or ¹³⁷Cs) and estimates that most of the excess cesium is due to direct discharge, we infer that much of the excess ¹²⁹I is from direct (non-atmospheric deposition) discharge. After taking into account oceanic transport, we estimate the direct discharge, i.e., that directly released into the ocean, off Fukushima to have been ∼1 kg ¹²⁹I. Although this small pulse is dwarfed by the ~90 kg of weapons-testing-derived ¹²⁹I that was released into the environment in the late 1950s and early 1960s, it should be possible to use Fukushima-derived ¹²⁹I and other radionuclides (e.g., ^{134, 137}Cs) to study transport and entrainment processes along and across the Kuroshio Current.