Radioactivity levels of radiocesium and 90Sr in the surface water of the low-latitude western-central North Pacific Ocean in 2017

Following the 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, establishing a robust post-event baseline for anthropogenic radionuclides in the open ocean is essential. Herein, we report ¹³⁷Cs, ¹³⁴Cs, and ⁹⁰Sr activities in 2017 surface seawater samples across a vast expanse of the low-latitude western-central North Pacific Ocean (9.98°N - 22.08°N, 120°E-178°W). ¹³⁴Cs was undetectable in all samples. ¹³⁷Cs (0.77-1.93 Bq/m³) and ⁹⁰Sr (0.11-1.72 Bq/m³) concentrations were almost consistent with the range of pre-Fukushima global fallout levels reported for the open North Pacific, indicating no detectable recent FDNPP-derived input. The ¹³⁷Cs/⁹⁰Sr ratio matched the well-established characteristic signature of global fallout from atmospheric weapons testing, confirming the dominance of this historical source. By 2017, concentrations in the study region had returned to levels characteristic of the pre-accident background, This dataset provides a valuable post-FDNPP baseline for assessing anthropogenic radioactivity in the North Pacific.