ABSTRACT

The ¹³⁵Cs/¹³⁷Cs atom ratio has been proved to be a reliable tracer for radiocesium source identification in the studies on the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. However, due to the technical challenge to measure ¹³⁵Cs, no ¹³⁵Cs data are available for Japanese river or ocean sediments. In the present study, the vertical distributions of ¹³⁴Cs and ¹³⁷Cs activities and their ratios in two marine sediment cores, collected offshore from the FDNPP site immediately after the accident, were measured by γ spectrometry. A conventional introduction system was replaced by an APEX-Q system, by which it was possible to get 6.5 times higher Cs intensities and then ¹³⁵Cs was analyzed by triple-quadrupole inductively coupled plasma-mass spectrometry. From the vertical distributions, it was seen that the deposition of ¹³⁴Cs and ¹³⁷Cs presented an increasing trend, which indicated continuous radiocesium input into the ocean and deposition from sea water onto the sea floor up to the collection date. Therefore, the ¹³⁴Cs/¹³⁷Cs activity ratios (0.866–1.16) and ¹³⁵Cs/¹³⁷Cs atom ratios (0.249–0.343) (all decay-corrected to March 11, 2011) in marine sediment core samples showed the fingerprints of radiocesium in the oceanic environment were mainly derived from the FDNPP.