Anthropogenic radionuclides in sediments in the NW Pacific Ocean and its marginal seas: results of the 1994–1995 Japanese–Korean–Russian expeditions

Abstract

Assessment of contamination of anthropogenic radionuclides from past dumping of radioactive waste in areas of the Okhotsk Sea, NW Pacific Ocean and the Sea of Japan/East Sea has been performed. Two joint Japanese–Korean–Russian scientific expeditions were carried out in 1994–1995, where seawater and seabed sediments were samples from 22 sites. Results of sediment analysis are reported here, where concentrations of 90Sr, 137Cs, 238Pu, 239,240Pu and 241Am in surface layer and bulk sediments showed on large spatial variations, ranging between <0.04 and 0.75 Bq kg −1 dry wt., <0.2 and 7.2 Bq kg −1 dry wt., <0.001 and 0.08 Bq kg −1 dry wt., 0.006 and 2.0 Bq kg −1 dry wt., 0.03 and 1.8 Bq kg −1 dry wt., respectively. However, the concentrations are comparable with those found in reference sites outside the dumping areas and they generally fall within ranges previously reported for non-dumping areas of the investigated seas. Estimates of sediment inventories indicated differences in radionuclide load between shelf/slope and basin type sediments as well as dependence on water depth. Except for the shallow areas, most of the inventories of 90Sr, 137Cs and Pu isotopes are still to be found in the water column. Total inventories (in water+sediment) show a surplus of 137Cs and Pu-isotopes compared to expected integrated global fall-out deposition, which is consistent with previous observations in non-dumping areas in the seas investigated. Analysis of sediment 238Pu/ 239,240Pu activity ratios showed values in accord with that of global fall-out. Analysis of radionuclide depth distributions in core samples from areas of the Sea of Okhotsk showed sedimentation rates of 0.2–0.4 g cm −2 year −1 and 0.03 g cm −2 year −1 for shelf and basin areas respectively, which is similar to values found in the Sea of Japan/East Sea. Depth profiles of 90Sr, 137Cs and Pu isotopes in cores of the basin area indicate a typical delay compared to the input records of global fall-out.