Sedimentary record of plutonium in the North Yellow Sea and the response to catchment environmental changes of inflow rivers

Abstract

Plutonium (Pu) isotopes were first determined in surface and core sediment samples collected from the northern North Yellow Sea (NYS) to elucidate their source terms and deposition process as well as the response to catchment environmental changes of inflow rivers. Pu/Pu atom ratios in all sediments showed the typical global fallout value of ∼0.18 without any influences from the nuclear weapons tests conducted recently in the North Korea or early in the Pacific Proving Ground. The large variation of Pu activities (0.022-0.515 mBq/g) observed in surface sediments should be mainly attributed to the re-suspension and transportation of fine sediments influenced by the Liaonan Costal Current. Based on the twoPu depth profiles with easily observed onset fallout levels (1952) and global fallout peaks (1963), Pu served as a valid time mark in the coastal sedimentary system. Riverine input Pu contributed only 15-27% to the total global fallout inventory (92.5-108.8 Bq/m) in the northern NYS, much lower than that in the Yangtze River estuary (77-80%), indicating a better soil conservation in the northeast China due to higher forest coverage compared to the Yangtze River's drainage basin. The increase of riverine input Pu after 1980s reflected the more intense soil erosion degree caused by the land use and cover change due to the increment of human activities in the northeast China at the same period. Our results demonstrated that plutonium is a good indicator for studying sedimentary process and its response to the environment in the coastal area.

Distribution and source of Pu in the sediments of the seas and estuaries of China—a review

“…By comparison, the 240 Pu/ 239 Pu atom ratios in the surface sediments of the northern Yellow Sea range from 0.175 to 0.190, with an average of 0.184 ± 0.005. Besides, in two sediment cores of the northern Yellow Sea (NYSC-01, NYSC-02), the 240 Pu/ 239 Pu atom ratios vary from 0.163 to 0.233, with an average of 0.194 ± 0.02, which are relative constant and similar to those determined in the surface sediments (Xu et al 2018). Thus, it is clear that the atom ratios of 240 Pu/ 239 Pu obtained in these sediment samples of the northern Yellow Sea agree very well with the average value of ∼0.18 for global fallout.…”

“…In the northern Yellow Sea, the distribution of 239+240 Pu activity in the surface sediments presents a westwards-increasing trend along the coast, and the highest value of 239+240 Pu activity appears near the northeast coast of Dalian Bay (Xu et al 2018). Chen et al (2013) have reported that after resuspension and transportation by the Liaonan Coastal Current, the Yalu River-derived fine-grained sediments are redeposited in water depths of 20-40 m, which is 180-300 km away from the Yalu River mouth and extends along the southeast coast of the Liaodong Peninsula between the northeast of Dalian Bay and southwest of the Changshan Islands.…”

“…With this method of purification, the overall chemical recoveries ranged from 48% to 86% (Bu et al 2014). In recent years, TEVA has been used to replace the anion exchange resin to reduce the interference of 238 U (Bu et al 2018;Xu et al 2018;Hou 2019). The obtained purified Pu solutions are often analyzed by SF-ICP-MS to improve the test accuracy (Zheng and Yamada 2006).…”

“…Moreover, 239+240 Pu activities near Bohai Strait are relatively lower (0.025-0.040 mBq/g) than other surrounding samples, which are likely to be related to the interaction influence of the Yellow Sea Warm Current and Yellow Sea Coastal Current. Therefore, the transport of coastal currents and sediment dynamics of fine particles might play an important role in controlling the spatial distribution of Pu isotopes in the surface sediment of the northern Yellow Sea (Xu et al 2018). In addition, the fine particles might absorb more total organic carbon content, and the total organic carbon content also has a certain influence on the distribution of 239+240 Pu activity (Wang et al 2019b).…”

“…There are two main input channels of Pu isotopes in the ocean: one is global fallout from atmospheric nuclear tests ( 240 Pu/ 239 Pu = 0.179 ± 0.019) (Kelley et al 1999), and the other is from the United States nuclear test sites in the Marshall islands in the Pacific Ocean (Pacific Proving Grounds, PPG) (Muramatsu et al 2001) ( 240 Pu/ 239 ). In the past decades, numerous studies have been carried out to investigate source terms, transport, and scavenging and deposition processes of Pu isotopes in an inland sea (Bohai Sea), three marginal seas (Yellow Sea, East China Sea, and South China Sea), and some estuaries of China (Huh and Su 1999;Su and Huh 2002;Lee et al 2003;Wang and Yamada 2005;Hong et al 2006;Zheng and Yamada 2006;Dong et al 2010;Liu et al 2011a;Pan et al 2011;Wu et al 2014Wu et al , 2020Wang et al 2017;Wang et al 2019aWang et al , 2019bXu et al 2018;Guan et al 2020;Zhang et al 2020) (Table 1). In particular, many previous studies have also reported that the sources of Pu in the marginal seas of China are mainly from extensive atmospheric nuclear weapons tests since 1945 and close-in fallout from the PPG (where a series of US nuclear tests were conducted in the 1940s-1950s) through ocean currents and water masses based on 240 Pu/ 239 Pu atom ratios (0.30-0.36) (Huh and Su 1999;Su and Huh 2002;Lee et al 2003;Wang and Yamada 2005;Hong et al 2006;Zheng and Yamada 2006;Dong et al 2010;Liu et al 2011a;Pan et al 2011;Wu et al 2014…”