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

: The coastal zone is the most concentrated area of human activities, and it is also the main accumulation zone of continental sediments, which is an ideal area for studying anthropocene sedimentary records. This study summarizes the distribution of 239 + 240 Pu activity, 239 + 240 Pu inventory, and 240 Pu/ 239 Pu atom ratios in the sediments of the seas and estuaries of China. Studies have shown that the distribution of 239 + 240 Pu activity in sediments is mainly influenced by sediment properties and ocean current dynamics. Furthermore, 239 + 240 Pu activity in sediment cores has obvious peak characteristics, which can be used in sediment dating. In fact, 240 Pu/ 239 Pu atom ratios indicate that the Pu in the sediments of the seas and estuaries of China mainly comes from global fallout and the Pacific Proving Grounds (PPG). Pu from the PPG enters the seas of China through the North Equatorial Current and Kuroshio intrusion current. And the contribution of Pu from the PPG in the East China Sea, the South China Sea, and the Yangtze estuary is over 40%. Moreover, Pu has been applied in the tracer of land – sea interactions and ocean dynamics, and it can be used as a background value to study the changes of Pu in the coastal zone of China in the future.


Introduction
Plutonium (Pu), an anthropogenic element, is harmful to the environment and human health, and has been released into the oceans since the 1940s due to nuclear bombs (Sholkovitz 1983), accidents (Zheng et al. 2012), and nuclear material plants (Dai et al. 2002(Dai et al. , 2005. The radionuclides 239 Pu (T 1/2 = 24 100 years) and 240 Pu (T 1/2 = 6560 years) have high radiological toxicity and long-term retention in the environment, and have been used as tracers to analyze artificial radionuclide inputs and redistributions, elucidate their source terms, to rebuild historical pollution events, to evaluate various marine processes, and to assess their environmental behavior and impacts (Lind et al. 2006;Zheng and Yamada 2006;Yamada and Zheng 2008;Lindahl et al. 2010;Tims et al. 2010;Hancock et al. 2011;Liu et al. 2011aLiu et al. , 2011bLiu et al. , 2013Pan et al. 2011;Liao et al. 2014;Wu et al. 2014Wu et al. , 2020Lal et al. 2017;Wang et al. 2017;Huang et al. 2019). In addition, Pu caused by nuclear weapon testing, accidental release, and discharges from nuclear fuel reprocessing sites and nuclear power plants could enter the environment through global atmospheric fallout and local regional fallout (Krey et al. 1976;Kelley et al. 1999;Hamilton 2005).
The ocean is the largest reservoir of artificial radionuclides, and according to Aarkrog (2003), by the year 2000 the 239+240 Pu inventory deposited into the ocean via global fallout is about 10 PBq (1 PBq = 1015 Bq), and the amount of 239+240 Pu inventory received in the Pacific and Indian oceans is about 6.3 PBq (Aarkrog 2003). 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 (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). However, according to 240 Pu/ 239 Pu atom ratio to indicate the Pu source, the impact of other local nuclear weapon tests and accidental releases of Pu isotopes in the marginal seas of China could be negligible. For example, weapon-grade Pu has a 240 Pu/ 239 Pu atom ratio of 0.01-0.07 (Lindahl et al. 2011), nuclear reactor-grade Pu has a 240 Pu/ 239 Pu atom ratio of 0.2-1.0 (Schneider et al. 2013), and the 240 Pu/ 239 Pu atom ratio from global fallout is approximately 0.178 ± 0.019 (Kelley et al. 1999). And the Pu caused by the Fukushima accident has the 240 Pu/ 239 Pu atom ratio of 0.30-0.38 (Zheng et al. 2012;Schneider et al. 2013).
Previous studies on Pu isotopes in the sediments of the marginal seas and estuaries of China show that the Pu sources are mainly from global fallout and the PPG (Dong et al. 2010;Wu et al. 2014). Meanwhile, the different distributions of Pu in the sediment and seawater further confirm the process of Pu scavenging process from the seawater to the sediment and accumulation processes in the sediment (Yamada and Zheng 2011;Wu et al. 2018). However, in the past 30 years, previous studies on Pu in the seas and estuaries of Table 1. 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in different seas and estuaries of China. Area 239+240 Pu activity (mBq/g) 240 Pu China mainly focused on specific regions, lacking the analysis and comparison of the overall distribution of Pu isotopes. Due to the complexity of the environment, it is also difficult to fully understand the Pu source in the seas and estuaries of China, their influencing factors, and their chemical behavior in different marine systems. The aim of this study was to make an in-depth comparative analysis of the 239+240 Pu activity and to elaborate the distribution of Pu isotopes in sediments and the dynamic influence on the Pu source in the seas and estuaries of China based on previously published data, which are very important to understand the fate of Pu in the marine environment. In addition, the isotopic data of Pu in the sediments help to establish background information for future assessment of the marine environment and to study the biogeochemical impact of Pu in the seas and estuaries of China. Further, the study of Pu isotopes in the seas and estuaries of China provides an important scientific basis for the study of sea-land interactions in the future. Moreover, it is worth noting that the distribution of Pu isotopes in sediment cores can be used for sediment dating, which is also the future development direction of Pu. Finally, future research on Pu isotopes in oceans and rivers in China and the significance as a tracer of sea-land interactions are prospected.

Sample area
We collected 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio data of 137 surface sediments ( Fig. 1) and 77 sediment cores from the seas and estuaries of China. These samples were mainly distributed between 15°N-41°N and 107°E-127°E, and cover a large proportion of China.

Analytical methods
To assess possible Pu isotopes contamination in the environment, the background distribution levels of Pu isotopes have to be known. However, due to the low concentration of Pu isotopes in the nature environment, special detection methods are needed to analyze the Pu isotopes. When using an α spectrometry to detect Pu isotopes in the environment, 239 Pu and 240 Pu cannot be resolved due to the close energies of their emitted particles, and only the sum activities of these two isotopes ( 239+240 Pu) can be obtained (Bu et al. 2018). In the past few decades, mass spectrometry has been favored for its high sensitivity, low detection limit, and short measurement time . Mass spectrometry is becoming more and more important in the fields of nuclear safeguards, radiation protection, environmental science, and geochemistry (Ketterer et al. 2004a(Ketterer et al. , 2004bLariviere et al. 2006;Ketterer and Szechenyi 2008;Boulyga et al. 2015;Bu et al. 2015Bu et al. , 2018Aggarwal 2016;Alewell et al. 2017;Croudace et al. 2017), and its main application is to detect relatively long-lived radionuclides. Inductively coupled plasma mass spectrometry (ICP-MS) has the advantages of suitable price, convenient operation, high sensitivity, and strong multi-element determination ability, and it has become the most commonly used mass spectrometry method in Pu isotopes analysis (Becker et al. 1999;Becker 2003Becker , 2005Lariviere et al. 2006;Croudace et al. 2017;Hou 2019). Zheng (2015) combined the sector field ICP-MS (SF-ICP-MS) with a high-efficiency introduction system APEX-Q and (or) Aridus II for the determination of Pu. Meanwhile, an APEX-Q sample introduction system with a membrane desolvation unit (ACM) and a conical concentric nebulizer were used as sample introduction systems to improve the sensitivity of the SF-ICP-MS. The detection limit of this method is 10-18 g, which is comparable or even better than that of accelerator mass spectrometry (AMS). In addition, mass spectrometry can be used to determine the atomic ratio, which is very important to determine the radioactive source (Bu et al. 2015;Hou 2019). AMS first appeared in nuclear physics laboratories in the late 1970s and was soon applied to the measurement of elements and radionuclides (Skipperud and Oughton 2004;Fifield 2008;Yang et al. 2016). AMS can be used for the determination of ultra-trace, long-lived radionuclides such as 236 U, 237 Np, 239 Pu, and 240 Pu (Povinec et al. 2013). This method can effectively eliminate and avoid the interference of polyatomic isobaric. However, the high price of AMS limits its widespread application (Yang et al. 2016). In addition, AMS has a bottleneck to distinguish isotopes with the same mass-charge ratio. Due to the interference of 238 U, it is impossible to measure 238 Pu with conventional AMS (Bu et al. 2018). Another common method for testing Pu isotopes is thermal ionization mass spectrometry (TIMS) Aggarwal 2016). With its outstanding high sensitivity and precision, TIMS is considered an authoritative analytical method and has been used as a reference technique to calibrate other atomic ratio measurements (Aggarwal 2016). Moreover, the atomic ratio of long-lived radionuclides is measured with precision better than 0.1% using TIMS, which has been used as a standard method (Beasley et al. 1998;Kelley et al. 1999).
Although there are various methods to analyze Pu isotopes in the environment, ICP-MS is commonly used to analyze Pu isotopes in sediment samples. Moreover, most of the published studies involving the analysis of Pu isotopes in sediments of the coastal areas of China use ICP-MS. Due to the low concentration of Pu isotopes in environmental sediments, the analysis of Pu isotopes in sediments by ICP-MS requires the separation and purification of Pu isotopes to obtain reliable analytical results. In the present study, the sediment samples were dried at 105°C for 24 h. Then they were calcinated in a muffle furnace at 450°C for 5 h to destroy any organic matter. Then 2-2.5 g samples were taken into Teflon cups with a yield monitor of 242 Pu (approximately 1 pg) as a chemical yield tracer. Subsequently, acid leaching (with 20 mL HNO 3 ) was performed on a hot plate for at least 4 h at 60°C. After this sequence of operations, a two-stage anion exchange chromatography method using AG 1-X8 and AG MP-1 M resins was used for separation and purification of the Pu isotopes ). The final sample solutions were diluted to 1 mL using 4% HNO 3 for Pu analysis. With this method of purification, the overall chemical recoveries ranged from 48% to 86% . 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 .
In addition, due to the relatively short half-life (14.4 years) of 241 Pu, its concentration in the environment of global fallout is currently very low (<0.5 fg g −1 in surface soil). However, the concentration of 241 Pu and the 241 Pu/ 239 Pu atom ratio in environment samples can indicate the radioactive source at the early stage of a nuclear accident. Therefore, more and more attention has been paid to the analysis of 241 Pu in environment samples, and it is also the development direction of Pu analysis in the future. The analysis method of 241 Pu is mainly SF-ICP-MS, but for marine sediment samples, more than 10 g is needed (Bu et al. 2018). To further purify 241 Pu from sediments, a TEVA + UTEVA + DGA procedure can be used . Moreover, this method could be used for the long-term monitoring of Pu contamination in environmental samples in the coastal areas of China.
3. The distributions of 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in the sediment of the seas and estuaries of China 3.1. The surface sediments 3.1.1. The spatial distribution of 239+240 Pu activity in the surface sediments 239+240 Pu activity in the surface sediments of different marginal seas is shown in Table 1 and Fig. 2. These reported data indicate that 239+240 Pu activity in the surface sediments of the seas and estuaries of China are inhomogeneous. The average 239+240 Pu activity in the surface sediments of Baohai Bay is 0.497 ± 0.269 mBq/g and in Liaodong Bay it is 0.343 ± 0.276 mBq/g (Zhuang et al. 2019). The average 239+240 Pu activity in the surface sediments of the Yellow Sea is 0.23 mBq/g (Nagaya and Nakamura 1992;Hong et al. 2006), which is lower than that of the Bohai Sea (Baohai Bay and Liaodong Bay). In addition, previous studies have reported that the average 239+240 Pu activity in the surface sediments in the southern South China Sea is 0.625 mBq/g Wang et al. 2019a), which is the highest 239+240 Pu activity in the seas of China. However, Dong et al. (2010) reported the 239+240 Pu activity in the surface sediment of core sediment PA-11 from the South China Sea to be 0.157 mBq/g. Besides, the reported 239+240 Pu activity in the surface sediments of the East China Sea is 0.118 ± 0.119 mBq/g , which is the lowest 239+240 Pu activity in the seas of China. Although the 239+240 Pu activities in the seas of China are different, they are relatively consistent. Furthermore, previous researches have studied the 239+240 Pu activity in the surface sediment of some estuaries, such as the Yangtze River estuary (0.405 mBq/g) (Liu et al. 2011a;Pan et al. 2011) and the Pearl River estuary (0.198 mBq/g) Wang et al. 2019a), and even in other estuaries, the 239+240 Pu activities in the surface sediments are obviously different. Compared with the 239+240 Pu activity in the Okinawa Trough (1.4-2.5 mBq/g, mean: 1.8 mBq/g) (Wang and Yamada 2005) and the northwest Pacific Ocean (0.15-5.4 mBq/g, mean: 2.4 mBq/g) (Moon et al. 2003), the 239+240 Pu activities in the surface sediments of the seas and some estuaries of China are lower. 239+240 Pu activities have different distributions in different seas. In the Bohai area, Zhuang et al. (2019) found that higher 239+240 Pu activity in the surface sediments were observed in the wetlands of the tidal flat covered by vegetation than in other selected samples and showed an increasing trend from land to sea. In addition, the distribution of 239+240 Pu activity in the surface sediment of Bohai Bay presented an increasing trend from north to south (Zhuang et al. 2019). According to the distribution of 239+240 Pu activity in the surface sediments of the Bohai area, on the one hand, they might be attributed to fine-grained sediments due to the high particle affinity of Pu (Xu et al. 2017). Hu et al. (2010) have reported that after transported by the counterclockwise gyre of the Yellow Sea Warm Current, the Luan River and Hai River-derived fine-grained sediments are deposited in the central belt from west to east in Bohai Bay. Overall, sand and silty sand cover the northern areas of Bohai Bay, and clay silt is distributed across the southeastern region. On the other hand, ocean current dynamics (Yellow Sea Warm Current) and fine-grained sediments both promote the process of Pu scavenging. 239+240 Pu activity in the surface sediments of the East China Sea is the lowest of the seas of China, and the lowest 239+240 Pu activities in the East China Sea are observed near the Yangtze estuary and Hangzhou Bay, with a range of 0.048-0.492 mBq/g . This is probably due to sediment being carried by the Yangtze River being deposited by ocean currents and water masses carried to the East China Sea (Liu et al. 2006). Moreover, earlier studies have shown deposition rates of >1.5 cm year −1 in these regions (e.g., Huh and Su 1999;Deng et al. 2006). So the relatively low 239+240 Pu activity relative to the rest of the ocean in China is likely related to dilution of massive influx of Pu-depleted riverborne sediments. Meanwhile, 239+240 Pu activities in the surface sediments of the northern East China Sea region are also relatively low (0.048-0.492 mBq/g) . In addition to the distributions of 239+240 Pu activities in these regions, 239+240 Pu activities in the surface sediments of the East China Sea present a decreasing trend from the offshore to the northwest of the East China Sea, which is consistent with the direction of the Yellow Sea Warm Current ).
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 . 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. Thus, the distribution trend of 239+240 Pu activity in the surface sediments of the northern Yellow Sea might be influenced by fine-grained deposition. The lowest 239+240 Pu activity in the east part of the northern Yellow Sea might be in relation to the dilution of sediments carried by the Yalu River and Dayang River. 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 ). 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).
In comparison, the distribution of 239+240 Pu activity in the surface sediments of the South China Sea is more complicated, and some studies have reported different distribution trends. For example, Wu et al. (2018) have reported that the distribution of 239+240 Pu activity in the surface sediments of the northern South China Sea shows an increasing trend from the outer shelf to the inner shelf, followed by a decreasing trend from the inner shelf toward the shore. Moreover, in the Pearl River estuary, the distribution of 239+240 Pu activity shows a gradual decreasing trend from the estuary to the land. However, Wang et al. (2019a) have reported that the distribution of 239+240 Pu activity in the surface sediments of the northern South China Sea presents an increasing trend from the shore to the inner shelf and a decreasing trend from the inner shelf toward the outer shelf and slope. In addition, the distribution of 239+240 Pu activity is also influenced by coastal current and sediment dynamics of fine particles. Previous studies have reported that the Guangdong Coastal Current mainly transports suspended sediments from the Pearl River estuary westward along the South China Sea shore (Yang et al. 2003;Ding et al. 2017). Moreover, the South China Sea Warm Current flows northeastward along the shelf during the winter and spreads over most parts of the shelf during the summer (Yang et al. 2003), which transports sediments from Hainan Island into the South China Sea. Furthermore, downwelling and upwelling of these coastal currents easily form mud areas in shallow areas and easily adsorb more Pu in fine-grained particles (Wang et al. 2019a).

Distribution of 240 Pu/ 239 Pu atom ratios in the surface sediments and sources of Pu in the seas and estuaries of China
Since the isotopic composition of Pu isotopes is related to their production and release, the atom ratio of 240 Pu/ 239 Pu in environmental samples could be used to identify the source term of Pu contamination. It has been well proved that global fallout has an average 240 Pu/ 239 Pu ratio of 0.178 ± 0.019 (Kelley et al. 1999;Warneke et al. 2002). 240 Pu/ 239 Pu atomic ratios for weapons-grade Pu are reported to be very low (<0.07) (Lee and Clark 2005), while much higher values are reported in materials of nuclear power reactors (0.2-0.8, depending on the reactor type and fuel burn-up) (Muramatsu et al. 2000;Warneke et al. 2002). It has also been found that different test series have different atom ratios. For example, 240 Pu/ 239 Pu atom ratios in the close-in fallout at the Nevada test site are generally very low with an average of 0.035 (Buesseler and Sholkovitz 1987), while the close-in fallout Pu from the PPG of the Marshall Islands in the 1950s is characterized by 240 Pu/ 239 Pu atom ratios of 0.30-0.36 (Koide et al. 1985;Buesseler 1997;Muramatsu et al. 2001). Thus, such characteristics of PPG-derived Pu have been intensively employed to trace transportation of PPG-derived pollution in the northern Pacific Ocean and Chinese marginal seas, including the East China Sea, Yellow Sea, and South China Sea. Also Bohai Bay, Yangtze River estuary, and Pearl River estuary have found higher 240 (Nagaya and Nakamura 1992;Huh and Su 1999;Fig. 3. 240 Pu/ 239 Pu atom ratio in the surface sediments of different seas and estuaries of China (background drawn using GeoMap software). Fig. 4. Relationship between 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in the surface sediments of different seas and estuaries of China. Su and Huh 2002;Lee et al. 2003;Wang and Yamada 2005;Hong et al. 2006;Zheng and Yamada 2006;Liu et al. 2011a;Pan et al. 2011;Wu et al. 2014;Wang et al. 2017;Wang et al. 2019b;Zhuang et al. 2019).
In the Bohai area, 240 Pu/ 239 Pu atom ratios in the surface sediments range from 0.192 to 0.236 (mean: 0.201 ± 0.015), and 240 Pu/ 239 Pu atom ratios in the surface sediment of Liaodong Bay range from 0.173 to 0.241 (mean: 0.190 ± 0.014). The 240 Pu/ 239 Pu atom ratios in the Bohai Bay are more or less higher than the global fallout values, implying that the Pu source in the Bohai Bay is mainly from global fallout and is also influenced by the contribution of Pu from the PPG. 240 Pu/ 239 Pu atom ratios in the Liaodong Bay are consistent with the values of global fallout, showing that the Pu source in the Liaodong Bay is mainly from global fallout. Besides, the distribution of 240 Pu/ 239 Pu atom ratios in the surface sediments of Bohai Bay also present an increasing trend from land to sea and from north to south. In addition, in the Liaohe estuary, the 240 Pu/ 239 Pu atom ratios show an overall decreasing trend from the estuary towards the land (Zhuang et al. 2019).
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 . 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. Although previous studies have shown that the Pu in the surface seawater samples of the central Yellow Sea and South Yellow Sea show some PPG-sourced Pu signals with relatively high ratios (0.18-0.33, with an average of 0.23) (Kim et al. 2004), the impact of close-in fallout Pu from the PPG on the Yellow Sea is still doubtful. Therefore, the influence of the PPG-derived Pu in the Yellow Sea, especially in the northern Yellow Sea, might be very limited ). However, the reported results of the 240 Pu/ 239 Pu atom ratio in the Yangtze River estuary is 0.238 ± 0.007, which indicates that the Pu might have received a contribution from the PPG (Liu et al. 2011a(Liu et al. , 2013Pan et al. 2011).
In the East China Sea area, 240 Pu/ 239 Pu atom ratios in the surface sediments range from 0.158 to 0.297 (mean: 0.238 ± 0.036), which is within the global fallout value (0.178 ± 0.019) (Kelley et al. 1999) and PPG close-in fallout value (0.33-0.36) (Buesseler 1997). Moreover, the distribution of 240 Pu/ 239 Pu atom ratios presents an increasing trend from offshore to the sea, which indicates that the source of Pu in the East China Sea is not only from global fallout but also from the PPG .
Compared to the 240 Pu/ 239 Pu atom ratios in the different sea areas discussed above, 240 Pu/ 239 Pu atom ratios in the surface sediments of the northern South China Sea are relatively high, ranging from 0.246 to 0.281, with an average of 0.264 ± 0.050. And the 240 Pu/ 239 Pu atom ratios in the surface sediments of the Pearl River estuary range from 0.186 to 0.244 (Wang et al. , 2019a. Similarly, the 240 Pu/ 239 Pu atom ratio is 0.227-0.30 in sediment core PA-11 from the South China Sea (Dong et al. 2010). Thus, these atom ratios are significantly higher than the value of global fallout (0.178 ± 0.019, 0°N-30°N) (Kelley et al. 1999), implying the surface sediments of the northern South China Sea have received Pu from the PPG. For the distribution of 240 Pu/ 239 Pu atom ratios in the surface sediment of the northern South China Sea, previous studies have reported different conclusions. For example, Wu et al. (2018) have reported that the distribution of 240 Pu/ 239 Pu atom ratios in the surface sediment show an increasing trend from the outer shelf to the inner shelf, and a decreasing trend from the inner shelf toward the shore. However, Wang et al. (2019a) have shown that the 240 Pu/ 239 Pu atom ratios in the surface sediment of the northern South China Sea present an increasing trend from the outer shelf to the inner shelf and a decreasing trend from the inner shelf toward the shore. And the 240 Pu/ 239 Pu atom ratios in the surface sediment of the Pearl River estuary show a decreasing trend from the estuary to the land. Moreover, Wang et al. (2019a) have clarified the retention of the old deposition information of Pu isotopes in the sediment in the northern South China Sea through the 240 Pu/ 239 Pu atom ratio and sediment dating.

The sediment cores
After reaching the stratosphere, under the influence of Earth's wind belt and pressure belt, 239+240 Pu from global fallout shows distribution characteristics with the change of latitude. According to the research of Krey et al. (1976)

239+240 Pu inventory in sediment cores from the seas and estuaries of China
The analyses and studies on these 77 sediment cores from the seas and estuaries of China show that the 239+240 Pu inventory in each sediment core varies greatly. Among them, the 239+240 Pu inventories of 43 sediment cores are significantly higher than the recommended values for global fallout at the same latitude. For example, 239+240 Pu inventories in the cores SC07 (Pan et al. 2011) and SC18 (Liu et al. 2011a) of the Yangtze River estuary, core A8 of the Pearl River estuary , and cores of the Okinawa trough (590-23, 590-16, 590-18, 590-22, 642-4, 679-7a) (Lee et al. 2004) (Fig. 5). In addition, for the other 34 sediment cores, the mean value of the 239+240 Pu inventory is 53.5 Bq/m 2 , which is slightly higher than the mean recommended value for global fallout at the range of 20°N-30°N (39 Bq/m 2 ) (UNSCEAR 2000). However, not all of the 34 sediment cores have higher 239+240 Pu inventories than those in the continental soils at the same latitude. For example, the 239+240 Pu inventories in cores F8 ), CB-7-2, CB-35 (Nagaya and Nakamura 1992), and 200908 (Zhuang et al. 2019) (cores F8 and CB-7-2 are from the South China Sea, core CB-35 is from the Yellow Sea, and core 200908 is from the Bohai Sea) are significantly less than the mean recommended value of 20 Bq/m 2 for global fallout at the same latitude (Fig. 5). The reason for the slightly lower 239+240 Pu inventory in these sediment cores may be related to the sedimentary environment (such as salinity, pH) and the content of fine particular matter and organic matter (Nagaya and Nakamura 1992;Dong et al. 2010;Wang et al. 2017;Zhuang et al. 2019). In addition, Pu from global fallout is not completely deposited to the sediments, and some of the Pu remains in the water. Moreover, part of the Pu in the sediments is transferred from the water to the sediments by Pu scavenging. Finally, erosion of the sediments on the seafloor by the coastal current and storm surge disturbances may also reduce or drain the 239+240 Pu inventory in these sediment cores.
For the Pu isotopes in these sediment cores from the seas and estuaries of China, previous studies have shown that they mainly come from global fallout and the PPG. In addition, some of the Pu isotopes in the sediments of seas and estuaries of China are imported by inland rivers. For example, Wang et al. (2017) have reported that the Yangtze River is the main source of Pu isotopes for the estuary area in the East China Sea, and the proportion of Pu isotopes carried by the river inputs in the sediments of the East China Sea is about 2.41% (according to the mass balance model of the 239+240 Pu inventory in sediment). Similarly, the proportion of Pu isotopes carried by river inputs in the Pear River estuary is about 13% (Wang et al. 2019a), which is lower than that carried by the river inputs in the Yangtze River estuary (77%-80%) (Liu et al. 2011a;Pan et al. 2011) and in Bohai Bay (32.8%) (Zhuang et al. 2019).

Distribution of 239+240 Pu activity in sediment cores from the seas and estuaries of China
Under the condition of a relatively stable marine sedimentary environment, the distribution of 239+240 Pu activity in the sediment cores from the seas and estuaries of China shows certain regularity. Meanwhile, in the offshore marine environment, the distribution of 239+240 Pu activity in sediment cores may be seriously disturbed if the sediment cores are under strong hydrodynamic conditions (Liu et al. 2013;Wang et al. 2017). Therefore, the sedimentary superposition effect of Pu from different sources (land-source input, direct global fallout, and sediment of seawater scavenging) leads to a peak profile of 239+240 Pu activity in these sediment cores from different seas and estuaries of China. This sedimentary superposition effect is influenced by the source of sediment and water depth (Lee et al. 2004;Wang and Yamada 2005;Deng et al. 2006;Wang et al. 2017). For the 77 sediment cores from the seas and estuaries of China, the distributions of 239+240 Pu activity show double peaks (multiple peaks), single peak, and no peak. The typical sedimentary environment in the estuary areas or the shallow water areas near the shore are dominated by continental sediment sources. Typical double peaks (multiple peaks) of 239+240 Pu activity in sediment cores can be directly used to determine the sediment dating year. For example, there are obvious double peaks in core SC07 from the Yangtze River estuary, and the maximum peak year can be directly judged as 1963 (Pan et al. 2011) (Fig. 6). While the secondary peak in the sediment core indicates a different sedimentary environment. As the Pu isotopes from different sources have different 240 Pu/ 239 Pu atom ratios, the dating year of Pu source can be determined by the change of the ratios accordingly. For example, the 240 Pu/ 239 Pu atom ratio in core SC07 changes with time, and the 240 Pu/ 239 Pu atom ratio at 100 cm depth is higher than that at a shallower depth, and these 240 Pu/ 239 Pu atom ratios at a shallower depth are close to 0.18. According to the 240 Pu/ 239 Pu atom ratios in core SC07, the secondary peak is judged to be at 1955. In addition, double-peak (multi-peak) distribution is also observed in sediment core SC18 from the Yangtze estuary (Liu et al. 2011a) and sediment core NYSC-02 from the northern Yellow Sea ).
In a shallow or deep-sea sedimentary environment, there is a certain proportion of continental sediments and marine sediments. Thus, typical one peak distribution of 239+240 Pu activity is observed in the following sediment cores in these sedimentary environments. For example, there is only one peak of 239+240 Pu activity in sediment cores 499-16 and 460-39 from the East China Sea, which are also the largest 239+240 Pu activities in these sediment cores, and the peak can be used as sediment dating, and the results of dating are consistent with those of 137 Cs and 210 Pb (Fig. 7) (Huh and Su 1999;Su and Huh 2002). Similarly, the distribution of 239+240 Pu activity in sediment core SN2 also shows a typical one peak feature, which is consistent with the peak feature of 137 Cs in the sediment core, and indicates that the sediment dating is 1963 (Fig. 6) (Guan et al. 2018). In the deep-sea sedimentary environment, the source of marine sediments is domination. There is no peak of 239+240 Pu activity in sediment cores, such as sediment core SST1 from the East China Sea (Wang and Yamada 2005) and sediment core PA-11 from the South China Sea (Dong et al. 2010) (Fig. 8). Because of the shallow burial depth of Pu, no peak of 239+240 Pu activity is found in either of the two sediment cores, and the maximum 239+240 Pu activity appears in the surface layer, so the distribution of 239+240 Pu activities in the two sediment cores cannot  240 Pu/ 239 Pu atom ratio distributions in sediment cores SN2 and SC07 (Pan et al. 2011;Guan et al. 2018). be used to judge the dating year. Although there is no peak in sediment cores SST1 and PA-11, the sedimentary environments of both are different. The inventory of 239+240 Pu activity in core PA-11 is 3.75 ± 0.29 Bq/m 2 (Dong et al. 2010), which is far lower than the 239+240 Pu inventory in the latitude range of 10°N-20°N (UNSCEAR 2000). While the inventory of 239+240 Pu in core SST1 (47.0 ± 1.2 Bq/m 2 ) (Wang and Yamada 2005) is higher than the 239+240 Pu inventory in the latitude range of 20°N-30°N (36 Bq/m 2 ) (UNSCEAR 2000). This is because sediment core PA-11 is located in the basin of the South China Sea at a depth of 4323 m, and the 239+240 Pu activity in the seawater is higher than that in the sediments. Sediment core SST1 is located in the Okinawa trough at a depth of 1080 m, and the 239+240 Pu activities are also relatively low. Furthermore, according to the 240 Pu/ 239 Pu atom ratios in the two sediment cores, it can be known that the Pu may come from global  499-16, 460-39, and A8 (Huh and Su 1999;Su and Huh 2002;Wu et al. 2014). fallout and the PPG. However, there is no peak in cores PA-11 and SST1 mainly because of the location of these sediment cores and the ocean current, such as Kuroshio (Wang and Yamada 2005;Dong et al. 2010).

Application of dating 239+240 Pu activity in marine sediment cores
In previous studies, 239+240 Pu activity in the sediment cores from the northern hemisphere shows three obvious peaks, with the highest peak year being 1963, the first secondary peak year being 1959, and the second secondary peak year being 1955. And the 239+240 Pu activity in the sediment cores of the seas of China also shows these peak characteristics. The contribution of the secondary peak is mainly due to the PPG (UNSCEAR 2000). For sediment dating, 210 Pbex, 137 Cs, and 239+240 Pu activities are usually used (Ketterer et al. 2002(Ketterer et al. , 2004bZheng et al. 2008;Guan et al. 2020). Meanwhile, the 240 Pu/ 239 Pu atom ratio can also be used as a reference to determine the year with 239+240 Pu. It is generally assumed that the formation of 137 Cs or 239+240 Pu sedimentary peaks is the result of continuous precipitation, rather than the peak formed by nuclide diffusion. In addition, it should be noted that the location of the maximum peak type is usually the first consideration for dating by 137 Cs or 239+240 Pu activity in the sediment cores, which is the most reliable time scale (1963 or 1964). Finally, if the peak types of 137 Cs or 239+240 Pu in sediment cores are not obvious, the initial sedimentary age can also be considered as the time scale ( 239+240 Pu is 1952, and 137 Cs is 1954-1955. There are usually three methods to determine the age by 239+240 Pu activity. The first is the time scale (start year and peak year) method of 239+240 Pu activity (Tims et al. 2010;Liu et al. 2011a;Pan et al. 2011), such as the dating study of the 239+240 Pu activity in sediment cores SC07 and SC18 from the Yangtze River estuary (Fig. 6). The second method is the 240 Pu/ 239 Pu atom ratio (Koide et al. 1985), and the final method is the 238 Pu/ 239+240 Pu ratio (Hancock et al. 2011). If the 240 Pu/ 239 Pu atom ratios in the sediment cores are the same, it will be invalid to judge the sedimentary age using this method. For example, there is no various peak of 240 Pu/ 239 Pu in the sediment core NYSC-01 from the northern Yellow Sea   (Fig. 9). And the 240 Pu/ 239 Pu atom ratios in core NYSC-01 are relatively consistent, losing the value of judging the time information for the Pu source. In addition, it is worth mentioning that the time scale of 137 Cs and 239+240 Pu activity might be inconsistent. For example, in sediment core SC18 from the Yangtze River estuary, the maximum peak of 137 Cs (141 cm) is above the maximum peak of 239+240 Pu activity (70 cm) (Liu et al. 2011a) (Fig. 10). However, sediment core 201026 from Bohai Bay has the opposite situation in that the maximum peak of 137 Cs (70 cm) is below the maximum peak of 239+240 Pu activity (42 cm) (Zhuang et al. 2019) (Fig. 10). Therefore, for such type of sediment core dating, a comprehensive judgment should be made based on the 240 Pu/ 239 Pu atom ratio and the distribution of 210 Pbex, rather than using the peak type of 137 Cs or 239+240 Pu activity as the dating year (1963 or 1964). 240 Pu/ 239 Pu atom ratio in sediment cores from the seas and estuaries of China In these 77 sediment cores, the distributions of 240 Pu/ 239 Pu atom ratios varied greatly with the depth of these sediment cores, but they were mainly divided into three types. One type is that the 240 Pu/ 239 Pu atom ratios are inconsistent in the upper and lower parts of the sediment cores, such as core SC07 from the Yangtze River estuary. In core SC07, the 240 Pu/ 239 Pu atom ratios in the bottom of the sediment core are higher than 0.18, while the ratios in the upper part of the sediment core are close to 0.18 (Tims et al. 2010;Pan et al. 2011) (Fig. 6, SC07). Another type is that the 240 Pu/ 239 Pu atom ratios are consistent in the upper and lower parts of the sediment cores, but they are significantly higher than the ratios of global fallout and lower than the ratios of the PPG, such as core A8 from the Pearl River estuary ) and core SN2 from Beibu Bay (Guan et al. 2018) (Figs. 6  and 7). Moreover, these significantly higher 240 Pu/ 239 Pu atom ratios indicate that the sediments in these cores received a contribution of Pu from the PPG during deposition. The final type is that the 240 Pu/ 239 Pu atom ratios are consistent in the upper and lower parts of the sediment cores but close to 0.18, such as cores NYSC-01 and NYSC-02 from the northern Yellow Sea   (Fig. 9). Furthermore, Xu et al. (2018) have reported that Pu in the sediments from the northern Yellow Sea is not affected by Pu from the PPG, and the Pu isotopes in the two sediment cores are from global fallout.

Long-range transport of Pu from the PPG
According to the reported values of 240 Pu/ 239 Pu atom ratios in the surface sediments in different seas and estuaries of China, it is easy to conclude that the sources of Pu may come  (Liu et al. 2011a;Zhuang et al. 2019). from global fallout and the PPG, rather than other nuclear accidents (Figs. 3 and 4) (Zheng et al. 2012;Oikawa et al. 2015;Casacuberta et al. 2017;Hain et al. 2017). Pu from the PPG can be transported and deposited to different seas and estuaries of China mainly by ocean currents and water mass transportation. To be more specific, the major current system in the western Pacific is the Kuroshio Current, which originates from the northward bifurcation of the Northern Equatorial Current flowing to the Philippine Sea (Fig. 11). The Kuroshio Current flows northward along the coast of Luzon to as far as the eastern part of Taiwan Island, resulting in the appearance of the South China Sea branch of the Kuroshio. In addition to the Kuroshio Current extension in the South China Sea, water transport also occurs via the South China Sea Warm Current. In the South China Sea, the northeasterly winds force the Guangdong Coastal Current flowing westward and eastward in winter and summer, and the South China Sea Warm Current flows northeastward along the shelf during the winter and spreads over most parts of the South China Sea during the summer. Meanwhile, the South China Sea branch of the Kuroshio Current flows southeastward along the edge of the South China Sea continental shelf. In the southern part of the East China Sea, there are two major northward currents: one is the Kuroshio Current flowing in a northwest direction along the edge of the East China Sea continental shelf, and the other is the Taiwan Warm Current that flows northward from the Taiwan Strait. Along the coast of the East China Sea, the southeastern northern Jiangsu Coastal Current flows to the northern Yangtze River mouth, while the Zhejiang-Fujian Coastal Current flows along the coast of Zhejiang and Fujian provinces, moving northward in summer and southward in winter (Yang et al. 2003;Jilan 2004;Liu et al. 2014;Ding et al. 2017;Wang et al. 2017). In addition, the East China Sea Coastal Current flows southward from the Yangtze River mouth, whereas the Changjiang Diluted Water extends eastward from the river mouth. Further, the Yellow Sea Warm Current, a branch derived from the Kuroshio Current that turns into the Yellow Sea, intersects with the Yellow Sea Coastal Current and the Taiwan Warm Current to form a counterclockwise circulation in the Yellow Sea (Su and Huh 2002).
The inland rivers are essential to supply sediments, and the particles of sediments are important to control the distributions of 239+240 Pu activity and 240 Pu/ 239 Pu atom ratios in sediments. Moreover, Pu in the estuarine areas is mainly deposited into the sediment from water through the Pu scavenging process. According to the idea of "source-sink", most rivers flow into the ocean, resulting in the accumulation of large amounts of sediments at the estuaries ). These sediments reach specific areas along with currents and water masses, absorbing the Pu from the PPG source through a series of processes to settle into the sediments. For example, the sediments from the Luan River and Hai River are deposited in the central Bohai Bay, and clay particles are derived to the southeastern region (Hu et al. 2010) where the 239+240 Pu activities are significantly higher than other areas (Zhuang et al. 2019). Moreover, the Yangtze River also carries a large amount of sediments deposited in the Yangtze River estuary, and forms two large-scale mud deposits (Changjiang Subaqueous Delta and Zhejiang-Fujian Coastal Mud Belt) (Gao et al. 2017). The two mud deposits have exceptionally higher 239+240 Pu activities and 240 Pu/ 239 Pu atom ratios than their surroundings. However, because of the dilution of the massive influx of Pu derived from river-borne sediments, the sediments in the Yangtze River estuary and Hangzhou Bay have the lowest 239+240 Pu activities than other areas in the East China Sea (Pan et al. 2011;Liu et al. 2014). In the South China Sea, the largest river input is the Pearl River, which carries a large amount of fine-grained sediments to form a mud delta in the northwest of the northern China Sea (Yang et al. 2003;Ding et al. 2017). And areas with relatively high 239+240 Pu activities and 240 Pu/ 239 Pu atom ratios is consistent with mud deltas (Wang et al. 2019a).

Estimation of Pu contributions in different seas and estuaries of China
In the seas and estuaries of China, Pu sources may come from global fallout and the PPG, but the contributions from these two sources are different. A simple two-end-member mixing model (Krey et al. 1976) was used to calculate the relative contributions of Pu from global fallout and the PPG in different seas and estuaries of China (Table 2). For example, the contribution of Pu from the PPG in the Bohai Bay ranged from 4.6% to 28.1% (mean value: 15.5%) (Zhuang et al. 2019). In the East China Sea, Pu contribution from the PPG was estimated to be 45%-52% , which is comparable to previous results obtained in the South China Sea (∼42%) Wu et al. 2018). However, these results were lower than that in the shelf sediments of the South China Sea (∼68%) ) and the basin sediments of the South China Sea (∼57%) (Dong et al. 2010). The contributions of Pu from the PPG in different estuarine sediments are different. For example, the contribution of Pu from the PPG in the Yangtze River estuary was 40%-47% (average 44%) (Liu et al. 2011a;Pan et al. 2011), which was higher than that in the Pearl River estuarine sediments (∼30%) ).

Summary and outlook
This study initially reviewed the current understanding of the distribution of 239+240 Pu activity and Pu sources in surface sediment and sediment cores from the seas and estuaries of China. 239+240 Pu activities in the surface sediments of different seas and estuaries of China are various and unevenly distributed, and the distribution of 239+240 Pu activity is more influenced by Pu scavenging processes, combined with the interactions of the sediments carried by inland rivers and ocean currents. Meanwhile, the distribution of 239+240 Pu activity in these sediment cores presents double peaks, single peak, and no peak, and these peaks can be used to determined the sediment dating and sediment source. 240 Pu/ 239 Pu atom ratios in the sediments of the seas and estuaries of China are similar, which indicates that the Pu may be from global fallout and the PPG, with the expection of Pu in the sediment cores of the northern Yellow Sea from global fallout. For the contributions of Pu from the PPG, different seas and estuaries receive different contribution degrees, and the degree in surface sediments is consistent with the degree in sediment cores. In addition, the higher degrees of Pu contributions from the PPG in some areas may be due to the precipitation of fine-grained sediments. Although Pu isotopes in the environment have been studied extensively, much science is still to be learned about Pu biogeochemistry and its fate in the environment. In addition, with the rapid development of the Chinese nuclear industry, it is necessary to establish a long-term monitoring program for Pu in the environment in the coastal areas of China to better assess the risks of environment pollution. The transport of Pu in the environment depends on its isotope distribution and transformation. In different environments (such as oxidation and reduction environments), the isotope morphology of Pu varies over time due to environmental complexities. Meanwhile, understanding the nature and morphology of Pu isotopes is helpful to understand the dynamic process of Pu in different environments. Understanding the biological formation and transformation process of Pu is helpful to accurately evaluate its ecological impact. Besides, the coastal areas have complex environments, and some coastal zone areas become Pu isotope sinks of inland rivers. The role of these hot spots of Pu pollution in the Pu migration process needs to be further studied. In addition, Pu isotopes are good tracers of ocean circulation, but their difficulty in measurement and modeling hinders the study of Pu in the marine environment. Finally, due to the limitation of testing and analyzing methods of Pu isotopes, the tracing of Pu needs to be studied by means of model simulation in the future to reveal the interaction mechanism and spatial and temporal evolution law of key elements such as water, soil, gas, and biology in the surface system, so as to improve the understanding of the structure and function of the land surface system.