Distribution and budget of 137Cs in the China Seas

Cesium–137 is one of the most abundant anthropogenic radionuclides released by atmospheric nuclear testing and nuclear accidents, and accordingly it may significantly impact the health of humans and marine environmental eco–systems. Documenting the distribution and inventory of 137Cs is thus a crucial task. In this study, we collected a large number of datasets with field observations of 137Cs in the China Seas, in order to provide an in–depth understanding of 137Cs budgets and distributions. The activity and inventory of 137Cs in China Seas’ sediments showed large spatial variations, related to the 137Cs source, sedimentation rates and the mineral composition of sediments. The 137Cs concentration in sediments decreased with distance from the shore, generally tracing the distribution of sedimentation rates. High 137Cs inventories in the water column indicated a high solubility and long mean residence times. The mean residence times of 137Cs in the China Seas were determined to be 45.6 ± 3.8 years for the South China Sea (SCS), 36.8 ± 3.1 years for the East China Sea (ECS), and 12.0 ± 1.0 years for the Yellow Sea (YS). A 137Cs mass balance suggests that oceanic input from the north Pacific is the dominant 137Cs source to the China Seas, contributing about 96.9% of this substance. Furthermore, the bulk of 137Cs remains dissolved in the SCS water column, while 137Cs is mostly deposited to the sediments of the ECS and the YS. This new compilation of the activity level and inventory of 137Cs help to establish background levels for future 137Cs studies in the China Seas.

exhibits a seasonal pattern, with a weaker intrusion into the SCS in summer than in winter 20 . The Kuroshio flows northeastward along Taiwan's eastern coast throughout the year, with a branch entering the ECS at the northeast corner of Taiwan while the mainstream continues to flow northeast to the eastern Japanese coast 21 (Fig. 1). In the YS, the Yellow Sea Cold Water Mass, which is the cold water located below the seasonal thermocline from spring to autumn, and tides, which are mixed diurnal or semi-diurnal 22,23 , are the most important hydrographic features 22 .
To our knowledge, reports on the spatiotemporal distribution of 137 Cs and its influencing factors in the China Seas as a whole are scarce, with most publications focusing on localized areas. For instance, the activity level and distribution of 137 Cs in ECS seawater were investigated after the FDNPP accident 24,25 . Zhang et al. 26 discussed the source and budget of 137 Cs in the ECS covering a small area based on a limited 137 Cs dataset. Hao et al. 27 investigated the activity level of 137 Cs in coastal seawater of the Bohai Sea. In the SCS, Yamada et al. 12 and Zhou et al. 28 investigated the horizontal and vertical distribution of 137 Cs in the water column. Additionally, 137 Cs in sediments of the ECS was investigated to calculate the apparent sedimentation rates 5 , while Zhuang et al. 29 studied the impact of tides on the distribution of 137 Cs in Bohai Sea sediment; in the SCS, 137 Cs in sediments was used to trace the transport of terrestrial particles 30 . However, the limited study areas present just a glimpse of 137 Cs behavior, which may lead to biased interpretations of 137 Cs behavior in the China Seas. Expanded 137 Cs datasets including both seawater and sediment are rarely reported, which limits our ability to fully understand the distribution of 137 Cs and discuss its important influencing factors in the China Seas. The objective of this study is to provide detailed insight into the distribution and budget of 137 Cs in the China Seas, by compiling expanded 137 Cs datasets that include both seawater and sediments and investigating the factors influencing them. This will also improve our general understanding of the fate of 137 Cs in marine environments. Furthermore, information about the activity and budget of 137 Cs also contributes important background information that can be used for future risk assessment in the China Seas, and for the study of 137 Cs biogeochemistry in marginal seas. Finally, the prospect of future 137 Cs studies in the China Seas is discussed.

Methods
Data source and treatment. We reviewed over 400 datasets of 137 Cs in the China Seas covering the past 30 years, which include surface measurements and profiles in both sediment and seawater. Detailed sample information is shown in the Supplementary Information (SI). These 137 Cs datasets were mainly extracted from bibliometric databases such as the Web of Science, Google Scholar and Scopus, using the keywords "East China Sea, South  5,13,[24][25][26]31,44,45,49 ; South China Sea 6,12,13,28,30,39,76 ; Yellow Sea 26,27,29 ). (b) a schematic chart of the China Seas surface current: the Kuroshio, and the North Equatorial Current (NEC), represented by the blue dotted lines. In the South China Sea (SCS), the red dashed and black solid lines indicate the anticyclonic gyre in summer and the cyclonic gyre in winter, respectively. In the East China Sea (ECS), the red dashed and black solid lines represent the coastal currents in summer and winter, respectively. We modified this pattern of ocean circluation according to Hu et al. 77 5,13,[24][25][26]31,44,45,49 ; South China Sea 6,12,13,28,30,71 ; Yellow Sea 14,26,27,29 ). This figure was drawn using the free software Ocean Data View (ODV 5.1.2) (Schlitzer, R., Ocean Data View, https://odv.awi.de, 2018). The relationships between 137 Cs activity and the salinity (a) and temperature (b) of surface seawater in the China Seas (data soured from previous studies 12,13,[25][26][27][28]31 ). This figure was prepared with Sigma-Plot professional 10.0 software. www.nature.com/scientificreports www.nature.com/scientificreports/ from global fallout is bound to soil and sediment particles, which are discharged into the China Seas due to soil erosion. Based on the "fingerprint" of Pu, Xu et al. 34 calculated the global fallout 239+240 Pu contribution to the ECS using a two end-member mixing model. They found that riverine input accounts for ~80% of ECS inventories and direct deposition accounts for ~20%. Here, we assumed that 137 Cs and 239+240 Pu originating from global fallout have a similar chemical behavior 35 (the riverine input flux of 137 Cs is also calculated later in discussion). On the other hand, abundant particles facilitate rapid scavenging of 137 Cs from the ECS water column, resulting in the lower 137 Cs activity observed in the Yangtze River mouth compared to the shelf or basin of the ECS. This is also consistent with the 137 Cs showing a stronger particle affinity in the low salinity or freshwater zones because the high salinity in seawater causes desorption of 137 Cs 36,37 . The 137 Cs activity in the China Seas gradually decreases with increasing latitude (e.g., from mid-latitude (SCS and ECS) to high-latitude (YS)), which is inconsistent with the 137 Cs deposition flux of global fallout, namely, the deposition flux of 137 Cs in high-latitudes is higher than that observed in low-latitudes since the atmospheric nuclear weapons testing in the early 1960s was mostly conducted in high latitude zones 3 . Such a difference is possibly caused by the different oceanic regimes. The SCS and ECS are deeper and most 137 Cs is preserved in the water column. The shallower YS experiences strong hydrodynamic conditions (e.g., storms, stronger winds, and intensified waves), which result in the resuspension of particles and vigorous mixing. Therefore, the 137 Cs in the YS is more readily scavenged from the water column and then deposited in the sediment.
Additionally, the relationship between temperature and 137 Cs was examined based on a large number of field observations. We found no correlation (Fig. 3b), suggesting that the seasonal variation of 137 Cs in the China Seas was minor. This may be related to the long half-life and residence time of 137 Cs in the China Seas (see discussion below) 38 .
Vertical distribution of 137 Cs. The 137 Cs activities in 33 water column locations of the China Seas were collected in previous studies [24][25][26]28,31,39 , and their vertical distributions are plotted in Fig. 4.
The vertical distributions of 137 Cs activity in the SCS are shown in Fig. 4b, displaying an initial increasing tendency with water depth and a maximum in the subsurface, followed by a more gradual decrease. This profile distribution of 137 Cs agrees with those obtained elsewhere 40 . For example, in the Pacific Ocean and the Atlantic Ocean, the vertical pattern of 137 Cs gradually increases from surface to subsurface (~150 m), which is followed by a slower decrease 3,11,41,42 . The high activity level of 137 Cs in the SCS subsurface seawater (~150 m) is potentially related to the input of Kuroshio (i.e., the SCS seawater exchanging with the Kuroshio through the Luzon Strait) and the difference of particle removal across this depth layer (namely, higher removal above and lower removal below). The vertical distribution of suspended matter indicates suspended particulate matter (SPM) from the SCS shelf through lateral transport was noticeable in this depth range, which would enhance 137 Cs scavenging 43 . In the middle deep layer, the concentration of SPM was low, and regeneration should be the dominant process because of SPM microbial decomposition during downward transport 43 .
In the ECS, the vertical distribution of 137 Cs activity was similar to the pattern in the SCS, namely, a remarkable maximum usually occurred in the subsurface at depths ranging from 100-600 m (Fig. 4c). Wide depth range of the ECS subsurface maxima is potentially due to the relatively wide spatial coverage of the collected samples. For example, the spatial coverage of sampling extends from the estuary to the shelf and the basin. In the YS, the vertical pattern of 137 Cs activity had no consistent distribution (Fig. 4d), which was related to the tidal effect (semi-diurnal and diurnal tides) and the sampling locations in the coastal area. Nevertheless, they overall showed the 137 Cs activity in the YS surface seawater was higher than that in the bottom water.
Sediment. Horizontal distribution of 137 Cs. The lateral distribution of 137 Cs activity in the China Seas sediment is plotted in Fig. 2b based on compilation of over 205 datasets. The 137 Cs activity of surface sediment in the China Seas varies widely from 0.06 to 5.55 Bq kg −1 , averaging 1.44 ± 1.07 Bq kg −114, 25,29,31,44,45 . Overall, the horizontal distribution of 137 Cs activity decreases from nearshore to offshore in the China Seas sediment. This pattern is mainly controlled by the 137 Cs source, transportation, sedimentation rate, mineral composition and particle size. Given that 137 Cs is mainly bound to riverine particles, it follows the deposition patterns of riverine particles. For example, high 137 Cs activity is observed in the Min-Zhe coastal zone and the northwest corner of Taiwan in the ECS. The high 137 Cs activity observed in the Min-Zhe coastal zone was caused by a large abundance of terrestrial particle-bound 137 Cs. The terrestrial particulate matters entrained by Changjiang diluted water (CDW) in winter, and the resuspended sediments generated by the typhoons in summer, eventually join the northeastwardly Kuroshio in the northern Taiwan Strait 46 . Therefore, the terrestrial particulate matters and resuspended sediments create favorable conditions for the resulting high 137 Cs activity in the northwest corner of Taiwan. In contrast, the distribution of 137 Cs activity in the northern SCS shelf shows a decrease from nearshore to offshore, and high 137 Cs activity is observed on both sides of the Pearl River Estuary (PRE). In the northern SCS, the Pearl River plume disperses southwestward in winter, but northeastward in summer 47 . The terrestrial particulate matter carried by the Pearl River plume is preferentially deposited along the dispersing direction of the plume and favors quick 137 Cs removal 26 . It is worth noting that we cannot present the spatial distribution with a high resolution because of the limited number of samples available. Finally, the distribution of 137 Cs activity in the YS is likely controlled by tides, ocean current, fluvial input, and resuspension. In the Yellow River estuary and the central YS, high 137 Cs activity was observed, while low 137 Cs activity was observed in the western YS. The Bohai sea, a semi-enclosed bay, connects Yellow River discharge upstream with the YS downstream. The estuarine circulation is mainly influenced by riverine influx and tides 48 . The semi-diurnal and diurnal tides around the Bohai bay are favorable for the resuspension of 137 Cs in the shallow area. High 137 Cs activity in the central YS is potentially related to the mineral composition of sediment (i.e., clay, silt and sand) 26 . Further examination of the relationship between 137 Cs activity and mineral composition/mean size of sediment in the China Seas is shown in Fig. 5. The sediments are mainly composed of silt (2-63 μm), sand (>63 μm) and clay (<2 μm), of which range from 6.7%-73.8% of silt (averaging Scientific RepoRtS | (2020) 10:8795 | https://doi.org/10.1038/s41598-020-65280-x www.nature.com/scientificreports www.nature.com/scientificreports/ 46.9%±20.1%, n = 148), 0.2%-89.7% of sand (averaging 34.1%±21.9%, n = 148), and 3.3%-48.6% of clay (averaging 19.0% ± 9.5%, n = 148) 26,30,45,49,50 . By analyzing a large number of field observation data, the 137 Cs activity showed linear positive correlations with the content of clay (R 2 = 0.3247) and silt (R 2 = 0.2027), and a negative correlation with the content of sand (R 2 = 0.2834) ( Fig. 5b-d). The mineral compositions of clay and silt mainly include illite, chlorite, kaolinite and smectite, which more easily adsorbs 137 Cs. In contrast, the mineral composition of sand is mainly silicon dioxide, which is unfavorable for 137 Cs adsorbed on the particles. Laboratory experiment also suggested the adsorption of 137 Cs in sediment depends on the grain size and have reported this type of empirical relationship 51 . Here, the relationship between 137 Cs activity and grain size was examined based on in-situ data, indicating the 137 Cs activity exponentially decreased with increasing particle size (Fig. 6). The 137 Cs is easily adsorbed and accumulated in the finer particles compared to the coarser particles 51 . Walling and Woodward (1992) 52 suggested the 137 Cs activity in the finer fractions of Jackmoor Brook catchment soil (Devon, UK) was several times higher than those in the coarser fractions. Indeed, the mineral composition and grain size in sediments of the China Seas has a significant influence on the distribution of 137 Cs activity. For example, high percentage of clay minerals (>25%) and small grain size in sediments of the Min-Zhe coastal zone corresponds to high 137 Cs activity 26 . Similar behavior is observed in the Yellow River estuary (percentage of clay >25%) and the central YS (percentage of clay >40%), where high clay content corresponds to high 137 Cs activity 26 .  Fig. 7.
The activity level of 137 Cs in the different sediment cores displays huge spatial variability with distance from the shore. In the nearshore, the 137 Cs activities show large fluctuations with respect to core depth (e.g., the coastal YS; Fig. 7d). In the shelf zone, the 137 Cs activity is well preserved in the sediment cores (e.g., the SCS and ECS shelf: Fig. 7b,c). The vertical distribution of 137 Cs in the shelf zone shows an initial increase down core, a prominent maximum appears in the mid-layer, and then a marked decline further down core. Indeed, the variation of 137 Cs activity with depth in well preserved sediment cores usually reflects the input and depositional history of 137 Cs, which is widely used to reconstruct sedimentary chronology in the marine environment 6,44,45 . For example, utilizing the time marker of 137 Cs fallout pulse input (i.e., the 137 Cs peak concentration in sediment cores corresponding to the global fallout maximum, circa 1963), Wu et al. 6 calculated the sedimentation rate in the northern SCS shelf to be ~0.328 cm yr −1 , which agrees with the rate calculated by an another independent natural  www.nature.com/scientificreports www.nature.com/scientificreports/ radionuclide-210 Pb ex (~0.337 cm yr −1 ). This indicates that biological perturbation in the northern SCS shelf is limited 5 . Therefore, the temporal change of 137 Cs input in the sediment core could be reflected by the sedimentary record.
Here, we discuss the temporal variation of 137 Cs in the northern SCS shelf (Fig. 8). It is of note that the influence of diffusion and mixing generally exists, although the dominant process on the shelf is sedimentation. Therefore, we cannot present the annual change of 137 Cs. Nevertheless, we can discuss the source influences and the features of 137 Cs on a ten-year timescale by subdividing the sedimentary record into four time periods: pre-1945, 1946-1965, 1965-1985  www.nature.com/scientificreports www.nature.com/scientificreports/ From 1965 to 1985, the 137 Cs activities showed a slight decrease from 1.76 to 1.56 Bq kg −1 . This decrease was not as sharp as expected as the global large scale atmospheric nuclear weapon testing had been banned in this period. This suggests a continuous input of 137 Cs derived from the PPG via the North Equatorial Current and Kuroshio transport. Post-1986, the 137 Cs activities showed a gradual decrease, which was inconsistent with the termination of atmospheric nuclear weapon testing during this period. The reason is similar as in the above discussion during the period of 1965-1985.
Based on the above method (using the 137 Cs time marker), the sedimentation rates in the ECS and the YS were also calculated. They ranged from 0.01 to 6.3 cm yr −1 (mean of 0.91 cm yr −1 ), which agreed with the results estimated with 210 Pb ex 26,44,53,54 . The lateral distribution of apparent sedimentation rates in the China Seas is plotted in Fig. 9. Overall, high sedimentation rates appeared in the river mouth and nearshore, and gradually decreased with increasing distance from the shoreline. For example, the very high apparent sedimentation rates observed in the estuary of Yellow River and Yangtze River can be attributed to large amounts of terrestrial particles discharged from the two major rivers. In the shelf, low sedimentation rates suggest the sedimentary process is dominant. In contrast, in the nearshore, riverine input of terrestrial particles is a major influencing factor on the sedimentation rates. For example, the distribution of sedimentation rates decreased southwards along the inner shelf and offshore, which is consistent with the dispersal of CDW carried terrestrial particles. The apparent sedimentation rates in the YS showed an increasing trend from east to west. Therefore, 137 Cs was a great tracer for sedimentary chronology.
The inventory of 137 Cs. The inventory of 137 Cs in the water column and the sediment cores is calculated by integrating the activity measured at each depth 24,55 . The 137 Cs inventory of seawater and sediment in the China Seas was reviewed and their spatial distributions are plotted in Fig. 10a,b, respectively.  www.nature.com/scientificreports www.nature.com/scientificreports/ In the ECS water column, the 137 Cs inventory varied from 3 to 908 Bq m −2 , averaging 88 Bq m −2 , which is lower than that expected solely from global fallout within the same latitudinal zone (20°-35°N: ~750 Bq m −2 ) 56 . Indeed, the quantity was lower than that of the Atlantic Ocean (~1190 Bq m −2 , 20°-35°N) and the Pacific Ocean (~1440 Bq m −2 , 20°-35°N) 3 . The low 137 Cs inventory was potentially due to the shorter turnover time of ESC water and rapid scavenging of 137 Cs compared with the open ocean. The water turnover time of ECS is estimated to be less than 3 months 24 , which makes preserving 137 Cs in the water column unfavorable in the ECS. Additionally, a large number of terrestrial particles discharged from the Yangtze River and the enhanced biomass in the ECS 57,58 create favorable conditions for 137 Cs scavenging in the water column 59 . Therefore, the bulk of 137 Cs is preferentially deposited into sediments, resulting in the high 137 Cs inventory of the ECS sediment 5,31,45 . For example, the 137 Cs inventory in the ECS sediment exhibited a wide range of 39-3732 Bq m −2 (average = 970 Bq m −2 ), which is over twice what is predicted solely from global fallout (~750 Bq m −2 ) 56 . The distribution of 137 Cs inventory in the ECS sediment decreased significantly away from the shore, indicating that the input of terrestrial particle is an important 137 Cs source (see in discussion below). High 137 Cs inventory also indicate elevated recent deposition of particles near the estuary, most likely, preferential deposition and accumulation. The distribution of apparent accumulation rates in the China Seas indicates a high sedimentation rate near the Yangtze River estuary with the value reaching up to 4 cm yr −1 , which gradually decreased with distance from the shoreline 5,53 . Overall, the distribution of 137 Cs inventory in the sediment of ECS is in agreement with the distribution of sedimentation rates.
It is worth noting that the reported 137 Cs inventory in the upper 600 m in the northeastern SCS and the Luzon Strait (0-600 m) represents only a part of the whole water column 28 . We thus carefully examined the literature data concerning the profiles of the 137 Cs activity in the Pacific Ocean and found that the spatial variation is limited where the typical percentage of the 137 Cs inventory at a depth interval of 0-600 m accounted for 62.9-76.5% (average of 69.4%, n = 36) of the whole water column 3,4,11,39 . Given that the depth profiles in the northeastern SCS in the upper 600 m were rather similar to the Pacific, we extrapolated the upper 600 m inventory to the whole water column using the percentage partitioning in the Pacific Ocean. Accordingly, the 137 Cs inventory in the northeastern SCS water column was calculated, varying from 49 to 1257 Bq m −2 (average = 867 Bq m −2 ) 28,39 , which is over two times what is predicted solely from global fallout (~412 Bq m −2 , 10-20°N) 56 . This 137 Cs inventory is significantly higher than that found in the ECS (~88 Bq m −2 ). Note that, in the northeastern SCS, the 137 Cs inventory in the water column was higher than that found in the sediment core (48-401 Bq m −2 , averaging 186 Bq m −2 ), which is different from the ECS. This indicates that, in the deep marginal sea (e.g., SCS), 137 Cs largely resides in the water column, in contrast to the ECS where 137 Cs is preferentially stored in the sediment. Therefore, for the deeper marginal sea or open ocean the 137 Cs inventory of the water column positively correlates with water depth, and the 137 Cs inventory of sediment cores negatively correlates with water depth.
The 137 Cs inventory in the YS water column varied from 0.3 to 36 Bq m −2 (average = 7 Bq m −2 ) 26 , which is three orders of magnitude lower than that expected solely from global fallout within the same latitudinal zone (30°-50°N: ~1016 Bq m −2 ) 56 . The reason for the low 137 Cs inventory in the YS water column is similar to that of the ECS, and depends on the input of terrestrial particles, water depth, the turnover time and quick of scavenging 137 Cs as discussed above. Accordingly, this would lead to higher 137 Cs inventory preserved in the YS sediment cores. The 137 Cs inventory of the YS sediment exhibited a wide range, from 240 to 5071 Bq m −2 , averaging 1736 Bq m −2 26,29 , which is nearly twice that expected solely from global fallout within the same latitudinal zone (30°-50°N: ~1016 Bq m −2 ) 56 . This 137 Cs inventory is also higher than those calculated for the ECS and the SCS, indicating the 137 Cs carried by the terrestrial particles is more quickly and easily scavenged and deposited into the sediment.  Cs inventories are cited from previous studies 5,6,14,24,26,28,29,31,45 . Note that, the 137 Cs inventory in sediment core of the SCS basin is calculated based on 239+240 Pu inventory and 137 Cs/ 239+240 Pu activity ratio. This map was drawn using the free software Ocean Data View (ODV 5. Cs is a good tracer to study water mass transport due to its high solubility in seawater 2 . In marine systems, the distribution coefficient (K d ) of 137 Cs in sediment is reported to be ~2000, and 137 Cs scavenging from the water column mainly depends on its diffusion and decay rate 60 . In general, 137 Cs activity in surface seawater decreases exponentially with time. The temporal change of 137 Cs activity in the ocean can be expressed by the following exponential function 61 , where R t is the 137 Cs activity during year t and R 0 is the 137 Cs activity at t = 1967. A previous study roughly estimated the effective environmental half-life of 137 Cs (T EF ) in the SCS and the ECS based on limited in situ 137 Cs data 13 . Here, the most comprehensive 137 Cs dataset in the China Seas was collected in order to further improve the accuracy or reduce the uncertainty of this estimate. The temporal change of 137 Cs activity in the China Seas surface seawater over the past 60 years, by expanding our dataset to include 137 Cs datasets from previous studies 12,13,25,26,28,31 , is shown in Fig. 11a-c (SCS: Fig. 11a, ECS: Fig. 11b, YS: Fig. 11c). According to this expanded 137 Cs dataset, fitted equations of 137 Cs activity with respect to time are shown in Fig. 11a-c. The T EF in the China Seas was then determined as 15.4 ± 1.3 years for the SCS, 13.8 ± 1.1 years for the ECS, and 6.5 ± 0.5 years for the YS. These estimated values are slightly lower than previous results based on the more limited 137 Cs datasets 13,26 . The longer T EF in the SCS indicates that a large amount of 137 Cs is preserved in the water column, which is consistent with the higher 137 Cs inventories found in the SCS compared to the ECS and YS. Our estimates were also lower than those calculated in the WNP at the same latitude (15-24 years) 61 and in the coastal water of Japan (~18.7 years) 62 . Additionally, the estimated T EF values in the China Seas were significantly lower than the 137 Cs half-life. T EF is related to the natural decay of 137 Cs and subsequent marine processes, including vertical and horizontal water mass movements and particle scavenging. Note that estimated values are usually larger than the real values since the fraction of radioactive decay and scavenging activity are ignored. Nevertheless, they are indicative of current fallout deposition and terrestrial inputs, although the upper limit of 137 Cs mean residence time (T M ) is a more realistic indicator 62 . The T M could be expressed using the following formula 62 , where T EF is the effective environmental half-life and T R represents the radiological half-life (T R = 30.17 years). Using Eq. (2), T M in the China Seas was calculated as 45.6 ± 3.8 years for the SCS, 36.8 ± 3.1 years for the ECS, and 12.0 ± 1.0 years for the YS. Our estimated residence times will help to understanding the turnover time of 137 Cs in the China Seas. Longer residence times indicate that 137 Cs activity decreases more slowly with time. For example, the 137 Cs residence time of SCS is longer than those in the ECS and the YS, indicating the activity and inventory of 137 Cs in the former is higher than the latter, which agrees with the above field observation results. In the future, the activity level of 137 Cs in the SCS would be still higher than that observed in the ECS and the YS with the assumption of no additional input of 137 Cs. Our estimated residence times of 137 Cs in the China Seas are slightly lower than those obtained in the Atlantic Ocean (~100 years) 63 34 , we calculated the 137 Cs inventory in the SCS basin to be ~75 Bq m −2 , assuming a similar behavior between 137 Cs and 239+240 Pu. On the SCS shelf, the 137 Cs inventory was estimated to be 207.9 ± 133.8 Bq m −2 (see details in Table S4 in the SI) 6 . The total inventory of 137 Cs was thus calculated to be (3.96 ± 1.34) ×10 14 Bq for the SCS. According to the distribution of sedimentation rates in the SCS, the burial flux of 137 Cs was further calculated to be (6.95 ± 2.35) ×10 12 Bq yr −1 for the SCS. The total burial flux of 137 Cs in the China Seas was calculated to be (17.06 ± 5.43) ×10 12 Bq yr −1 . We point out that these estimates are subject to large uncertainties due to the limited collection of water column and sediment field data.
There are many rivers discharging a large quantity of terrestrial particulates into the China Seas as a result of soil erosion in the river drainage area. Riverine input is thus an important 137 Cs source to the China Seas. Previous studies suggest the input of 137 Cs from the Yangtze River and Yellow River is a major source to the ECS and YS, Scientific RepoRtS | (2020) 10:8795 | https://doi.org/10.1038/s41598-020-65280-x www.nature.com/scientificreports www.nature.com/scientificreports/ respectively 5,26 . The precise estimate of the total riverine input of 137 Cs to the China Seas needs a large number of in-situ 137 Cs data for each river. However, this is an extensive work to carry out, and currently only very limited 137 Cs data in Chinese rivers are available. Here, we highlight the three major rivers in China: namely, the Yangtze River, Pearl River and Yellow River. The following equation can be used to roughly estimate the riverine input of 137 Cs to estuaries with large drainage basin to estuarine area ratios 65,66 , and has been successfully applied in the ECS and the YS 5,26 .
where A d is the area of the drainage basin, I f is the 137 Cs inventory in the soil of the river drainage basin and f e is the fraction of 137 Cs inventory eroded each year from the watershed (f e = ln2/residence time of 137 Cs in the watershed). The collective inventory of 137 Cs in soil cores from river drainage basins around the China Seas are shown in Table S5 (SI). The average inventory of 137 Cs was calculated based on the three major river systems flowing into the SCS (Pearl River), the ECS (Yangtze River) and the YS (Yellow River). The residence times of 137 Cs in various global river drainage basins vary greatly, ranging from 800 to 4100 years 5,26,[67][68][69] . Considering that the drainage area of the Yangtze River is among the largest in the world, taking the upper limit of residence time (4100 years) is reasonable 26 . The residence time of 137 Cs in other rivers is shown in Table S6 (SI). Accordingly, the calculated riverine input of 137 Cs is (1.66 ± 0.63) × 10 11 Bq yr −1 for the SCS, (5.01 ± 0.83) × 10 11 Bq yr −1 for the ECS, and (5.82 ± 1.72) × 10 11 Bq yr −1 for the YS, resulting in a total riverine input of 137 Cs into the China Seas of (12.49 ± 3.18) ×10 11 Bq yr −1 . The input of 137 Cs discharged from the three major rivers (Yangtze River, Yellow River and Pearl River) is calculated to be (7.35 ± 1.69) ×10 11 Bq yr −1 , accounting for ~60% of the riverine input to the China Seas. These three major rivers account for ~70% of the terrestrial particulate matter discharged into the China Seas 70-73 , which is slightly higher than the 137 Cs fraction contributed by riverine inputs (~60%).
It is well known that the large-scale global atmospheric nuclear weapons testing conducted in the 1950s and the early 1960s resulted in the worldwide deposition of 137 Cs. The peak 137 Cs deposition flux appeared in 1963 74 , and then gradually decreased with time (with the exception of the additional depositional influence of the Chernobyl nuclear accident in 1986) due to the global ban of atmospheric nuclear weapons testing. At present, the 137 Cs deposition of global fallout in the China Seas is mainly originating from the resuspension and transport of East Asian dust packaged 137 Cs 74 . Through long-time series observation at the Japan Meteorological Station (36.05° N, 140.13° E), the deposition flux of 137 Cs is calculated to be 0.22 ± 0.09 Bq m −2 yr −1 74 . At another observation station in Shanghai (31.23° N, 121.40° E), the deposition flux of 137 Cs is estimated to be 0.33 ± 0.20 Bq m −2 yr −1 49 . For this study, we used the mean value (0.28 ± 0.08 Bq m −2 yr −1 ) at the two observation stations as the deposition flux of 137 Cs in the China Seas, as the China Seas are located between them. Then we calculated the direct 137 Cs deposition of global fallout to be (9.80 ± 2.80) × 10 11 Bq yr −1 for the SCS, (2.16 ± 0.62) × 10 11 Bq yr −1 for the ECS, and (1.34 ± 0.38) × 10 11 Bq yr −1 for the YS. Our calculated result in the ECS is higher than the previously reported value 26 , since our study area is significantly larger than their area. The total 137 Cs deposition flux of global fallout in the China Seas is calculated to be (13.30 ± 3.80) ×10 11 Bq yr −1 . Finally, according to the Eq. (3), the net 137 Cs flux exchange between the China Seas and North Pacific is ~8.04 × 10 13 Bq yr −1 . The total 137 Cs inventory in the China Seas is roughly estimated to be 5.4 × 10 15 Bq, which is less than 1.0% of the 137 Cs inventory in the global ocean. The 137 Cs contribution to the China Seas from the oceanic input is estimated to be about 96.9%: the dominant 137 Cs source. This result is consistent with the above discussion.
Overall, the total 137 Cs inventories in water column and sediment core of China Seas are calculated to be (29.5 ± 6.3) × 10 14 Bq and (17.6 ± 12.4) × 10 14 Bq (data see in Tables S3 and S4 in the SI), accounting for 62.6% and 37.4%, respectively. In detail, the total 137 Cs inventories in water column and sediment of the SCS are calculated to be (28.05 ± 5.32) × 10 14 Bq and (3.96 ± 1.34) × 10 14 Bq (data see in Tables S3 and S4 in the SI), respectively, accounting for 87.6% and 12.4%. This indicates that most of the 137 Cs is well preserved in the SCS water column. In contrast, the percentage of the 137 Cs inventory in water column and sediment of the ECS are about 20.6% and 79.4%, respectively, suggesting most 137 Cs is deposited in the sediment. In the YS, the water column and sediments contain approximately 0.8% and 99.2% of the 137 Cs inventory, respectively, suggesting that most of it is deposited in the sediment. Therefore, the bulk of 137 Cs remains in the SCS water column, in contrast to the ECS and the YS where most of 137 Cs is deposited in the sediments.
Additional work is needed to fully understand radio-cesium biogeochemistry and its fate in the environment. Accurate determination of radio-cesium isotopic composition could further help to identify its source; the 134 Cs/ 137 Cs isotopic ratio is widely used. However, this ratio is unavailable in the China Seas due to the decay of 134 Cs to undetectable levels because of its short half-life (~2.06 years) and the absence of recent inputs. The 135 Cs/ 137 Cs isotopic ratio is a potential alternative chronometer-tracer to investigate 137 Cs source contributions in the marine environment 75 . 135 Cs is difficult to measure, complicating the study of its transport and fluctuations in the ocean. Therefore, developing methods for the determination of 135 Cs and the 135 Cs/ 137 Cs isotopic ratio in the China Seas is of future research interest, which may help assess the environmental risk of Chinese nuclear power plants in the future. Lastly, understanding the biological speciation and transformation processes of 137 Cs would also be useful for accurately evaluating its ecological impact.