Contrasting Behaviors of 210Pb and 210Po in the Productive Shelf Water Versus the Oligotrophic Water

Introduction

Naturally occurring ²¹⁰Pb (half-life = 22.3 years) is produced from ²²²Rn (half-life = 3.8 days), belonging to the ²³⁸U decay series, and it produces ²¹⁰Po (half-life = 138.4 days). In oceanic environments, ²¹⁰Pb originates mainly from the atmospheric deposition in the upper ocean and in situ production from ²²⁶Ra (half-life = 1,600 years) decay, via ²²²Rn, in the deep ocean. Since the production of ²¹⁰Po from ²¹⁰Pb is negligible in the atmosphere, most ²¹⁰Po in the ocean is assumed to be produced from ²¹⁰Pb decay. Both ²¹⁰Pb and ²¹⁰Po are known to be particle reactive in aqueous systems, but ²¹⁰Po tends to be preferentially assimilated by marine biota (Fisher et al., 1983; Wei and Murray, 1994; Hung and Chung, 1998; Stewart and Fisher, 2003; Carvalho, 2011). This efficient biological removal of ²¹⁰Po results in disequilibria between ²¹⁰Pb and ²¹⁰Po in the upper ocean, and this principle has been used to determine particulate organic carbon export (Bacon et al., 1976; Friedrich and van der Loeff, 2002; Stewart et al., 2007; Tang and Stewart, 2019).

Although the removal of ²¹⁰Po is known to be related to marine productivity, previous studies have reported a large deficiency of ²¹⁰Po in oligotrophic oceans (Nozaki et al., 1990a; Kim, 2001; Chung and Wu, 2005). Nozaki et al. (1990a) attributed such a distinct deficiency to the significant atmospheric input of ²¹⁰Pb. However, Kim (2001) suggested that an unusually large deficiency in the oligotrophic ocean could be due to the efficient uptake of ²¹⁰Po by cyanobacteria and then transfer to higher trophic levels along marine food chains rather than downward settling. On the contrary, in the eutrophic ocean, ²¹⁰Po may reside for a much longer time in the non-settling organic pool as it is taken up by free-living bacteria (Kim, 2001). This hypothesis was further supported by Chung and Wu (2005) in the South China Sea.

The East China Sea (ECS) shelf, including the Yellow Sea and the southern sea of Korea, is one of the largest continental shelves in the world. It has a total area of 3.6 × 10⁵ km² with a mean depth of 70 m (Fang et al., 2009; Dong et al., 2011). This sea receives great amounts of nutrients from the Changjiang (e.g., 1.1 × 10¹¹ mol year^–1 for dissolved inorganic nitrogen; Dai et al., 2011) and also shows high primary productivity (510–580 mg C m^–2 day^–1), which is 2.7 times higher than that in the adjacent Kuroshio water (Hama et al., 1997; Gong et al., 2000). As very oligotrophic Kuroshio water in the northwestern Pacific Ocean (NWPO) flows into the East Sea (Japan Sea) through this ECS shelf, significant biogeochemical alterations occur in the shelf water (Kim et al., 2018; Cho et al., 2019). The East Sea is a semi-enclosed marginal sea that has a total area of 1.0 × 10⁶ km² with a maximum depth of over 3,500 m. This interconnected system (the NWPO–the ECS shelf water–the East Sea) may provide an ideal opportunity to study how chemical species behave in different biogeochemical conditions. However, only a few studies have examined the behaviors of ²¹⁰Pb and ²¹⁰Po according to the change in environmental conditions in this region. Thus, in this study, we aim to (1) investigate the behaviors of ²¹⁰Pb and ²¹⁰Po in different oceanic settings (productive shelf water versus oligotrophic water) and (2) understand the scavenging mechanisms of both radionuclides in association with colloids.

Materials and Methods

Sampling

Sampling was conducted over two periods in the southern sea of Korea and the East Sea: from May 8 to 22, 2005, on the R/V Tamgu (Stn. C-1, C-2, C-3, and C-4), and from January 26 to February 2, 2018, on the R/V ISABU (Stn. S1, S4, E1, E5, and E8; Figure 1). Seawater samples for total (∼10–20 L) and size fractionation (∼40 L) were collected directly from Niskin bottles. To collect the size-fractionated samples, seawater was filtered through the 0.2-μm cartridge filter using a peristaltic pump. The pre-filtered (<0.2 μm) samples were separated into the truly dissolved (<10 kDa) and colloidal phases (10 kDa–0.2 μm) using a tangential flow filtration system (PLCGC, Pellicon, Millipore, Burlington, MA, United States), which was pre-cleaned with 1 M HCl, 10 L deionized water, and 0.5 M NaOH (Guéguen et al., 2002; Baskaran et al., 2003; Kim and Kim, 2012, 2014). This filtration procedure was completed within 10 h after collection to avoid adsorption of particles onto the bottles. The filtered samples were acidified with 8 M nitric acid (pH ∼1) and transferred into 40-L plastic buckets.

FIGURE 1

Figure 1. Locations of the sampling stations in this study and previous studies in the northwestern Pacific Ocean (NWPO) and its adjacent marginal seas.

²¹⁰Pb and ²¹⁰Po Analysis

The analytical methods for ²¹⁰Pb and ²¹⁰Po were adapted from Kim and Kim (2012). Briefly, the ²⁰⁹Po spike (1 dpm), Pb²⁺ carrier (20 mg), and Fe³⁺ carrier (50 mg) were added to the seawater samples. After the equilibration, the pH was raised to 8 using NH₄OH to precipitate Fe(OH)₃, together with ²¹⁰Pb and ²¹⁰Po. The precipitates were allowed to settle for 4 h. After the supernatants were siphoned off, the precipitates were filtered using Whatman 41 grade paper. The precipitates and any organic matters in the sample were fully digested with the mixed solution of concentrated HNO₃, HCl, and HF and then converted to 0.5 M HCl. The samples were heated to 80°C after adding ascorbic acid (0.5 g) to reduce Fe³⁺ to Fe²⁺. Po from the heated samples was simultaneously plated onto a silver disk while rotating the disk for 3 h using a magnetic stirrer. The ²¹⁰Po sources were counted using alpha spectrometry with a passivated implanted planar silicon detector (Alpha Analyst, Canberra, Australia). The remaining solution was further purified for ²¹⁰Pb analysis. Concentrated HNO₃ was added to the solution and heated to oxidize the ascorbic acid. After the solution was converted to 9 M HCl, it was loaded onto the pre-conditioned anion exchange column (AG 1 × 8 resin, Bio-Rad Laboratories, Hercules, CA, United States) to separate Pb. The eluents (Pb) were stored for more than 6 months for the ingrowth of ²¹⁰Po. The ²¹⁰Pb activity was determined via the ingrown ²¹⁰Po activity using the same Po plating and alpha counting procedures. The recovery of ²¹⁰Pb was obtained by measuring stable Pb in ²¹⁰Pb solutions using a magnetic sector field inductively coupled plasma mass spectrometer (ICP-MS; Element 2, Thermo Scientific, Waltham, MA, United States).

Results

In order to examine the behaviors of ²¹⁰Pb and ²¹⁰Po in different oceanic settings, our results are compared with previously published data in the northwestern Pacific seas. The data are sorted into three regions: the NWPO (Tsunogai and Nozaki, 1971; Nozaki and Tsunogai, 1976; Nozaki et al., 1990a), the ECS shelf water (Lee et al., 1996; Hong et al., 1999; Su et al., 2017; this study), and the East Sea (Kim and Kim, 2012; this study; Figure 1). Data on the truly dissolved and colloidal phases of ²¹⁰Pb and ²¹⁰Po are only available in this study (ECS shelf water) and in Kim and Kim2012; (East Sea). Although the stations of Lee et al. (1996) and stations E1, S1, and S4 in this study are located in the southwestern part of the East Sea, they are included in the ECS shelf water data because they are located downstream of the ECS shelf water flows (Morimoto and Yanagi, 2001; Chang et al., 2004, 2016). Some data near the Changjiang (Hong et al., 1999; Su et al., 2017) and the southern Yellow Sea (Hong et al., 1999), which show unusually high activities (²¹⁰Po/²¹⁰Pb ratios > 1), are excluded since they are considered to be significantly influenced by terrestrial sources. All the analytical results of our experiments are given in Supplementary Table 1.

In the surface layer (0–25 m), the activities of total ²¹⁰Pb were lower in the ECS shelf water (14 ± 3 dpm 100 L^–1) relative to the NWPO (20 ± 6 dpm 100 L^–1) and the East Sea (17 ± 4 dpm 100 L^–1; Figure 2A). The distributions of total ²¹⁰Pb generally showed maximum values in the surface layer and decreased with depth in the three regions, as observed in other major oceans (e.g., Nozaki et al., 1980; Kim, 2001; Rigaud et al., 2015; Horowitz et al., 2020). For example, the activities of total ²¹⁰Pb decreased by approximately 50, 20, and 50% from 0 to 150 m in the ECS shelf water, the NWPO, and the East Sea, respectively. In the ECS shelf water, several stations showed maximum ²¹⁰Pb activities near the bottom sediments, which were 1.5–2.0 times higher than those in the surface layer. Of the total activities, the proportions of truly dissolved, colloidal, and particulate ²¹⁰Pb in the ECS shelf water were about 32 ± 13%, 27 ± 14%, and 41 ± 24%, whereas those in the East Sea were about 34 ± 7%, 51 ± 8%, and 16 ± 6%, respectively (Figure 3A). There was no significant difference in the activities of truly dissolved and colloidal phases between the ECS shelf water and the East Sea. However, the activities of particulate ²¹⁰Pb in the ECS shelf water were about four times higher than those in the East Sea (Supplementary Table 1).

FIGURE 2

Figure 2. Vertical distributions of ²¹⁰Pb (A), ²¹⁰Po (B), and ²¹⁰Po deficiency (C; ²¹⁰Pb-²¹⁰Po) in the northwestern Pacific Ocean (NWPO; Tsunogai and Nozaki, 1971; Nozaki and Tsunogai, 1976; Nozaki et al., 1990a), the East China Sea (ECS) shelf water (Lee et al., 1996; Hong et al., 1999; Su et al., 2017; this study), and the East Sea (Kim and Kim, 2012; this study).

FIGURE 3

Figure 3. Proportions of ²¹⁰Pb (A) and ²¹⁰Po (B) in truly dissolved (<10 kDa), colloidal (10 kDa–0.2 μm), and particulate (>0.2 μm) phases in the East China Sea (ECS) shelf water and the East Sea.

For total ²¹⁰Po activities, there was no distinct difference in the surface layer (0–25 m) among the ECS shelf water (11 ± 3 dpm 100 L^–1), the NWPO (11 ± 5 dpm 100 L^–1), and the East Sea (10 ± 2 dpm 100 L^–1; Figure 2B). As such, there was no clear trend with depth. However, there was a significant difference in the deficiency of ²¹⁰Po (²¹⁰Pb-²¹⁰Po) in the three regions (Figure 2C). The most oligotrophic NWPO showed a relatively larger deficiency (9 ± 4 dpm 100 L^–1) in the upper ocean (0–25 m) compared with those in the East Sea (7 ± 3 dpm 100 L^–1) and the ECS shelf water (3 ± 3 dpm 100 L^–1). The largest deficiency of ²¹⁰Po in the NWPO was observed at 200 m, while it was observed in the surface layer and decreased with depth in the ECS shelf water and the East Sea. Of the total activities, the proportions of truly dissolved, colloidal, and particulate ²¹⁰Po in the ECS shelf water were about 48 ± 19%, 31 ± 8%, and 24 ± 21%, whereas those in the East Sea were about 21 ± 7%, 36 ± 6%, and 47 ± 8%, respectively (Figure 3B). The activities of ²¹⁰Po in the ECS shelf water were approximately 3.8, 1.7, and 1.8 times higher than those in the East Sea for the truly dissolved, colloidal, and particulate phases, respectively (Supplementary Table 1).

Discussion

Steady-State and Non-steady-State Scavenging Models for ²¹⁰Pb and ²¹⁰Po

A steady-state (SS) scavenging model is used to estimate the scavenging rates of ²¹⁰Pb and ²¹⁰Po in this study, as utilized by previous studies in open ocean (e.g., Bacon et al., 1976; Obata et al., 2004; Murray et al., 2005). At steady state (∂A/∂t = 0), if advection and diffusion are neglected, the scavenging rate constants of ²¹⁰Pb and ²¹⁰Po can be calculated using the following equations:

For ²¹⁰Pb:

$$\frac{\partial A_{\text{Pb}}^{\text{t}}}{\partial t}={\mathrm{\lambda }}_{\text{Pb}}\times \left(A_{\text{Ra}}-A_{\text{Pb}}^{\text{t}}\right)+F_{\text{Atm}}-A_{\text{Pb}}^{\text{t}}{k}_{\text{Pb}}^{\text{t}}=0(1)$$

$$\frac{\partial A_{\text{Pb}}^{\text{td}}}{\partial t}={\mathrm{\lambda }}_{\text{Pb}}\times \left(A_{\text{Ra}}-A_{\text{Pb}}^{\text{td}}\right)+F_{\text{Atm}}-A_{\text{Pb}}^{\text{td}}{k}_{\text{Pb}}^{\text{td}}=0(2)$$

$$\frac{\partial A_{\text{Pb}}^{\text{c}}}{\partial t}={\mathrm{\lambda }}_{\text{Pb}}\times \left(-A_{\text{Pb}}^{\text{c}}\right)+A_{\text{Pb}}^{\text{td}}{k}_{\text{Pb}}^{\text{td}}-A_{\text{Pb}}^{\text{c}}{k}_{\text{Pb}}^{\text{c}}=0(3)$$

$$\frac{\partial A_{\text{Pb}}^{\text{p}}}{\partial t}={\mathrm{\lambda }}_{\text{Pb}}\times \left(-A_{\text{Pb}}^{\text{p}}\right)+A_{\text{Pb}}^{\text{c}}{k}_{\text{Pb}}^{\text{c}}-A_{\text{Pb}}^{\text{p}}{k}_{\text{Pb}}^{\text{p}}=0(4)$$

For ²¹⁰Po:

$$\frac{\partial A_{\text{Po}}^{\text{t}}}{\partial t}={\mathrm{\lambda }}_{\text{Po}}\times \left(A_{\text{Pb}}^{\text{t}}-A_{\text{Po}}^{\text{t}}\right)-A_{\text{Po}}^{\text{t}}{k}_{\text{Po}}^{\text{t}}=0(5)$$

$$\frac{\partial A_{\text{Po}}^{\text{td}}}{\partial t}={\mathrm{\lambda }}_{\text{Po}}\times \left(A_{\text{Pb}}^{\text{td}}-A_{\text{Pb}}^{\text{td}}\right)-A_{\text{Po}}^{\text{td}}{k}_{\text{Po}}^{\text{td}}=0(6)$$

$$\frac{\partial A_{\text{Po}}^{\text{c}}}{\partial t}={\mathrm{\lambda }}_{\text{Po}}\times \left(A_{\text{Pb}}^{\text{c}}-A_{\text{Pb}}^{\text{c}}\right)+A_{\text{Po}}^{\text{td}}{k}_{\text{Po}}^{\text{td}}-A_{\text{Po}}^{\text{c}}{k}_{\text{Po}}^{\text{c}}=0(7)$$

$$\frac{\partial A_{\text{Po}}^{\text{p}}}{\partial t}={\mathrm{\lambda }}_{\text{Po}}\times \left(A_{\text{Pb}}^{\text{p}}-A_{\text{Pb}}^{\text{p}}\right)+A_{\text{Po}}^{\text{c}}{k}_{\text{Po}}^{\text{c}}-A_{\text{Po}}^{\text{p}}{k}_{\text{Po}}^{\text{p}}=0(8)$$

where λ, A, F_Atm, and k are the decay constant (day^–1), inventory of element (dpm m^–2), atmospheric depositional flux of ²¹⁰Pb (55 dpm m^–2 day^–1; Nozaki et al., 1973; Turekian et al., 1977), and the scavenging rate constant (year^–1), respectively. t, td, c, and p represent the total, truly dissolved, colloidal, and particulate phases, respectively. To obtain the activities of ²²⁶Ra in the ECS shelf water, the empirical relationship with salinity is used for the southern sea of Korea (Stn. C-1, C-2, C-3, and C-4; Yang et al., 1992, 1996), and the data from Wang et al. (2018c) are used for the other ECS shelf regions. The activities of ²²⁶Ra in the NWPO and the East Sea are from previously published results (Nozaki and Tsunogai, 1976; Chung and Craig, 1980; Harada and Tsunogai, 1986; Nozaki et al., 1990b). The riverine inputs are neglected because ²¹⁰Pb is almost completely trapped in the Changjiang estuary (Wang et al., 2018b). The atmospheric input of ²¹⁰Po is also neglected since the ²¹⁰Po/²¹⁰Pb ratios in precipitation in this study region are lower than 0.2 (Kim et al., 2005a; Yan et al., 2012).

The hydrological conditions and biogeochemical processes in the ECS shelf water are significantly affected by the intrusion of the NWPO (Wang et al., 2018a; Zuo et al., 2019; Liu et al., 2021). The chemical properties of the intruded water undergo rapid changes during the water residence times in the ECS shelf. Therefore, the removal fluxes of ²¹⁰Pb and ²¹⁰Po in the ECS shelf water can be calculated using the non-steady-state (NSS) model, accounting for the residence times of the shelf water. The removal fluxes of ²¹⁰Pb and ²¹⁰Po in the ECS shelf water are expressed by the following equations:

$$\frac{\partial A_{\text{Pb}}^{\text{ECS}}}{\partial t}={\mathrm{\lambda }}_{\text{Pb}}\times \left(A_{\text{Ra}}^{\text{ECS}}-A_{\text{Pb}}^{\text{ECS}}\right)+F_{\text{Atm}}-R_{\text{Pb}}^{\text{ECS}}+F_{\text{lateral},\mathrm{Pb}}(9)$$

$$\frac{\partial A_{\text{Po}}^{\text{ECS}}}{\partial t}={\mathrm{\lambda }}_{\text{Po}}\times \left(A_{\text{Pb}}^{\text{ECS}}-A_{\text{Po}}^{\text{ECS}}\right)-R_{\text{Po}}^{\text{ECS}}+F_{\text{lateral},\mathrm{Po}}(10)$$

where R and F_lateral are the removal flux (in disintegrations per minute per square meter per day) and the lateral transport of the radionuclide by current, respectively. Assuming that A_Ra, F_Atm, R, and F_lateral are constant during a given time interval, the solutions for Eqs 9, 10 are expressed as follows (based on the work by Friedrich and van der Loeff, 2002):

$$R_{\text{Pb}}^{\text{ECS}}=F_{\text{atm}}+F_{\text{lateral},\mathrm{Pb}}+{\mathrm{\lambda }}_{\text{Pb}}{A}_{\text{Ra}}^{\text{ECS}}+\frac{{\mathrm{\lambda }}_{\text{Pb}}}{1-{\text{e}}^{-{\mathrm{\lambda }}_{\text{Pb}}\mathrm{\Delta }t}}(A_{\text{Pb},t1}{e}^{-{\mathrm{\lambda }}_{\text{Pb}}\mathrm{\Delta }t}-A_{\text{Pb},t2})(11)$$

$$R_{\text{Po}}^{\text{ECS}}=F_{\text{lateral},\mathrm{Po}}+\frac{{\mathrm{\lambda }}_{\text{Po}}}{1-e^{-{\mathrm{\lambda }}_{\text{Po}}\mathrm{\Delta }t}}[\frac{{\mathrm{\lambda }}_{\text{Pb}}{A}_{\text{Ra}}^{\text{ECS}}+F_{\text{atm}}+F_{\text{lateral},\mathrm{Pb}}-R_{\text{Pb}}^{\text{ECS}}}{{\mathrm{\lambda }}_{\text{Pb}}}\left\{\frac{{\mathrm{\lambda }}_{\text{Po}}}{{\mathrm{\lambda }}_{\text{Po}}-{\mathrm{\lambda }}_{\text{Pb}}}\left(e^{-{\mathrm{\lambda }}_{\text{Po}}\mathrm{\Delta }t}-e^{-{\mathrm{\lambda }}_{\text{Pb}}\mathrm{\Delta }t}\right)+(1-e^{-{\mathrm{\lambda }}_{\text{Po}}\mathrm{\Delta }t})\right\}$$

$$+A_{\text{Pb},t1}\frac{{\mathrm{\lambda }}_{\text{Po}}}{{\mathrm{\lambda }}_{\text{Po}}-{\mathrm{\lambda }}_{\text{Pb}}}(e^{-{\mathrm{\lambda }}_{\text{Pb}}\mathrm{\Delta }t}-e^{-{\mathrm{\lambda }}_{\text{Po}}\mathrm{\Delta }t})+A_{\text{Po},t1}{\text{e}}^{-{\mathrm{\lambda }}_{\text{Po}}\mathrm{\Delta }t}-A_{\text{Po},t2}](12)$$

In this calculation, we assume that the water of the NWPO enters the ECS shelf at time t₁ and stays for an amount of time Δt (t₂–t₁). Therefore, Δt is the residence time of the ECS shelf water. A_t1 and A_t2 are the activities of radionuclides in the NWPO and the ECS shelf water, respectively. The lateral transport term (F_lateral) is calculated by multiplying the current velocity of ∼20 cm s^–1 (Ichikawa and Beardsley, 2002; Lee et al., 2014) by the activity gradient between the NWPO and the ECS shelf water along the 400-km distance (Yangtze River mouth–Jeju Island). The calculated results using Eqs 11, 12 according to the change in residence times of the ECS shelf water are shown in Figure 4. The SS model, which accounts for the advection of water, is given for comparison.

FIGURE 4

Figure 4. Net removal flux of total ²¹⁰Pb (in disintegrations per minute per square meter per day; A) and net input flux of total ²¹⁰Po (in disintegrations per minute per square meter per day; B) from the upper 0–50 m in the East China Sea (ECS) shelf water, calculated based on the steady-state and non-steady-state models accounting for the advection of water. The dotted lines represent the range of the previously reported residence times in the ECS shelf water.

Behaviors of ²¹⁰Pb and ²¹⁰Po

The scavenging rate constant of total ²¹⁰Pb in the ECS shelf water (3.0 ± 0.6 year^–1) was relatively higher than that in the NWPO (2.2 ± 0.4 year^–1) and the East Sea (2.6 ± 0.4 year^–1) based on the SS model (Table 1). The corresponding values of truly dissolved and colloidal ²¹⁰Pb in the ECS shelf water were also 1.2–2.0 times higher than those in the East Sea. However, the scavenging rate constant of particulate ²¹⁰Pb in the ECS shelf water was approximately 3.8 times lower than that in the East Sea, perhaps associated with the higher activities of particulate ²¹⁰Pb in the ECS shelf water. Given that the maximum activities of particulate ²¹⁰Pb were observed in the surface layer at C3 and the bottom layer at C4, respectively, such high activities of particulate ²¹⁰Pb might be attributed to the lateral input from land or the resuspension from the bottom sediments. This could occur due to the shallow water depth and by episodic storm events (Choi et al., 2004, 2010). In both cases, the SS model is not appropriate for calculating the scavenging rate constant and removal flux in the ECS shelf water.

TABLE 1

Table 1. Scavenging rate constants (per year) of total, truly dissolved (<10 kDa), colloidal (10 kDa–0.2 μm), and particulate (>0.2 μm) ²¹⁰Pb and ²¹⁰Po in the upper 0–50 m of the northwestern Pacific Ocean (NWPO), the East China Sea (ECS) shelf water, and the East Sea.

Based on the NSS model, total ²¹⁰Pb was more effectively removed in the ECS shelf water (128–137 dpm m^–2 day^–1) relative to the NWPO (55 dpm m^–2 day^–1) and the East Sea (55 dpm m^–2 day^–1; Figure 5). The removal flux of total ²¹⁰Pb showed differences from 1 to 9% between the NSS and SS models within the previously reported residence times of the ECS shelf water (0.5–3.1 years; Figure 4A; Nozaki et al., 1989; Chen, 1996; Tsunogai et al., 1997; Wang et al., 2018c). These high scavenging rate and removal flux of ²¹⁰Pb in the ECS shelf water might be due to the higher concentrations of the suspended particulate matters in the ECS shelf water (6.8–15.0 mg L^–1; Bi et al., 2020) compared with those in the NWPO (0.1–0.3 mg L^–1; Hung and Chan, 1998) and the East Sea (0.2–0.6 mg L^–1; Chen et al., 1996; Hong et al., 2008).

FIGURE 5

Figure 5. A schematic box model (non-steady-state) of total ²¹⁰Pb and ²¹⁰Po accounting for the lateral transport of water from the northwestern Pacific Ocean (NWPO) to the East Sea through the East China Sea (ECS) shelf. The uncertainty of each source and sink term accounting for variations in the current velocity and atmospheric input of ²¹⁰Pb is below 15%, except for the net input of ²¹⁰Po in the ECS shelf water (28%).

For ²¹⁰Po, the scavenging rate constant of total ²¹⁰Po in the ECS shelf water (0.3 ± 0.2 year^–1) was much lower than that in the NWPO (1.3 ± 0.5 year^–1) and the East Sea (1.8 ± 0.3 year^–1), which is opposite to the ²¹⁰Pb trend (Table 1). Especially, the truly dissolved and colloidal ²¹⁰Po in the ECS shelf water approached equilibrium with ²¹⁰Pb. The calculated results of the NSS model showed the net input of ²¹⁰Po in the ECS shelf water (7–10 dpm m^–2 day^–1), opposite to the net removal in the NWPO (20 dpm m^–2 day^–1) and the East Sea (18 dpm m^–2 day^–1; Figure 5). The net input flux of ²¹⁰Po increased with the residence times of the ECS shelf water and showed a difference from 13 to 35% between the NSS and SS models (Figure 4B).

These distinctively low scavenging rate constant and net input flux of ²¹⁰Po have been reported in the ECS (Nozaki et al., 1991), Gulf of Lion (Radakovitch et al., 1998), and northwestern Mediterranean Sea (Masqué et al., 2002). Nozaki et al. (1991) and Radakovitch et al. (1998) argued that this trend might be due to the extensive inputs of suspended particulate matters from river discharges with ²¹⁰Po/²¹⁰Pb ratios of ∼1. On the other hand, Masqué et al. (2002) suggested that the incorporation of ²¹⁰Po into the organic matter following the uptake by buoyant particles resulted in the low scavenging rate constant of ²¹⁰Po. In this study, we discount the riverine sources as the main reason for the low scavenging rate constant of ²¹⁰Po because the influence of river discharge with high ²¹⁰Po/²¹⁰Pb ratios was excluded in the comparison. In addition, the ²¹⁰Po/²¹⁰Pb ratios were ∼0.5 for the particulate phase and ∼1 for the total dissolved (truly dissolved + colloidal) phases in the ECS shelf water.

In general, low contents of organic carbon (0.2–1.0%) were observed in the sinking particles and the bottom sediments in the ECS shelf, associated with rapid organic carbon remineralization (Lin et al., 2000, 2002; Iseki et al., 2003). The results of sediment core incubation also showed that the organic carbon remineralization rate in the bottom sediments accounted for 12–24% of the primary productivity in this highly eutrophic ECS shelf water (Song et al., 2016), indicating the efficient regeneration of organic matter in the bottom sediments. In addition, hypoxia occurred intermittently in the bottom of ECS shelf water (dissolved oxygen concentrations <2–3 mg L^–1; Chen et al., 2007), which can also increase the net input flux of ²¹⁰Po as insoluble Po(IV) can be reduced to soluble Po(II) (Balistrieri et al., 1995; Kim et al., 2005b; Kim and Kim, 2014). Considering the relatively higher proportions of the total dissolved (truly dissolved + colloidal) ²¹⁰Po in the ECS shelf water (>80%), the colloidal matter might be associated with the net input of ²¹⁰Po due to regeneration from the sinking particles and in the bottom sediments.

Thus, our results suggest that ²¹⁰Po, a proxy for sulfur group elements (S, Se, and Te), may reside longer in the water column because of active regeneration, while ²¹⁰Pb and other particle-reactive elements would be removed in association with the large population of the sinking particles in the shelf water.

Conclusion

The scavenging rate constant and removal flux of ²¹⁰Pb and ²¹⁰Po were estimated using a geochemical scavenging model in the productive shelf system connected to the oligotrophic NWPO. The scavenging rate constant of ²¹⁰Pb was relatively higher (3.0 ± 0.6 year^–1) in the ECS shelf water, whereas that of ²¹⁰Po was lower (0.3 ± 0.2 year^–1) than that in the NWPO and the East Sea. In addition, a NSS model accounting for the ocean currents showed the effective removal of ²¹⁰Pb in the ECS shelf water (128–137 dpm m^–2 day^–1), whereas ²¹⁰Po showed the net input from the sinking particles and the bottom sediments (7–10 dpm m^–2 day^–1). Given that the largest proportions of ²¹⁰Pb and ²¹⁰Po were particulate and total dissolved (truly dissolved + colloidal) phases, respectively, these opposite behaviors could be due to the efficient scavenging of ²¹⁰Pb versus the efficient regeneration of ²¹⁰Po from organic matter in the ECS shelf water. Thus, our results suggest that particle-reactive elements would be effectively removed in the shelf water, whereas sulfur group elements (S, Se, and Te) might be supplied from the sinking particles and the bottom sediments by efficient regeneration.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

Author Contributions

GK contributed to the conceptualization of the study. HS and DJ performed field sampling and analyses. HS and GK were involved in the data interpretation and writing of the manuscript. All authors contributed to the final version of the manuscript.

Funding

This research was a part of the project titled “Deep Water Circulation and Material Cycling in the East Sea (20160040),” funded by the Ministry of Oceans and Fisheries, South Korea, and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT; 2018R1A2B3001147).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank the crew members of the R/V Tamgu and R/V Isabu for their help with field sampling. We also thank all lab members for their assistance with sampling and analyses.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2021.701441/full#supplementary-material

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