Tracing the Atmospheric Input of Seawater-Dissolvable Pb Based on the Budget of 210Pb in the East Sea (Japan Sea)

In order to determine the atmospheric input of ²¹⁰Pb and seawater-dissolvable Pb in the East Sea (Japan Sea), we measured the concentrations of total ²¹⁰Pb and dissolved Pb (<0.2 μm) in seawater and ²¹⁰Pb and ²²⁶Ra in sinking particles. The East Sea is deep (∼3700 m) and enclosed by surrounding continents except for the shallow sills (<150 m). Since the East Sea is located off the East Asian continent under the westerlies, the concentrations of ²¹⁰Pb and dissolved Pb in this sea are significantly affected by terrestrial sources through the atmosphere. The vertical profiles of total ²¹⁰Pb and dissolved Pb generally showed a surface maximum and then decreased with depth. The concentrations of dissolved Pb in the surface water were 2 and 3 times higher than those in the North Pacific and North Atlantic Oceans, respectively. Using an independent box model (upper 1000 m or 2000 m), we estimate the atmospheric input of ²¹⁰Pb to be 1.46 ± 0.25 dpm cm⁻² y ^{− 1}, which is within the range of published results from the land-based sites (0.44–4.40 dpm cm⁻² y ^{− 1}) in South Korea, China, and Japan. Based on this flux, the residence time of total ²¹⁰Pb in the East Sea is calculated to be approximately 7.1 ± 1.6 years, which is twice lower than the previous estimation. Combining the residence time of ²¹⁰Pb and the inventory of dissolved Pb, the atmospheric input of seawater-dissolvable Pb is estimated to be 0.98 ± 0.28 nmol cm⁻² y ^{− 1}. This flux is approximately 25% of the Pb flux through the wet deposition (acid-leachable fraction). Thus, our results suggest that the flux and fate of atmospheric Pb in the ocean can be successfully determined using an accurate mass balance model of naturally occurring ²¹⁰Pb.

Introduction

The naturally occurring radionuclide ²¹⁰Pb (t_1/2 = 22.3 years), belonging to the ²³⁸U decay series, is produced from ²²⁶Ra (t_1/2 = 1600 years), via ²²²Rn (t_1/2 = 3.8 days) and other short-lived radionuclides (t_1/2 < 30 min). In the upper ocean, most of the ²¹⁰Pb originates from atmospheric deposition, while that in the deep ocean is mainly produced by in situ decay of ²²⁶Ra. Therefore, an excess of ²¹⁰Pb over ²²⁶Ra (²¹⁰Pb_ex: ²¹⁰Pb-²²⁶Ra) is observed in the upper ocean, except for the polar regions, where the atmospheric input of ²¹⁰Pb is small (Elsässer et al., 2011; Persson and Holm, 2014; Baskaran and Krupp, 2021). ²¹⁰Pb is rapidly removed by particles in the surface water, so the residence time of ²¹⁰Pb in the surface water is short: 1–3 years in the North Pacific and North Atlantic Oceans (Nozaki and Tsunogai, 1973, 1976; Bacon et al., 1976; Nozaki et al., 1976; Rigaud et al., 2015). The occurrence of ²¹⁰Pb_ex decreases toward the continent as enhanced scavenging occurs at the ocean margin (Bacon et al., 1976; Nozaki et al., 1976; Anderson et al., 1994; Seo et al., 2021).

²¹⁰Pb has been useful in tracing the behavior of anthropogenic Pb in marine environments. For example, the atmospheric input of Pb was estimated using Pb/²¹⁰Pb ratios in the rain and the steady-state ²¹⁰Pb flux (Settle et al., 1982). The decreasing trend of anthropogenic Pb flux to the Sargasso Sea in response to decline in the emission of United States leaded gasoline was revealed based on the reduced Pb/²¹⁰Pb ratios in the surface water from 1979 to 1987 (Boyle et al., 1986; Shen and Boyle, 1988; Sherrell et al., 1992). Those studies also combined the Pb/²¹⁰Pb ratios with ³H-³He thermocline ventilation model (Jenkins, 1980) to reveal the importance of isopycnal transport on Pb distributions in that region. Sherrell et al. (1992) also suggested that dissolved Pb and particulate Pb were in equilibrium within the residence time of particulate matter based on the Pb isotope ratios (²⁰⁶Pb/²⁰⁷Pb) and ²¹⁰Pb in suspended particulate matter. However, these ²¹⁰Pb applications included significant uncertainties since it is difficult to constrain the actual inputs of atmospheric ²¹⁰Pb to the specific ocean region. In previous studies, the atmospheric input of ²¹⁰Pb has been estimated by sampling the aerosol and/or rain from the land (islands or coastal sites) (e.g., Turekian et al., 1983; Turekian, 1989), sampling the aerosol and/or rain from the ocean via research cruise (e.g., Rengarajan and Sarin, 2004; Niedermiller and Baskaran, 2019), and modeling (e.g., Turekian et al., 1977; Feichter et al., 1991; Balkanski et al., 1993). However, the land-based sampling methods cannot cover vast areas of the ocean and the sampling during cruise covers only a limited time period. The models also suffer from the lack of measured data (Nozaki et al., 1998), although their results with the measured data can be more representative.

The East Sea (Japan Sea) is an enclosed deep marginal sea in the northwestern Pacific Ocean, with a maximum depth of 3700 m and a surface area of 1.0 × 10 ⁶ km ². This sea has a deep water formation and a meridional overturning circulation similar to the global ocean (Ichiye, 1984; Gamo, 1999; Kim et al., 2001), although the turnover time of deep water (∼100 years) is shorter than the global circulation time (Watanabe et al., 1991; Tsunogai et al., 1993; Kumamoto et al., 1998). Since the East Sea does not have a large discharge from rivers and is located off the eastern part of the Asian continent, large amounts of lithogenic and anthropogenic elements enter the upper ocean through atmospheric deposition (Park et al., 2006; Jo et al., 2007; Kim et al., 2011; Yan and Kim, 2015; Seo and Kim, 2020). The deep water of the East Sea is almost disconnected from the North Pacific Ocean, except through four shallow sills (<150 m). Thus, the East Sea is an ideal site to study the atmospheric inputs of various elements and their behaviors within a closed system.

In this study, we attempt to estimate the atmospheric input of seawater-dissolvable Pb in the East Sea using the inventory of dissolved Pb in the water column and the residence time of ²¹⁰Pb. The residence time of ²¹⁰Pb is calculated using the mass balance of the input terms (atmospheric deposition; ingrowth from ²²⁶Ra in seawater) and the output terms (decay of ²¹⁰Pb; removal by sinking particles). For this mass balance estimation, we measured the distributions of total ²¹⁰Pb in seawater, dissolved Pb in seawater, and settling fluxes of ²²⁶Ra and ²¹⁰Pb through 1000 m and 2000 m from sediment trap samples. In addition, we compile previously published ²²⁶Ra and ²¹⁰Pb data including the Japan side for more accurate estimation.

Materials and Methods

Sampling

For the measurements of total ²¹⁰Pb in seawater, sampling was conducted from April 6 to May 3 in 2015 on R/V Akademik M.A. Lavrentyev (Figure 1). Seawater samples (10 L, n = 29) were collected in high-density polyethylene (HDPE) bottles from Niskin samplers. The samples were acidified with 12 M HCl (pH < 2) within 1 hour to prevent ²¹⁰Pb from sorbing onto the bottles and then stored until analysis.

FIGURE 1

Figure 1. Sampling locations for total ²¹⁰Pb and dissolved Pb in seawater and ²¹⁰Pb and ²²⁶Ra in sinking particles.

For the measurements of dissolved Pb in seawater, ultra-clean sampling was conducted from January 26 to February 2 in 2018 on R/V Isabu and from October 26 to November 22 in 2019 on R/V Akademik Oparin, respectively (Figure 1). We used PRISTINE ultra-clean CTD (UCC) and Teflon-coated Niskin-X samplers on the R/V Isabu and R/V Akademik Oparin, respectively. All procedures, including cleaning and sampling, followed the GEOTRACES protocol (Cutter et al., 2017). Seawater samples (125 mL, n = 64) were collected in pre-cleaned low-density polyethylene (LDPE) bottles from PRISTINE or Niskin-X samplers through pre-cleaned polyethersulfone capsule filters (0.2-μm pore size; AcroPak-200, Pall). The samples were acidified to pH ∼1.8 within 1 hour after sampling using 12 M HCl (ultra-high pure grade, ODLAB) and stored for laboratory analysis. Milli-Q water (18.2 MΩ, n = 15) was used as a procedural blank.

For ²²⁶Ra and ²¹⁰Pb in sinking particles, the conical type sediment traps (MARK7G-21, McLane) were deployed at 1000 m and 2000 m depths from December 1998 to December 1999 (Figure 1). All sample cups were filled with HgCl₂ solutions before deployment to prevent the samples from bacterial degradation. After the recovery of the sediment traps, samples were kept below 4°C until they were transported to the land-based laboratory for further analysis.

Analytical Methods

The method for measuring the total phase of ²¹⁰Pb in seawater in this study was similar to that of previous studies (Kim and Kim, 2012; Seo et al., 2021). Briefly, all seawater samples for total ²¹⁰Pb were stored for more than 2 years to allow for the equilibrium between ^{2 10}Pb and ²¹⁰Po. ²⁰⁹Po spike (1.5 dpm) and Fe ^{3 +} (50 mg) carrier were added to the samples and left to equilibrate for 24 hours. Ammonium hydroxide was used to adjust the pH to ∼8 for the co-precipitation of ²¹⁰Po and Fe(OH)₃. The supernatants were removed and then the precipitates were filtered. The precipitates were digested with a mixture solution of concentrated HNO₃ and HCl (1:1, v/v) to remove any organic matter in the samples. The mixture solution was dried down and then re-dissolved in 50 mL of 0.5 M HCl. After adding ∼0.5 g of ascorbic acid to reduce Fe ^{3 +}, Po was plated onto a silver disk at a temperature of ∼80°C with stirring for 3 hours. ²¹⁰Po activities on the silver disks were counted by using alpha spectrometry (Alpha Analyst, Canberra). The measured counts were corrected for the background of the alpha spectrometry, decay of ²¹⁰Po during counting, recovery of ²⁰⁹Po spike, decay of ²¹⁰Pb from sampling to plating, and the reagent blank (Church et al., 2012). The reagent blank for ²¹⁰Pb was 0.0175 ± 0.004 dpm (n = 5), which is comparable with those in Roca-Martí et al. (2021). The blank accounted for 1.1–4.6% (average: 2.7 ± 1.0%, n = 35) of the total ²¹⁰Pb in this study.

The concentrations of dissolved Pb were determined using an online pre-concentration system (seaFAST SP3; Elemental Scientific) coupled to a sector-field inductively coupled plasma mass spectrometry (ICP-MS; Element 2, Thermo Fisher Scientific). Approximately 10 mL of sample was buffered to pH ∼6.2 with a 4 M ammonium acetate buffer. The sample was loaded onto the seaFAST column filled with Nobias-chelate PA1 resin (200 μL), subsequently rinsed with a mixed solution of Milli-Q water and buffer to remove the salt. Then, Pb was eluted with 1.6 M HNO₃ (ultra-high pure grade, ODLAB). During the analysis, rhodium was used for an internal standard to correct the changes in ICP-MS sensitivity for each sample. The procedural blank and detection limit of this method was 3.9 pmol kg ^{− 1} and 2.8 pmol kg ^{− 1}, respectively. The accuracy of the measurement was determined by analyzing the certified reference materials (CASS-6 and NASS-7; National Research Council of Canada) and GEOTRACES reference standards (GSC: bottle number 97 and GSP: bottle number 36).

For ²¹⁰Pb and ²²⁶Ra analyses in sinking particles, sediment trap samples were filtered through a 1 mm nylon mesh to separate swimmers and then freeze-dried. The freeze-dried samples were packed into gamma vials and sealed to avoid the loss of ²²²Rn. After more than 3 weeks for the secular equilibrium between ²²⁶Ra and its daughter (²¹⁴Pb and ²¹⁴ Bi), the activities of ²¹⁰Pb and ²²⁶Ra were measured using a gamma counter, with a high-purity germanium well detector (HPGe, Canberra), for the gamma-ray energy of each isotope (46.5 keV for ²¹⁰Pb; 351.9 keV for ²¹⁴Pb; 609.3 keV for ²¹⁴ Bi). All analytical results are summarized in Supplementary Tables 1–4.

Results

The vertical profiles of total ²¹⁰Pb in the East Sea showed the highest activities in the surface and decreased with depth, as observed in other major non-polar open oceans (Nozaki et al., 1980; Cochran et al., 1990; Kim, 2001; Rigaud et al., 2015; Tang et al., 2018; Horowitz et al., 2020; Figure 2A). The activities of total ²¹⁰Pb ranged from 9.3 to 16.4 dpm 100 L ^{− 1} (average: 12.8 ± 2.8 dpm 100 L ^{− 1}, n = 8) in the surface water (0–100 m) and decreased to a range from 4.2 to 6.6 dpm 100 L ^{− 1} (average: 5.0 ± 0.9 dpm 100 L ^{− 1}, n = 7) in the deep water (2500–3200 m). The activities of total ²¹⁰Pb in the surface water of the East Sea (0–100 m) were comparable with those in the North Pacific and North Atlantic Oceans, whereas the activities of total ²¹⁰Pb in the deep East Sea (2000 m) were approximately 4.8 and 2.2 times lower than those in the North Pacific and North Atlantic Oceans, respectively (Nozaki and Tsunogai, 1976; Nozaki et al., 1980; Rigaud et al., 2015).

FIGURE 2

Figure 2. Vertical profiles of (A) total ²¹⁰Pb (dpm 100 L ^{− 1}) and (B) dissolved Pb (pmol kg ^{− 1}) in the East Sea.

The vertical profiles of dissolved Pb in the East Sea were similar to those of total ²¹⁰Pb, showing the surface maximum and decrease with depth, except at station E1 (Figure 2B). This distribution pattern differed from those in other open oceans, such as the central North Pacific, North Atlantic, and South Atlantic Oceans, which displayed a sub-surface maximum (Noble et al., 2015; Zurbrick et al., 2017, 2018; Schlosser et al., 2019; Zheng et al., 2019). The concentrations of dissolved Pb ranged from 45 to 107 pmol kg ^{− 1} (average: 73 ± 15 pmol kg ^{− 1}, n = 27) in the surface water (0–100 m) and decreased to a range from 4 to 15 pmol kg ^{− 1} (average: 8.5 ± 3.7 pmol kg ^{− 1}, n = 6) in the deep water (1500–2200 m). The concentrations of dissolved Pb in the surface water of the East Sea (0–100 m) were approximately 1.8 and 3.2 times higher than those in the North Pacific and North Atlantic Oceans, respectively. On the other hand, the concentrations of dissolved Pb in the deep East Sea (2000 m) were approximately 2.1 and 3.1 times lower than those in the North Pacific and North Atlantic Oceans, respectively (Noble et al., 2015; Zheng et al., 2019; Jiang et al., 2021). At station E1 (water depth: 190 m), the concentrations of Pb were ∼56 pmol kg ^{− 1} in the surface (0–100 m) and decreased to ∼26 pmol kg ^{− 1} in the sub-surface (100–150 m), followed by an increase to 38 pmol kg ^{− 1} near the bottom sediments.

The total particle flux through the sediment traps ranged from 0.09 to 1.27 g m⁻² d⁻¹ (average: 0.40 ± 0.23 g m⁻² d⁻¹, n = 52) and 0.15 to 0.84 g m⁻² d⁻¹ (average: 0.36 ± 0.16 g m ⁻² d⁻¹, n = 57) at 1000 and 2000 m, respectively (Figure 3A). The total particle flux at 1000 m showed peaks in March (0.78 g m⁻² d⁻¹), April (1.03 g m⁻² d⁻¹), and November (1.27 g m⁻² d⁻¹). At 2000 m, the seasonal variation of total particle flux was similar to that at 1000 m. The total particle flux at 1000 m in the East Sea was approximately 4 and 9 times higher than that in the North Pacific (Moored depth: 1004–1264 m; Tsunogai et al., 1982; Harada and Tsunogai, 1986b) and North Atlantic Oceans (Moored depth: 1500 m; Hong et al., 2013), respectively.

FIGURE 3

Figure 3. One-year time series data (December 1998 to December 1999) of (A) total particulate flux (g m⁻² d⁻¹) and (B) ²¹⁰Pb (dpm m⁻² d⁻¹) flux in the sinking particles.

The sinking flux of particulate ²¹⁰Pb through the sediment traps ranged from 15 to 97 dpm m⁻² d⁻¹ (average: 46 ± 19 dpm m⁻² d⁻¹, n = 52) and 17 to 115 dpm m⁻² d⁻¹ (average: 57 ± 20 dpm m⁻² d⁻¹, n = 57) at 1000 and 2000 m, respectively (Figure 3B). The ²¹⁰Pb flux at 1000 m exhibited peaks in February (83 dpm m⁻² d⁻¹), April (73 dpm m⁻² d⁻¹), and November (97 dpm m⁻² d⁻¹), similar to total particle flux, whereas 2000 m data showed peaks in February (107 dpm m⁻² d⁻¹) and March (106 dpm m⁻² d⁻¹). The sinking flux of particulate ²¹⁰Pb at 1000 m in the East Sea was approximately 5 and 3 times higher than that in the North Pacific (Moored depth: 1004–1264 m; Tsunogai et al., 1982; Harada and Tsunogai, 1986b) and North Atlantic Oceans (Moored depth: 1500 m; Hong et al., 2013), respectively. The activities of ²²⁶Ra were ∼6% of ²¹⁰Pb in the sinking particles.

Discussion

Budget of ²¹⁰Pb in the East Sea

The budget of ²¹⁰Pb in the East Sea is estimated using two different boxes of a steady-state scavenging model (0–1000 m or 0–2000 m). At steady state (∂A/∂t = 0), by neglecting advection and diffusion, the mass balance of ²¹⁰Pb can be calculated as follow:

$$\frac{\partial {\text{A}}_{\text{Pb}-210}}{\partial \text{t}}={\mathrm{\lambda }}_{\text{Pb}-210}\times \left({\text{A}}_{\text{Ra}-226}-{\text{A}}_{\text{Pb}-210}\right)$$

$$+{\text{F}}_{\text{Atm}}-{\text{k}}_{\text{Pb}-210}{\text{A}}_{\text{Pb}-210}=0(1)$$

where λ, F_Atm, k, and A represent the decay constant of ²¹⁰Pb (y ^{− 1}), atmospheric depositional flux of ²¹⁰Pb (dpm cm⁻² y ^{− 1}), first-order scavenging rate constant (y ^{− 1}), and inventory of each radionuclide (dpm cm⁻²) in the 0–1000 m and 0–2000 m, respectively. For the inventory of total ²¹⁰Pb, we compile our measured data with previously published data from the East Sea (Nozaki et al., 1973; Kim and Kim, 2012; Seo et al., 2021; this study). The inventory of ²²⁶Ra in the East Sea is from the published results (Harada and Tsunogai, 1986a; Inoue et al., 2015). The first-order scavenging flux (k_Pb–210A_Pb–210) is measured using the ²¹⁰Pb_ex flux through the sediment traps. The sediment trap samples were not available in several periods (November 11–December 11 in 1999 at 1000 m; December 6–13 in 1998 and April 25–29 in 1999 at 2000 m). In order to estimate the annual ²¹⁰Pb_ex flux, we interpolate the ²¹⁰Pb_ex flux for the missing-sample periods using the significant correlation between the monthly average ²¹⁰Pb_ex fluxes at 1000 m and 2000 m (r ² = 0.85; Supplementary Figure 1). In addition, we note that sediment trap samples of this study were obtained 16 years before the seawater sampling for ²¹⁰Pb. However, the activities of ²¹⁰Pb in seawater were similar from the 1970s to the 2010s in the East Sea (Nozaki et al., 1973; Kim and Kim, 2012; Seo et al., 2021; this study). Thus, we assume that there was no temporal change in ²¹⁰Pb in the East Sea over a few decades.

In Eq. (1), the only unknown term is the atmospheric input of ²¹⁰Pb into the ocean (F_Atm). The atmospheric input of ²¹⁰Pb should be balanced by the in situ production from ²²⁶Ra, in situ decay of ²¹⁰Pb, and settling flux to the deeper layer (Figure 4). Then, the atmospheric input of ²¹⁰Pb is calculated to be 1.37 ± 0.22 and 1.55 ± 0.27 dpm cm⁻² y ^{− 1} according to the water boxes of 0–1000 m and 0–2000 m, respectively. The results are consistent despite the different depths of both boxes, indicating no significant effect of the lateral transport of total ²¹⁰Pb. The atmospheric input of ²¹⁰Pb in the East Sea (1.46 ± 0.25 dpm cm⁻² y ^{− 1}) is similar to the average of previously reported values (1.64 ± 1.10 dpm cm⁻² y ^{− 1}) in this sea based on the land-based measurements or the numerical modeling, which showed large variations depending on the measurement sites and periods (0.44–4.40 dpm cm⁻² y ^{− 1}; Tokieda et al., 1996; Henderson and Maier-Reimer, 2002; Hirose et al., 2004; Yi et al., 2007; Akata et al., 2008; Du et al., 2008, 2015; Tateda and Iwao, 2008; Table 1). It is reasonable to assume that the atmospheric input of ²¹⁰Pb calculated from the ²¹⁰Pb budget in the ocean represents the actual atmospheric flux over the annual to decadal time scales. The atmospheric ²¹⁰Pb flux in the East Sea is 2–4 times higher than that in the major open oceans, such as the North Pacific (0.22–0.30 dpm cm⁻² y ^{− 1}; Turekian, 1989), equatorial Pacific (0.11–0.51 dpm cm⁻² y ^{− 1}; Murray et al., 2005), North Atlantic (0.40–0.69 dpm cm⁻² y ^{− 1}; Turekian et al., 1983; Kim et al., 1999), and Indian Oceans (0.73 dpm cm⁻² y ^{− 1}; Sarin et al., 1999), respectively. However, the atmospheric input of ²¹⁰Pb in the East Sea approaches the upper limit of that in the corresponding latitudinal belt of global fallout curve (average: 0.96 ± 0.58 dpm cm⁻² y ^{− 1}, 30–40°N, Baskaran, 2011). This higher flux in the study region is known to be due the elevated emanation of ²²²Rn from the Asian continent (Baskaran, 2011; Zhang et al., 2021).

FIGURE 4

Figure 4. A schematic box model accounting for the ingrowth, decay, export, and atmospheric flux of ²¹⁰Pb in the East Sea (dpm cm⁻² y ^{− 1}).

TABLE 1

Table 1. Comparison of atmospheric depositional fluxes of ²¹⁰Pb (dpm cm⁻² y ^{− 1}) around the East Sea.

The residence time of ²¹⁰Pb in the water column can be calculated using Eq. (2):

$$\mathrm{\tau }=\frac{1}{{\text{k}}_{\text{Pb}-210}}=\frac{{\text{A}}_{\text{Pb}-210}}{{\text{A}}_{\text{Ra}-226}{\mathrm{\lambda }}_{\text{Pb}-210}-{\text{A}}_{\text{Pb}-210}{\mathrm{\lambda }}_{\text{Pb}-210}+{\text{F}}_{\text{Atm}}}(2)$$

where τ is the residence time of total ²¹⁰Pb. The residence times of total ²¹⁰Pb are estimated to be 4.8 ± 1.2 years and 7.1 ± 1.6 years in the 0–1000 m and 0–2000 m, respectively. The calculated result in this study (7 years, 0–2000 m) is approximately 2.3 times lower than that in the Nozaki et al. (1973) (15 years; 0–2000 m) from the same region. The different residence time of ²¹⁰Pb by Nozaki et al. (1973) is associated with the different atmospheric input term. Nozaki et al. (1973) assumed that approximately 2.0 dpm cm⁻² y ^{− 1} of ²¹⁰Pb entered the East Sea from the atmosphere, and only a quarter of that (0.5 dpm cm⁻² y ^{− 1}) was transported into the deeper layer. It resulted in the net removal flux of ²¹⁰Pb at 2000 m to be 0.8 dpm cm⁻² y ^{− 1}, which is 2.5 times lower than the measured flux from the sediment trap (moored depth: 2000 m) (2.0 dpm cm⁻² y ^{− 1}; this study) or sedimentation rates in this region (water depth: ∼2200 m) (1.79 to 2.70 dpm cm⁻² y ^{− 1}; Hong et al., 1997; Hong et al., 1999). Thus, our estimated residence time of ²¹⁰Pb in the East Sea, which is much shorter than the previous estimation, appears to be more reliable. The residence time of ²¹⁰Pb in the East Sea is 1.5–15 times lower than that in the major oceans, such as the North Pacific (54 years, ∼1800–4000 m, Craig et al., 1973; 96 years, ∼1000–4000 m, Nozaki and Tsunogai, 1976), southeastern Pacific (95 years, ∼0–3700 m, Niedermiller and Baskaran, 2019), North Atlantic (15–22 years, ∼0–3000 m, Cochran et al., 1990), and Indian Oceans (10–15 years, ∼500–4000 m, Obata et al., 2004). The shorter residence time of ²¹⁰Pb in the water column of this sea seems to result in the lower concentrations of ²¹⁰Pb and Pb in the deep ocean, relative to other major oceans, although their atmospheric input fluxes were higher (e.g., Akata et al., 2008; Sakata and Asakura, 2009; Du et al., 2015). This was also evidenced by the fractionations of rare earth elements (Seo and Kim, 2020). The higher removal rates have been attributed to higher fluxes of sinking particles, which mainly consist of lithogenic materials and opal (>80%) (Kim et al., 2020).

Atmospheric Input of Seawater-Dissolvable Pb

In order to calculate the atmospheric input of seawater-dissolvable Pb in the East Sea, we apply the residence time of dissolved ²¹⁰Pb in this study. Assuming that dissolved ²¹⁰Pb is about 80–90% of total ²¹⁰Pb in the East Sea (Kim and Kim, 2012), the residence times of dissolved ²¹⁰Pb in this sea are estimated to be 4.0 ± 1.0 years and 5.8 ± 1.3 years in the 0–1000 m and 0–2000 m, respectively. The average annual atmospheric depositional flux of seawater-dissolvable Pb can be obtained by dividing the inventory of dissolved Pb by the residence time of dissolved ²¹⁰Pb. The Pb data of station E1 is excluded from this calculation because of the distinctly low concentrations in the 100–150 m layer, which might be due to boundary scavenging.

The atmospheric input of seawater-dissolvable Pb is calculated to be 0.98 ± 0.28 nmol cm⁻² y ^{− 1}. Although Pb can be introduced into the East Sea from the adjacent continental shelf, including the East China Sea and the Yellow Sea, we exclude this source since the concentrations of dissolved Pb (0–100 m) in the southern East Sea (stations S1, S4, E5, and E8; ∼69 pmol kg ^{− 1}) were lower than those in the northern East Sea (stations M9, 40, 41, 134-3, 134-9, 144, and 12; ∼78 pmol kg ^{− 1}). The atmospheric input of seawater-dissolvable Pb in the East Sea is distinctively higher than wet deposition of Pb in the remote oceans, including the North Pacific (0.05–0.08 nmol cm⁻² y ^{− 1}; Settle et al., 1982; Duce et al., 1991), North Atlantic (0.03–0.46 nmol cm⁻² y ^{− 1}; Duce et al., 1991; Helmers and Schrems, 1995; Kim et al., 1999), and North Indian Oceans (0.01 nmol cm⁻² y ^{− 1}; Duce et al., 1991). Our calculated Pb flux is approximately 25% of the previously published fluxes around this region, which used the leaching method with nitric acid for precipitation samples collected on land (Sakata et al., 2006, 2008; Sakata and Asakura, 2009; Table 2). Diluted nitric or hydrochloric acid has been widely used to desorb Pb from the particles in precipitation. We believe that the leaching method could overestimate the atmospheric depositional flux of seawater-dissolvable Pb since the much lower pH in this process than the actual pH of seawater can affect the solubility of Pb (Chester et al., 2000; Martín-Torre et al., 2015). Our results suggest that the application of ²¹⁰Pb provides a useful tool to estimate the flux of actual seawater-dissolvable Pb in the ocean. However, the estimated flux of seawater-dissolvable Pb in this study cannot distinguish the relative contribution of different origins of Pb (leaded gasoline, coal burning, and dust). Thus, future studies are necessary to determine the solubility of atmospheric Pb in the ocean according to its origins.

TABLE 2

Table 2. Comparison of atmospheric depositional fluxes of Pb (nmol cm⁻² y ^{− 1}) around the East Sea.

Conclusion

The budget of ²¹⁰Pb in the East Sea is determined by measuring the activities of ²¹⁰Pb in seawater and sinking particles. Based on the different depths (1000 m or 2000 m) of the scavenging box model, the atmospheric input of ²¹⁰Pb is estimated to be 1.46 ± 0.25 dpm cm⁻² y ^{− 1}. Based on this atmospheric input of ²¹⁰Pb, the residence time of ²¹⁰Pb in the East Sea (0–2000 m) is calculated to be 7.1 ± 1.6 years, which is an order of magnitude lower than that in the major open oceans due to the efficient Pb removal in the East Sea. Combining this residence time and the concentrations of dissolved Pb, the atmospheric input of seawater-dissolvable Pb is calculated to be 0.98 ± 0.28 nmol cm⁻² y ^{− 1}, which is ∼25% lower than the previous wet deposition results in this region. Thus, our results suggest that our approach, measuring the flux of seawater dissolvable Pb using the ²¹⁰Pb budget in the ocean, can be successfully used for other major oceans.

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.

Author Contributions

GK conceptualized the study. HS, Y-IK, and IK performed the field sampling and analyses. HS and GK interpreted the data and wrote the manuscript. All authors contributed to the final version of the manuscript.

Funding

This research was supported by the project titled “Deep Water Circulation and Material Cycling in the East Sea (20160400),” 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.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

We thank the crew members of R/V Akademik M.A. Lavrentyev and R/V Isabu for helping with the sampling. We also thank all lab members for their assistance.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2021.756076/full#supplementary-material

References

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