Front. Mar. Sci. Frontiers in Marine Science Front. Mar. Sci. 2296-7745 Frontiers Media S.A. 10.3389/fmars.2021.756076 Marine Science Original Research Tracing the Atmospheric Input of Seawater-Dissolvable Pb Based on the Budget of 210Pb in the East Sea (Japan Sea) Seo Hojong 1 Kim Guebuem 1 * Kim Young-Il 2 Kim Intae 3 1School of Earth and Environmental Sciences, Research Institute of Oceanography, Seoul National University, Seoul, South Korea 2East Sea Research Institute, Korea Institute of Ocean Science and Technology, Uljin, South Korea 3Marine Environmental Research Center, Korea Institute of Ocean Science and Technology, Busan, South Korea

Edited by: Selvaraj Kandasamy, Xiamen University, China

Reviewed by: Edward Boyle, Massachusetts Institute of Technology, United States; Mark Baskaran, Wayne State University, United States

 *Correspondence: Guebuem Kim, gkim@snu.ac.kr

This article was submitted to Marine Biogeochemistry, a section of the journal Frontiers in Marine Science

 28 10 2021 2021 8 756076 10 08 2021 05 10 2021 Copyright © 2021 Seo, Kim, Kim and Kim. 2021 Seo, Kim, Kim and Kim

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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.

 East Sea Japan Sea 210Pb seawater-dissolvable Pb atmospheric input Introduction

The naturally occurring radionuclide 210Pb (t1/2 = 22.3 years), belonging to the 238U decay series, is produced from 226Ra (t1/2 = 1600 years), via 222Rn (t1/2 = 3.8 days) and other short-lived radionuclides (t1/2 < 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 (210Pbex: 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 210Pbex 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.

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 10Pb 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).

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.

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 <sup>210</sup>Pb 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:

∂ ⁡ A Pb - 210 ∂ ⁡ t = λ Pb - 210 × ( A Ra - 226 - A Pb - 210 ) + F Atm - k Pb - 210 ⁢ A Pb - 210 = 0

where λ, FAtm, 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 (kPb–210APb–210) is measured using the 210Pbex 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 210Pbex flux, we interpolate the 210Pbex flux for the missing-sample periods using the significant correlation between the monthly average 210Pbex 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 (FAtm). 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).

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

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

Study area Lat. Long. Collection period Method 210Pb flux (dpm cm−2 y−1) References
Average Range
Rokkasho, Japan 41.0°N 141.4°E July 2000–March 2006 Bulk deposition (no leaching) 4.38 1.10–55.3 Akata et al., 2008
Tsukuba, Japan 36.1°N 140.1°E January 2000–December 2001 Bulk deposition (no leaching) 1.10 0.62–2.26 Hirose et al., 2004
Nagasaki, Japan 32.8°N 129.8°E January 2000–December 2000 1.39 0.07–5.62
Odawa Bay, Japan 35.2°N 139.6°E September 1997–August 1998 Bulk deposition (leaching with HNO3) 0.47 0.07–2.04 Tateda and Iwao, 2008
Tsuyazaki, Japan 33.8°N 130.5°E 1.17 0.26–3.47
Akajima, Japan 26.2°N 127.3°E 0.51 0.04–2.08
Hakodate, Japan 41.8°N 140.7°E January 1990–June 1991 Wet deposition (no leaching) 1.61 0.22–3.69 Tokieda et al., 1996
Xiamen, China 24.4°N 118.1°E March 2004–April 2005 Bulk deposition (leaching with 0.2 M HCl) 1.13 0.07–1.86 Yi et al., 2007
Shanghai, China 31.2°N 121.4°E November 2005–October 2006 Bulk deposition (leaching with HNO3) 2.88 0.66–7.30 Du et al., 2008
Shanghai, China 31.2°N 121.4°E December 2005–December 2013 Bulk deposition (leaching with 0.2 M HNO3) 2.30 0.51–5.99 Du et al., 2015
East Sea Atmospheric model (222Rn) 0.99–1.39 Henderson and Maier-Reimer, 2002
East Sea Water column mass balance 1.37–1.55 This study

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

τ = ⁢ 1 k Pb - 210 = ⁢ A Pb - 210 A Ra - 226 ⁢ λ Pb - 210 - A Pb - 210 ⁢ λ Pb - 210 + F Atm

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.

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

Study area Fraction Collection period Pb flux (nmol cm−2 y−1) References
Japan (10 sites) Wet deposition (leaching with 0.3 M HNO3) December 2003–November 2004 2.26 ± 1.50 Sakata et al., 2006
Tokyo Bay Wet deposition (leaching with 0.3 M HNO3) December 2003–November 2005 3.14 ± 1.28 Sakata et al., 2008
Sea coast of Japan (Noshiro) Wet deposition (leaching with 0.3 M HNO3) December 2002–March 2006 4.34 ± 1.42 Sakata and Asakura, 2009
Sea coast of Japan (Nakanoto) 4.82 ± 0.95
Sea coast of Japan (Matsuura) 3.32 ± 0.44
East Sea Actual dissolvable fraction into the seawater – 0.98 ± 0.28 This study
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.

 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

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 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).

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

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