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Article

The Atmospheric Input of Dissolvable Pb Based on the Radioactive 210Pb Budget in the Equatorial Western Indian Ocean

1
Department of Ocean Science, University of Science and Technology (UST), Daejeon 34113, Republic of Korea
2
Marine Environmental Research Department, Korea Institute of Ocean Science and Technology (KIOST), Busan 49111, Republic of Korea
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(6), 1120; https://doi.org/10.3390/jmse11061120
Submission received: 28 April 2023 / Revised: 15 May 2023 / Accepted: 24 May 2023 / Published: 25 May 2023
(This article belongs to the Section Marine Environmental Science)

Abstract

:
To estimate the atmospheric deposition flux of 210Pb in the equatorial western Indian Ocean, we determined the dissolved (<0.45 μm) and particulate 210Pb (>0.45 μm) in the water column. In addition, we calculated the atmosphere-derived dissolvable Pb in seawater using the budget of 210Pb. The dissolved 210Pb and total 210Pb were higher in the surface layer and, overall, showed a decreasing distribution with depth. In particular, radioactive 210Pb activities in the surface-to-upper layer (<1000 m depth) were 1.5 to 2 times higher than those reported in the 1970s (in nearby regions), suggesting that there has been additional 210Pb input in recent years. Based on the mass balance of the total 210Pb budget in the water column, we estimated the atmospheric deposition flux of 210Pb and the residence time of Pb for the first time in this region. The atmospheric deposition flux of 210Pb was estimated to be 0.1–0.5 dpm cm−2 yr−1, and these values agreed with the general global estimations for the major oceans (0.1–0.7 dpm cm−2 yr−1). Considering the residence time of 210Pb (29–41 years) in the water column (estimated from the 210Pb inventory and 234Th-based Pb scavenging rate), the atmospheric input of seawater-dissolvable Pb was quantified to be 0.08–0.1 nmol cm−2 yr−1, which is about eight times higher than the estimated input in the early 1990s in the region. Therefore, these results imply that radioactive 210Pb could be a useful tracer for quantifying Pb flux in seawater.

1. Introduction

As a naturally occurring radionuclide from the 238U decay series, 210Pb (half-life (T1/2) = 22.3 years) is mostly derived from the decay of 222Rn in the atmosphere and is mainly generated via the in situ decay of 226Ra in deep water [1]. 210Pb shows the highest concentration at the atmosphere–ocean interface layer due to dry deposition (in the form of particles) and wet deposition (directly supplied to the upper ocean via precipitation) [2,3]. 210Pb has a half-life suitable for tracing the behavior of particulate matter and is used to estimate the biogeochemical cycle of chemical species due to its adsorption properties towards particles in an aquatic environment [4,5,6,7,8,9].
Although the sources of 210Pb and Pb could be different, it has been suggested that they have similar behaviors and removal mechanisms in seawater [3]. Therefore, 210Pb can be used to understand the observed behavior of Pb in the ocean. 210Pb has been studied to trace the behavior of anthropogenic Pb in the marine environment using ratios of Pb/210Pb [10,11,12,13,14]. Recently, seawater-dissolvable Pb was quantified using the scavenging rate of 210Pb [14]. Despite these various studies, Pb is known to have a wide range of solubility in seawater (13–90%) [15]. Therefore, the determination of Pb solubility in seawater remains a challenging issue, and thus, Pb solubility-related studies in various environments and geographical locations are required.
The Indian Ocean differs from the Pacific and Atlantic Oceans in that it accounts for 30% of the global oceans, but it has a range of only 25° N latitude [16]. Due to these geographic characteristics and its complex seafloor topography, this region has distinct and complex physical features, such as seasonal climate variations (monsoons) and various patterns of ocean currents and circulations [17]. The Indian Ocean is surrounded by rapidly developing countries such as India and South Africa. As a result of the increased high-temperature industrial activities, the late phase-out of leaded gasoline (in the late 1990s and mid-2000s), and the weak environmental regulations of these countries [18,19,20,21,22], Pb emissions in the Indian Ocean have increased over the past few decades. For example, a recent study reported that the Pb emissions from coal combustion have increased by almost 15 times in India [23]. In addition, the concentrations of Pb in seawater were extremely high in the Indonesian coastal region (range of 600–2900 nM) [24,25,26] and Indian Ocean coast near Kenya (35–340 nM) [27]. Moreover, wildfires from Australia and Indonesia may have transported atmospheric Pb into the ocean through an easterly wind [28]. The Indian Ocean is one of the areas with scarce Pb data and is the least explored compared to the Pacific and Atlantic oceans [23], although Pb inventories in this region have likely been increasing compared to the past.
Therefore, in this study, (i) we investigated the distributions of radioactive 210Pb in the water column of the equatorial Indian Ocean and compared them to those reported in the past (before the 1980s) [29] to evaluate the changes in Pb inventories in this region, and (ii) we evaluated the atmospheric inputs of 210Pb based on the mass balance of the 210Pb in the water column of the Indian Ocean. Then, (iii) we also quantified the atmospheric seawater-dissolvable Pb inputs coupled with recently reported Pb concentrations (inventory) in our study area, since Pb solubility in the ocean is still controversial.

2. Materials and Methods

2.1. Sampling

To determine the dissolved and particulate 210Pb in seawater, water samples from the equatorial western Indian Ocean were obtained during a research cruise in April 2018 (Figure 1). Seawater samples (8 L, n = 28) for 210Pb analysis were collected in high-density polyethylene (HDPE) bottles from Niskin samplers from 3 stations. The samples were filtered (0.45 μm, polycarbonate, Millipore), and the dissolved water samples were acidified with 6 N HCl (pH < 2) immediately after sampling to prevent 210Pb from adsorbing onto the bottle; then, the filtered samples were stored in petri dishes at room temperature until analysis.

2.2. Analytical Procedure

In this study, the 210Pb in seawater was measured using 210Po, which was analyzed according to a previously published protocol [31]. Briefly, a 209Po spike (1 dpm g−1), stable Pb (for monitoring the chemical yield) (1 dpm g−1), and an Fe3+ (100 mg g−1) carrier were added to all samples and stirred for 6 h. 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 digested with HNO3 and HCl to remove any organic matter in the samples. The particulate (filtered) samples were digested with a solution of concentrated HNO3 and HCl (1:1 v/v) and repeatedly heated until the sample was completely dissolved.
All the samples were dried down after rinsing with 0.5 M HCl, and then, 100 mL of 0.5 HCl and 0.5 g of ascorbic acid (to reduce Fe3+) were added to the samples. Po was plated on a silver (Ag) planchet (Φ 24.1 mm and 0.15 mm thickness) (99.9% Ag, Aldrich, Burlington, MA, USA) coated with commercial nail polish on one side for 15 h with stirring. The 210Po activities on the silver planchet were counted using alpha spectrometry (Alpha Analyst, Mirion Technology, (Former Canberra, Australia), Canada, USA). The measured counts were corrected for the background of the alpha spectrometry, the decay of 210Po during counting, the recovery of the 209Po spike, the decay of 210Pb from sampling to plating, the recovery of the 209Po spike, the decay of 210Pb from sampling to plating, and the reagent blank.
After removing the silver plate on which the 210Po was adsorbed, 210Pb analysis was performed using the remaining solution. The samples were heated while adding a sufficient amount of conc. HNO3 to the sample to decompose any ascorbic acid contained in the solution. The samples were dried after rinsing with 9 M HCl, and 5 mL of 9 M HCl was added to the samples. To separate 210Pb from 210Po, 50 mL of 9 M HCl was conditioned by passing it through a column (~2.5 cm length of quartz wool, 5–6 cm length of resin, and some glass wool) filled with an anion exchange resin (AG1-ⅹ8, 100–200 mesh, Bio-Rad Laboratories, Inc., Hercules, CA, USA), followed by passing the samples and washing them 4 times with 5 mL of 9 M HCl. The eluted samples were stored in vials, and then, a 209Po tracer (1 dpm g−1) was added to the samples and they were incubated for at least 6 months to generate 210Po, a daughter nuclide of 210Pb. The generated 210Po was measured through the same process. The concentration of 210Pb was calculated using the measured 210Po concentration, incubation time, and recovery rate, which was calculated through the Pb concentration, measured using an inductively coupled plasma mass spectrometer (ICP-MS) (Element 2, Thermo Fisher Scientific, Waltham, MA, USA). To calculate the recovery of the Pb carrier added to the sample, the standardization of Pb was performed using a 500-fold dilution (0–50 ppb) of the Pb carrier in Milli-Q water. The Pb carrier for standardization and the diluted samples for the calculation of Pb recovery were measured using an ICP-MS. The recovery of Pb was calculated using a calibration curve between the measured sample and the result of the Pb carrier dilution (count s−1). 210Pb activity was calculated from the measured 210Po activity, which revealed that the average chemical yield of stable Pb was 84.07 ± 15.07 % (n = 87).

3. Results and Discussion

3.1. Hydrological Properties

Potential temperature and salinity data were used to identify major water masses along the occupied transect (Figure 2). In this study region, the Indian Ocean possesses different water mass characteristics, such as temperature and salinity, and we observed various water masses in this study: ITW (Indonesian Throughflow Water), STUW (South Indian Subtropical Underwater), SICW (South Indian Central Water), ROSW (Red Sea Overflow Water), AAIW (Antarctic Intermediate Water), IDW (Indian Deep Water), and CDW (Circumpolar Deep Water). The ITW, STUW, SICW, SAMW, and RSOW are observed in the surface-to-upper intermediate layer (0–1000 m), the AAIW and IDW are observed in the intermediate layer (1000–2000 m), and the CDW is observed in the deep layer (2000–4000 m). The water masses observed in this study were defined according to previous similarly conducted studies for this study region, e.g., [32,33,34,35,36].

3.2. Distribution of Radioactive 210Pb in the Indian Ocean

The horizontal distributions of dissolved 210Pb, particulate 210Pb, and total 210Pb (dissolved + particulate) in the surface layer (0–20 m) in this study are shown in Figure 1b,c. All the 210Pb data are presented in Table 1.
Dissolved 210Pb and total 210Pb showed slightly higher activities in the western part (60.00° E) (21.47 dpm 100 L1 and 22.44 dpm 100 L1, respectively) than stations in the eastern region (st. 19 and st. 34) (16.60 ± 3.76 dpm 100 L1 and 17.49 ± 3.65 dpm 100 L1, respectively). These results may be attributed to the more lithogenic materials and/or matter originating from land from the African continent and Mascarene Plateau (Figure 1) [37] along the South Equatorial Current (SEC, orange arrow in Figure 1a).
The vertical distributions of dissolved 210Pb and total 210Pb in the water column are shown in Figure 3. The vertical distributions of both dissolved 210Pb and total 210Pb in the Indian Ocean showed higher activities in the surface layer and lower activities in the deeper layer. The average activities of dissolved 210Pb and total 210Pb were 11.32 ± 0.56 dpm 100 L1 and 13 ± 1.21 dpm 100 L1, respectively. The activities of both dissolved 210Pb and total 210Pb slightly increased in the middle layer (1000–1500 m) (average of 14.23 ± 1.72 dpm 100 L1 and 15.1 ± 1.79 dpm 100 L1, respectively).
In the surface layer, the dissolved and total 210Pb activities were about 1.9 times higher than those reported in a previous study [29] (Geochemical Ocean Sections Study (GEOSECS) cruise data from the 1970s) (Figure 3). In the same study area, dissolved Pb metal concentrations were also about 2.3 times higher than those reported in previous studies [30,38].
The total and dissolved 210Pb activities in the surface-to-intermediate layer (<1000 m) in this study were consistently 1.5 to 2 times higher than those in previous observations (in the 1970s GEOSECS data) of the equatorial western Indian Ocean. The 210Pb activities in this study and previous measurements of 210Pb activity data from various ocean/seawater samples are presented in Table 2. The total and dissolved 210Pb activities (<1000 m) in this study were also similar to or relatively higher than those in the north Atlantic, some Pacific regions, and the Antarctic. These results may be due to the following reasons. First, 210Pb may be introduced by the long-range transport of coastal water masses, such as from the Indonesian coastal region (600–2900 nM dissolved Pb) [24,25,26] and African coast (near Kenya) (35–340 nM dissolved Pb) [27] along the Indian Ocean subtropical gyre. Second, the increased Pb may come from modern anthropogenic sources (e.g., coal combustion, mining and smelting operations, etc.) from various industries in the surrounding countries. For example, Schaule and Patterson [39] suggested that the Pb increase is congruent with that observed for 210Pb concentrations in the same water sampled at the same times. Recently, Witt et al. [40] suggested that the ratios of stable Pb isotopes (206,207,208Pb) are consistent with coal combustion and its increased importance as a source of Pb around the Indian Ocean. Moreover, Lee et al. [22] suggested that the Pb in the Chagos coral (located near the study area) reflects the predominance of India’s industrial (gasoline and coal) Pb in this region. Third, Pb released into the atmosphere from wildfires in southeast Asia and Australia may have been transported by seasonal and easterly winds and deposited into the Indian Ocean. Das et al. [41] determined that the ratios of Pb isotopes (208Pb/207Pb and 206Pb/207Pb) increased during wildfire haze periods in Indonesia, and suggested that the suspension of the crustal material was the dominant emission source of total suspended particulate (TSP) matter. Fourth, it could be the result of the additional dissolution of particulate Pb from the artificial origin (i.e., especially fine particulate forms smaller than 50 μm, which are dissolved very efficiently), which is continuously introduced to the water column on the way to being exported to the deeper layer. Overall, the 210Pb in the upper layer (<1000 m) in the water column of in this study region was consistently increased.

3.3. 210Pb Budget

The budget of 210Pb in the Indian Ocean is estimated using the box of a steady-state scavenging model (0–300 m). At steady state (∂A/∂t = 0), by neglecting advection and diffusion, the rate of change of 210Pb activity can be expressed as following Equation (1):
A 210 P b t = λ 210 P b A 226 R a A 210 P b + F A t m k 210 P b A 210 P b = 0  
where A is the inventory of each radionuclide (dpm cm2) in the 0–300 m depth water column, and λ, FAtm, and k represent the decay constant of 210Pb (0.0311 yr1), the atmospheric depositional flux of 210Pb (dpm cm2 yr1), and the first-order scavenging rate constant (yr1), respectively. The atmospheric input of 210Pb should be balanced with the in situ-production from mother nuclei, 226Ra (FIngrowth), the in situ decay of 210Pb (Fdecay), and the settling of flux to the deeper layer (Fexport) (Figure 4). We estimated FIngrowth by multiplying the 226Ra inventory and the decay constant of 210Pb, and Fdecay by multiplying the 210Pb inventory and the decay constant of 210Pb. The inventory of 226Ra in the south Indian Ocean was taken from recently published results [48]. Fexport was calculated by multiplying the 210Pb inventory and the first-order scavenging rate constant (yr1). The first-order scavenging rate constant was obtained from previous published results (k210Pb; 0.02–0.07 yr1 in 0–300 m of water column), which was calculated using the 234Th-based export flux of Pb [37] in the same station (also using the same sample) in this study. The unknown constant FAtm was calculated by assuming a steady state. Each calculated term was schematized as a box model in Figure 4. The FAtm (atmospheric depositional flux of 210Pb) was calculated to be 0.10 ± 0.05–0.50 ± 0.16 dpm cm2 yr1. The atmospheric depositional flux of 210Pb estimated in this study was comparable to those in the major oceans, such as the North Pacific (0.22–0.30 dpm cm2 yr1 [49]), the Equatorial Pacific (0.11–0.51 dpm cm2 yr1 [50]), the North Atlantic (0.4–0.69 dpm cm2 yr1 [51,52]), the northeasternmost part of the Indian Ocean (~0.4 dpm cm2 yr1 [53]), and the Arabian Sea (0.73 dpm cm2 yr1 [54]) near the northern Indian Ocean.
From the 210Pb budget, the residence time of total 210Pb in the water column was calculated using the following Equation (2):
τ = 1 k 210 P b = A 210 P b A 226 R a λ 210 P b A 210 P b λ 210 P b + F A t m
Here, τ is the residence time of the total 210Pb, and it was estimated to be in the range of 28.7–40.9 (average: 36.07 years) in the 0–300 m layer. The calculated residence time, about 28.7–40.9 years, in this study is comparable with that in the North Pacific (54–96 years) [55,56], Southeastern Pacific (95 years) [47], and Atlantic Oceans (15–22 years) [57].

3.4. Atmospheric Input of Seawater-Dissolvable Pb

In order to calculate the atmospheric input of seawater-dissolvable Pb in the Indian Ocean, we used the residence time of dissolved 210Pb in this study. The residence time of dissolved 210Pb in this region was estimated to be 36.5 ± 6.6 years at a water column depth of 0–300 m using the activity of dissolved 210Pb. The average annual atmospheric depositional flux of seawater-dissolvable Pb can be calculated by dividing the inventory of dissolved Pb by the residence time of dissolved 210Pb, resulting in 0.08 ± 0.02 nmol cm2 yr1 in this study (Figure 5 and Table 3). The residence time of 234Th or particulate Pb in the upper layer of nearby regions ranges from tens of days to 1 year [30,58,59], which is significantly shorter than the residence time of the dissolved Pb estimated in this study. In this study, the dissolvable Pb flux in the upper layer may be higher because the fine Pb particles that first enter the ocean are rapidly scavenged, and therefore, not detectable in the deeper layers.
The calculated soluble Pb flux (0.08 ± 0.02 nmol cm2 yr1) from the atmosphere in this study was about eight times higher than the previously estimated total Pb flux (0.019 nmol cm2 yr1) in the northern Indian Ocean [15], see Table 3). This result implies that there has been an increase in the atmospheric input of Pb into the Indian Ocean to this day, in contrast to the Pacific and Atlantic oceans, where Pb inventories are now decreasing due to the ban on the use of leaded gasoline. We also noted that modern Pb input from the atmosphere (unlike the lithogenic dust with coarse particles) has a relatively smaller particle size due to its artificial origin (fine particle sources measuring <1~50 μm (e.g., PM10, PM2.5, etc.)), and seems to be more soluble in seawater. In addition, the soluble Pb fluxes (0.08 ± 0.02 nmol cm2 yr1) in this study were also higher than the fluxes of wet deposition of Pb in remote oceans, including the North Pacific (0.05–0.08 nmol cm2 yr1 [10,15]) and North Atlantic (average: 0.4 nmol cm−2 yr−1 [15,52,60,61]), and also comparable with the marginal sea region near the continent, such as the Arabian Sea (in the Indian Ocean, ~0.12 nmol cm2 yr1 [62]) (Table 3). These results imply that radioactive 210Pb could be a useful tracer for quantifying actually dissolvable fractions of atmospheric depositional Pb flux into seawater.
Table 3. Comparison of depositional fluxes of Pb (nmol cm2 yr1) with other oceans.
Table 3. Comparison of depositional fluxes of Pb (nmol cm2 yr1) with other oceans.
Study AreaCollection PeriodFractionPb Flux
(nmol cm−2 yr−1)
Reference
This study2018.04Actual dissolvable fraction into the seawater0.08 ± 0.02
North Indian Ocean
(Arabian Sea)
1986.10–1986.11Total deposition flux0.483[62]
Soluble atmospheric flux~0.116
North Indian Ocean1985Total deposition flux0.024[15]
Dry deposition flux0.005
Wet deposition flux0.019
South Indian Ocean (Kerguelen)2008.11–2010.10Total deposition flux0.003 ± 0.000[63]
South Indian Ocean (Crozet)2010.01–2010.11Total deposition flux0.009 ± 0.001
North Pacific Ocean1979Wet deposition flux0.05–0.08[10,15]
Northwestern
Atlantic Ocean
1996.06Dry deposition flux0.277[60]
1997.050.913
1996.06Wet deposition flux0.022
1997.051.095
Northwestern
Atlantic Ocean
1985–1990Wet deposition flux0.030–0.460[61]
Western
Mediterranean
2011.09–2012.08Dry deposition flux0.483–1.448[64]
Wet deposition flux~3.089
Northwestern
Mediterranean
1986–1992Total deposition flux of dissolved Pb0.440–0.881[65]
Red Sea
(Gulf of Aqaba)
2003–2005Dry deposition flux0.063–0.384[66]
East Sea/ Sea of Japan 2018.01–2018.02Actual dissolvable fraction into the seawater0.98 ± 0.28[14]
Japan2008.02–2008.05Total deposition flux0.25–3.29[67]

4. Conclusions

For the first time, this study quantified the 210Pb budget using the mass balance of total 210Pb in the equatorial western Indian Ocean, where Pb is expected to be introduced through various pathways (e.g., industrial activities, the late phase-out of leaded gasoline, wildfires from Australia and Indonesia, etc.). Compared with data from the 1970s (the only published 210Pb data from the Indian Ocean), 210Pb in the Indian Ocean has increased 1.5- to 2-fold in the upper layer (<1000 m depth) of the water column. This suggests a continuous input of Pb of anthropogenic origin into this region. We estimated that the atmospheric deposition flux of 210Pb was 0.1–0.5 dpm cm−2 yr−1, based on the 210Pb budget in our study area. Based on this atmospheric input of 210Pb, the residence time of 210Pb in the Indian Ocean (0–300 m layer) was calculated to be 29–41 years. Applying this 210Pb residence time, seawater-dissolvable Pb was quantified to be 0.08–0.10 nmol cm−2 yr−1. However, since this estimate was calculated using 226Ra data from nearby areas, 226Ra analysis should be performed at this study site in the future in order to calculate a more accurate value. Moreover, in order to better understand the behavior of trace elements, including 210Pb, it is necessary to analyze stable Pb isotopes or the particulate fraction of Pb to identify the source of anthropogenic Pb, in order to quantify Pb solubility, and to estimate the input of terrestrial and/or lithogenic materials by investigating radioactive 226Ra in the Indian Ocean.

Author Contributions

H.L. (Huisu Lee) and I.K. designed the research and wrote the manuscript. H.L. (Hyunmi Lee) and I.K. conducted the field campaign. J.L., H.L. (Huisu Lee), and H.L. (Hyunmi Lee) conducted the lab experiment and analysis. J.L. significantly contributed to the discussion. All authors have read and agreed to the published version of the manuscript.

Funding

This research was a part of the project titled ‘KIOS (Korea Indian Ocean Study): Korea-US Joint Observation Study of the Indian Ocean’, funded by the Korean Ministry of Oceans and Fisheries (20220548, PM63470).

Data Availability Statement

All data sets used in this paper are available upon request from the corresponding author (ikim@kiost.ac.kr).

Acknowledgments

We thank all the crew members of R/V Isabu who helped with the field sampling. We also thank D.J. Kang and S.H. Kim, who provided constructive comments on this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Sampling locations (square symbols) for 210Pb (stations 13, 19, and 34) and dissolved Pb (1, 5, 13, 19, 24, 29, 34, and 38; data from [30]) together with all stations covered by the previous (a) dissolved 210Pb and (b) total 210Pb [29] (from GEOSECS data in 1970s) data (triangle symbols), and (c) Pb data from the Indian Ocean. (a) Sampling locations for dissolved 210Pb and surface activities of dissolved 210Pb (dpm 100 L−1). (b) Sampling locations for total 210Pb and surface activities of total 210Pb. (c) Sampling locations for dissolved Pb, including various mean annual surface currents (colored arrows) and wind directions (black arrows) during the sampling period (April to May) in the western Indian Ocean.
Figure 1. Sampling locations (square symbols) for 210Pb (stations 13, 19, and 34) and dissolved Pb (1, 5, 13, 19, 24, 29, 34, and 38; data from [30]) together with all stations covered by the previous (a) dissolved 210Pb and (b) total 210Pb [29] (from GEOSECS data in 1970s) data (triangle symbols), and (c) Pb data from the Indian Ocean. (a) Sampling locations for dissolved 210Pb and surface activities of dissolved 210Pb (dpm 100 L−1). (b) Sampling locations for total 210Pb and surface activities of total 210Pb. (c) Sampling locations for dissolved Pb, including various mean annual surface currents (colored arrows) and wind directions (black arrows) during the sampling period (April to May) in the western Indian Ocean.
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Figure 2. T–S diagram indicating the identified water masses in the equatorial western Indian Ocean and major water masses (left figure) and distributions of dissolved Pb by depth (data from Kim et al. [30]) in this study area (modified from recent work of Kim et al. [30]) (right figure). Isopycnals are shown as gray lines. Abbreviations: ITW (Indonesian Throughflow Water), STUW (South Indian Subtropical Underwater), SICW (South Indian Central Water), ROSW (Red Sea Overflow Water), AAIW (Antarctic Intermediate Water), IDW (Indian Ocean Deep water), and CDW (Circumpolar Deep Water).
Figure 2. T–S diagram indicating the identified water masses in the equatorial western Indian Ocean and major water masses (left figure) and distributions of dissolved Pb by depth (data from Kim et al. [30]) in this study area (modified from recent work of Kim et al. [30]) (right figure). Isopycnals are shown as gray lines. Abbreviations: ITW (Indonesian Throughflow Water), STUW (South Indian Subtropical Underwater), SICW (South Indian Central Water), ROSW (Red Sea Overflow Water), AAIW (Antarctic Intermediate Water), IDW (Indian Ocean Deep water), and CDW (Circumpolar Deep Water).
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Figure 3. Vertical profiles of (a) dissolved 210Pb and (b) total 210Pb in the Indian Ocean. The GEOSECS data from the 1970s [29] obtained from nearby stations in our study area are shown for comparison (see Figure 1).
Figure 3. Vertical profiles of (a) dissolved 210Pb and (b) total 210Pb in the Indian Ocean. The GEOSECS data from the 1970s [29] obtained from nearby stations in our study area are shown for comparison (see Figure 1).
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Figure 4. A schematic box model accounting for the ingrowth, decay, export, and atmospheric flux of 210Pb (dpm cm−2 yr−1) in the equatorial western Indian Ocean.
Figure 4. A schematic box model accounting for the ingrowth, decay, export, and atmospheric flux of 210Pb (dpm cm−2 yr−1) in the equatorial western Indian Ocean.
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Figure 5. A schematic box model accounting for residence time of dissolved Pb, Pb inventory, and atmospheric flux of seawater-dissolved Pb (nmol cm2 yr1) in the equatorial western Indian Ocean.
Figure 5. A schematic box model accounting for residence time of dissolved Pb, Pb inventory, and atmospheric flux of seawater-dissolved Pb (nmol cm2 yr1) in the equatorial western Indian Ocean.
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Table 1. Activities of 210Pb in the equatorial western Indian Ocean.
Table 1. Activities of 210Pb in the equatorial western Indian Ocean.
StationDepth (m)Dissolved 210Pb (dpm 100 L−1)Particulate 210Pb (dpm 100 L−1)Total 210Pb (dpm 100 L−1)
5.00° S, 60.00° E (2018.04.11)
st. 13021.47 ± 0.790.97 ± 0.1722.44 ± 0.81
5012.05 ± 0.440.31 ± 0.212.36 ± 0.48
1008.71 ± 0.450.44 ± 0.099.14 ± 0.46
20010.54 ± 0.60.6 ± 0.0611.14 ± 0.6
3008.48 ± 0.461.20 ± 0.119.68 ± 0.47
5009.19 ± 0.430.6 ± 0.079.79 ± 0.44
100014.46 ± 0.60.68 ± 0.0815.14 ± 0.6
20009.9 ± 0.490.87 ± 0.0810.77 ± 0.49
300010.96 ± 0.461.81 ± 0.1212.77 ± 0.48
403510.67 ± 0.460.99 ± 0.0811.66 ± 0.46
5.27° S, 67.90° E (2018.04.15)
st. 19013.94 ± 0.550.96 ± 0.1114.9 ± 0.56
5010.85 ± 0.591.07 ± 0.2511.92 ± 0.64
1008.39 ± 0.40.71 ± 0.129.10 ± 0.41
2008.76 ± 0.40.8 ± 0.099.56 ± 0.41
3008.96 ± 0.40.78 ± 0.099.73 ± 0.41
5009.46 ± 0.410.84 ± 0.1210.3 ± 0.43
100012.21 ± 0.540.88 ± 0.1513.10 ± 0.56
200014.48 ± 0.640.73 ± 0.0715.21 ± 0.64
300611.64 ± 0.491.70 ± 0.1613.34 ± 0.52
20.00° S, 67.05° E (2018.04.23)
st. 34019.25 ± 0.710.82 ± 0.0720.07 ± 0.72
5020.36 ± 0.671.42 ± 0.1121.78 ± 0.68
10019.17 ± 0.720.88 ± 0.1620.05 ± 0.73
20016.03 ± 0.580.79 ± 0.116.82 ± 0.59
30013.37 ± 0.550.82 ± 0.0814.19 ± 0.55
50011.55 ± 0.481.02 ± 0.1112.57 ± 0.49
100013.87 ± 0.50.87 ± 0.0914.74 ± 0.51
150016.38 ± 0.721.06 ± 0.117.44 ± 0.72
204011.64 ± 0.442.37 ± 0.1214.01 ± 0.45
Table 2. Comparison of 210Pb activities in seawater from various ocean/marginal sea regions.
Table 2. Comparison of 210Pb activities in seawater from various ocean/marginal sea regions.
SiteDateDepth (m)210Pb Activity (dpm 100 L−1)Reference
TotalDissolved
This study2018.04<100013.74 (9.10–22.44) (n = 28)12.91 (8.39–21.47) (n = 28)
>100013.60 (10.77–17.44) (n = 28)12.24 (9.90–16.38) (n = 28)
Indian Ocean1970s<10009.51 (4.95–13.46) (n = 28)9.35 (4.50–15.70) (n = 36)[29]
>100011.16 (5.51–14.51) (n = 70)10.47 (4.50–13.70) (n = 82)
North Pacific2009.05<100019.25 (11.64–31.23) (n = 11)18.96 (11.40–31.10) (n = 11)[42]
>100026.26 (23.89–28.62) (n = 2)25.95 (23.60–28.30) (n = 2)
North Atlantic2010.10<100012.93 (6.56–24.10) (n = 57)12.36 (6.18–23.37) (n = 64)[43]
>100011.38 (5.10–17.72) (n = 25)10.56 (1.63–22.30) (n = 40)
South Antarctic2007.07<10004.12 (0.72–12.20) (n = 170)3.03 (0.24–10.28) (n = 161)[44]
Black Sea1988.06<10004.22 (0.89–22.4) (n = 49)2.53 (0.50–21.88) (n = 50)[45]
>10003.96 (3.40–5.09) (n = 7)1.52 (1.15–2.97) (n = 7)
Pacific Ocean marginal sea
(ECS: East China Sea)
2013.10<3503.09 (1.49–6.95) (n = 27)2.01 (0.90–2.99) (n = 27)[46]
East Pacific Zonal Transect (EPZT)2013.11<100011.90 (5.51–40.20) (n = 63)11.57 (5.35–39.80) (n = 63)[47]
>100016.92 (5.12–26.30) (n = 64)15.45 (1.87–25.20) (n = 64)
East Sea/Sea of Japan2015.05<10009.66 (5.20–16.40) (n = 16) [14]
>10005.21 (3.80–7.20) (n = 13)
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Lee, H.; Lee, J.; Lee, H.; Kim, I. The Atmospheric Input of Dissolvable Pb Based on the Radioactive 210Pb Budget in the Equatorial Western Indian Ocean. J. Mar. Sci. Eng. 2023, 11, 1120. https://doi.org/10.3390/jmse11061120

AMA Style

Lee H, Lee J, Lee H, Kim I. The Atmospheric Input of Dissolvable Pb Based on the Radioactive 210Pb Budget in the Equatorial Western Indian Ocean. Journal of Marine Science and Engineering. 2023; 11(6):1120. https://doi.org/10.3390/jmse11061120

Chicago/Turabian Style

Lee, Huisu, Jaeeun Lee, Hyunmi Lee, and Intae Kim. 2023. "The Atmospheric Input of Dissolvable Pb Based on the Radioactive 210Pb Budget in the Equatorial Western Indian Ocean" Journal of Marine Science and Engineering 11, no. 6: 1120. https://doi.org/10.3390/jmse11061120

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