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

: To estimate the atmospheric deposition ﬂux of 210 Pb in the equatorial western Indian Ocean, we determined the dissolved (<0.45 µ m) and particulate 210 Pb (>0.45 µ m) in the water column. In addition, we calculated the atmosphere-derived dissolvable Pb in seawater using the budget of 210 Pb. The dissolved 210 Pb and total 210 Pb were higher in the surface layer and, overall, showed a decreasing distribution with depth. In particular, radioactive 210 Pb 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 210 Pb input in recent years. Based on the mass balance of the total 210 Pb budget in the water column, we estimated the atmospheric deposition ﬂux of 210 Pb and the residence time of Pb for the ﬁrst time in this region. The atmospheric deposition ﬂux of 210 Pb 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 210 Pb (29–41 years) in the water column (estimated from the 210 Pb inventory and 234 Th-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 210 Pb could be a useful tracer for quantifying Pb ﬂux in seawater.


Introduction
As a naturally occurring radionuclide from the 238 U decay series, 210 Pb (half-life (T 1/2 ) = 22.3 years) is mostly derived from the decay of 222 Rn in the atmosphere and is mainly generated via the in situ decay of 226 Ra 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 210 Pb and Pb could be different, it has been suggested that they have similar behaviors and removal mechanisms in seawater [3].Therefore, 210 Pb 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/ 210 Pb [10][11][12][13][14]. Recently, seawater-dissolvable Pb was quantified using the scavenging rate of 210 Pb [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 hightemperature 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 210 Pb 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 210 Pb based on the mass balance of the 210 Pb in the water column of the Indian Ocean.Then, (iii) we also quantified the atmospheric seawaterdissolvable Pb inputs coupled with recently reported Pb concentrations (inventory) in our study area, since Pb solubility in the ocean is still controversial.

Sampling
To determine the dissolved and particulate 210 Pb 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 210 Pb 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 210 Pb from adsorbing onto the bottle; then, the filtered samples were stored in petri dishes at room temperature until analysis.

Analytical Procedure
In this study, the 210 Pb in seawater was measured using 210 Po, which was analyzed according to a previously published protocol [31].Briefly, a 209 Po spike (1 dpm g −1 ), stable Pb (for monitoring the chemical yield) (1 dpm g −1 ), and an Fe 3+ (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 210 Po and Fe(OH) 3 .The supernatants were removed, and then, the precipitates were digested with HNO 3 and HCl to remove any organic matter in the samples.The particulate (filtered) samples were digested with a solution of concentrated HNO 3 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 Fe 3+ ) 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 210 Po 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 210 Po during counting, the recovery of the 209 Po spike, the decay of 210 Pb from sampling to plating, the recovery of the 209 Po spike, the decay of 210 Pb from sampling to plating, and the reagent blank.

Sampling
To determine the dissolved and particulate 210 Pb 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 210 Pb 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 210 Pb from adsorbing onto the bottle; then, the filtered samples were stored in petri dishes at room temperature until analysis.After removing the silver plate on which the 210 Po was adsorbed, 210 Pb analysis was performed using the remaining solution.The samples were heated while adding a sufficient amount of conc.HNO 3 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 210 Pb from 210 Po, 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 209 Po tracer (1 dpm g −1 ) was added to the samples and they were incubated for at least 6 months to generate 210 Po, a daughter nuclide of 210 Pb.The generated 210 Po was measured through the same process.The concentration of 210 Pb was calculated using the measured 210 Po 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 210 Po activity, which revealed that the average chemical yield of stable Pb was 84.07 ± 15.07 % (n = 87).

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].

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].

Distribution of Radioactive 210 Pb in the Indian Ocean
The horizontal distributions of dissolved 210 Pb, particulate 210 Pb, and total 210 Pb (dissolved + particulate) in the surface layer (0-20 m) in this study are shown in Figure 1b,c.All the 210 Pb data are presented in Table 1.

Distribution of Radioactive 210 Pb in the Indian Ocean
The horizontal distributions of dissolved 210 Pb, particulate 210 Pb, and total 210 Pb (dissolved + particulate) in the surface layer (0-20 m) in this study are shown in Figure 1b,c.All the 210 Pb data are presented in Table 1.Dissolved 210 Pb and total 210 Pb showed slightly higher activities in the western part (60.00 • E) (21.47 dpm 100 L −1 and 22.44 dpm 100 L −1 , respectively) than stations in the eastern region (st.19 and st.34) (16.60 ± 3.76 dpm 100 L −1 and 17.49 ± 3.65 dpm 100 L −1 , 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 210 Pb and total 210 Pb in the water column are shown in Figure 3.The vertical distributions of both dissolved 210 Pb and total 210 Pb in the Indian Ocean showed higher activities in the surface layer and lower activities in the deeper layer.The average activities of dissolved 210 Pb and total 210 Pb were 11.32 ± 0.56 dpm 100 L −1 and 13 ± 1.21 dpm 100 L −1 , respectively.The activities of both dissolved 210 Pb and total 210 Pb slightly increased in the middle layer (1000-1500 m) (average of 14.23 ± 1.72 dpm 100 L −1 and 15.1 ± 1.79 dpm 100 L −1 , respectively).
In the surface layer, the dissolved and total 210 Pb 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 210 Pb 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 210 Pb activities in this study and previous measurements of 210 Pb activity data from various ocean/seawater samples are presented in Table 2.The total and dissolved 210 Pb 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, 210 Pb 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 210 Pb concentrations in the same water sampled at the same times.Recently, Witt et al. [40] suggested that the ratios of stable Pb isotopes ( 206,207,208 Pb) 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 ( 208 Pb/ 207 Pb and 206 Pb/ 207 Pb) 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 210 Pb in the upper layer (<1000 m) in the water column of in this study region was consistently increased.The total and dissolved 210 Pb 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 210 Pb activities in this study and previous measurements of 210 Pb activity data from various ocean/seawater samples are presented in Table 2.The total and dissolved 210 Pb 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, 210 Pb 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 210 Pb concentrations in the same water sampled at the same times.Recently, Witt et al. [40] suggested that the ratios of stable Pb isotopes ( 206,207,208 Pb) 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 ( 208 Pb/ 207 Pb and 206 Pb/ 207 Pb) 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

210 Pb Budget
The budget of 210 Pb 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 210 Pb activity can be expressed as following Equation (1): where A is the inventory of each radionuclide (dpm cm −2 ) in the 0-300 m depth water column, and λ, F Atm , and k represent the decay constant of 210 Pb (0.0311 yr −1 ), the atmospheric depositional flux of 210 Pb (dpm cm −2 yr −1 ), and the first-order scavenging rate constant (yr −1 ), respectively.The atmospheric input of 210 Pb should be balanced with the in situ-production from mother nuclei, 226 Ra (F Ingrowth ), the in situ decay of 210 Pb (F decay ), and the settling of flux to the deeper layer (F export ) (Figure 4).We estimated F Ingrowth by multiplying the 226 Ra inventory and the decay constant of 210 Pb, and F decay by multiplying the 210 Pb inventory and the decay constant of 210 Pb.The inventory of 226 Ra in the south Indian Ocean was taken from recently published results [48].F export was calculated by multiplying the 210 Pb inventory and the first-order scavenging rate constant (yr −1 ).The first-order scavenging rate constant was obtained from previous published results (k 210Pb ; 0.02-0.07yr −1 in 0-300 m of water column), which was calculated using the 234 Th-based export flux of Pb [37] in the same station (also using the same sample) in this study.The unknown constant F Atm was calculated by assuming a steady state.Each calculated term was schematized as a box model in  From the 210 Pb budget, the residence time of total 210 Pb in the water column was ca culated using the following Equation ( 2  From the 210 Pb budget, the residence time of total 210 Pb in the water column was calculated using the following Equation (2): Here, τ is the time of the total 210 Pb, 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 28.7-40.9years, 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].

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 210 Pb in this study.The residence time of dissolved 210 Pb 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 210 Pb.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 210 Pb, resulting in 0.08 ± 0.02 nmol cm −2 yr −1 in this study (Figure 5 and Table 3).The residence time of 234 Th 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 cm −2 yr −1 ) from the atmosphere in this study was about eight times higher than the previously estimated total Pb flux (0.019 nmol cm −2 yr −1 ) 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 cm −2 yr −1 ) 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 cm −2 yr −1 [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 cm −2 yr −1 [62]) (Table 3).These results imply that radioactive 210 Pb could be a useful tracer for quantifying actually dissolvable fractions of atmospheric depositional Pb flux into seawater.The calculated soluble Pb flux (0.08 ± 0.02 nmol cm −2 yr −1 ) from the atmosphere in this study was about eight times higher than the previously estimated total Pb flux (0.019 nmol cm −2 yr −1 ) 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 con-

Conclusions
For the first time, this study quantified the210 Pb budget using the mass balance of total 210 Pb 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

Figure 1 .
Figure 1.Sampling locations (square symbols) for 210 Pb (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 210 Pb and (b) total 210 Pb [29] (from GEOSECS data in 1970s) data (triangle symbols), and (c) Pb data from the Indian Ocean.(a) Sampling locations for dissolved 210 Pb and surface activities of dissolved 210 Pb (dpm 100 L −1 ).(b) Sampling locations for total 210 Pb and surface activities of total 210 Pb.(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 2 .
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 .
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).

J 13 Figure 3 .
Figure 3. Vertical profiles of (a) dissolved 210 Pb and (b) total 210 Pb 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 .
Figure 3. Vertical profiles of (a) dissolved 210 Pb and (b) total 210 Pb 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 4 .
Figure 4.A schematic box model accounting for the ingrowth, decay, export, and atmospheric flu of 210 Pb (dpm cm −2 yr −1 ) in the equatorial western Indian Ocean.
is the residence time of the total 210 Pb, and it was estimated to be in the rang of 28.7-40.9(average: 36.07 years) in the 0-300 m layer.The calculated residence tim about 28.7-40.9years, in this study is comparable with that in the North Pacific (54-9 years)[55,56], Southeastern Pacific (95 years)[47], and Atlantic Oceans (15-22 years) [57

Figure 4 .
Figure 4.A schematic box model accounting for the ingrowth, decay, export, and atmospheric flux of 210 Pb (dpm cm −2 yr −1 ) in the equatorial western Indian Ocean.

13 Figure 5 .
Figure 5.A schematic box model accounting for residence time of dissolved Pb, Pb inventory, and atmospheric flux of seawater-dissolved Pb (nmol cm −2 yr −1 ) in the equatorial western Indian Ocean.

Figure 5 .
Figure 5.A schematic box model accounting for residence time of dissolved Pb, Pb inventory, and atmospheric flux of seawater-dissolved Pb (nmol cm −2 yr −1 ) in the equatorial western Indian Ocean.

Table 1 .
Activities of 210 Pb in the equatorial western Indian Ocean.

Table 1 .
Activities of 210 Pb in the equatorial western Indian Ocean.

Table 2 .
Comparison of 210 Pb activities in seawater from various ocean/marginal sea regions.