Earth and Planetary Science Letters 398 (2014) 37–47

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Earth and Planetary Science Letters

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Coral-based history of lead and lead isotopes of the surfaceIndian Ocean since the mid-20th century

Jong-Mi Lee a,∗, Edward A. Boyle a, Intan Suci Nurhati b,c, Miriam Pfeiffer d,Aron J. Meltzner e, Bambang Suwargadi f

a Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USAb Center for Environmental Sensing and Modeling, Singapore–MIT Alliance for Research and Technology, Singaporec Center for Oceanography and Marine Technology, Surya University, Indonesiad Institute of Geology and Paleontology, RWTH Aachen University, Aachen, Germanye Earth Observatory of Singapore, Nanyang Technological University, Singaporef Research Centre for Geotechnology, Indonesian Institute of Sciences, Bandung, Indonesia

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 December 2013Received in revised form 19 April 2014Accepted 21 April 2014Available online xxxxEditor: G.M. Henderson

Keywords:leadlead isotopesPbPb isotopesIndian Oceancoral

Anthropogenic lead (Pb) from industrial activities has greatly altered the distribution of Pb in the present-day oceans, but no continuous temporal Pb evolution record is available for the Indian Ocean despiterapidly emerging industries around the region. Here, we present the coral-inferred annual history of Pbconcentration and isotope ratios in the surface Indian Ocean since the mid-20th century (1945–2010).We analyzed Pb in corals from the Chagos Archipelago, western Sumatra and Strait of Singapore – whichrepresent the central Indian Ocean via nearshore sites. Overall, coral Pb/Ca increased in the mid-1970s atall the sites. However, coral Pb isotope ratios evolve distinctively at each site, suggesting Pb contaminationarises from different sources in each case. The major source of Pb in the Chagos coral appears to be India’sPb emission from leaded gasoline combustion and coal burning, whereas Pb in western Sumatra seemsto be largely affected by Indonesia’s gasoline Pb emission with additional Pb inputs from other sources.Pb in the Strait of Singapore has complex sources and its isotopic composition does not reflect Pb fromleaded gasoline combustion. Higher 206Pb/207Pb and 208Pb/207Pb ratios found at this site may reflectthe contribution of Pb from coals and ores from southern China, Indonesia, and Australia, and local Pbsources in the Strait of Singapore. It is also possible that the Pb isotope ratios of Singapore seawater wereelevated through isotope exchange with natural fluvial particles considering its delta setting.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Lead (Pb) in our present-day oceans is dominated by Pb pro-duced by human activities, mainly from leaded gasoline com-bustion and high-temperature industries such as smelting, min-ing, and coal combustion. Anthropogenic Pb emissions have in-creased more than an order of magnitude above natural levelsduring the past century (Nriagu, 1979; Wolff and Peel, 1985;Shotyk et al., 1998), increasing surface ocean Pb concentrations byalmost 6-fold in the western North Atlantic (Kelly et al., 2009) andby 3-fold in the western Pacific (Inoue et al., 2006) as a conse-quence. Previous studies have shown that surface ocean Pb concen-trations and isotope ratios have varied in time and space, reflectingthe changes in the amount of inputs and sources of anthropogenic

* Corresponding author. Tel.: +1 617 253 5739; fax: +1 617 253 3388.E-mail address: jm_lee@mit.edu (J.-M. Lee).

http://dx.doi.org/10.1016/j.epsl.2014.04.0300012-821X/© 2014 Elsevier B.V. All rights reserved.

Pb (Schaule and Patterson, 1981, 1983; Flegal and Patterson, 1983;Boyle et al., 1986; Helmers, 1996; Veron et al., 1998; Wu andBoyle, 1997a; Alleman et al., 1999; Weiss et al., 2003).

Long-term monitoring of surface ocean Pb concentrations andisotope ratios thus enables us to assess the impact of anthro-pogenic Pb inputs to the ocean and relative importance of variousPb sources, which is informative for marine environmental man-agement. Moreover, by knowing the surface ocean Pb transient, wecan interpret the present distribution of Pb in the ocean interiorin a historical context, deriving information on the transport andfate of the anthropogenic Pb in the ocean. For example, Pb in thesurface North Atlantic Ocean has evolved reflecting historical Pbemissions in North America and western Europe (Schaule and Pat-terson, 1983; Boyle et al., 1986; Shen and Boyle, 1988a; Veron etal., 1998; Wu and Boyle, 1997a; Kelly et al., 2009), and this surfacePb transient has been used to trace the transport of North Amer-ican Pb to the eastern North Atlantic Ocean (Veron et al., 1994;Helmers, 1996; Hamelin et al., 1997), intrusion of anthropogenic

38 J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47

Pb to the deep North Atlantic Ocean and its southward transport(Veron et al., 1998; Alleman et al., 1999, 2001a, 2001b), and toestimate the time scales of those processes (Boyle et al., 1986;Shen and Boyle, 1988a).

However, data on surface ocean Pb is still quite limited, mak-ing it difficult to evaluate the impact of anthropogenic Pb in-puts in our global oceans. Because of the low concentrations(10−10–10−12 mol kg−1) of Pb in seawater, analysis of Pb requiresstrict control of Pb contamination during sampling and analysis aswell as advanced analytical technique and instruments with lowdetection limits (e.g., Schaule and Patterson, 1981; Bruland andCoale, 1985; Wu and Boyle, 1997b; Lohan et al., 2005; Sohrin etal., 2008). Because of this challenge, reliable measurement of Pbin seawater became available only after 1976 (Schaule and Patter-son, 1981), and even after then, most of seawater Pb data wereobtained in the North Atlantic and North Pacific Oceans wherePb pollution was most significant during the 20th century. Withthe absence of long direct monitoring, the history of the surfaceocean Pb longer than a few decades has been reconstructed fromannual-banded hermatypic corals, but coral-based Pb records existonly for the western North Atlantic (Shen and Boyle, 1987; Reuer,2002; Kelly et al., 2009), Caribbean (Dodge and Gilbert, 1984;Desenfant et al., 2006), and western Pacific (Inoue et al., 2006;Inoue and Tanimizu, 2008). Therefore, more direct observationsand coral-based reconstructions of surface ocean Pb are necessaryto access the evolving global imprint of anthropogenic Pb pollu-tion.

The Indian Ocean is one of the most Pb data-deficient areas inthe world ocean. Pb emissions in this region are expected to haveincreased over the past few decades, however, as a result of therapid development and urbanization in the countries around theIndian Ocean. Moreover, most countries around the Indian Oceanphased out leaded gasoline in the 2000s (UNEP, 2007), whereasphasing out of leaded gasoline began in the early 1970s and 1980sin most of the developed countries in North America and Europe.There were indications that increased Pb pollution causes a serioushealth problem in the countries of this region (e.g., George, 1999;Nriagu et al., 1997), and high Pb concentrations have been foundin coastal seawaters off India (Rejomon et al., 2010) as well asthe marine aerosols collected over the Indian Ocean (Chester etal., 1991; Witt et al., 2006; Balasubramanian et al., 2013). Thefirst reliable Indian Ocean seawater Pb data were published re-cently, and the results showed that surface Pb concentrations,particularly in the northern Indian Ocean, are higher than thosein the present-day Atlantic or Pacific Oceans, implying large an-thropogenic Pb inputs to the Indian Ocean (Vu and Sohrin, 2013;Echegoyen et al., submitted for publication).

The goal of this study is to reconstruct historical changes of Pbconcentration and isotope ratios of the surface Indian Ocean in re-sponse to the increased anthropogenic Pb emissions in the region.For this purpose, we used annually-banded corals collected fromthree locations: Chagos Archipelago, western Sumatra, and Straitof Singapore. We analyzed Pb/Ca and Pb isotope ratios from thesecorals and generated high-resolution 30–50 year-long histories ofPb in the surface Indian Ocean since the mid-20th century. Weidentify potential sources of Pb in the region by comparing ourcoral Pb/Ca record to historical Pb emissions from the nearby coun-tries, and also by comparing our coral Pb isotope ratios to those ofthe anthropogenic Pb sources in those countries.

2. Materials and methods

2.1. Coral collection

The first coral (core GIM; referred to as Chagos coral hereafter)was collected in February 1996 in the lagoon of Peros Banhos Atoll

Fig. 1. Sampling locations for Chagos, Western Sumatra, and Jong Island corals.

(5◦15.8′S, 71◦45.3′E), which is the NW-most atoll of the ChagosArchipelago in the central Indian Ocean (Fig. 1). The coral core wasretrieved from a Porites solida colony living at a 3-m depth in achannel between Lle Diamant and Grand Ile Mapou, where tidalcurrents allow good water exchange with the open ocean (Pfeifferet al., 2009). The coral chronology was assigned based on its an-nual density bands and the seasonal cycle of the coral Sr/Ca ratiosreported in Pfeiffer et al. (2009). The Chagos coral is relativelyshort (∼30 cm) and covers the period of 1973–1996.

The second coral (core LKP-1; referred to as Sumatra coral here-after) was collected from the northwestern coast of Simeulue Is-land (2◦51.7′N, 95◦45.8′E), which is located in the west of Sumatra,Indonesia. The coral core was collected from a Porites sp. microa-toll, which died in 2004 because of the Sumatra–Andaman earth-quake. After cutting the coral into slabs, the chronology was de-termined by counting annual density bands backwards from 2004(Meltzner et al., 2010). The coral spans the period of 1945–2004.

The third coral was collected from a Porites lutea colony at JongIsland (1◦12.9′N, 103◦47.2′E) located in the Strait of Singapore inDecember 2010. The coral has visible brown bands that coincidewith high luminescence bands when examined under UV light.Bright luminescence in corals has been linked to the incorpora-tion of terrestrial humic acids into the coral skeleton (Isdale, 1984;Scoffin et al., 1989; Lough, 2011). In the case of the Jong Is-land coral, its bright luminescent bands correspond to summerwhen there is higher accumulation of riverine organic materialsin the strait. The chronology of the Jong Island coral was deter-mined based on these bands, and the coral spans the period of1960–2010.

2.2. Coral cleaning and analysis

At MIT, coral slabs were physically cleaned with a brush anddistilled water, and then approximately 100 mg of samples werecut from each band of the corals using a diamond-coated cuttingwheel and Dremel tool. The most recent layers were not pro-cessed due to potential contamination from organic tissues. Surfacecontaminants and organic materials of the corals were removedusing a method modified from Shen and Boyle (1988b), which in-cludes crushing corals into small fragments (2–4 mm for primaryand 280–700 μm for final cleaning), and cleaning the corals withoxidant (alkaline H2O2), reductant (ammonia, hydrazine, and cit-ric acid), and strong acid (HNO3). During the cleaning procedure,crushed coral samples were divided into three subsamples, eachwith approximately 30 mg of coral.

After cleaning, corals were dissolved in high-purity HNO3. Analiquot of this solution was transferred to another vial and di-luted for Pb and Ca analysis, and the rest of the solution wasstored for Pb isotope analysis. The Pb concentration was mea-

J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47 39

sured by isotope dilution ICP-MS (VG Plasma Quad 2+) after spik-ing the sample with a 204Pb enriched spike (Oak Ridge NationalLaboratory; calibrated with a gravimetric Pb concentration stan-dard). The Ca concentration was measured by flame AAS (Perkin–Elmer 403). Pb and Ca were measured in all three subsamples,and the average standard deviation of the triplicate Pb/Ca mea-surement was ∼0.8 nmol/mol for the samples with Pb/Ca lowerthan 10 nmol/mol, and ∼1.6 nmol/mol for the samples with Pb/Cahigher than 20 nmol/mol.

The remaining aliquots of the dissolved coral solutions wereused for Pb isotope analysis. Pb isotope ratios were measured onlyin one subsample with the lowest Pb/Ca ratios among the threesubsamples, because this arguably is likely to be the least contam-inated during sample processing. Dissolved coral solutions wereevaporated to dryness on a clean hot plate in a recirculating cleanair fume hood and re-dissolved in HBr, followed by the purificationusing HBr–HCl anion exchange chromatography (Kraus and Moore,1953; Strelow, 1978). Pb stable isotopes (204Pb, 206Pb, 207Pb, and208Pb) were measured in the purified samples by multiple collectorICP-MS (Micromass/GV IsoProbe). Data processing and correctionswere performed as in Boyle et al. (2012), and the details are de-scribed in Appendix A. In this manuscript, we will mainly discuss206Pb/207Pb and 208Pb/207Pb ratios, as 206Pb/204Pb measurementwas not as precise as we desire. All the coral data, Pb/Ca and Pbisotope ratios including 206Pb/204Pb, are provided in SupplementalMaterial I.

2.3. Seawater and aerosol analysis

In order to verify the sources of Pb in corals, Pb concentra-tion and isotope ratios were measured in the seawater samplescollected at the Jong Island coral site and in the eastern SouthChina Sea (6 samples from 18–21◦N and 116–119◦W), and inthe aerosol samples collected from the roof of a building in thecampus of National University of Singapore in Singapore (see Ap-pendix B for sampling method). Seawater Pb concentrations weremeasured using single batch nitrilotriacetate (NTA) resin extrac-tion and isotope-dilution ICP-MS (Lee et al., 2011), and Pb isotoperatios were measured by multiple collector ICP-MS after concen-trating samples using double Mg(OH)2 precipitation and HBr–HClanion exchange chromatography (Boyle et al., 2012). Aerosol sam-ples were leached 1N HNO3–1.75N HCl acid mixture (Graney et al.,1995), and Pb isotope ratios were measured in the leachate afterHBr–HCl anion exchange column purification.

2.4. Regional settings

All corals used in this study lie in the tropical Indian Ocean,near the equator. The equatorial regime of the Indian Ocean lacksthe sustained equatorial easterly winds that are characteristic ofthe Atlantic and Pacific Oceans (Tomczak and Godfrey, 2003). In-stead, winds and circulation of the tropical Indian Ocean varyseasonally and interannually due to the influence of the regionalmonsoons and El Niño-Southern Oscillation (ENSO) (Godfrey, 1996;Potemra and Lukas, 1999; Schott et al., 2009). In general, bothChagos Island and the margin sites (Sumatra and Singapore) expe-rience monsoonal winds that blow from the north–northeast dur-ing boreal winter monsoon and from the south during the borealsummer monsoon (Schott et al., 2002, 2009; Tomczak and Godfrey,2003). The air in the southern Indian Ocean is relatively pristine, somost pollutants to the central Indian Ocean (Chagos region) are de-livered from South Asia during the winter monsoon, whereas theSumatra and Singapore sites would have pollutants mostly fromSoutheast Asia and possibly local sources near the sites.

2.5. Historical Pb emissions from countries around the Indian Ocean

In order to find out how major sources contribute to anthro-pogenic Pb in the Indian Ocean, we estimated Pb emission historiesfrom the countries around the Indian Ocean. In the North AtlanticOcean, the dominant source of anthropogenic Pb was found to beautomobile exhausts (from the use of leaded gasoline), and Pb/Caratios analyzed in Bermuda corals from 1940–2000 closely followthe trend of Pb emissions from leaded gasoline consumption inNorth America and Europe (Shen and Boyle, 1987; Reuer, 2002;Kelly et al., 2009). On the other hand, coal combustion was sug-gested as an important source of Pb to the western Pacific (Inoueand Tanimizu, 2008; Gallon et al., 2011), and the same may be truefor the Indian Ocean. Therefore, we estimated the amount of Pbemitted from automobile exhausts and coal burning in the coun-tries around the Indian Ocean, particularly those that have highGDP and/or are close to the sample collection area.

Historical Pb emissions from automobile exhausts were esti-mated using the following formula (Li et al., 2012):

Egasoline = Qgasoline × (Pb)gasoline × EF (1)

where Egasoline is the Pb emission from gasoline (t), and Qgasoline isthe road sector gasoline consumption (kilotons of oil). (Pb)gasoline isthe Pb content in gasoline used in corresponding years (g Pb/l ofgasoline), and it changes with time as leaded gasoline is phasedout stepwise. EF is the emission factor which is the fractionof the Pb in petrol emitted to the atmosphere, which was as-sumed to be 76% as in Li et al. (2012). For the unit conversion,0.74 kg/l was used for the density of gasoline (usually ranging0.71–0.77 kg/l). Qgasoline data for the years 1971–2009 were avail-able from World Bank database (http://data.worldbank.org), and(Pb)gasoline was taken from literature (e.g. Hilton, 2006) or as-sumed based on the environmental regulations implemented ineach country (see Supplemental Material II). If both leaded andunleaded gasoline were in use, the market share of the leadedgasoline was taken into account. If leaded gasoline was phased outon a city-scale, like in India and Indonesia, market share of theleaded gasoline was calculated based on the number of cars (In-dia) or the population (Indonesia) of the cities.

Coal is used in various industries and also domestically forcooking and heating, but most of the coal is used to generate elec-tricity (e.g. Nriagu and Pacyna, 1988). The amount of Pb emittedfrom coal burning in power plants was estimated based on thefollowing equation (Li et al., 2012):

Ecoal = Pcoal × C × (Pb)coal × EF (2)

where Ecoal is the Pb emission from coal combustion (t), Pcoal is theelectricity produced from coal sources (kWh, data available fromWorldBank database), C is the amount of coals used to generate1 kWh electricity (kg), (Pb)coal is the Pb content in coals (g Pb/kgcoal), and EF is the emission factor. C is assumed to be 0.472 kgper kWh (U.S. Energy Information Administration, www.eia.gov),although it may vary depending on coal Pb content and the ef-ficiency of the power plant. (Pb)coal has a large range (3 to over300 mg/kg) depending on the origin of coals, and 15 mg/kg wasused in this study, which is an average Pb content for the coalsfrom Australia, India, Indonesia, and China (Masto et al., 2007;Li et al., 2012; Diaz-Somoano et al., 2009). EF also varies depend-ing on the types of coal-burning stoves used at the power plants,and was assumed to be 80% in this study (Li et al., 2012). Coalcombustion devices used for other industrial purposes or domes-tic use have no lead removal devices (i.e. dust removal and fluegas desulfurization devices), and the emission efficiencies will behigher. Results of the calculations are shown in Figs. 2 and 3, andalso provided numerically in Supplement Material II.

40 J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47

Fig. 2. Historical Pb emissions (t) from vehicle gasoline combustion in the countries around the Indian Ocean. Dotted lines are for the years where data are unavailable.

Fig. 3. Historical Pb emissions (t) from coal burning for electricity production. Singapore, Sri Lanka, and most countries in the Middle East and Africa did not use coal toproduce power during the period of 1971–2008. Namibia and Tanzania began coal-based power production in the 1990s, but Pb emitted from them was below 1 t. Coalconsumed by other industrial facilities and domestic use are not included.

J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47 41

Fig. 4. Time series of Pb/Ca (F), 206Pb/207Pb (P), and 208Pb/207Pb (2) ratios in theChagos coral. Error bars of the Pb/Ca ratios represent the standard deviation of thetriplicates. Single measurement was made for Pb isotopes in the sample with thelowest Pb/Ca among the triplicates.

3. Results

3.1. Pb/Ca ratios of the corals

Pb/Ca ratios in all corals start to increase in the mid-1970s(Figs. 4–6). Chagos Pb/Ca starts to rise in 1975 (∼5 nmol/mol) un-til it reaches ∼11 nmol/mol in 1979. After a slight decrease in1980–1984, it increases drastically after 1985 with about 3.5-foldincrease between 1985 and 1994 (Fig. 4). Unusually high Pb/Capeaks appear in 1986–1987 and 1989, which are 2.5-fold and1.6-fold higher than the previous years, respectively. Such rapidchanges in the surface ocean Pb concentrations seem unlikely,however, and we have to at least consider the possibility that theymight be the result of sample contamination. In the Sumatra coral,Pb/Ca stays relatively constant at 4.3±0.3 (1σ ) nmol/mol between1951 and 1977 and starts to increase from 1978 (Fig. 5). Pb/Caslightly decreases from 6.7 to 5.0 in 1984–1993 and then increasesto ∼9 nmol/mol in 2004. Pb/Ca of the Jong Island coral is almostconstant at 23 ± 2 (1σ ) nmol/mol during the period of 1962–1974(Fig. 6). Despite the large inter-annual variability, Jong Pb/Ca gener-ally increases from 1975 to reach 45 nmol/mol in 2002–2003 andthen rapidly drops to 21 nmol/mol in 2008.

These results indicate that Pb contamination of the surfaceIndian Ocean began to occur at a significant level from themid-1970s. Among the three studied sites, the contamination ef-fect is most prominent in the Chagos coral given its rapid increasesin Pb/Ca ratios (7-fold increase between 1975 and 1994), proba-bly because of the low background of Pb at this remote location.A similar result was found in a preliminary study by Shen andBoyle (1987) for Pb/Ca ratios in a coral from Mauritius. They re-ported only 6 datapoints spanning from 1968 to 1978, and sawincreased Pb/Ca ratios in samples from 1974 to 1978. However,because of the unusually high Pb/Ca values of these two samples(∼3.5 fold higher than the samples from 1968 to 1972), the au-

Fig. 5. Time series of Pb/Ca (F), 206Pb/207Pb (P), and 208Pb/207Pb (2) ratios in thewestern Sumatra coral.

Fig. 6. Time series of Pb/Ca (F), 206Pb/207Pb (P), and 208Pb/207Pb (2) ratios in theJong Island coral. Pb isotope ratios of the seawater (e) collected at the coral growthsite are shown together.

thors also cautioned us to be aware of the possible influence ofuncleaned refractory phase contaminant Pb that was not removedduring cleaning.

The partition coefficient of Pb (D p= (Pb/Ca)coral/(Pb/Ca)seawater)for the Jong Island coral was calculated to be 3.8, using the coralPb/Ca ratio in 2009 (no data from 2010) and the Pb concentration(67.2 pmol/kg) in seawater collected near the coral site in 2010(this study). Based on this D p , the highest seawater Pb concentra-tion appeared in the Jong Island region in 2002–2003 is estimatedto be ∼120 pmol/kg. The Pb/Ca ratios of the Chagos and Suma-tra corals are lower than the Jong Island coral. However, this doesnot necessarily indicate lower surface water Pb concentrations in

42 J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47

the Chagos and Sumatra region because D p for these two coralsare unknown as no comparable seawater Pb concentration dataare available. A range of 2.4–3.7 has been reported for the D p ofPb for various genus and species of corals (Shen and Boyle, 1987;Kelly et al., 2009). The corals used in this study are all Poritesspecies, but D p can also vary within one genus (Shen and Boyle,1987).

3.2. Pb isotope ratios of the corals

Pb isotope ratios found in the Jong Island coral in 2009 (thereare no data from 2010) agrees with the Pb isotope ratios of theseawater collected at the coral site in 2010 (206Pb/207Pb = 1.190and 208Pb/207Pb = 2.471, this study). Thus, the Pb isotope vari-ation in the coral records the changes of the Pb isotope ratio inambient seawater. No seawater data is available for Chagos andwestern Sumatra regions for similar assessment. The Pb isotope ra-tios in three corals appear different from each other, implying thatthe sampling locations were influenced by different sources of Pb,and the isotope composition of the Pb affecting each region haschanged over time.

Pb isotope ratios of the Chagos coral are lower than those in theSumatra and Jong corals, ranging from 1.138–1.154 for 206Pb/207Pband 2.414–2.429 for 208Pb/207Pb (Fig. 4). Both isotope ratios de-crease from 1973 to 1985, and increase after 1985 to ∼1.153for 206Pb/207Pb and 2.415 for 208Pb/207Pb in 1990 and retain theslightly higher isotope ratios until 1994. This change in the Pb iso-tope ratios around 1985 coincides with the rapid increase in Pb/Caof the Chagos coral after 1985. This indicates that the main sourceof anthropogenic Pb affecting this region changed around 1985,from sources with low isotope ratios to the ones with higher iso-tope ratios.

In the western Sumatra coral, rather constant isotope ratiosare found between 1950 and 1973, 206Pb/207Pb around 1.18 and208Pb/207Pb around 2.47 (Fig. 5). After 1973, both isotope ratiosgenerally decrease to 1.147 (206Pb/207Pb) and 2.440 (208Pb/207Pb)in 2004, indicating that the Pb polluting the Sumatra region hadlow isotope ratios.

Pb isotope ratios in the Jong Island coral show high variabil-ity like its Pb/Ca ratios (Fig. 6). The Pb isotope ratios are rel-atively low in the 1960s and the early 1970s, with 206Pb/207Pb= 1.179 ± 0.003 (1σ ) and 208Pb/207Pb = 2.464 ± 0.003 (avg.1962–1974). Then, both isotope ratios generally increase to highervalues (206Pb/207Pb = 1.192 ± 0.005 and 208Pb/207Pb = 2.477 ±0.005 in 1995–2009), indicating additional Pb sources with highisotopic ratios. The highest 206Pb/207Pb and 208Pb/207Pb ratios ap-pear in 1997, and the second highest ratios appear in 2002–2003,corresponding to the peak in the Jong coral Pb/Ca.

4. Discussion

4.1. Pb in Chagos coral and its potential sources

Chagos coral Pb/Ca ratios increase rapidly from 1985. Given theprevailing wind direction, it is most likely that anthropogenic Pbin this area originated from South Asia. The largest Pb-emittingcountry in South Asia is India (Figs. 2 and 3), which has thelargest population and GDP in South Asia. The number of vehiclesin India started to increase from the early 1980s, and Pb emis-sions from the automobile exhausts also increased rapidly fromthen (Fig. 2). In addition, India is burning millions of tons of coalannually to supply its abundant power plants and heavy metalindustries (Lelieveld et al., 2001; Ramanathan et al., 2001). Thecoal-based power generation in India has grown much over thepast three decades, and Pb emissions from this source are compa-rable to those from leaded gasoline exhausts in India (Fig. 3).

Fig. 7. (a) Total Pb emissions (gasoline and coal combustion) from India in com-parison with the Chagos coral Pb/Ca ratios, when 0.5 and 4 years of lag-times areassumed between the Pb emission and the coral-Pb calcification. Unusually highcoral Pb/Ca in 1986, 1987, and 1988 (open E) may be the result of sample contam-ination. (b) R-squared values from the linear regression between India’s Pb emissionand the Chagos coral Pb data when a 0–6 year of lag time is assumed, for all data(1) and for the data excluding the years 1986, 1987, and 1988 (2).

Chagos coral Pb/Ca ratios are well correlated (r2 = 0.89) withthe total Pb emissions (sum of vehicle Pb emissions and coal Pbemissions) from India when a 0.5-year lag time is assumed be-tween the Pb emission and the coral Pb/Ca (Fig. 7). Excluding thethree datapoints (from 1986, 1987, and 1989) that might possiblyrepresent sample contamination, a higher correlation (r2 = 0.93) isachieved between the two data when a 3.5–4 year lag time is as-sumed. The residence time of dissolved Pb has been estimated tobe a few years in the oligotrophic surface ocean based on 210Pbstudies (e.g., ∼2 years in the Atlantic and Pacific Oceans; Bacon etal., 1976; Nozaki et al., 1976). Thus, it is reasonable to expect thatany changes in the atmospheric Pb flux (e.g., changes in India’sPb emissions) would have observable consequences in the upperocean (as observed in the Chagos coral) with a few years of de-lay, although the residence time of Pb is unknown for the centralIndian Ocean yet. Thus, the increase in the Chagos coral Pb/Ca ap-pears to be mostly affected by Pb emissions from India’s gasolineand coal combustion. There could be minor contribution of the Pbfrom India’s other sources or from other countries, which may ex-plain the small differences between India’s gasoline and coal Pbemissions and the coral record, e.g., the abrupt increase in thecoral Pb/Ca around 1985 compared to the increase in India’s Pbemissions in 1980–1983.

Pb isotope ratios in the Chagos coral (Fig. 8) are in the samerange as the Pb in aerosols collected from India in 1995 (Bollhöferand Rosman, 2001), but 206Pb/207Pb is higher (and 208Pb/206Pb islower) than the urban aerosols from Sri Lanka, Thailand, Indone-sia and Australia in the 1990s (Mukai et al., 1993; Bollhöfer andRosman, 2000, 2001), supporting the inference that the Pb emittedfrom India is the largest contributor to the Pb in Chagos region.The Chagos coral Pb isotope ratios appear close to the mixing

J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47 43

Fig. 8. Triple isotope plot for the Pb in the Chagos and Sumatra corals in comparisonwith various Pb ores (Sangster et al., 2000; Inoue and Tanimizu, 2008) and coals(Diaz-Somoano et al., 2009) and the aerosols collected from the countries in Southand Southeast Asia (Bollhöfer and Rosman, 2000, 2001; Mukai et al., 1993). Blackcircles represent typical Australian (Broken Hill) type and US (Mississippi Vally) typePb ores used in alkyl lead (Cooper et al., 1969; Doe, 1970; Cummings and Ricards,1975).

line of Broken Hill (Australia) type Pb ores and Mississippi Val-ley (US) type Pb ores (Fig. 8), which are the main Pb ores thatwere used by the world’s two largest alkyl-lead suppliers, Associ-ated Octel (Broken Hill type Pb) and Ethyl Corp. (Mississippi Valleytype Pb) (Bollhöfer and Rosman, 2000). This implies that leadedgasoline combustion is most likely the major source of the Pbin this region. However, the Chagos coral Pb has slightly higher206Pb/207Pb (lower 208Pb/206Pb) than this mixing line and the Pbin Indian aerosols, which may reflect the input of coal-derived Pb,because coals generally have high 206Pb/207Pb (low 208Pb/206Pb)(Diaz-Somoano et al., 2009). The Pb isotopic signatures of Indiancoals are unknown. However, coals imported by India between1988 and 2000 were mainly from Australia (69%) and Indonesia(10%) (COMTRADE, 2009), and mixing with the Pb from these coalscan increase the 206Pb/207Pb and 208Pb/206Pb in the observed di-rection (Fig. 8).

The drastic change in the Chagos coral Pb isotope ratios around1985 (Fig. 4) could possibly be caused by increased use of thealkyl-lead supplied by Ethyl Corp. after 1985 compared to thosesupplied by Associate Octel, but we need further information onthe market share of these companies in India during that period toverify this possibility. The increased 206Pb/207Pb and 208Pb/207Pbafter 1985 could be also due to the increased coal burning inIndia (and other countries), as coals have high 206Pb/207Pb and208Pb/207Pb ratios (Diaz-Somoano et al., 2009), or could be thecombined effect of both mechanisms.

Local Pb sources may need to be considered for the Chagoscoral record. On the south of the Banhos Atoll (coral site) lies thelargest island of the Chagos Archipelago, Diego Garcia. This islandcontains a large joint UK–US naval support facility with about 1500military personnel and 2000 civilian contractors, and the activi-ties of this island were suggested to cause high Polychlorinatedbiphenyls (PCBs) levels in air samples from the Chagos Archipelago(Wurl et al., 2006). The activities on the Diego Garcia may havesupplied Pb to the Chagos coral region, but we cannot assess itsimpact because the amount and the isotope signals of the Pb usedon that island are unknown. We suspect this impact would be

minimal, though, because the coral Pb record from North RockBermuda appear to be unaffected by local sources (Shen and Boyle,1987; Kelly et al., 2009), although Bermuda was inhabited by morethan 60,000 people and housing a US military base until 1995.

4.2. Pb in western Sumatra coral and its potential sources

Significant Pb contamination started in the western Sumatra re-gion around the mid-1970s given the increasing Pb/Ca ratios anddecreasing 206Pb/207Pb and 208Pb/207Pb ratios around that time(Fig. 5). Assuming that the “background Pb” of this region is theaverage of the coral Pb values in 1950–1970, when Pb/Ca andPb isotope ratios are relatively invariable, we calculated the iso-tope ratios of the anthropogenic Pb (RAnthro) that contaminated theSumatra region after the 1970s using a mixing equation (Shirahataet al., 1980):

RAnthro = (RCoral × CCoral) − (RBackground × CBackground)

CCoral − CBackground(3)

where R and C denote the Pb isotope ratio and Pb/Ca of the coraland the background Pb. The calculated RAnthro for the years af-ter the 1970 are relatively uniform around 206Pb/207Pb = 1.12 and208Pb/207Pb = 2.41 (208Pb/206Pb = 2.15). On the triple-isotope plot,these isotope ratios fall onto the range of Indian Pb ores (Sangsteret al., 2000) and Pb in aerosols found in Indonesia and Thailandin 1987–1989 (Mukai et al., 1993) and 1995–1998 (Bollhöfer andRosman, 2000) (Fig. 8), which were mainly from leaded gasolinecombustion. The use of Indian Pb ores was negligible in the coun-tries around the Sumatra region (COMTRADE, 2009). Leaded gaso-line was phased out in Thailand in the early 1990s (Hirota, 2006),whereas it was widely used until 2006 in Indonesia (Hosono etal., 2011). Thus, the dominant source of Pb affecting the westernSumatra region seems to be the leaded gasoline emissions fromIndonesia.

The Sumatra coral Pb/Ca record does not exactly correspondto the vehicle Pb emissions transient from Indonesia, however,because Indonesian Pb emissions decrease in the early 2000s,whereas coral Pb/Ca continues to increase during that period. Thedecreasing Pb emissions may not have resulted in the decrease incoral Pb/Ca yet because of a lag time between the two records as-sociated with the residence time of Pb in the waters off Sumatra. Itis also possible that the decrease of the vehicle Pb emissions waspartly compensated by Pb emissions from coal burning and otherindustrial sources, although we should note that coal Pb emissionsfrom the countries near Sumatra were almost an order of magni-tude smaller than vehicle Pb emissions (Figs. 2 and 3). The coralPb 206Pb/207Pb and 208Pb/207 ratios also did not increase to highervalues as they would if coal Pb inputs were large. The continualincrease of coral Pb/Ca in the 2000s might be also partly ascribedto the upwelling off western Sumatra. Mixing with subsurface wa-ter with higher Pb concentrations might have supplied additionalPb to the surface water, although direct Pb inputs from the atmo-sphere declined.

4.3. Pb in Jong Island coral and its potential sources

Although the data show high variability, Pb/Ca ratios of the JongIsland coral generally increase from the early 1970s to 2002 anddecrease after 2003. This overall trend can be attributable to an-thropogenic Pb inputs from the countries surrounding the Strait ofSingapore. Pb emissions (gasoline exhausts and coal combustion)from Indonesia, Malaysia, and Singapore were high in the 1980sand early 1990s and decreased in the 2000s after leaded gasolinewas phased out in these countries (Afroz et al., 2003; Hilton, 2006;Hosono et al., 2011) (Figs. 2 and 3). However, the detailed pat-tern of the coral Pb/Ca does not correspond to the Pb transient

44 J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47

Fig. 9. Triple isotope plot for the Pb in the Jong Island coral in comparison withvarious Pb ores, coals, and aerosols collected from the countries in Southeast Asia.References are same as Fig. 8, and the Singapore aerosols are from this study.

from a single source. Rather, it seems to be the result of Pb inputsfrom multiple sources from multiple countries, including industrialsources other than leaded gasoline and coal combustion.

In the triple Pb isotope plot (Fig. 9), Pb in the Jong Islandcoral falls on a straight line between 206Pb/207Pb = 1.175–1.20 and208Pb/206Pb = 2.07–2.10, consistent with a hypothesis of two end-member mixing. Pb with low 206Pb/207Pb and high 208Pb/206Pbmight be from the gasoline Pb used in the countries around theStrait of Singapore. Extrapolating the mixing line of the coral Pboverlaps with the Pb in Indonesia, Malaysia, and Thailand aerosolscollected in the early 1990s, which were dominated by gasolinePb (Bollhöfer and Rosman, 2000; Mukai et al., 1993). Relativelylow coral 206Pb/207Pb and 208Pb/207Pb in the years prior to 1995(Fig. 6) might be also attributed to the higher gasoline Pb emis-sions during that period than the late 1990s and the 2000s (Fig. 2).However, the overall impact of local gasoline Pb to the Singaporeregions is probably small, because the 206Pb/207Pb (208Pb/206Pb) ra-tios of the Jong Island coral are all much higher (lower) than thegasoline Pb-containing aerosols (Fig. 9). Pb with high isotope ra-tios could be from the coals from Indonesia and Australia and thePb ores from southern China (Fig. 9) that were used in the coun-tries around the Singapore Strait. For example, during 1990–2010,Malaysia imported 67% of its coal from Indonesia and 24% fromAustralia; Indonesia imported 37% of its coal from Australia and35% from China (COMTRADE, 2009); and Singapore imported 73%of unwrought Pb from China (UNEP, 2011).

However, a question arises on how this anthropogenic Pb wasdelivered to the surface water of the Strait of Singapore. It hasbeen known that the Pb in open ocean surface waters is mainlysupplied by atmospheric deposition (e.g. Schaule and Patterson,1981). However, in the Jong Island region, the seawater Pb isotopicsignature, which is also recorded in the local coral, does not agreewith that of the local aerosol pollutant Pb. As shown previously,the 206Pb/207Pb ratios of the aerosols collected from Malaysia col-lected in 1995, Indonesia in 1989 (Mukai et al., 1993) and 1995(Bollhöfer and Rosman, 2000), and Singapore in 2011–2012 (thisstudy, see Supplemental Material III), are all considerably lower(208Pb/207Pb ratios are higher) than the Jong Island coral Pb iso-tope ratios for the corresponding years (Fig. 9).

There is a possibility that aerosols used in this study were dom-inated by local sources and do not represent the overall isotoperatios of the Pb emitted from the region. For example, Mukai et al.(1993) show that Pb isotope ratios can vary significantly depend-ing on collection site in some Asian countries. Pb isotope ratiosin aerosols collected from a site with heavy traffic (206Pb/207Pb =1.14) were different from those from a site with low traffic andother industries (206Pb/207Pb = 1.10), although the two sites wereonly ∼10 km apart. It is less likely that the aerosol samples usedin this study were all influenced by such localized inputs. However,aerosol sampling covering a wider area, particularly the main in-dustrial areas, is necessary before concluding that aerosols are notthe major source of Pb in the Jong Island region.

If not due to local atmospheric deposition, Pb in the Straitof Singapore might have been advectively transported. The Straitof Singapore is a mixture of waters from the South China Sea,Java Sea, and eastern Indian Ocean. Pb isotope ratios of the sur-face waters in the eastern South China Sea collected in 2000 are206Pb/207Pb = 1.156 ± 0.015, 208Pb/207Pb = 2.444 ± 0.017 (thisstudy, Supplemental Material III). No seawater data is available forthe Java Sea, but a coral sample collected off Jakarta Bay, whoseage is from 1998–2001, had Pb isotope ratios of 206Pb/207Pb =1.158 and 208Pb/207Pb = 2.430 (Inoue et al., 2006). Although wedon’t have direct information for the eastern Indian Ocean seawa-ter, surface seawater Pb isotope ratios in the open ocean sectionof the Bay of Bengal (8◦31′N, 86◦01′E, collected in 2010) were206Pb/207Pb = 1.148, 208Pb/207Pb = 2.431 (Lee, 2013). The Pb iso-tope ratios of these waters are too low to be the source of Pb inthe Strait of Singapore.

One possible process of changing the Pb isotope ratios in thewaters of the Strait of Singapore is through isotope exchange be-tween seawater and natural fluvial particles. Most of the dissolved210Pb in rivers is scavenged in estuaries (e.g. Turekian, 1977). How-ever, reversible exchange equilibrium may occur between the Pbisotopes in riverine particles and seawaters, delivering the river-ine Pb isotopic signals to the ocean without increasing seawaterPb concentrations. This process has been suggested to occur for Srand Nd isotopes (Oelkers et al., 2011, 2012; Jones et al., 2012a,2012b), and it is also consistent with the similar Pb isotope val-ues found in dissolved and suspended particulate Pb in the watercolumn near Bermuda (Sherrell and Boyle, 1992). Many rivers runinto the Malacca Strait and southern South China Sea, or directlyinto the Strait of Singapore (e.g. the Johor River). Sediment dis-charges from the rivers in this region are particularly large becauseof mountainous terrain, erodible strata, and seasonally heavy rain-fall (Douglas, 1996; Milliman and Meade, 1983; Milliman et al.,1987). For example, sediment discharge from Sumatra, Java, andBorneo (2.02 × 109 t/yr) accounts for 10–12% of the total sedi-ment discharge to the global ocean (Milliman et al., 1999). Little isknown about the Pb isotope ratios of the riverine particles of thisregion, but sediment samples and K-feldspar grains in the rivers ofthe Indochina Peninsula have been found to have high 206Pb/207Pb(>1.18) and 208Pb/207Pb ratios (>2.47) (Bodet and Schärer, 2001;Millot et al., 2004). Thus, if isotopic exchange occurs with theseparticles, 206Pb/207Pb and 208Pb/207Pb ratios in the seawater wouldincrease to higher values, as observed in the Jong Island coral.Seawaters and rivers (waters and particles) close to the Strait ofSingapore need to be further examined for Pb isotope ratios in or-der to prove this hypothesis.

There also could be sources of Pb that are directly injected tothe Jong Island region. For example, scrapings from boat paintsthat leach out from transport vessels were suggested as a causeof high Cu and Zn found in the sediments of this region (Gohand Chou, 1997), and they may also release Pb to the seawater.The Strait of Singapore is one of the busiest shipping routes in the

J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47 45

world, and Jong Island is in the vicinity (within 5 km) of the Portof Singapore.

5. Conclusions

This study provides the first data on the evolution of Pb con-centrations and isotope ratios of the Indian Ocean over the past30–50 years, where such data has been unavailable in spite of in-creasing anthropogenic Pb emissions in the region. As expected,unlike the North Atlantic Ocean, where surface Pb concentrationsdecreased by almost 10 fold after the 1970s (Kelly et al., 2009),Pb levels in the surface Indian Ocean began to increase from themid-1970s (all sites), continuously until the early 2000s (Sumatraand Singapore sites). Pb in the Chagos coral reflects dominance ofIndia’s industrial (gasoline and coal) Pb in this region. The Suma-tra coral record reflects the impact of the gasoline-Pb emissionsfrom Indonesia, with some inputs from unidentified sources. Pb inthe Singapore Strait Jong Island coral appears to be affected by Pbemitted from nearby countries, although the major sources couldnot be identified clearly in this study. In the Jong Island region, Pbcould be mainly supplied by advection rather than aerosol depo-sition, possibly with an increased Pb isotope ratios resulted fromthe reversible isotope exchange with fluvial particles. Further stud-ies on the Pb in other industrial sources, aerosols, and rivers arerequired to identify sources and pathways of the Pb in this region.

Among the three corals studied, historical changes in the Pband Pb isotope ratios of the surface Indian Ocean would prob-ably be best represented by the Chagos coral given its location.The features in the surface Pb transient revealed from the Cha-gos coral, such as a rapid increase in Pb/Ca and drastic changesin Pb isotope ratios around 1985, provide an important markerfor estimating the age of Pb in the Indian Ocean water column.Continuous monitoring of surface seawater and analysis of a coralwith a longer time span would provide more information on thechanges of the surface ocean Pb that resulted from the phasing outof leaded gasoline and the increased use of industrial Pb.

Acknowledgements

We thank Kerry Sieh for arranging for the Sumatra coral col-laboration, Jani Tanzil for assistance in Jong Island coral collection,and Rick Kayser and Jessica Fitzsimmons for ICP-MS maintenance.The work on the Sumatra coral is Earth Observatory of Singaporecontribution 65. We also thank Steve Galer and two anonymousreviewers for their constructive criticism. This research was sup-ported by the Singapore National Research Foundation through agrant to CENSAM/SMART.

Appendix A. Pb isotope analysis

Coral, seawater, and aerosol samples that were purified by HCl–HBr anion exchange columns were dried on a hot plate in a re-circulating fume hood in a positive-pressure clean lab. On theday of the ICP-MS run, dried samples were re-dissolved in dilute(0.2 M) nitric acid. After spiking the samples with Tl, stable Pbisotopes (204Pb, 206Pb, 207Pb, and 208Pb) in the samples were de-termined by multiple collector ICP-MS (Micromass/GV IsoProbe).204Pb was measured with an ion-counting Daly detector with theWARP (Wide Aperture Retarding Potential) filter, with calibrationfor deadtime (50 ns) and ion-counting efficiency, whereas othermasses were measured on Faraday cups. Data processing and cor-rections were performed as in Boyle et al. (2012), which is themethod modified from Reuer et al. (2003). The method includes:(1) elimination of the isobaric interferences of 204Hg, (2) expo-nential “beta” mass fractionation correction normalized with a205Tl/203Tl spike, (3) a tailing correction derived from a curve fit

to 209Bi at half-mass intervals, and (4) on-peak-zero correctionsfor instrumental hardware blanks using nitric acid (for Tl isotopes)and Tl-spiked nitric acid (for Hg and Pb isotopes). Within-day Dalycounter efficiency variations were monitored by measuring NBSSRM-981 and an independent internal lab standard every 3–5 sam-ples, as well as the beginning and end of each session. The Pbisotope ratios were finally re-normalized to the estimated absolutevalues reported for NBS SRM-981 (Galer and Abouchami, 1998;Thirlwall, 2000; Baker et al., 2004) to correct for minor differencesin Faraday cup efficiencies and isotope fractionation that is not ac-counted for by Tl correction.

External reproducibility of a Pb standard that has Pb signal in-tensities comparable to samples was 0.2 per mil for 206Pb/207Pband 0.3 per mil for 208Pb/207Pb (1σ , n = 51, measured on 4 dif-ferent days). No replicate measurement was made for the samples,but reproducibility of the samples should be on the same magni-tude as those of the standard, although samples with lower signalintensities may have higher errors due to the uncertainties in theblank correction. The 206Pb/204Pb was not as precise as 206Pb/207Pband 208Pb/207Pb, e.g., external reproducibility of a standard mea-surement was 66 per mil for 206Pb/204Pb ratios (1σ , n = 12).

Appendix B. Seawater and aerosol sampling

Seawater samples were collected using a “pole sampler” (sam-ple bottle mounted in clean plastic holder at the end of a pole)or MITESS “ATE” sampler (Bell et al., 2002). Samples were filteredthrough 0.4 μm Nuclepore® filters and acidified with 4× vycor-distilled 6N HCl to pH 2 upon retrieval.

Aerosol samples were collected using a method modified fromBollhöfer et al. (1999). The aerosols were collected on PTFE fil-ters (37 mm 0.45 μm front and 30–60 μm back PTFE filters,both from Savillex) fitted to the plastic monitors (Millipore Cat.#M000-O37AO) and suctioned via flexible tubing connected to anelectric diaphragm pump. A total of 18 samples were collectedover a period of 10 months, each with 2 days to 4 weeks-longsampling. The PTFE filters were cleaned by first leaching in ethanolfor ∼6 h at room temperature to remove possible organic lead con-tamination, rinsing with distilled water, and leaching in 1M HCl foranother day at 60 ◦C. After rinsing the filters five times with dis-tilled water, they were left to dry under a HEPA filtered air streamfor one day prior to mounting into the monitors.

Appendix C. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.04.030.

References

Afroz, R., Hassan, M.N., Ibrahim, N.A., 2003. Review of air pollution and health im-pacts in Malaysia. Environ. Res. 92 (2), 71–77.

Alleman, L.Y., Veron, A.J., Church, T.M., Flegal, A.R., Hamelin, B., 1999. Invasion ofthe abyssal North Atlantic by modern anthropogenic lead. Geophys. Res. Lett. 26(10), 1477–1480.

Alleman, L.Y., Church, T.M., Ganguli, P., Veron, A.J., Hamelin, B., Flegal, A.R., 2001a.Role of oceanic circulation on contaminant lead distribution in the South At-lantic. Deep-Sea Res., Part II 48 (13), 2855–2876.

Alleman, L.Y., Church, T.M., Veron, A.J., Kim, G., Hamelin, B., Flegal, A.R., 2001b.Isotopic evidence of contaminant lead in the South Atlantic troposphere andsurface waters. Deep-Sea Res., Part II 48 (13), 2811–2827.

Bacon, M.P., Spencer, D.W., Brewer, P.G., 1976. Pb-210/Ra-226 and Po-210/Pb-210disequilibria in seawater and suspended particulate matter. Earth Planet. Sci.Lett. 32, 277–296.

Baker, J., Peate, D., Waight, T., Meyzen, C., 2004. Pb isotopic analysis of standardsand samples using a Pb-207–Pb-204 double spike and thallium to correct formass bias with a double-focusing MC-ICP-MS. Chem. Geol. 211 (3–4), 275–303.

Balasubramanian, R., Karthikeyan, S., Potter, J., Wurl, O., Durville, C., 2013. Chemicalcharacterization of aerosols in the equatorial atmosphere over the Indian Ocean.Atmos. Environ. 78, 268–276.

46 J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47

Bell, J., Betts, J., Boyle, E.A., 2002. MITESS: a moored in situ trace element serialsampler for deep-sea moorings. Deep-Sea Res., Part I 49, 2103–2118.

Bodet, F., Schärer, U., 2001. Pb isotope systematics and time-integrated Th/U ofSE-Asian continental crust recorded by single K-feldspar grains in large rivers.Chem. Geol. 177 (3–4), 265–285.

Bollhöfer, A., Rosman, K.J.R., 2000. Isotopic source signatures for atmospheric lead:the Southern Hemisphere. Geochim. Cosmochim. Acta 64 (19), 3251–3262.

Bollhöfer, A., Rosman, K.J.R., 2001. Isotopic source signatures for atmospheric lead:the Northern Hemisphere. Geochim. Cosmochim. Acta 65 (11), 1727–1740.

Bollhöfer, A., Chisholm, W., Rosman, K.J.R., 1999. Sampling aerosols for lead isotopeson a global scale. Anal. Chim. Acta 390, 227–235.

Boyle, E.A., Chapnick, S.D., Shen, G.T., Bacon, M.P., 1986. Temporal variability of leadin the Western North-Atlantic. J. Geophys. Res., Oceans 91 (C7), 8573–8593.

Boyle, E.A., John, S., Abouchami, W., et al., 2012. GEOTRACES IC1(BATS)contamination-prone trace element isotopes Cd, Fe, Pb, Zn, Cu, and Mo Inter-calibration. Limnol. Oceanogr., Methods 10, 653–665.

Bruland, K.W., Coale, K.H., 1985. Analysis of seawater for dissolved cadmium, copper,and lead: an intercomparison of voltammetric and atomic absorption methods.Mar. Chem. 17, 285–300.

Chester, R., Berry, A.S., Murphy, K.J.T., 1991. The distributions of particulate atmo-spheric trace metals and mineral aerosols over the Indian Ocean. Mar. Chem. 34,251–290.

COMTRADE (Commodity Trade Statistics Database), 2009. United Nations StatisticsDivision. Available from: http://comtrade.un.org/db/default.aspx.

Cooper, J.A., Reynolds, P.H., Richards, J.R., 1969. Double-spike calibration of the Bro-ken Hill standard lead. Earth Planet. Sci. Lett. 6, 467–478.

Cummings, G.L., Ricards, J.R., 1975. Ore lead isotope ratios in a continuously chang-ing earth. Earth Planet. Sci. Lett. 28, 155–171.

Desenfant, F., Veron, A.J., Camoin, G.F., Nyberg, J., 2006. Reconstruction of pollutantlead invasion into the tropical North Atlantic during the twentieth century. CoralReefs 25 (3), 473–484.

Diaz-Somoano, M., Kylander, M.E., López-Antón, M.A., Suárez-Ruiz, I., Martínez-Tarazona, M.R., Ferrat, M., Kober, B., Weiss, D.J., 2009. Stable lead isotope com-positions in selected coals from around the world and implications for presentday aerosol source tracing. Environ. Sci. Technol. 43 (4), 1078–1085.

Dodge, R.E., Gilbert, T.R., 1984. Chronology of lead pollution contained in bandedcoral skeletons. Mar. Biol. 82 (1), 9–13.

Doe, B.R., 1970. Lead Isotopes. Springer Verlag, Berlin, Heidelberg, New York.Douglas, I., 1996. The impact of land-use changes, especially logging, shifting culti-

vation, mining and urbanization on sediment yields in humid tropical SoutheastAsia: a review with special reference to Borneo. In: Proceedings of the ExeterSymposium. July. In: IAHS Publ., vol. 236, pp. 463–471.

Echegoyen, Y., Boyle, E.A., Lee, J.M., Gamo, T., Obara, H., Norisuye K., submitted forpublication. Recent distribution of lead in the Indian Ocean – the impact ofregional anthropogenic activities.

Flegal, A.R., Patterson, C.C., 1983. Vertical concentration profiles of lead in the Cen-tral Pacific at 15N and 20S. Earth Planet. Sci. Lett. 64, 19–32.

Galer, S.J.G., Abouchami, W., 1998. Practical application of lead triple spiking forcorrection of instrumental mass discrimination. Mineral. Mag. A 62, 491–492.

Gallon, C., Ranville, M.A., Conaway, C.H., Landing, W.M., Buck, C.S., Morton, P.L., Fle-gal, A.R., 2011. Asian industrial lead inputs to the North Pacific evidenced bylead concentration and isotopic compositions in surface waters and aerosols.Environ. Sci. Technol. 45 (23), 9874–9882.

George, A.M., 1999. The George Foundation Project lead free: a study of lead poi-soning in major cities. In: International Conference on Lead Poisoing Preventionand Treatment, Bangalore, India, February 8-10..

Godfrey, J.S., 1996. The effect of the Indonesian throughflow on ocean circulationand heat exchange with the atmosphere: a review. J. Geophys. Res., Oceans 101(C5), 12217–12237.

Goh, B.P.L., Chou, L.M., 1997. Heavy metal levels in marine sediments of Singapore.Environ. Monit. Assess. 44 (1–3), 67–80.

Graney, J.R., Halliday, A.N., Keeler, G.J., Nriagu, J.O., Robbins, J.A., Norton, S.A., 1995.Isotopic record of lead pollution in lake sediments from the northeastern UnitedStates. Geochim. Cosmochim. Acta 54 (9), 1715–1728.

Hamelin, B., Ferrand, J.L., Alleman, L., Nicolas, E., Veron, A., 1997. Isotopic evidenceof pollutant lead transport from North America to the subtropical North Atlanticgyre. Geochim. Cosmochim. Acta 61 (20), 4423–4428.

Helmers, E., 1996. Trace metals in suspended particulate matter of Atlantic Oceansurface water (40 degrees N to 20 degrees S). Mar. Chem. 53 (1–2), 51–67.

Hilton, F.G., 2006. Poverty and pollution abatement: evidence from lead phase-out.Ecol. Econ. 56, 125–131.

Hirota, K., 2006. Review of lead phase out for air quality improvement in the ThirdWorld Cities. J. Stud. Reg. Sci. 36 (2), 527–541.

Hosono, T., Su, C.C., Delinom, R., Umezawa, Y., Toyota, T., Kaneko, S., Taniguchi, M.,2011. Decline in heavy metal contamination in marine sediments in JakartaBay, Indonesia due to increasing environmental regulations. Estuar. Coast. ShelfSci. 92 (2), 297–306.

Inoue, M., Tanimizu, M., 2008. Anthropogenic lead inputs to the western Pacific dur-ing the 20th century. Sci. Total Environ. 406 (1–2), 123–130.

Inoue, M., Hata, A., Suzuki, A., Nohara, M., Shikazono, N., Yim, W.W.S., Hantoro, W.S.,Sun, D.H., Kawahata, H., 2006. Distribution and temporal changes of lead in thesurface seawater in the western Pacific and adjacent seas derived from coralskeletons. Environ. Pollut. 144 (3), 1045–1052.

Isdale, P., 1984. Fluorescent bands in massive corals record centuries of Coastal Rain-fall. Nature 310 (5978), 578–579.

Jones, M.T., Pearce, C.R., Oelkers, E.H., 2012a. An experimental study of the interac-tion of basaltic riverine particulate material and seawater. Geochim. Cosmochim.Acta 77, 108–120.

Jones, M.T., Pearce, C.R., Jeandel, C., Gislason, S.R., Eiriksdottir, E.S., Mavromatis, V.,Oelkers, E.H., 2012b. Riverine particulate material dissolution as a significantflux of strontium to the oceans. Earth Planet. Sci. Lett. 355–356, 51–59.

Kelly, A.E., Reuer, M.K., Goodkin, N.F., Boyle, E.A., 2009. Lead concentrations and iso-topes in corals and water near Bermuda, 1780–2000. Earth Planet. Sci. Lett. 283(1–4), 93–100.

Kraus, K.A., Moore, G.E., 1953. Anion exchange studies 6. The divalent transitionelements manganese to zinc in hydrochloric acid. J. Am. Chem. Soc. 75 (6),1460–1462.

Lee, J.-M., 2013. Evolution of anthropogenic Pb and Pb isotopes in the deep NorthAtlantic Ocean and the Indian Ocean. Ph.D. thesis. Massachusetts Institute ofTechnology.

Lee, J.-M., Boyle, E.A., Echegoyen-Sanz, Y., Fitzsimmons, J.N., Zhang, R., Kayser, R.A.,2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batchnitrilotriacetate resin extraction and isotope dilution inductively coupled plasmamass spectrometry. Anal. Chim. Acta 686, 93–101.

Lelieveld, J., Crutzen, P.J., Ramanathan, V., et al., 2001. The Indian Ocean experiment:widespread air pollution from South and Southeast Asia. Science 291 (5506),1031–1036.

Li, Q., Cheng, H.G., Zhou, T., Lin, C.Y., Guo, S., 2012. The estimated atmospheric leademissions in China, 1990–2009. Atmos. Environ. 60, 1–8.

Lohan, M.C., Aguilar-Islas, A.M., Franks, R.P., Bruland, K.W., 2005. Determination ofiron and copper in seawater at pH 1.7 with a new commercially available chelat-ing resin, NTA Superflow. Anal. Chim. Acta 530, 121–129.

Lough, J.M., 2011. Measured coral luminescence as a freshwater proxy: comparisonwith visual indices and a potential age artifact. Coral Reefs 30 (1), 169–182.

Masto, R.E., Ram, L.C., Selvi, V.A., Jha, S.K., Srivastava, N.K., 2007. Soil contamina-tion and human health risks in coal mining environs. In: Proceedings of the1st International Conference on Managing the Social and Environmental Conse-quences of Coal Mining in India. New Delhi, India.

Meltzner, A.J., Sieh, K., Chiang, H.W., Shen, C.C., Suwargadi, B.W., Natawidjaja, D.H.,Philibosian, B.E., Briggs, R.W., Galetzka, J., 2010. Coral evidence for earthquakerecurrence and an A.D. 1390–1455 cluster at the south end of the 2004 Aceh–Andaman rupture. J. Geophys. Res., Solid Earth 115, B10402. http://dx.doi.org/10.1029/2010JB007499.

Milliman, J.D., Meade, R.H., 1983. World-wide delivery of river sediment to theoceans. J. Geol. 91 (1), 1–21.

Milliman, J.D., Qin, Y.S., Ren, M.E., Saito, Y., 1987. Mans influence on the erosion andtransport of sediment by Asian Rivers – the Yellow-River (Huanghe) example.J. Geol. 95 (6), 751–762.

Milliman, J.D., Farnsworth, K.L., Albertin, C.S., 1999. Flux and fate of fluvial sedi-ments leaving large islands in the East Indies. J. Sea Res. 41 (1–2), 97–107.

Millot, R., Allegre, C.-J., Gaillardet, J., Roy, S., 2004. Lead isotopic systematic of majorriver sediments: a new estimate of the Pb isotopic composition of the UpperContinental Crust. Chem. Geol. 203, 75–90.

Mukai, H., Furuta, N., Fujii, T., Ambe, Y., Sakamoto, K., Hashimoto, Y., 1993. Charac-terization of sources of lead in the urban air of asia using ratios of stable leadisotopes. Environ. Sci. Technol. 27 (7), 1347–1356.

Nozaki, Y., Thomson, J., Turekian, K.K., 1976. The distribution of Pb-210 and Po-210in the surface waters of the Pacific Ocean. Earth Planet. Sci. Lett. 32, 304–312.

Nriagu, J.O., 1979. Global inventory of natural and anthropogenic emissions of tracemetals to the atmosphere. Nature 279, 409–411.

Nriagu, J.O., Pacyna, J.M., 1988. Quantitative assessment of worldwide contaminationof air, water and soils by trace-metals. Nature 333 (6169), 134–139.

Nriagu, J., Jinabhai, C.C., Naidoo, R., Coutsoudis, A., 1997. Lead poisoning of childrenin Africa, II. Kwazulu/Natal, South Africa. Sci. Total Environ. 197, 1–11.

Oelkers, E.H., Gislason, S.R., Eiriksdottir, E.S., Jones, M., Pearce, C.R., Jeandel, C., 2011.The role of riverine particulate material on the global cycles of the elements.Appl. Geochem. 26, S365–S369.

Oelkers, E.H., Jones, M.T., Pearce, C.R., Jeandel, C., Eiriksdottir, E.S., Gislason, S.R.,2012. Riverine particulate material dissolution in seawater and its implicationsfor the global cycles of the elements. C. R. Géosci. 344, 646–651.

Pfeiffer, M., Dullo, W.C., Zinke, J., Garbe-Schonberg, D., 2009. Three monthly coralSr/Ca records from the Chagos Archipelago covering the period of 1950–1995AD: reproducibility and implications for quantitative reconstructions of sea sur-face temperature variations. Int. J. Earth Sci. 98 (1), 53–66.

Potemra, J.T., Lukas, R., 1999. Seasonal to interannual modes of sea level variabilityin the western Pacific and eastern Indian Oceans. Geophys. Res. Lett. 26 (3),365–368.

Ramanathan, V., Crutzen, P.J., Lelieveld, J., et al., 2001. Indian Ocean experiment: anintegrated analysis of the climate forcing and effects of the great Indo-Asianhaze. J. Geophys. Res., Atmos. 106 (D22), 28371–28398.

J.-M. Lee et al. / Earth and Planetary Science Letters 398 (2014) 37–47 47

Rejomon, G., Kumar, P.K.D., Bahulayan, N., 2010. Biogeochemistry of lead in theEastern Arabian Sea and Western Bay of Bengal. Environ. Forensics 11 (3),223–236.

Reuer, M.K., 2002. Centennial-scale elemental and isotopic variability in the trop-ical and subtropical North Atlantic Ocean. Ph.D. thesis. Massachusetts Instituteof Technology.

Reuer, M.K., Boyle, E.A., Grant, B.C., 2003. Lead isotope analysis of marine carbonatesand seawater by multiple collector ICP-MS. Chem. Geol. 200 (1–2), 137–153.

Sangster, D.F., Outridge, P.M., Davis, W.J., 2000. Stable lead isotope characteristics oflead ore deposits of environmental significance. Environ. Rev. 8, 115–147.

Schaule, B.K., Patterson, C.C., 1981. Lead concentrations in the Northeast Pacific –evidence for global anthropogenic perturbations. Earth Planet. Sci. Lett. 54 (1),97–116.

Schaule, B.K., Patterson, C.C., 1983. Perturbations of the natural lead profile in theSargasso Sea by industrial lead. In: Wong, C.S. (Ed.), Trace Metals in Sea Water.Plenum, New York, pp. 487–504.

Schott, F.A., Dengler, M., Schoenefeldt, R., 2002. The shallow overturning circulationof the Indian Ocean. Prog. Oceanogr. 53 (1), 57–103.

Schott, F.A., Xie, S.P., McCreary, J.P., 2009. Indian Ocean circulation and climatevariability. Rev. Geophys. 47 (1). http://dx.doi.org/10.1029/2007RG000245.

Scoffin, T.P., Tudhope, A.W., Brown, B.E., 1989. Fluorescent and skeletal densitybanding in porites-lutea from Papua New-Guinea and Indonesia. Coral Reefs 7(4), 169–178.

Shen, G.T., Boyle, E.A., 1987. Lead in corals – reconstruction of historical industrialfluxes to the Surface Ocean. Earth Planet. Sci. Lett. 82 (3–4), 289–304.

Shen, G.T., Boyle, E.A., 1988a. Thermocline ventilation of anthropogenic lead in theWestern North-Atlantic. J. Geophys. Res., Oceans 93 (C12), 15715–15732.

Shen, G.T., Boyle, E.A., 1988b. Determination of lead, cadmium and other trace-metals in annually-banded corals. Chem. Geol. 67 (1–2), 47–62.

Sherrell, R.M., Boyle, E.A., 1992. Isotopic equilibration between dissolved and sus-pended particulate lead in the Atlantic-Ocean – evidence from Pb-210 andstable Pb isotopes. J. Geophys. Res., Oceans 97 (C7), 11257–11268.

Shirahata, H., Elias, R.W., Patterson, C.C., Koide, M., 1980. Chronological variations inconcentrations and isotopic compositions of anthropogenic atmospheric lead insediments of a remote subalpine pond. Geochim. Cosmochim. Acta 44, 149–162.

Shotyk, W., Weiss, D., Appleby, P.G., Cheburkin, A.K., Frei, R., Gloor, M., Kramers,J.D., Reese, S., Van der Knaap, W.O., 1998. History of atmospheric lead depo-sition since 12,370 C-14 yr BP from a peat bog, Jura Mountains, Switzerland.Science 281 (5383), 1635–1640.

Sohrin, Y., Urushihara, S., Nakatsuka, S., Kono, T., Higo, E., Minami, T., Norisoye,K., Umetani, S., 2008. Multielemental determination of GEOTRACES key trace

metals in seawater by ICPMS after preconcentration using an ethylenediamine-triacetic acid chelating resin. Anal. Chem. 80, 6267–6273.

Strelow, F.W.E., 1978. Distribution coefficients and anion-exchange behavior of someelements in hydrobromic nitric acid mixtures. Anal. Chem. 50 (9), 1359–1361.

Thirlwall, M.F., 2000. Inter-laboratory and other errors in Pb isotope analysesinvestigated using a 207Pb–204Pb double spike. Chem. Geol. 163, 299–322.

Tomczak, M., Godfrey, J.S., 2003. Regional Oceanography: An Introduction. DayaPublishing House, Delhi.

Turekian, K.K., 1977. The fate of metals in the oceans. Geochim. Cosmochim.Acta 41, 1139–1144.

UNEP, 2007. The global campaign to eliminate leaded gasoline: progress as ofJanuary 2007. http://www.unep.org/transport/pcfv/pdf/leadreport.pdf.

UNEP, 2011. Study on the possible effects on human health and the environment inAsia and the Pacific of the trade of products containing lead, cadmium and mer-cury. http://www.unep.org/hazardoussubstances/Portals/9/Lead_Cadmium/docs/Trade_Reports/AP/UNEPLeadPb-CaicedoCompilation110601.pdf.

Veron, A.J., Church, T.M., Flegal, A.R., 1998. Lead isotopes in the western NorthAtlantic: transient tracers of pollutant lead inputs. Environ. Res. 78 (2), 104–111.

Veron, A.J., Church, T.M., Patterson, C.C., Flegal, A.R., 1994. Use of stable leadisotopes to characterize the sources of anthropogenic lead in North-Atlanticsurface waters. Geochim. Cosmochim. Acta 58 (15), 3199–3206.

Vu, H.T.D., Sohrin, Y., 2013. Diverse stoichiometry of dissolved trace metals in theIndian Ocean. Sci. Rep. 3, 1745. http://dx.doi.org/10.1038/srep01745.

Weiss, D., Boyle, E.A., Wu, J.F., Chavagnac, V., Michel, A., Reuer, M.K., 2003. Spa-tial and temporal evolution of lead isotope ratios in the North Atlantic Oceanbetween 1981 and 1989. J. Geophys. Res., Oceans 108 (C10). http://dx.doi.org/10.1029/2000JC000762.

Witt, M.L.I., Baker, A.R., Jickells, T.D., 2006. Atmospheric trace metals over the At-lantic and South Indian Oceans: investigation of metal concentrations and leadisotope ratios in coastal and remote marine aerosols. Atmos. Environ. 40 (28),5435–5451.

Wolff, E.W., Peel, D.A., 1985. The record of global pollution in polar snow and ice.Nature 313, 535–540.

Wu, J., Boyle, E.A., 1997a. Lead in the western North Atlantic Ocean: completedresponse to leaded gasoline phaseout. Geochim. Cosmochim. Acta 61 (15),3279–3283.

Wu, J., Boyle, E.A., 1997b. Low blank preconcentration technique for the determi-nation of lead, copper, and cadmium in small-volume seawater samples byisotope dilution ICPMS. Anal. Chem. 69, 2464–2470.

Wurl, O., Potter, J.R., Obbard, J.P., Durville, C., 2006. Persistent organic pollutants inthe equatorial atmosphere over the open Indian Ocean. Environ. Sci. Technol. 40(5), 1454–1461.