Concentration and isotopic composition of dissolved Pb in surface waters of the modern global ocean

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Concentration and isotopic composition of dissolved Pb in surface waters of the modern global ocean Paulina Pinedo-Gonza´lez a,⇑, A. Joshua West a, Antonio Tovar-Sanchez b, Carlos M. Duarte c, Sergio A. San˜udo-Wilhelmy a,d b

a Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA Department of Ecology and Coastal Management, Andalusian Institute for Marine Science, ICMAN (CSIC), Puerto Real, Ca´diz, Spain c Red Sea Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia d Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA

Received 24 August 2017; accepted in revised form 3 May 2018;

Abstract Several years have passed since the global phase-out of leaded petrol use. Nonetheless, emissions from anthropogenic activities remain the principal source of Pb to the oceans. The distribution of elemental Pb and its stable isotopes throughout the surface ocean provide information on the source and transport of these anthropogenic inputs. This study presents dissolved Pb concentrations and isotopic distributions from 110 surface water samples collected during the Malaspina 2010 Circumnavigation Expedition. Dissolved Pb concentrations ranged from 10 pM to 49 pM across the sampling stations covering all major ocean basins. The highest concentrations were found in the northeast Atlantic Ocean and the lowest in both the south Pacific and south Atlantic Oceans. Lead concentrations measured in the north Pacific Ocean, near Hawaii, were compared to previously published data from the same region. That comparison showed that Pb concentration has decreased 40% since 1975, although the rate of decrease has slowed in the past two decades. The overall decline in concentration probably has been induced by the cessation of leaded gasoline use in North America. The temporal evolution of stable Pb isotopes in this region shows a shift from dominant North American-like composition in 1979 towards a more Asian-like composition in later years. More widely, the distribution of Pb and Pb isotopes measured in the Malaspina sample set of global surface waters were compared with previously published ratios of aerosols and other atmosphere-derived Pb sources from the countries surrounding the different ocean basins. This comparison identified the potential Pb sources to each ocean basin, providing new insights into the transport and fate of Pb in the oceans. Ó 2018 Elsevier Ltd. All rights reserved. Keywords: Lead; Lead isotopes; Lead concentration; Surface waters global ocean; Anthropogenic emissions

1. INTRODUCTION Lead (Pb) is a natural constituent of the Earth’s crust that is commonly found in soils, plants, and even at trace levels in surface waters of the world ocean (e.g., Chow and Patterson, 1962; Boyle et al., 1986; Pais and Jones, ⇑ Corresponding author.

E-mail address: [email protected] (P. Pinedo-Gonza´lez). https://doi.org/10.1016/j.gca.2018.05.005 0016-7037/Ó 2018 Elsevier Ltd. All rights reserved.

1997; Boyle et al., 2005). For the past two centuries, however, anthropogenic Pb emissions to the atmosphere from high-temperature industrial activities (e.g., coal burning, cement production, smelting of Pb and other metals) and from the combustion of leaded gasoline have dominated over natural Pb emissions (Nriagu, 1979; Boyle et al., 2014). Global natural Pb emissions are estimated to be 2  106 kg/year (Nriagu, 1989) while anthropogenic Pb emissions are 3.6  106 kg/year (Liang and Mao, 2015). The

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oceans receive much of these Pb emissions and provide a sensitive recorder of their changes over time and space. Spatial and temporal variability of Pb fluxes to the surface ocean have been studied directly from seawater measurements (e.g., Schaule and Patterson, 1981; Boyle et al., 1986, 2005) as well as indirectly from Pb measurements in corals (Desenfant et al., 2006; Inoue et al., 2006; Kelly et al., 2009; Lee et al., 2014) and sediments (Trefry et al., 1985; Veron et al., 1987; Hamelin et al., 1990). In general, these previous studies have shown that in surface waters of some oceanic basins such as the Atlantic Ocean, the concentration of Pb increased rapidly with the onset of the industrial revolution and the combustion of leaded gasoline, and then decreased quickly after the phase-out of leaded gasoline in North America and Europe in the late 1970s and early 1980s (Shen and Boyle, 1988; Wu and Boyle, 1997, Shotyk et al., 2005; Kelly et al., 2009). In addition to increasing the concentration of Pb in the ocean, anthropogenic inputs have also modified the stable Pb isotopic composition of seawater. Many different types of ore deposits and other anthropogenic sources of Pb have distinct isotopic signatures that depend on when and where they were formed. Radiogenic isotopes 206Pb, 207Pb, and 208 Pb are products of radioactive decay of 238U, 235U, and 232Th, respectively. Their abundances in Pb ores and minerals varies due to the different U and Th decay constants and initial concentrations. Therefore, depending on the age of formation and the initial U and Th concentrations, the Pb isotope ratios of rocks and minerals can differ significantly from one location to another (Dickin, 1995; Koma´rek et al., 2008). The isotopic composition of Pb is not affected by physical or chemical fractionation processes, so since different sources of Pb have their own isotopic signature, it is possible to identify the source of Pb by matching the Pb isotopic composition of seawater with that of potential sources (e.g., Sangster et al., 2000; Koma´rek et al., 2008). Source apportionment can even be quantified in cases where all isotopically distinct sources of Pb can be characterized (e.g., Atwood, 2013). Altogether, the combination of Pb concentrations and isotopic signatures provides a powerful tool for interpreting changes in anthropogenic Pb inputs and sources to oceanic surface waters. Although in the last three decades significant advances have been made in documenting the Pb distribution and isotopic composition of oceanic waters (e.g., Flegal et al., 1984; Shen and Boyle, 1988; Veron et al., 1994; Wu et al., 2010; Noble et al., 2015, Bridgestock et al., 2016), our understanding of the sources and transport of anthropogenic Pb is still limited due to the lack of data from many oceanic regions. So far, Pb concentrations and isotope ratios have been most studied in the North Atlantic Ocean, with some data from the North Pacific and North Indian Ocean (e.g., San˜udo-Wilhelmy and Flegal, 1994; Gallon et al., 2011; Lee et al., 2015). Other areas of the world’s oceans have remained largely uncharacterized. Surface water samples collected during the Malaspina 2010 Circumnavigation Expedition (MCE), which covered all of the world’s major marine basins, provide an opportunity to fill this gap. The main objective of this study is to

determine the concentration and isotopic composition of dissolved Pb from surface waters collected in different oceanic basins during the MCE. These measurements comprise one of the most globally comprehensive surveys to date of current Pb levels in the surface ocean, providing an opportunity to build on the work of previous oceanographic campaigns and improve our understanding of some understudied areas of the world ocean. Furthermore, the data provide the basis for evaluating the evolution of Pb concentration over time in selected ocean regions where historical data are available. Finally, the geographical gradients in Pb isotopic composition make it possible to consider how potential sources of Pb differ for the different oceanic basins sampled during the MCE. 2. MATERIALS AND METHODS 2.1. Malaspina circumnavigation expedition Surface water samples were collected during the MCE aboard the R/V Hespe´rides from December 2010 to July 2011 (Fig. 1 and Table S1 in the supporting information). The MCE consisted of six oceanic transects: (1) a meridional transect from Cadiz, Spain, to Rio de Janeiro, Brazil (Stations 19–36), from December 2010 to January 2011, (2) a transect from Brazil to Cape Town, South Africa (Stations 37–51), from January to February 2011, (3) a transect in the Indian Ocean from South Africa to Perth, Australia (Stations 52–69), from February to March 2011, (4 & 5) two transects in the Pacific Ocean, from Auckland, New Zealand, to Honolulu, Hawaii (Stations 70–86), from April to May 2011, and from Hawaii to Panama (Station 87 to 110), from May to June 2011, and (6) a final transect back to Spain across the subtropical Atlantic, from Cartagena de Indias, Colombia to Cartagena, Spain (Stations 1 to 18), from June to July 2011. 2.2. Collection and analysis of samples for Pb and Pb isotopes Samples were collected using a Teflon tow-fish sampling system deployed at approximately 3 m depth utilizing established trace metal-clean techniques (e.g., Bruland et al., 2005; Berger et al., 2008). After sample collection, seawater was filtered on board through acid-washed 0.2 lm filter cartridges and acidified using Optima grade HCl to a pH < 2. Sample bottles containing dissolved samples (<0.2 lm) were double bagged in polyethylene bags and shipped to the trace metal clean laboratories at the University of Southern California in Los Angeles, where 1 L of sample was pre-concentrated using the technique described in Bruland et al. (1985). Procedural blanks for this technique are 0.05 ± 0.025 ng Pb (n = 33). The amount of Pb extracted per sample ranged from 3 to 9 ng. Levels of Pb were quantified by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) on a Thermo Element 2 HR-ICP-MS, using external calibration curves and an internal indium standard. The accuracy of our analytical procedure was verified by analysis of a certified seawater reference material (SRM), CASS-5, for

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Fig. 1. Map of sampling stations during the 2011 Malaspina Circumnavigation Expedition (MCE; black dots), and the distribution of dissolved Pb concentrations, 206Pb/207Pb ratios, and 208Pb/206Pb ratios measured in the global ocean from these samples.

which good agreement with the certified values were obtained as shown in Table S2 in the supporting information. Reproducibility of the Pb concentration measurements was calculated by measuring 5 different samples in quadruplicate per analytical session. These measurements yielded a reproducibility (2 s.d.) of 3–7%. The detection limit of the analytical method is 0.7 ng L 1. Other trace element concentration data from these samples were reported in Pinedo-Gonza´lez et al. (2015). Lead isotopic ratios (206Pb, 207Pb, and 208Pb) in the samples were also measured by HR-ICP-MS. Details on the instrumental operating conditions and measuring parameters are given in Table S3 in the supporting information. Nickel sample and skimmer cones and an ESI-PC3 Peltier cooled spray chamber were employed. Instrument tuning and optimization were performed daily using a 10 ppb multi-element solution containing Pb. Isotope ratios were calibrated with concurrent measurements of NIST SRM 981 National Common Lead Isotopic Standard. The isotopic composition in the SRM 981 standard was measure after every 5 samples, providing a means to make the small mass bias corrections required. Precision (2r) obtained from 10 consecutive measurements of SRM 981 was 0.48–0.52‰ and 0.42–0.47‰ for 206Pb/207Pb and 208Pb/206Pb ratios, respectively. Replicate analyses of seawater samples

(3 replicates per sample) had average standard errors (2r) of 2‰ for 206Pb/207Pb and 208Pb/206Pb. 3. RESULTS The distribution of dissolved Pb and Pb isotopes is presented in Figs. 1–4 and Table S1. Because distinct processes influence each ocean basin (e.g., dust and aerosol inputs, surface ocean currents, etc.), Pb concentrations and Pb isotopic composition are described separately for each region and/or transect. The hydrographic data generated during the MCE were reported and discussed elsewhere (PinedoGonza´lez et al., 2015), and are referred to here for reference. 3.1. Surface distributions of dissolved Pb and Pb isotopes 3.1.1. Overview of surface distribution of dissolved Pb and Pb isotopes in the global ocean Dissolved Pb concentrations in surface waters of the global ocean ranged from 10 pM to 49 pM (Fig. 1). The highest concentrations were found in the North Atlantic close to Europe. However, the median dissolved Pb concentration for the Atlantic basin (21 pM) was similar to the median concentrations measured in the Pacific (24 pM) and Indian

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Fig. 2. Map of sampling stations (black dots) and the distribution of dissolved Pb concentrations, 206Pb/207Pb ratios, and 208Pb/206Pb ratios in the Indian Ocean.

(21 pM) oceans. The lowest concentrations were found in the southern hemisphere close to New Zealand (15 pM) and in the South Atlantic (10 pM). This geographical distribution is not surprising because historically, Pb concentrations in the northern hemisphere have been higher since measurements began in the 20th Century, due to the larger number of industrialized countries surrounding the northern ocean basins. It is noteworthy that dissolved Pb concentrations measured in the samples collected during the MCE are on the order of tens of picomol L 1, while in the past, when leaded gasoline was intensively used, typical concentrations measured in surface waters of the central ocean gyres were about an order of magnitude higher (e.g., 160 pM in the North Atlantic Ocean, Schaule and Patterson, 1983; 65 pM in the North Pacific Ocean, Schaule and Patterson, 1981; Flegal et al., 1984). Similar to dissolved Pb concentrations, Pb isotopes also varied geographically in surface waters of the world ocean (Figs. 1–4). Global 206Pb/207Pb and 208Pb/206Pb ratios

ranged from 1.134 to 1.184 and from 2.071 to 2.111, respectively (Fig. 1). The highest 206Pb/207Pb ratios were found in the northwestern Atlantic Ocean, while the lowest ratios were found in the Indian Ocean. In contrast, the highest 208 Pb/206Pb ratios were found in the Indian Ocean close to the Australian coast and the lowest 208Pb/206Pb ratios in the North Atlantic and the northeastern Pacific Ocean. 3.1.2. Surface distribution of dissolved Pb and Pb isotopes in the Indian Ocean Dissolved Pb concentrations in the Indian Ocean ranged from 17 pM to 37 pM with a median value of 21 pM (Fig. 2). The highest concentrations were found near coastal areas, especially in the eastern side of the transect. Away from the coastal zones, in the subtropical gyre (between 60° and 105°E), the concentration of Pb ranged between 17 and 22 pM. Dissolved Pb concentrations measured in the Indian Ocean during the MCE were similar to those measured previously in surface waters of the

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Fig. 3. Map of sampling stations (black dots) and the distribution of dissolved Pb concentrations, 206Pb/207Pb ratios, and 208Pb/206Pb ratios in the Atlantic Ocean.

western region at about 31°S (21 pM; Echegoyen et al., 2014) (Table 1). 206Pb/207Pb and 208Pb/206Pb ratios in surface waters of the Indian Ocean ranged from 1.134 to 1.150 and 2.110 to 2.127, respectively (Fig. 2). These ratios are similar to recent measurements of Pb isotopic composition in surface waters of the southwestern Indian Ocean (206Pb/207Pb ratios 1.1398 to 1.1502; 208Pb/206Pb ratios 2.1114 to 2.1221; Lee et al., 2015) (Table 1).

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Fig. 4. Map of sampling stations (black dots) and the distribution of dissolved Pb concentrations, 206Pb/207Pb ratios, and 208Pb/206Pb ratios in the Pacific Ocean.

3.1.3. Surface distribution of dissolved Pb and Pb isotopes in the Atlantic Ocean Dissolved Pb levels along the longitudinal transect that started in Cartagena the Indias, Colombia and ended in Cartagena, Spain ranged from 19 pM to 41 pM with a median of 25 pM. Along this transect, 208Pb/206Pb isotope ratios increased towards the east from 2.071 to 2.093 while

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Table 1 Comparison between MCE concentration and Pb isotope data and literature values. Basin

Pb (pM)

206

208

Reference

Indian Ocean Indian Ocean (31°S) Southwestern Indian Ocean Colombia to Spain (Atlantic Ocean) Spain to Brazil (Atlantic Ocean) Brazil to South Africa (Atlantic Ocean) Western tropical Atlantic (18°N to 21°N) Off coast Africa North Atlantic Ocean New Zealand to Hawaii (Pacific Ocean) Hawaii to Panama (Pacific Ocean) Off Hawaii (Pacific Ocean) Off Hawaii (Pacific Ocean)

17–37 21 21–42 19–41 49–17 10–20 19–22 11–20 10–40 13–31 17–33 30 32

1.134–1.150 x 1.139–1.150 1.173–1.184 1.172–1.181 1.161–1.179 1.178–1.181 1.165–1.174 1.172–1.192 1.156–1.167 1.164–1.174 1.164–1.167 1.164–1.169

2.110–2.127 x 2.111–2.122 2.071–2.093 2.077–2.095 2.076–2.102 2.081–2.086 2.087–2.096 2.063–2.089 2.097–2.111 2.082–2.095 2.095–2.097 2.093–2.103

This study Echegoyen et al. (2014) Lee et al. (2015) This study This study This study Bridgestock et al. (2016) Bridgestock et al. (2016) Noble et al. (2015) This study This study This study Gallon et al. (2011)

the 206Pb/207Pb ratios decreased from 1.184 to 1.173 (Fig. 3). The 206Pb/207Pb and 208Pb/206Pb range found in the western samples along this transect are within the range of surface water samples reported by Bridgestock et al. (2016) for samples collected in the western tropical Atlantic between 18°N and 21°N in 2010 along the GEOTRACES section GA02 (206Pb/207Pb = 1.178–1.181; 208Pb/206Pb = 2.081–2.086) (Table 1). Dissolved Pb concentrations along the transect from Spain to Brazil exhibited a strong latitudinal gradient, with concentrations decreasing from 49 pM close to the coast of Spain to 17 pM in the southern hemisphere close to Brazil (Fig. 3). The high concentrations observed between 0° and 25°N may be associated with aeolian inputs of mineral dust from the Sahel. Chiapello et al. (1997) showed that during the northern hemisphere winter (MCE samples were collected in December and January) the low altitude trade winds deposit large quantities of dust in surface waters of the eastern Atlantic. Furthermore, Bridgestock et al. (2016) showed that mineral dust accounts for up to 30– 50% of the total Pb delivered to surface waters of the tropical north Atlantic. Finally, below the equator, Pb concentrations dropped down to <20 pM. 206Pb/207Pb isotope ratios along this transect ranged from 1.172 to 1.176 with the exception of samples collected close to the coast of Africa, which showed higher values of about 1.181. 208 Pb/206Pb ratios showed the same variability, with ratios ranging from approximately 2.077 in the southern part of the transect to 2.085 close to the coast of Africa and then increasing again to 2.095 close to Europe (Fig. 3). The 206Pb/207Pb and 208Pb/206Pb range found in the northern samples along this transect are within the range reported by Noble et al. (2015) for surface waters collected in 2010 along a more northward transect (206Pb/207Pb = 1. 172–1.192; 208Pb/206Pb = 2.063–2.089) (Table 1). The 206 Pb/207Pb range found in the samples close to the coast of Brazil (>1.174) is around 6% higher than the range of surface waters reported by Bridgestock et al. (2016) for samples collected in 2011 along a latitudinal transect off the coast of Africa (GEOTRACES section GA06; 206 Pb/207Pb = 1.165–1.174) (Table 1). This offset may be attributed to the spatial and temporal differences between both studies; the samples from Bridgestock et al. (2016) were collected farther north than those from the MCE,

Pb/207Pb

Pb/206Pb

and at a different time to year. However, we cannot rule out the possibility of contamination during sampling, which could affect the reported isotope ratios. Future measurements in this region might explore whether there is seasonal or subtle spatial variability. Lead levels along the longitudinal transect from Brazil to South Africa were the lowest Pb concentrations measured in the Atlantic Ocean during the MCE (median: 14 pM), with concentrations ranging from 20 pM close to the eastern and western margins to 10 pM in the oligotrophic gyre. 206Pb/207Pb ratios along this transect decreased from 1.179 in the west side of the transect to 1.161 in the east side close to South Africa, while 208 Pb/206Pb increased from 2.076 to 2.102 (Fig. 3). 3.1.4. Surface distribution of dissolved Pb and Pb isotopes in the Pacific Ocean Lead concentrations along the transect from New Zealand to Hawaii exhibited a strong latitudinal gradient, with concentrations increasing from around 13 pM to 31 pM close to Hawaii (Fig. 4). This spatial gradient is likely related to the distribution of anthropogenic Pb emissions to the atmosphere. In general, Pb emissions are highest within the mid-latitudes of the northern hemisphere, and this Pb is subsequently transported across the North Pacific by prevailing Westerlies (Schaule and Patterson, 1981; Settle and Patterson, 1991). In contrast, Pb concentrations of 14–17 pM measured in the South Pacific at 14°–19°S (Fig. 4) are similar to the pre-industrial surface water Pb concentration range of 16–19 pM inferred from measurements of corals from Fiji (Shen and Boyle, 1987), implying that Pb concentrations in this area of the Pacific Ocean, although never heavily impacted by anthropogenic Pb emissions, have returned to natural pre-industrial levels. Lead concentrations along the Hawaii to Panama transect ranged from 17 pM to 34 pM with a median of 25 pM. The highest concentrations were found in both the east (34 pM) and west (30 pM) sides of the transect, while the median Pb concentration in the middle of the transect was 21 pM (Fig. 4). Fig. 4 shows that the isotopic composition of Pb in surface waters of the Pacific Ocean defines 3 isotopically distinct regions – (i) the Southern Pacific, with 206Pb/207Pb and 208Pb/206Pb ratios of about 1.158 and 2.105

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respectively, (ii) the northwestern Pacific with 206Pb/207Pb and 208Pb/206Pb ratios of 1.165 and 2.095 respectively, and finally (iii) the northeastern section (east of 130°W) showing the highest 206Pb/207Pb (1.172) and lowest 208 Pb/206Pb (2.085) ratios measured in this basin. This distribution reflects the influence of different Pb sources, discussed in more detail below. In general, the isotopic ratios measured during the Pacific portion of the MCE are similar to other recent measurements in the central Pacific Ocean; the isotopic ratios measured near Hawaii (206Pb/207Pb = 1 .164–1.167; 208Pb/206Pb = 2.095–2.097) are analogous to those of surface water samples collected in the North Pacific (30°N, 140°W) in 2004 (206Pb/207Pb = 1.159; 208Pb/206Pb = 2.114) (Wu et al., 2010) and 2011 (206Pb/207Pb = 1.164–1. 169; 208Pb/206Pb = 2.093–2.103) (Gallon et al., 2011) (Table 1). 4. DISCUSSION 4.1. Temporal evolution of Pb concentrations and Pb isotopes in the North Pacific Ocean The data derived from samples collected in the north Pacific Ocean, in particular close to the Hawaiian Islands, provide the opportunity to explore the temporal variability in Pb concentrations and isotopic composition in surface

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waters of this region, by comparing our results with those measured around the same area over the last 30 years (Fig. 5). Comparison of Pb concentrations measured near the Hawaiian Islands in May 2011 during the MCE and those measured around the same area in 1978 (Nu¨rnberg et al., 1983), 1979 (Schaule and Patterson, 1981, Flegal et al., 1984), 1980 (Flegal and Patterson, 1983), 1997 and 1999 (Boyle et al., 2005), 2002 (Gallon et al., 2011), and 2005 (Wu et al., 2010) shows that Pb concentrations in this region of the Pacific Ocean have been steadily decreasing over the last 33 years from about 65 pM measured in 1978 to about 30 pM measured in 2011 during the MCE cruise (Fig. 5A; Table 2). Surface water Pb levels decreased very rapidly between 1977 and 1995 (Fig. 5A), attributed to the complete elimination of leaded gasoline in Japan (1986), the United States (1996), and Canada (1993) (Boyle et al., 2005). Although leaded gasoline was not completely phased out in China until the year 2000 (Flegal et al., 2013), gasoline consumption in China was small (9%) by comparison to Pb gasoline utilization in the United States (Thomas, 1995). The rapid decrease in Pb levels in surface waters of the Pacific Ocean near Hawaii seems to have slowed down in the mid-1990s (Fig. 5A). Dissolved Pb concentrations during the 21-yr period from 1976 to 1997 decreased by half (65 pM to 39 pM), with a slope of about 1.5 pM per year, while in the following 15 years, from 1997 to 2011, Pb concentrations decreased only by about 25% (39 pM to 30 pM), with an average slope of 0.5 pM per year. We hypothesize that the difference in rate of decrease is due to a switch in the emission sources of Pb. After the complete phase-out of leaded gasoline in the surrounding industrialized countries, the dominant sources of Pb to the North Pacific Ocean became coal combustion and other hightemperature industrial activities, primarily taking place in China, Japan, and Korea. Increasing importance of these sources appears to be consistent with the Pb emission patterns mapped by Pacyna et al. (1995) and with the temporal shift in the Pb isotope signatures measured in that area (Fig. 5B). During earlier decades (e.g., the 1970s), 206 Pb/207Pb ratios were dominated by United Statesderived Pb, with isotopic values trending more toward a US end-member in excess of 1.2 (Flegal et al., 1984). Over time, that ratio has decreased, from 1.196 in 1979 to 1.159–1.166 in the 2000s, reflecting both an increasing relative influence of lower 206Pb/207Pb Asian-derived Pb (with typical 206Pb/207Pb aerosol ratios of 1.141–1.166), and a decrease in fluxes from United States sources (with typical 206 Pb/207Pb aerosol ratios of 1.173–1.223) caused by the phase out of leaded gasoline. 4.2. Pb sources to surface waters of the world ocean

Fig. 5. (A) Dissolved lead concentrations and (B) associated isotopic composition of surface waters of the North Pacific Ocean near Hawaii for the past 35 years. 1977 – Schaule and Patterson, 1981, 1978 – Nu¨rnberg et al., 1983, 1979 – Flegal et al., 1984, 1980 – Flegal and Patterson, 1983, 1997 and 1999 – Boyle et al., 2005, 2002 – Gallon et al., 2011, 2005 – Wu et al., 2010, 2011 – This study.

The surface water distribution of Pb and Pb isotopes reflects recent fluxes to the surface ocean because the Pb residence time in surface waters is 2 years (Bacon et al., 1976). The isotopic ratios of Pb ores are unaltered by smelting and other manufacturing processes (Flegal and Smith, 1995), so concentrations and stable isotope information can be effective tracers of atmospheric emission sources

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Table 2 Historical Pb concentrations and isotopic composition of surface waters of the North Pacific Ocean near Hawaii. Concentration [pM]

Degrees latitude

65.6 62.0 63.2 54.5 39.4 34.2 32.8 34.3 30.1

24.33 11.91 19.00 20.00 22.00 22.00 22.45 22.00 21.27

Degrees longitude 154.50 150.09 158.00 160.00 148.00 148.00 158.00 158.00 157.86

and transport processes in marine environments (Flegal and Patterson, 1983; Ve´ron et al., 1987; Shen and Boyle, 1988; Flegal et al., 1989; Lambert et al., 1991; San˜udoWilhelmy and Flegal, 1994). Here we describe the potential sources of Pb to the different regions of the world oceans based on the MCE data. 4.2.1. Pb sources to the Indian Ocean Fig. 6A shows a plot of 206Pb/207Pb vs. 208Pb/206Pb ratios measured in surface waters from the South Africa to Australia transect in the Indian Ocean along with previously published isotopic ratios for surface waters (Lee et al., 2015), aerosols (Bollho¨fer and Rosman, 2000; Witt et al., 2006), and other atmosphere-derived Pb sources from the countries surrounding the Indian Ocean. It has been reported that the sources of natural Pb to the southern section of the Indian Ocean are continental dust from the arid

Year 1977 1978 1979 1980 1997 1999 2002 2005 2011

206

Pb/207Pb

208

Pb/206Pb

1.196

2.044

1.166 1.159 1.164

2.093 2.114 2.095

Reference Schaule and Patterson (1981) Nu¨rnberg et al. (1983) Flegal et al. (1984) Flegal and Patterson (1983) Boyle et al. (2005) Boyle et al. (2005) Gallon et al. (2011) Wu et al. (2010) This study

regions of South Africa (e.g., the Kalahari and Namib deserts) and Australia (Hovan and Rea, 1992). This source of natural Pb has 206Pb/207Pb and 208Pb/206Pb ratios of 1. 203 ± 0.005 and 2.069 ± 0.008, respectively, as inferred from ferromanganese nodules deposited in the Indian Ocean (Vlaste´lic et al., 2001; Fig. 6A). The anthropogenic Pb reaching the southern section of this ocean basin below 20°S originates mainly from the southern parts of Africa and Australia. The influence of the Inter-Tropical Convergence Zone (ITCZ), located between 0° and 10°S, effectively removes aerosols by wet deposition and limits the southward transport of aerosols from the northern hemisphere (Wilcox and Ramanathan, 2004). In addition, significant transport of Australian dust and aerosols into the eastern part of the Indian Ocean occurs during DecemberFebruary (when MCE samples were collected) due to the prevailing southeasterly winds (Rajeev et al., 2000). Lead

Fig. 6. (A) Comparison in triple isotope space between surface water samples collected during the MCE transect in the Indian Ocean (yellow filled circles), surface water samples collected in the southwestern Indian Ocean (white circles) (Lee et al., 2015), aerosols collected in the major cities of Australia (green filled triangles), New Zealand (blue filled triangles), South Africa (purple filled triangles), and the mid-Indian Ocean (red filled triangles) (Bollho¨fer and Rosman, 2000; Witt et al., 2006), and the average of Pb in Fe-Mn deposits in the Indian Ocean basin (Vlaste´lic et al., 2001) and (B) 1/Pb versus 206Pb/207Pb in surface waters of the Indian Ocean. The high correlation coefficient suggests a mixing of two end members. The data not fitting this trend correspond to the samples collected close to the coast of South Africa and Australia, which have elevated Pb concentrations due to their proximity to urban areas. Data used are provided in Supplementary Table S4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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from southern Africa reaches the Indian Ocean through the semi-permanent subtropical continental anticyclones and westerly disturbances that transport dust and aerosols from South Africa to the southeast over the Indian Ocean (Garstang et al., 1996). Anthropogenic Pb in aerosols from both South Africa and Australia has lower 206Pb/207Pb and higher 208Pb/206Pb ratios than natural Pb (Fig. 6A). Hence, our data are consistent with the premise that anthropogenic Pb inputs from South Africa and Australia dominate the southern portion of the Indian Ocean, rather than natural dust from these regions as invoked by Hovan and Rea (1992). This conclusion is substantiated (i) by the fact that the Pb isotopic composition of our samples falls on the two end-member mixing line described by these two sources (Fig. 6A) and (ii) by the high linear correlation (R2 = 0.90) observed in a plot of the inverse of Pb concentration vs. the 206Pb/207Pb isotopic signature (Fig. 6B). The two samples that deviate from the linear trend shown in Fig. 6B correspond to the samples collected close to the coast of South Africa and Australia, which have elevated Pb concentrations. 4.2.2. Pb sources to the Atlantic Ocean Fig. 7A shows the isotopic composition of Pb measured in surface water samples collected in the 3 different transects of the MCE in the Atlantic Ocean along with previously published isotopic composition of Pb in surface waters (Noble et al., 2015; Bridgestock et al., 2016), preanthropogenic/Holocene sediments (Sun, 1980), Pb isotope ratios from annually-banded corals that grew in Mona Island in the Caribbean basin (Desenfant et al., 2006),

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and the main potential Pb sources to the Atlantic Ocean: anthropogenic Pb transported by easterly winds originating from Africa and Europe (Bollho¨fer and Rosman, 2000, 2001; Witt et al., 2006; Bridgestock et al., 2016), anthropogenic Pb transported by westerly winds originating from North and Central America (Bollho¨fer and Rosman, 2000, 2001; Noble et al., 2015), African mineral dust (Abouchami and Zabel, 2003), and riverine inputs from the Amazon basin (Asmerom and Jacobsen, 1993; Alle`gre et al., 1996). This figure shows that Pb in samples collected in the southern hemisphere portion of this basin seem to be grouped in two main clusters: the samples close to Africa with low 206 Pb/207Pb ratios and the samples close to South America with higher 206Pb/207Pb ratios. In the context of these endmembers (Fig. 7A), samples from the eastern side of the South Atlantic contain the highest proportions of anthropogenic African Pb (with low 206Pb/207Pb ratios) transported by easterly winds. This is consistent with the extremely low estimates of desert dust inputs (with higher 206 Pb/207Pb ratios) to the South Atlantic region (Mahowald et al., 2005) and studies illustrating the importance of the easterlies in the overall elemental composition of aerosols delivered to the South Atlantic (Swap et al., 1996). In contrast, the western side of the South Atlantic seems to be affected by a Pb source with higher 206Pb/207Pb ratios originating from anthropogenic emissions in South America, carried by the south Westerlies that transport Pb from South America to the northeast (Church et al., 1990; Helmers et al., 1990). Identifying the potential sources of Pb to surface waters of the north Atlantic is more complicated. This ocean basin

Fig. 7. (A) Comparison in triple isotope space between surface water samples collected during the MCE in the Atlantic Ocean and surface waters collected in the North Atlantic by Noble et al. (2015) and in the tropical Atlantic by Bridgestock et al. (2016); anthropogenic emissions carried by the Easterlies and Westerlies (Bollho¨fer and Rosman, 2000, 2001; Witt et al., 2006; Noble et al., 2015; Bridgestock et al., 2016); African dust (Abouchami and Zabel, 2003); Holocene sediments (Sun, 1980); corals from the Caribbean (Desenfant et al., 2006); riverine inputs from the Amazon basin (Asmerom and Jacobsen, 1993; Alle`gre et al., 1996); and coals from North America and South Africa (Dı´azSomoano et al., 2009). Data used to assess the isotopic composition of the colored fields are provided in supplementary Table S5. (B) 1/Pb versus 206Pb/207Pb in surface waters collected along the transect from Spain to Brazil in the Atlantic Ocean. The distinct trends, each with high correlation coefficients, suggest different Pb sources between the Northern and Southern hemisphere.

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is influenced by several wind regimes: the Westerlies that transport Pb from North America, the Easterlies that transport European Pb to the central and north Atlantic, and the Trade Easterlies that transport large amounts of dust from the great deserts in North Africa (Sahel and Sahara) to the west (Church et al., 1990; Helmers et al., 1990; Bridgestock et al., 2016). The influence of the Westerlies is evident in the western north Atlantic, where surface waters with high 206 Pb/207Pb and low 208Pb/206Pb ratios are found (Figs. 3 and 7A). This corresponds to an increase in the contribution of north/central American anthropogenic Pb (Fig. 7A). Another potential source of Pb with higher 206 Pb/207Pb and lower 208Pb/206Pb ratios to the western north Atlantic is the Amazon River. A significant Amazon source is consistent with (i) the low salinity found in these samples (Pinedo-Gonza´lez et al., 2015), and (ii) the seasonality of the North Brazil Current. MCE samples from this region were collected in late May to early June, when the North Equatorial Counter Current surface flow is discontinuous and North Brazil current waters (chemically modified by the Amazon and Orinoco River waters) (Fig. S1) enter this area between Trinidad and Barbados (Farmer et al., 1993; Longhurst, 1998). Although it has been argued that the Amazon River is not a significant source of natural Pb to the Atlantic because most Pb is removed from river water during mixing with seawater (Bridgestock et al., 2016), our results suggest that the influence of the Amazon River to this area of the Atlantic Ocean merits further investigation, especially because the Pb isotope ratios from annually-banded corals that grew in the Mona Island in the Caribbean basin show that the relative contribution of anthropogenic Pb sources to this area of the Atlantic Ocean is decreasing (Desenfant et al., 2006). Fig. 7A shows that the isotopic ratios measured in surface waters of the western North Atlantic during the MCE (206Pb/207Pb = 1.180– 1.182; 208Pb/206Pb = 2.071–2.079) are much closer to the 1914 ratios of Mona Island coral (206Pb/207Pb = 1.188; 208 Pb/206Pb = 2.069) than those from 1997 (206Pb/207Pb = 1.167; 208Pb/206Pb = 2.076). Additionally, the 1914 isotopic ratios of this coral are similar to those of quaternary marine sediments from the same basin (206Pb/207Pb = 1.207 ± 0.0 05; 208Pb/206Pb = 2.062 ± 0.007; Sun, 1980). In the eastern North Atlantic, the lowest 206Pb/207Pb ratios from the MCE samples are found close to Europe (Fig. 3). Based on the endmembers shown in Fig. 7A, samples from this area contain the highest proportions of anthropogenic European Pb, consistent with previously reported atmospheric transport patterns in this part of the Atlantic Ocean (Easterlies carrying Pb aerosols from western Europe with lower 206Pb/207Pb ratios). South of this area, 206Pb/207Pb ratios are higher and more uniform (Fig. 3). These high ratios are associated with aeolian inputs from the North African Trade Easterlies that carry Saharan dust with high 206Pb/207Pb ratio (1.200–1.208) (Bridgestock et al., 2016). Furthermore, a plot of the inverse of Pb concentration vs. the 206Pb/207Pb isotopic ratios (Fig. 7B) shows that surface waters in the latitudinal transect from Spain to Brazil are grouped in two main clusters: waters from the southern hemisphere with lower concentrations and a narrow range of 206Pb/207Pb ratios, and waters from

the northern hemisphere with higher concentrations but a much wider isotopic range. In the northern hemisphere, the high linear correlation between the inverse of Pb concentration and the 206Pb/207Pb ratios suggest that these samples are mixtures of two main endmembers. In the northern part of this transect, Canary current waters influenced by Pb aerosols from western Europe (with lower 206 Pb/207Pb ratios) flow southward parallel to the African coastline, and at around 15°N these waters begin to flow west under the influence of the Equatorial Countercurrent, whose waters have higher 206Pb/207Pb ratios due to the influence of the North African trade easterlies. The mixture of these 2 water masses may explain the observed isotopic ratios in the eastern North Atlantic. 4.2.3. Pb sources to the Pacific Ocean Fig. 8A shows the 206Pb/207Pb vs. 208Pb/206Pb ratios measured in surface waters sampled during the MCE along with previously published data for northwest Pacific surface waters (Gallon et al., 2011), urban aerosols from China, Australia, New Zealand and North America (Gao et al., 2004; Lee et al., 2007; Ewing et al., 2010; Zhu et al., 2010a, 2010b, 2013; Xu et al., 2011, 2012; Hu et al., 2014, Gai et al., 2014; Bollho¨fer and Rosman, 2001; Zhao et al., 2015; Dewan et al., 2016), and coals and ores from China (Dı´az-Somoano et al., 2009; Cheng and Hu, 2010; Fu et al., 2010; Zhu et al., 2010a, 2010b, 2012; Huang et al., 2012; Li et al., 2012a, 2012b; Lee et al., 2014; Bi et al., 2017). This figure shows that the major potential Pb sources to the Pacific Ocean have distinguishable isotopic compositions. For example, ratios of 208Pb/206Pb in aerosols from China are higher at a given 206Pb/207Pb value than those from the southern hemisphere (Australia and New Zealand) and North America. These distinct mixing trends make it possible to trace the different Pb sources to the Pacific Ocean. The transect from New Zealand to Hawaii is characterized by a rapid latitudinal change in isotopic composition between the northern and southern hemisphere, suggesting different Pb sources to each hemisphere and low northsouth interaction (Fig. 4). In the context of the endmembers identified in Fig. 8A, samples from the Southern hemisphere contain the highest proportions of anthropogenic Australian and New Zealand Pb (with low 206Pb/207Pb ratios). These observations are consistent with (i) previously reported atmospheric transport patterns over the Pacific Ocean (Merrill, 1989), where air from the west transports Australian/New Zealand pollutant Pb to the eastern South Pacific, and (ii) the limited inter-hemispheric advection of industrial Pb aerosols due to the influence of the ITCZ (Uematsu et al., 1985), which brings intense and frequent rainfall in April and May, enhancing particle removal from the atmosphere by scavenging. Fig. 8B and C show the inverse of Pb concentrations vs. the 206Pb/207Pb and 208 Pb/206Pb ratios respectively, for the 3 different sections identified in Fig. 4. The high linear correlation found in the southern hemisphere suggests that this area of the Pacific Ocean is a mixture of 2 endmembers. Water from the southernmost part of this transect comes from the East Australian Current. This current forms in the western edge

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Fig. 8. (A) Comparison in triple isotope space between surface water samples collected during the MCE in the Pacific Ocean and surface waters collected in the NW Pacific Ocean by Gallon et al. (2011); aerosols from Australia, New Zealand, North America, and China (Bollho¨fer and Rosman, 2001; Gao et al., 2004; Lee et al., 2007; Ewing et al., 2010; Zhu et al., 2010a, 2010b, 2013; Xu et al., 2011, 2012; Hu et al., 2014, Gai et al., 2014; Zhao et al., 2015; Dewan et al., 2016); and Chinese ores and coals (Dı´az-Somoano et al., 2009; Cheng and Hu, 2010; Fu et al., 2010; Zhu et al., 2010a, 2010b, 2012; Huang et al., 2012; Li et al., 2012a, 2012b; Lee et al., 2014; Bi et al., 2017). Data used to assess the isotopic composition of the colored fields and the Chinese lead line (CLL) are provided in supplementary Table S6. (B) 1/Pb versus 206 Pb/207Pb and (C) 208Pb/206Pb in surface waters collected in the Pacific Ocean.

of the South Pacific gyre, where the concentration of Pb is very low (close to pre-industrial levels as suggested by measurements in corals; Shen and Boyle, 1987). As it skirts along the east coast of Australia into the Tasman Front, it would collect Pb with the distinct isotopic signature of Australian sources (low 206Pb/207Pb and high 208Pb/206Pb ratios, Fig. 8A), which is the observed endmember north of New Zealand (206Pb/207Pb = 1.156; 208Pb/206Pb = 2.110). The other endmember must have higher 206Pb/207Pb and lower 208Pb/206Pb ratios, like those measured in surface waters just below the equator. These waters are influenced by the South Equatorial current which is driven by the trade winds. The trade winds blow from east to west, carrying large amounts of anthropogenic South American Pb with higher 206Pb/207Pb and lower 208Pb/206Pb ratios, similar to the isotopic composition of North American aerosols (Fig. 8A). Although this pattern of deposition and redistribution by ocean circulation would explain the two-endmember mixing observed in Fig. 8B and 8C, it is important to note that recent measurements of the Pb isotopic composition of South American aerosols are not available in the literature, and it is possible that the Pb isotopic signature of these anthropogenic sources is outdated and no longer representative. In terms of the Pb isotopic composition of surface waters, the northwest Pacific is the most homogeneous area found in the MCE (Fig. 4). Fig. 8A shows that samples collected in this area fall in the range of isotopic values that describe modern Chinese aerosols. Although China banned the production of leaded gasoline in 2000, aerosol Pb emissions in China are nearing their pre-2000 levels due to the high number of metal smelters and intensive coal consumption in the country (Li et al., 2012a, 2012b; Bi et al., 2017). In addition to high Pb concentrations in Chinese aerosols,

the prevailing winds over China aid the transport of Pb to the North Pacific Ocean, and it has even been suggested that modern Asian Pb emissions are impacting atmospheric Pb concentrations in USA and Canada (Zdanowicz et al., 2006; Osterberg et al., 2008). The MCE results are consistent with these previous studies. Furthermore, the linear correlation found between the inverse of Pb concentrations and Pb isotopic composition in the northwest Pacific again suggests a mixture of 2 reservoirs of seawater with distinct Pb isotope compositions. Gallon et al. (2011) found that Chinese industrial Pb emissions are the dominant source of Pb inputs to the western North Pacific. Thus, Chinese Pb deposited in these waters seems to be distributed east by the north Pacific current, which flows west-to-east. Towards the east, the 206Pb/207Pb ratios of surface waters decrease due to an influence of waters from the eastern side of the gyre. These east-gyre waters are influenced by the California current that carries the distinct North American isotopic signature (higher 206Pb/207Pb and lower 208 Pb/206Pb ratios; Fig. 8A). Finally, the eastern side of the transect from Hawaii to Panama shows a more heterogeneous isotopic distribution (Fig. 4). The samples toward the east have the highest 206 Pb/207Pb and lower 208Pb/206Pb ratios measured in the Pacific Ocean during the MCE, suggesting that these waters are influenced by North American Pb sources (Fig. 8A). Although this observation is consistent with studies illustrating the importance of the northern hemisphere Westerlies for the transport of anthropogenic Pb to the North Pacific (e.g., Flegal et al., 1984), the correlation between the inverse of Pb concentrations and Pb isotope ratios is not as strong as that observed in the other two areas of the Pacific (Fig. 8B and C). Thus, it may be possible that more than two sources contribute to the Pb measured in

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these waters, for example a mixture of Central and North American sources combined with an Asian source. In addition, the elevated 206Pb/207Pb ratios near the coast of Central America can also be explained by the contribution of natural radiogenic inputs, for example African dust carried into the Caribbean and across the Isthmus of Panama (Duce and Tindale, 1991). This hypothesis is coherent with the high Fe concentrations measured at the same stations (Pinedo-Gonza´lez et al., 2015). 5. CONCLUSIONS This study reported dissolved Pb concentrations and isotopic compositions in surface waters of the global ocean, providing one of the most globally comprehensive surveys conducted to date and including new data from previously unexplored regions. These data provide an unprecedented opportunity to evaluate Pb sources and a baseline to assess changes over time. In the central Pacific Ocean (specifically close to Hawaii), previously published data show a dramatic drop in Pb concentrations from 1976 to 1997, followed by a more gradual reduction in Pb concentrations over the following 15-year period. 206Pb/207Pb ratios in this region also decreased over the same time period, reflecting an increasing relative influence of Asian-derived Pb (with lower 206Pb/207Pb) and a decrease in fluxes from North American sources caused by the phase out of leaded gasoline. Although additional data are needed (particularly measurements of the isotopic composition of modern aerosols), and the interpretations presented here are far from definitive, the new Pb isotope information provides opportunities for identifying sources across wider swaths of the world oceans than had previously been possible. Based on the results from the MCE samples presented here, the most likely main Pb sources to surface waters of the Southern Indian, Atlantic and Pacific Oceans are consistent with previously published ratios in aerosols and other atmosphere-derived Pb sources from the countries surrounding the different ocean basins. ACKNOWLEDGEMENTS This research was partially supported by NSF OCE (award #1335269), a USC Graduate School Dissertation Completion Fellowship, and the Spanish Ministry of Economy and Competitiveness through the Malaspina 2010 expedition project (Consolider-Ingenio 2010, CSD2008-00077). We thank L. Pinho, E. Mesa, H. Marota and A. Dorsett for help with metal sampling, and the captain and crew of R/V Hespe´rides for help during the circumnavigation. Three reviewers and the Associate Editor are thanked for constructive comments that helped improve the manuscript. This work was presented in the 2015 AGU trace metal session entitled as an ‘‘unofficial” tribute to Russ Flegal. This work is dedicated to Russ.

APPENDIX A. SUPPLEMENTARY MATERIAL Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j. gca.2018.05.005.

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