Historic and Industrial Lead within the Northwest Pacific Ocean

Jan 3, 2017 - nation of the Northwest Pacific Ocean in 2002 and present the ... continent. Assuming Pb isotope compositions in the North Pacific Ocean...
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Historic and industrial lead within the Northwest Pacific Ocean evidenced by lead isotopes in seawater Cheryl M Zurbrick, Céline Gallon, and A. Russell Flegal Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04666 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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Historic and Industrial Lead within the Northwest Pacific

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Ocean Evidenced by Lead Isotopes in Seawater

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Cheryl M. Zurbrick α,

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α

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Santa Cruz, California 95064, United States

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β

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California 95064, United States

γ, *

, Céline Gallon β, δ, A. Russell Flegal α, β

WIGS, Environmental Toxicology, University of California Santa Cruz, 1156 High Street,

Institute of Marine Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz

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KEYWORDS: Lead, Lead isotopes, North Pacific Ocean

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TOC/ABSTRACT GRAPHIC:

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ABSTRACT

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We report the continued lead (Pb) contamination of the Northwest Pacific Ocean in 2002 and

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present the first comprehensive Pb isotope dataset for that region. In the upper ocean, a Pb

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concentration maxima (64 – 113 pmol kg-1) extended throughout the entire North Pacific

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Subtropical Gyre (NPSG). We determined most of the Pb in this feature was from industrial

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emissions by many nations in the 1980s and 1990s, with the largest contributions from leaded

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gasoline emissions. In contrast, the deep water (> 1000 m) Pb concentrations were lower (6 – 37

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pmol kg-1), and constituted a mix of background (natural) Pb and anthropogenic Pb inputs from

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preceding decades. Deep water below the Western Subarctic Gyre (WSAG) contained more

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industrial Pb than below the NPSG, which was attributed to a calculated 60-fold greater flux of

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particulate Pb to abyssal waters near the Asian continent. Assuming Pb isotope compositions in

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the North Pacific Ocean were homogenous prior to large-scale 20th century anthropogenic inputs,

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this evidence suggests a relatively faster change in Pb isotope ratios of North Pacific deep water

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below the WSAG versus the NPSG.

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INTRODUCTION

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With the nearly global phase-out of leaded gasoline over the past four decades, there has been a

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dramatic decrease in lead (Pb) concentrations in North Pacific surface waters similar to that

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observed in the North Atlantic Ocean. 1–3 In the North Atlantic, surface ocean Pb concentrations

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decreased from an average of 160 pmol kg-1 in 1979 to 30 pmol kg-1 in 1998.4, 5 Concurrently,

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the central North Pacific surface ocean Pb concentrations decreased from an average of 64 pmol

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kg-1 in 1977 to 34 pmol kg-1 in 1997 . 2, 6 The residence time of Pb in North Pacific surface

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waters is 6 - 20 months, 7 and so these declines were primarily attributed to the elimination of

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leaded gasoline in both Japan in the 1980s and North America in the 1990s. 2, 8, 9

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Although the central North Pacific Subtropical Gyre (NPSG) surface water Pb

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concentrations declined by almost 50% over two decades, the decreasing trend has stagnated

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since then. Previous cruises to the central NPSG documented similar Pb concentrations in 1997 –

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1999 (34 ± 5 pmol kg-1), 2 2002 (44 ± 8 pmol kg-1), 1 and 2004 – 2005 (35 ± 11 pmol kg-1). 10

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Studies have shown that atmospheric deposition is the major source of Pb to North Pacific

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waters. 11, 12 Between 30˚ and 60˚ N the dominant wind direction is from the west and as a result

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large quantities of aerosols (natural and anthropogenic) are blown from upwind terrigenous

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sources into the North Pacific Ocean. 13, 14 The persistence of Pb in central NPSG surface waters

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can be, in part, attributed to the fact that many windward countries (e.g., Indonesia and China)

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did not complete the phase-out of their leaded gasoline until mid-2000 and Russia did not until

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2003 (Figure 1a). 15, 16 However, the relatively short residence time of Pb in oceanic surface

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waters and the systematic phase-out of leaded gasoline in Asia which began in the 1990s

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suggests elevated Pb concentrations in the North Pacific Ocean must result from more than

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dwindling leaded gasoline inputs.

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4 An alternative hypothesis is that the decrease in leaded gasoline consumption was

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accompanied by a simultaneous increase in industrial atmospheric Pb emissions from booming

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economies in Asian nations as evidenced by increased coal consumption and metal smelting

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(Figure 1b, c). Atmospheric Pb measurements support this hypothesis. Chinese aerosols from

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1994 (before phase-out) had comparable Pb concentrations in 2002 (24 months after phase-out).

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17 - 19

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non-ferrous metal smelting, and other industrial activities that produce metallurgic dust. In 2002,

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aerosol Pb concentrations were collected at sea between Japan and the central NPSG; based on

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enrichment factors and aerosol back trajectories, the Pb in those marine aerosols was attributed

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to industrial Pb emissions from Asia. 1

The sustained atmospheric Pb concentrations were found to be a result of coal combustion,

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In addition, seawater data collected for this region in 2002 – 2004 support the proposal

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that on-going inputs of industrial Pb emissions are comparable to previous inputs from leaded

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gasoline emissions in the North Pacific Ocean. Gallon et al. 1 reported isotope ratios (206Pb/207Pb

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= 1.157 – 1.161) in the surface waters of the Northwest Pacific collected in 2002, which closely

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matched that of aerosols measured in 2001 over mainland China (206Pb/207Pb ≈ 1.16). 17, 18, 19 In

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addition, they found Asian Pb isotope signatures in surface waters of the Western Subarctic Gyre

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(WSAG; 206Pb/207Pb = 1.157 – 1.162) and the central NPSG (206Pb/207Pb = 1.163 – 1.169). In

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2004, Wu et al. documented a similar Pb isotope ratio (206Pb/207Pb = 1.159) in NPSG surface

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water. 10 These Pb isotope ratios are very different from the surface water isotope ratios of the

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NPSG measured in 1979 (206Pb/207Pb = 1.184 – 1.196). 20 At that time, the major sources of Pb in

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that region were attributed to emissions of leaded gasoline combustion in North America and

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Asia.

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In the ocean interior, Pb is typically representative of when that water mass last

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ventilated. 21, 22 For deep water in the North Pacific, this is ~1000 years ago. 23 However, the lone

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stable Pb isotope profile in the central NPSG, collected in 2004, found that deep water Pb isotope

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ratios were significantly different from pre-industrial Pb isotope ratios. 10 Wu et al. created a

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mixing model to show that vertical Pb inputs of modern anthropogenic emissions over the past

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100 years have begun to impact abyssal waters. 10 This isotope change is possible because of low

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background Pb concentrations and the rapid rate at which particles sink (70 – 300 m d-1) 24, 25 as

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compared with horizontal advection to the NPSG (~ 60 m d-1). 10, 23 The majority of particle-

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bound Pb does not solubilize in the deep water column before settling on the ocean floor, so

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temporal changes in Pb isotope ratios of deep waters are much slower than in upper ocean

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waters. Therefore we anticipated that the deep water samples would be mostly preindustrial Pb

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(i.e., background Pb from natural Asian loess deposition and benthic sediments) with a small

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contribution from vertical inputs of particulate anthropogenic Pb.

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The goal of this project was to determine the spatial distribution of Pb within the

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Northwest Pacific Ocean at a point in time when atmospheric emissions of Pb were changing in

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Asia, both to investigate anthropogenic perturbations to the North Pacific Ocean and to establish

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a baseline for future studies of environmental change. We analyzed the Pb concentrations and

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isotope ratios at 9 vertical profiles collected in 2002, near the beginning of a massive

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industrialization period in Asia. These measurements were compared with the isotope signatures

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of potential Pb sources and atmospheric Pb emissions estimates for the prior 30 years.

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EXPERIMENTAL SECTION

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Seawater samples were collected from 1 May to 3 June, 2002, aboard the R/V Melville during

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the Intergovernmental Oceanographic Commission’s 4th Global Investigation of Pollution in the

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Marine Environment expedition (Figure 2). The voyage began in the Kuroshio Current near

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Japan and had stations in both the WSAG and the NPSG. During the transect, waters were also

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sampled in the Oyashio Current and Kuroshio Extension. 26 The warm Kuroshio Current flows

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northward along the coast of Japan and the cold Oyashio Current flows southward from the

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WSAG. Where the two currents meet the Oyashio subducts under the Kuroshio and they jet

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eastward as the Kuroshio Extension. 26 The subsurface waters follow the same flow path, moving

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at a reduced rate, 27 whereas deep waters move at a much slower rate. 23, 28

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Seawater was collected at 9 vertical profile stations using 30 L GO-FLO ™ bottles,

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which were modified for trace metal clean sampling and hung on a Kevlar ® line. 29 Unfiltered

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subsamples were taken in an onboard trace metal clean laboratory under HEPA filtered air (Class

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100) and collected in acid-cleaned 2 L low-density polyethylene bottles. Samples were acidified

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with 8 mL ultrapure (sub-boiling quartz distilled) HCl and stored until processing and analysis

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(2009 – 2012). Lead is highly contamination prone, but the agreement between this dataset and

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those values reported for the North Pacific previously (see Figure 3c) demonstrates the high

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quality of these samples.

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The total dissolvable (dissolved + particles) Pb concentrations in seawater were measured

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at the UC Santa Cruz Marine Analytical Lab using an on-line chelating resin extraction system 30

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connected to a Thermo ELEMENT XR™ magnetic sector high resolution inductively coupled

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plasma mass spectrometer (HR ICP-MS) as detailed in Zurbrick et al. 31 The Pb procedural

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blanks, which were analyzed concurrently, were less than 6 pmol kg-1, the precision was within

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5%, and the detection limit was 5 pmol kg-1. All sample concentrations reported here are

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corrected for the analytical blank by subtraction of the average blank on the corresponding

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analytical day. For 6 samples, this blank accounted for 25% - 50% of the total Pb analyzed,

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which is relatively high, but not enough to preclude the utility of these data. Lead isotopes in these samples were also processed and subsequently analyzed at UC

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Santa Cruz using the method of Zurbrick et al. 32 Briefly, Pb was extracted from seawater (50 –

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800 mL) using Toyopearl AF-Chelate 650 M ™ ion-exchange resin and eluted in a 1 mL extract

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of ultrapure quartz-distilled 1.5 N HNO3; the Pb concentration in the extracts was ~ 5 nmol kg-1.

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Extraction blanks comprised of ultra-high purity (18 MΩ cm) water (Millipore Milli-Q ®

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Academic with Ultrapure Ionex Cartridge) acidified to pH ≈ 1.8 were less than 30 pg (0.1 – 1.3

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pmol kg-1). The extracts were analyzed with HR ICP-MS in counting mode. Measurements of

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204

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natural abundances of mercury isotopes (204Hg/202Hg = 0.22988). Isotope ratios were calibrated

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with concurrent measurements of National Institute of Standards and Technology (NIST)

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standard reference material 981 (common lead) as listed in the Supplemental Information (SI,

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available online). Replicate extractions and analyses of seawater samples (n = 2 – 9) had average

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standard errors (2 σ) of 2‰ for 206Pb/207Pb and 4‰ for 208Pb/207Pb`. This precision agreed well

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with our previously reported reproducibility for GEOTRACES intercalibration seawater. 32 The

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206

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in this work, but the values are included for reference in the SI.

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Pb were corrected for isobaric interferences from 204Hg by monitoring 202Hg and assuming

Pb/204Pb ratios measured had standard errors (2 σ) of ~ 74 ‰. This large error precluded usage

Atmospheric Pb emissions from coal combustion, vehicular gasoline and metal smelting

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were estimated for relevant countries. Details of methodology and the estimated values are

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presented in the SI, part 2. In brief, the data for coal and road sector gasoline consumption were

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taken from the World Bank (data.worldbank.org). Lead from coal was calculated as in Lee et al.

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33

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quantity of coal burned, the average Pb content of coal was calculated for the available Asian

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countries (China, Indonesia and India), 15, 34 - 35 and atmospheric Pb emissions from power plants

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was assumed to be 80%. 15 Lead from gasoline was calculated in a similar manner, with

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consideration for each nation’s annual gasoline consumption and the corresponding Pb-gas

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concentration at that point in time and an atmospheric emission from tailpipes of 76%. 33 Smelter

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emissions were estimated based on each nation’s annual plant production of Pb, Zn, Ni and Cu as

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summarized by the USGS Commodity Statistics and Information (http://minerals.usgs.gov/

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minerals/pubs/commodity/) and emission factors from 1995 for Asia calculated by Pacyna and

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Pacyna. 36 Emissions factors were unavailable for Ni production, so we assumed the most

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conservative value of emissions, equal to Zn smelter emissions.

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RESULTS & DISCUSSION

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All total dissolvable Pb concentration and isotope ratios discussed are provided in Table S1-1. In

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this data set there were two outliers. The 20 m depth sample at Station 5 had the highest Pb

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concentration of the entire dataset, and anomalous 206Pb/207Pb and 208Pb/206Pb ratios. Similarly,

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the deepest sample (5,400 m) at Station 7 had an elevated Pb concentration and isotope ratios

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which were most similar to those in the surface mixed layer. We found the isotope ratios of these

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two samples were not an analytical artifact through replicate measurements, and decided the

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distinct values were a result of bottle or sampling contamination. Therefore, these samples were

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not included in the interpretation of these data.

where each country’s annual eletricity production from coal combustion was converted to

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Upper Ocean Water Across the transect, the mixed layer depth varied from 12 – 110 m. In the

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Kuroshio Current mixed layer (0 to 48 m), 26 Pb concentrations were ~105 pmol kg-1 and in the

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WSAG mixed layer (0 to 110 m) Pb concentrations were 64 – 72 pmol kg-1 (Figure 3a, b). In

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contrast, the NPSG mixed layer (0 to 49 m) had lower concentrations (44 – 51 pmol kg-1; Figure

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3c), which agrees well with measurements from the 1990s and mid-2000s. 2, 10 Overall, the

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mixed layer Pb distributions are in agreement with dust flux models which show that more

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atmospheric-borne dust lands in the Kuroshio and relatively less undergoes long-range transport

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to the NPSG. 13, 37 – 40 In addition, the isotope ratios of WSAG and Kuroshio waters (206Pb/207Pb

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= 1.159 – 1.170; 208Pb/206Pb = 2.095 – 2.113) were different from the NPSG surface mixed layer

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(206Pb/207Pb = 1.171 – 1.180; 208Pb/206Pb = 2.080 – 2.095). While the Pb concentrations imply

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there was less Pb input to the NPSG than the more coastal regions, the isotope ratios highlight

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the relatively greater input of anthropogenic Pb to the coast. The surface mixed layer findings are

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in good agreement with the surface ocean samples, collected on this same expedition, previously

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discussed by Gallon et al. 1 The most striking feature of the Pb distribution was the subsurface Pb maxima that

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persisted throughout most of the section at σƟ = 25.4 – 26.5 (250 – 500 m depth range; Figure

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S1-2). The highest concentrations were observed in the Kuroshio Current near the coast of Japan

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(Station 1: 78 – 113 pmol kg-1), and elevated Pb concentrations were observed offshore, south of

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the Kuroshio Extension (Stations 4 – 6: 70 – 85 pmol kg-1) and in the NPSG (Stations 7, 8, 9: 75

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– 86 pmol kg-1). Despite the differences in concentrations across the section, Pb isotope ratios

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were similar throughout the subsurface Pb maxima feature (206Pb/207Pb = 1.160 – 1.170,

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Pb/206Pb = 2.099 – 2.112), indicating a similar source of Pb to the water mass.

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The WSAG was the only region surveyed where a subsurface Pb maxima was not

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observed, and instead Pb concentrations were highest near the surface (σƟ = 26.3 – 26.5). The

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lack of subsurface maxima is less likely a result of the very sparse sampling resolution within the

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250 – 500 m depth range and more likely a result of several physical processes. Located close to

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land, the WSAG receives relatively large atmospheric inputs of Pb, 1 but we also hypothesize

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lateral continental margin inputs increase surface Pb concentrations. Previous studies have

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documented plumes of both particulate and dissolved trace metals (e.g., Fe, Mn, Al) advecting

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laterally near ~ 150 m into the WSAG which originated in the Sea of Okhotsk or at the Kuril-

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Kamchatka margin. 41 - 43 Deep winter convection then brings entrained particles and dissolved

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metals to the surface ocean, obscuring a distinct lateral input signal. Nishioka et al. 43 found that

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dissolved Fe from this advective process was comparable in magnitude to atmospheric Fe

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deposition. Nagaoka et al. 11 conducted a time series study of particulate Pb at the same location

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as our Station 2 from 2005 – 2007 (Figure 3b); they found the majority of particulate Pb at 770

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m and 5100m was of anthropogenic origin (78 – 90%), with the remaining fraction likely from

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the continental shelf. These parallel lines of evidence support our dual input hypothesis, although

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we are unable to assign definite contributions from the atmospheric and continental margin

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sources for these samples.

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The lateral input of Pb from the continental margin potentially extends far beyond the

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WSAG, as the shelf-derived plume is comprised of “very small particles, whose inherent sinking

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rates are slow” (Lamborg et al. 42, pp 1564). Surface water of the WSAG flows southeast and

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subducts south of the SubArctic Front, forming North Pacific Intermediate Water. 44, 45 The

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subducted WSAG water lies at a depth just below the subsurface Pb maxima observed in the

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NPSG. Although the exported WSAG water is lower in concentration than the NPSG subsurface

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maxima, it is still relatively elevated in Pb concentration (63 – 83 pmol kg-1) and its isotope

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signatures (206Pb/207Pb = 1.158 – 1.170, 208Pb/206Pb = 2.095 – 2.117) are indistinguishable from

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the subsurface maxima feature. This isotope similarity highlights that WSAG Pb inputs were

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either 1) dominated by atmospheric inputs with minor margin input, or 2) continental shelf

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sediments are highly contaminated with anthropogenic Pb and so the contributions of

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atmospheric versus advected Pb cannot be determined – but regardless the majority of the Pb is

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from human activities.

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We used our Pb concentration data in conjunction with those of Wu et al. 10 and the

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approximate water mass ages to assess the extent of Pb contamination in the upper ocean from

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the 1970s until the mid-2000s. Details are presented in the SI, part 3. In brief, the North Pacific

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was divided into four sub-regions: the Kuroshio Current and Extension, WSAG, central NPSG,

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and eastern NPSG (Figure S1-1). For each region the water column was partitioned into layers

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based on water density (σƟ) which corresponded with CFC-derived ages (Figure S1-2). 27, 44

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“Excess Pb” for each layer was calculated by subtracting the background Pb concentration at

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1500 m (σƟ = 27.6) from the average Pb concentration within each layer. This background is 100

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– 200 years old, 46 and was considered a conservative over-estimate of pre-industrial Pb

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concentrations because deep water Pb concentrations slowly increase over time as small

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quantities of modern Pb attached to sinking particles rain down through the deep ocean. 10

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Therefore, the resulting excess Pb estimate is likely an underestimate of pre-industrial Pb

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concentrations. Although this is not the ideal way to calculate background, it serves as a

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conservative estimate with the available information. Excess Pb for each layer was integrated

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over the volume of water for that sub-region (Tables S1-6, -7) and the excess Pb quantities were

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then compared against possible emissions sources from the preceding decades.

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The upper ocean Pb concentration maxima (4 – 12 years old) had a much larger Pb

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excess (132 Gg) compared to the mixed layer depth waters (0 – 5 years old; 69 Gg). However,

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normalized to years of input, both the mixed layer and shallower portion of the maxima (4 -7

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years old) had similar Pb excesses (11 – 14 Gg year-1) in contrast with the deeper portion of the

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maxima (7 – 12 years old; 20 Gg year-1). The older waters (12 – 35 years old; 3 – 10 Gg year-1)

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had less excess Pb which corresponds to scavenging of Pb and export over time, lower

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atmospheric Pb emissions during the 1970s – 1980s, or a combination of the two.

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Sources of Lead Natural and anthropogenic Pb sources and their isotope ratios were compared

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against the calculated excess Pb and the seawater Pb isotope ratios (Figure 4). In the mixed layer

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Pb had integrated over a 5 year time period so we focused on the aerosol data available for the

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years 1997 - 2002. Regional aerosol signatures are the integration of multiple Pb sources

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including naturally occurring Asian loess and industrial emissions from coal combustion and

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metal smelting. Prior to the phase-out of leaded gasoline, tetraethyl Pb additives also contributed

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to the aerosol Pb isotope composition. Individual source identities were analyzed where data was

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available.

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Asian loess is the main natural Pb contributor to the North Pacific surface water. As

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expected, loess isotope ratios are an end-member in the mixing line of possible Pb sources to the

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seawater in this work (Figure 4a). –47 - 49 World inventories estimate 4 – 9% of total atmospheric

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Pb is from natural sources. 36, 50 However, these inventories were based on total atmospheric Pb

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emissions during the peak of leaded gasoline consumption, biasing the former natural

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percentages toward the low-end in present times. Settle and Patterson 51 previously calculated the

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natural aeolian Pb flux to the North Pacific as, at most, 3 ng cm-2 yr-1, or about 2 Gg each year

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(North Pacific ≈ 43 million km2). In near-surface waters (< 200 m) this Pb equates to less than

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20% of the excess Pb, ruling out a much dustier climate over the past 20 years as the main source

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of sustained excess Pb. A low flux of natural Pb to the North Pacific is further supported by large

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aerosol Pb enrichment factors measured concurrent to our sample collection, 1 as well as large

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enrichment factors for other industrial pollutants like Ag and Se. 14

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In evaluating the anthropogenic sources of Pb to the North Pacific, we assumed

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deposition to the ocean was 30% of industrial atmospheric emissions as in Nriagu 52 and Rauch

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and Pacyna. 53 The isotope composition of Pb aerosols of several countries (Malaysia,

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Philippines, South Korea, Vietnam and Singapore) were similar to Pb signatures of near-surface

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seawater in the North Pacific, 33, 54 - 56 but those nations were determined to be minor sources

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because their industrial Pb emissions were far less (< 4%) than the calculated excess Pb in the

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upper water column (Figure 4a; SI). Other countries (e.g., Mongolia, Thailand) had neither the

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mass of industrial Pb emissions nor similar aerosol isotope ratios to be considered of importance.

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54, 56

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North Pacific was also not considered to be of importance because its landmass is mostly south

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of 30˚ N and prevailing winds carry the majority of those emissions to the Indian Ocean. 33, 57

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Consequently, only three countries were determined to have both similar aerosol Pb isotope

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compositions and substantial (> 10%) industrial Pb inputs to the North Pacific: Japan, Russia and

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China.

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Despite large atmospheric Pb emissions in India, its contributions of industrial Pb to the

The calculated Japanese emissions in 1997 – 2002 were 10 ± 3% of the excess Pb in

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near-surface waters. Their emissions were largely influenced by industrial activities as opposed

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to gasoline (unleaded) or other combustion sources (Figure 1). Japan was the first nation to

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completely phase-out its use of leaded gasoline, in 1986. 8 As a result, atmospheric Pb emissions

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from 1997 – 2002 were dominated by metal smelting (∑ =15 Gg Pb; 206Pb/207Pb = 1.182), 58 coal

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combustion (∑ = 7.6 Gg Pb; 206Pb/207Pb = 1.157 – 1.209), 59 and influenced to a much lesser

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extent by municipal solid waste incineration (MSW; 0.6 Gg Pb; 206Pb/207Pb = 1.160). 11, 59 The

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integrated atmospheric Pb isotope composition (206Pb/207Pb = 1.156 – 1.162) 55 and each

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individual source are indistinguishable from the near-surface waters of this study, indicating

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Japanese Pb aerosols contribute to the Pb found in the North Pacific (Figure 4b). In addition, the

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close proximity of Japan to the North Pacific Ocean means a 30% deposition rate of aerosols to

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the ocean is likely an underestimate of what was transported to the North Pacific. This

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determination is supported by the elevated Pb concentrations in surface waters by its coast. 1 If

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the Japanese industrial Pb deposition rate was 50%, its contribution would have been 17 ± 4 % of

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the excess Pb, which is still far less than the contributions from Russia and China.

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Russian emissions of atmospheric Pb from 1997 – 2002 were 20 ± 4% of the excess Pb in

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near-surface waters, and the main contributor to aerosol Pb was the continued use of leaded

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gasoline (∑ = 33 Gg Pb; 206Pb/207Pb = 1.135 – 1.149). 60 Much smaller contributions were made

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by coal combustion (∑ = 5.6 Gg Pb; 206Pb/207Pb = 1.103 – 1.179) 61 and metal smelting (∑ = 6.6

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Gg Pb; 206Pb/207Pb = 1.202 – 1.212). 61 These three sources bracketed the isotope composition of

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both the aerosols measured in Russia during the late 1990s (206Pb/207Pb = 1.146 – 1.163) 55, 61 and

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the near-surface seawater samples of this work (Figure 4b). By coupling the atmospheric

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emissions estimate with the similarity of Pb isotopes in aerosols and seawater it is apparent that

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the Pb in the mixed layer water was, in part, a reflection of Russian leaded gasoline emissions

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and industrial aerosol inputs.

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Atmospheric Pb emissions from China in 1997 – 2002 were 44 ± 10% of the excess Pb in near-surface waters, and were derived from a mix of leaded gasoline and industrial activities.

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Atmospheric Pb emissions estimates were similar for leaded gasoline (∑ = 41 Gg Pb) and coal

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combustion (∑ = 35 Gg Pb) due to the ongoing phase-out of leaded gasoline through 2000 and

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increased use of coal at the end of the 20th century (SI Tables S1 – 3). In addition, metal smelting

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increased during this time period (∑ = 24 Gg Pb). The isotope composition of Chinese coal

304

(206Pb/207Pb = 1.14 – 1.22) 34 was a range around that of atmospheric Pb measured over eastern

305

China (Shanghai) in 1999 – 2002 (206Pb/207Pb = 1.16) 18, 19 and mixed layer water (Figure 4b).

306

Leaded gasoline (206Pb/207Pb = 1.10 – 1.12) 18 and Pb ores (206Pb/207Pb = 1.02 – 1.18) 58 fall along

307

the mixing line of potential Pb sources to these waters. The integrated Pb isotope composition of

308

these sources (i.e., eastern Chinese aerosols) was indistinguishable from the near-surface

309

seawater Pb isotope ratios. This similarity supports our determination that a large portion of the

310

Pb originated from Chinese emissions.

311

Due to overlapping isotope signatures among possible sources, isotope ratios alone

312

cannot uniquely pinpoint the dominant sources of Pb to the water in this work. By coupling

313

isotopic data with atmospheric emissions estimates it becomes clear that the major sources of Pb

314

in near-surface waters were leaded gasoline from Russia and China, on-going coal combustion in

315

China, and on-going metal smelting in Japan and China. Collectively, all 12 countries evaluated

316

in this work accounted for 96 ± 20 % of the excess Pb in near-surface waters. Possible sources of

317

error in this figure include: no estimate for natural Pb advected from the continental margin,

318

assumed solubility of dust near 100%, no estimate for Pb losses in the water column due to

319

scavenging and export, the errors associated with the terrestrial emission versus atmospheric

320

deposition rate, and over-estimates of leaded gasoline emissions.

321

The atmospheric Pb emissions inventories were repeated for 1990 – 1998 and compared

322

to the subsurface Pb concentration maxima at σƟ = 25.4 – 26.5. During this earlier time period,

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annual atmospheric Pb emissions were 10 – 20% higher than in 1997 – 2002 (SI, part 1) due to a

324

much larger consumption of leaded gasoline in China and Russia. 15 Limited aerosol Pb isotope

325

signatures from China and Russia in the mid-1990s (206Pb/207Pb ≈ 1.150) 17, 18, 61 were lower than

326

at the turn of the 21st century (206Pb/207Pb ≈ 1.160), presumably due to a greater use of leaded

327

gasoline. 17, 18, 55 The Pb isotope composition of the subsurface Pb maxima (206Pb/207Pb = 1.163 ±

328

0.002) reflected this enriched 206Pb aerosol input, and was isotopically distinguishable from the

329

near-surface waters (206Pb/207Pb = 1.169 ± 0.006), particularly in the NPSG (Figure 3). Therefore

330

we believe the majority of the Pb in this subsurface feature originated from leaded gasoline

331

emissions in Russia and China during the 1990s.

332 333

Deep Water In deep water (operationally defined as > 1000 m), Pb concentrations decreased

334

from 40 pmol kg-1 to less than 10 pmol kg-1 (Figure 3, Table S1-1). The 206Pb/207Pb ratios

335

increased with depth from 1.172 to greater than 1.185 at the three deepest stations, while the

336

208

337

concentrations agree within 7 pmol kg-1 of other profiles taken in 1977 and 2004 (Figure 3c), 6, 10

338

due to the long (~ 100 years) residence time of Pb in the ocean interior. 46

339

Pb/206Pb ratios decreased from 2.089 to less than 2.065. In the central NPSG, these Pb

The isotope ratios of the deep water were a mix of background and anthropogenic Pb.

340

Ferromanganese (FeMn) nodules and benthic sediments, dated to more than 200 years old, along

341

with Asian loess were used as representative end-members of background Pb. The isotope ratios

342

of seawater in depths > 1500 m (206Pb/207Pb = 1.172 – 1.194) were similar, but slightly lower,

343

than those of North Pacific FeMn nodules (1.194 ± 0.004), 11, 62 benthic sediments (1.191 ±

344

0.006) 63 and Asian loess (1.197 ± 0.007; Figure 4). 47 - 49As proposed by Wu et al., 10 mixing

345

small quantities of surface-derived anthropogenic Pb over a long period of time could have

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produced the Pb isotope ratios observed in the North Pacific deep water. In the WSAG where

347

particle export is high, 64 the effect in deep water could be amplified more so than in the open

348

ocean. Atmospheric Pb emissions from 1971 – 2002 were dominated by leaded gasoline (540

349 350

Gg Pb; SI) followed by coal combustion (155 Gg Pb) and smelter emissions (98 Gg Pb) (Tables

351

S1-3, 4, 5). Each of these sources has an isotope ratio capable of altering the deep water Pb

352

signature away from the background composition. Many nations contributed to these emissions,

353

and it is clear that leaded gasoline has impacted both near-surface and abyssal North Pacific

354

waters.

355 356

Spatial Trend In the deepest waters, there is a spatial trend in the isotope ratios from west to

357

east, despite the fact that they were in the same oceanic water mass. In the WSAG (Station 2), Pb

358

concentrations were 2 – 6 times higher than in the NPSG (Stations 7 and 9) at depths greater than

359

1500 m (Figure 3). Additionally, the 206Pb/207Pb ratios increased (1.172 to 1.193) from west to

360

east in these profiles. This observation hinges on just a handful of data points, which we justify

361

in the following discussion.

362

As demonstrated in this work and by Wu et al., 10 Pb in the deep water of the North

363

Pacific is dependent on both vertical inputs and horizontal ones. Lead concentration and isotope

364

composition differences between deep water below the WSAG and NPSG are due to differential

365

Pb fluxes to the deep water. First, the WSAG receives 2 – 10 times more atmospheric dust input

366

than the central NPSG due to its proximity to land. 13, 38 – 40, 51 The plume from the continental

367

margin, concentrated with particles, also impacts the WSAG more than the NPSG. 42 The greater

368

particulate Pb input to upper ocean water in the WSAG means there is a greater pool of Pb

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available to be exported to those deep waters. Second, scavenging and subsequent particle export

370

is also greater in the WSAG than the NPSG. During the VERTIGO program, Lamborg et al. 64

371

documented that the total mass flux of particles in the WSAG (230 – 960 mg m-2 d-1) was greater

372

than in the NPSG (50 – 80 mg m-2 d-1). The enhanced scavenging and export flux in the WSAG

373

can be attributed to relatively higher primary productivity in surface waters resulting from the

374

upwelling conditions in this region. Third, the flux attenuation in the upper 500 m of the WSAG

375

(20 – 54%) was lower than in the NPSG (60 – 80%), possibly due to differing particulate

376

composition. 11, 64 - 66 Together, the flux and attenuation measurements indicated two to three

377

times as many particles sank below 500 m in the WSAG than in the NPSG. These indirect

378

measurements are supported by the greater particle Pb concentrations (1.4 ± 0.5 pmol kg-1) in the

379

WSAG relative to the NPSG (0.6 ± 0.1 pmol kg-1) measured on particles (>0.4 um) at depths

380

greater than 1000 m from this cruise (P.L. Morton, personal communication). Using particle flux as a proxy for Pb inputs to the deep ocean, we estimate the differential

381 382

particulate Pb fluxes to the deep water as: ∆F = ∆atmos • ∆p • ∆sink

383 384

where ∆F is the difference in Pb flux to waters below 500 m between the WSAG and NPSG,

385

∆atmos is the ratio of aerosol inputs to the surface waters of the WSAG versus the NPSG (= 2 – 3),

386

37, 40

387

9 ± 6), 64 and ∆sink is the ratio of particles that sink below 500 m without remineralizing in the

388

WSAG versus the NPSG (= 2.7 ± 1.3). 64, 65

∆p is the ratio of particle flux in the lower euphotic zone of the WSAG versus the NPSG (=

389

The calculated difference in particulate Pb flux to deep waters (∆F) is 60 times greater in

390

the WSAG than the NPSG. The relatively greater flux does not equate with a 60 fold increase in

391

the concentration of Pb in those waters, as the majority of the Pb remains in the particulate phase

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and settles onto the ocean floor. 11 Rather, the elevated flux in the WSAG results in a larger pool

393

of particulate Pb relative to the NPSG. These particles, with anthropogenic isotope signatures,

394

increase total dissolvable Pb concentrations and have a greater Pb isotope effect in WSAG deep

395

water relative to the NPSG.

396

We hypothesize that Pb isotope compositions of abyssal water in the WSAG can change

397

measurably on the timescale of 1 – 2 decades based on these seawater observations in

398

conjunction with the estimated 60-fold difference in Pb flux to deep water and Wu et al.’s model

399

for the NPSG. 10 In that model, Wu et al. showed vertical Pb inputs in the central NPSG have

400

doubled the concentration of Pb in the deep waters and changed the 206Pb/207Pb isotope ratios by

401

14‰ over the past century which agrees closely with these NPSG measurements. Assuming Pb

402

in the North Pacific Ocean was homogenous in isotope composition prior to large-scale

403

anthropogenic inputs, 10 the change in 206Pb/207Pb isotope ratios for the WSAG would be 25‰,

404

implying an accelerated change in Pb isotope ratios of WSAG deep water relative to the NPSG.

405

We expect the Pb isotope ratios of the deep waters will increasingly reflect the aerosol inputs

406

from ongoing industrial Pb emissions, particularly in the WSAG. With Pb isotope ratio detection

407

limits as low as 1‰, 67 we anticipate changes in Pb isotope ratios of the WSAG abyssal water to

408

be detectable on decadal intervals.

409

In the 14 years since these samples were collected, several changes to atmospheric Pb

410

emissions have happened. Russia has eliminated use of leaded gasoline, and rapid

411

industrialization among many Asian countries has continued to grow dramatically. Countries

412

such as South Korea and Indonesia have each doubled their coal consumption between 2002 and

413

2016, and in 2014 China was responsible for 50% of the total coal consumed globally (BP:

414

www.bp.com/statisticalreview). In addition, metal smelting in Malaysia doubled, and it tripled in

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China in the same time period (USGS: http://minerals.usgs.gov/minerals/pubs/commodity/).

416

These activities have the potential to elevate Asian atmospheric Pb emissions above their pre-

417

2000 levels. 68 Therefore, future studies of the Northwest Pacific Ocean are warranted both to see

418

how continued emissions are impacting the deep ocean and to monitor on-going sources of Pb

419

contamination in near-surface waters. This work provides documentation of the vast

420

contamination of the North Pacific Ocean during the previous three decades, and serves as a

421

crucial baseline against which future studies can interpret their findings.

422

423

AUTHOR INFORMATION

424 425 426 427 428 429 430 431 432 433 434

* Corresponding Author: Tel.: +1 617 324 4984; fax: +1 617 253 8630. E-mail address: [email protected] (C.M. Zurbrick). γ

Present Address: Dept. of Earth Atmosphere and Planetary Sciences, E25-612; MIT; 45 Carleton Street; Cambridge, MA 02142, United States Present Address: California Regional Water Quality Control Board; 320 W. 4th Street #200; Los Angeles, CA 90013, United States. δ

Notes: The authors declare no competing financial interest.

435

ACKNOWLEDGEMENTS

436

We thank R. Franks for his analytical assistance, K. Bruland and C. Edwards for their insights,

437

P.L. Morton for sharing his particulate Pb data, the Captain and crew of the R/V Melville for

438

making this research possible, the science party leaders C. Measures, W. Landing, and G. Cutter

439

for organizing the expedition and M. Ranville, K.C. Filipino, W. Landing, J. Donat, and R.

440

Powell for collecting these samples. We also thank Stephen Galer and several anonymous

441

reviewers for their suggestions which vastly improved this paper. We thank the

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Intergovernmental Oceanographic Commission for funding this baseline cruise, as well as the

443

National Science Foundation for its financial support through the Graduate Research Fellowship

444

Program and grant OCE- 0751681.

445 446

SUPPLEMENTAL INFORMATION

447

SI. Includes table of all seawater data presented in this work. Contains methods for calculating,

448

and the estimated values of, leaded gasoline, coal, and metal smelting emissions from Asian

449

nations. Details “excess Pb” box model calculations and values.

450

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Schlitzer, R.; Ocean Data View, 2015, odv.awi.de

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Table of Contents/Abstract Graphic Table of Contents/Abstract Gra 84x46mm (300 x 300 DPI)

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Figure 1. Atmospheric Pb emission estimates for a) gasoline, b) industrial coal combustion and c) smelting of Pb, Ni, Cu and Zn (data and references in SI: Tables S1-3, -4, -5). Figure 1 158x197mm (300 x 300 DPI)

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Figure 2. Map of IOC-4 expedition with station numbers. The warm Kuroshio Current flows northward along the coast of Japan and the cold Oyashio Current flows southward from the Western Subarctic Gyre. Where the two currents meet the Oyashio subducts under the Kuroshio and they jet eastward as the Kuroshio extension. 26 Figure made in Ocean Data View. 69 Figure 2 236x144mm (300 x 300 DPI)

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Figure 3. Lead concentrations and isotope ratios in a) the Kuroshio Current and Extention, b) the WSAG, and c) NPSG. Particulate Pb throughout the column of the WSAG (collected in 2005 – 2007) is isotopically similar to surface waters. 11 Previous Pb profiles in the central NPSG are similar to data presented here, 2, 6, 10 although the Pb isotope ratios have changed in the surface waters. 20 Hatched bars represent Pb isotope composition of North Pacific sediments and FeMn nodules. 11, 62, 63 Figure 3 222x279mm (300 x 300 DPI)

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Figure 4. Triple isotope plots of Stations 1 – 9 (shallow < 200 m; deep > 200 m) along with a) global aerosol signatures and b) probable sources which contributed to the Pb in these seawater samples. Figure 4 254x250mm (300 x 300 DPI)

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