Multiple sources of lead in soils from a Hawaiian ...

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Chemical Geology 209 (2004) 215–231

Multiple sources of lead in soils from a Hawaiian chronosequence

Valerie Monastraa,*,1, Louis A. Derrya, Oliver A. Chadwickb

aDepartment of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, NY 14853, USAbDepartment of Geography, University of California, Santa Barbara, CA, USA

Received 16 July 2003; accepted 29 April 2004

Abstract

We report lead isotopic ratios and concentrations from a basaltic soil chronosequence in Hawaii. Substrate ages at the sites

range from 300 years to 4.1 million years. All soils show a net addition of Pb above the contributions made by basaltic

weathering, indicating that atmospheric deposition of natural or pollutant sources is important across the sequence. Isotopic

ratios from the chronoseqence soils indicate a mixture of basalt, mineral aerosol, and anthropogenic Pb sources derived from

both long range and local inputs. Locally derived anthropogenic Pb dominates young soils. Intermediate age soils in remote

locations have a strong mineral aerosol signature and likely contain some anthropogenic lead of Asian origin. Older soils have

undergone extensive leaching of both basaltic and eolian lead. While the anthropogenic inputs in most sites are from less

radiogenic local and Asian sources, we find evidence of ‘‘J-type’’ lead of North American origin in a remote soil on Kauai. The

diversity of anthropogenic lead sources suggests that at least two different weather patterns dominate atmospheric Pb transport

across the Hawaiian island chain.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Hawaii; Lead; Aerosol; Atmospheric transport; Isotopes; Anthropogenic

1. Introduction ic lead provide information on the origin of the Pb

Long-range atmospheric transport and deposition

of particulate matter is an increasingly recognized

geochemical pathway particulates can be deposited in

geochemically relevant quantities thousand of kilo-

meters from their place of origin. As a result of its

volatility during industrial processes, anthropogenic

lead has become globally distributed (Chow and

Patterson, 1962). Isotopic signatures of anthropogen-

0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemgeo.2004.04.027

* Corresponding author.

E-mail addresses: [email protected] (V. Monastra),

[email protected] (L.A. Derry).1 Tufts University, Program in Environmental Policy.

pollution and help establish the pathway of its long-

range transport (Sturges and Barrie, 1987; Church et

al., 1990; Veron et al., 1992). Although the addition

of anthropogenic Pb to soils has only become a

globally important source in the last 100 years, it

has become the dominant source in many localities

(Turekian and Cochran, 1981; Ng and Patterson,

1981; Maring and Duce, 1990). For instance, there

has been a 6 to 20-fold increase of Pb in

Greenland ice cores (Boutron et al., 1995) and

Japanese bay sediments (Hirao et al., 1983) since

the late 1800s, with the highest influx occurring

during the 1970s. Most anthropogenic Pb is released

to the environment through combustion of leaded

Fig. 1. Location map of study area in Hawaiian Islands with

substrate ages for individual soil sites.

V. Monastra et al. / Chemical Geology 209 (2004) 215–231216

gasoline and as an industrial pollutant from processes

such as smelting of ores.

Soils receive and store significant amounts of

natural mineral aerosols and their components (Simon-

son, 1995), including Pb (Boutron et al., 1995).

Despite its remoteness, soils and sediments in Hawaii

have been impacted by atmospheric deposition of

mineral aerosols (Jackson et al., 1972; Spencer et al.,

1995; DeCarlo and Spencer, 1997; Kurtz et al., 2001)

and dissolved ions (Capo et al., 1998; Kennedy et al.,

1998; Stewart et al., 2001). Weathering of the local

basaltic substrate provides the most important source

of metals for younger soils, while natural atmospheric

deposition from both marine and mineral aerosols

become increasingly important with time (Kennedy

et al., 1998; Kurtz et al., 2001). For elements that have

very low concentrations in surface seawater, such as

Nd and Pb, marine aerosols are not a significant source

relative to mineral aerosols. Anthropogenic sources of

lead are also important in Hawaiian soils (Spencer et

al., 1995; Teutsch et al., 1999). In this study, we

investigate the long-range atmospheric transport of

Pb to soils developed on different age volcanoes along

the Hawaiian Island chain. We measured Pb concen-

tration and 207Pb/206Pb and 208Pb/206Pb isotopic ratios

to identify the sources of Pb in soil profiles sampled

along a chronosequence spanning recent lava flows

and pumice deposits of Kilauea to highly weathered

ones on Kauai. Our results demonstrate changes in the

dominant source of Pb to soils depending on their

weathering status, proximity to local pollution sources

and the impact of atmospheric circulation patterns.

2. Methods and assumptions

2.1. Site characteristics and field sampling

We sampled soil horizon from six sites located in

remote areas of the rainforests on the islands of

Hawaii, Molokai, and Kauai (Fig. 1). The soils formed

in mixed lava and pumice parent material that range in

age: 0.3 ka (Kilauea, Thurston lava tube), < 2.1 ka

(Kilauea, Ola’a rainforest), 20 ka (Laupahoehoe), 150

ka (Kohala), 1400 ka (Molokai), and 4100 ka (Kauai)

(Crews et al., 1995). The sites are on minimally eroded

gently sloping shield surfaces at 1200-m elevation

with mean annual temperature of 15 jC. They are

exposed to the northeast trade winds and receive

approximately 2500 mm of rainfall annually. The soils

support forests that are dominated by Metrosideros

polymorpha (O’hia). The two youngest sites are locat-

ed on tholeites whereas the older sites are on more

alkalic lavas (i.e., alkali basalt, hawaiite, or mugearite)

(Kurtz et al., 2001). Although the modern climate at

each sample locality is similar, the four older sites have

experienced different environmental conditions during

Pleistocene glacial periods (Hotchkiss et al., 2000).

The older sites (z 20 ky) have, on average, experi-

enced drier and cooler conditions than present. With

appropriate consideration for differences in initial

conditions and paleoclimate, we can interpret these

sites as representing a progression in lava weathering

state and soil development.

Soil pits were hand-dug, described by genetic

horizon to unweathered rock or to a depth of 1 m,

and each horizon was channel sampled to form an

integrated homogenous unit. Each soil profile is

represented by six to eight horizons. Four to six

profiles were sampled at each site, although for this

study, Pb has been measured for one profile with the

exception of the Kauai site where two nearby profiles

(10 m apart) were measured. The soils have been well

characterized for ecological, mineralogical, and major

element properties (Crews et al., 1995; Vitousek and

Farrington, 1997; Chadwick et al., 1999) and trace

element distributions (Kurtz et al., 2000, 2001).

Soils younger than 20,000 years still contain some

primary basaltic volcanic minerals such as plagioclase,

V. Monastra et al. / Chemical Geology 209 (2004) 215–231 217

pyroxene, olivine, and volcanic glass. Primary miner-

als in soils older than 20,000 years have been replaced

by secondary minerals such as ferrihydrite, allophane,

and imogolite. These are replaced by kaolinites, goe-

thite, and hematite in soils older than 1 million years

(Vitousek and Farrington, 1997; Chadwick et al.,

1999).

2.2. Analytical procedures

Samples were air-dried, sieved to 0.297 mm, and

then dried at 75 jC for 2–4 h. Approximately 0.1 g of

each sample was weighed exactly and exposed to

cycles of ultraclean hydrochloric, nitric (HNO3),

hydrofluoric, and perchloric acids, and hydrogen

peroxide to digest silicates and organic compounds.

After the treatment with acids, samples were dried

down, dissolved in diluted HNO3, and centrifuged.

For concentration measurements samples were then

diluted in f 5% HNO3. Niobium data were obtained

on sample splits digested by Li-metaborate fusion and

analyzed by ICP-OES.

Lead from samples for isotope ratio determination

was purified by ion exchange chromatography using

Dowex AG1-x8 resin and hydrochloric and hydro-

bromic acids following the separation technique rou-

tinely used in the laboratory for mafic rocks (White et

al., 1990). After the samples were passed through the

columns, they were dried, redissolved, and diluted in a

5% HNO3 solution.

Analyses of Pb concentration and isotopic compo-

sition were performed on a Finnigan Element 2 high

resolution inductively coupled plasma mass spectrom-

eter (HR-ICP-MS). Concentrations were determined

by external calibration against a High PurityR Pb

standard diluted with f 5% HNO3 to make working

standards with concentrations ranging between 0.2–

20 ppb Pb. Total procedural blank measurements

were made following the same protocols as the

unknown soil and rock samples. A 2% (v/v) HNO3

acid was used as a blank for the standards. The total

procedural blank Pb content was f 0.1–0.2 ng vs.

total typical sample size of 200 ng Pb, although in a

few cases of low concentration samples, the blank

contribution was approximately 1% of the measured

Pb signal. Isotope ratio measurements were compared

to the National Bureau of Standards 981 (NBS-981)

lead isotopic standard, which was diluted with f 5%

HNO3, and spiked with thallium (Tl) to yield con-

centrations of 4.03 ppb Pb and 2.07 ppb Tl for isotope

measurements.

Samples were spiked with f 1–2 ppb 205Tl to

correct for drift and matrix effects in concentration

measurements and to correct for mass fractionation in

the isotope analyses. To check the accuracy of the

concentration data measured using our technique 10

samples were reanalyzed, but this time the concen-

trations were determined by Pb isotope dilution using

an enriched 208Pb tracer. Concentrations determined

by isotope dilution agreed with those determined by

external calibration within 5%.

Isotope ratio measurements collected data on206Pb, 207Pb, and 208Pb, as well as 203Tl and 205Tl.

After correcting for background and signal intensity

drift, a mass bias factor was determined from the205Tl/203Tl ratio using an ‘‘exponential law’’ and

assuming 205Tl/203Tl = 2.3871 (Dunstan et al.,

1980). A correction of this form has been widely

used in ICP-MS analysis of Pb isotope ratios (e.g.,

White et al., 2000 and references therein). Analysis

of the NBS-981 standard under similar conditions

over the 3-month interval of the analyses yielded

mean 207Pb/206Pb = 0.9148 and 208Pb/206Pb = 2.166

with a 2r uncertainty of 0.069% for 207Pb/206Pb

and 0.052% for 208Pb/206Pb (n = 16). Repeat analysis

of eight samples (n = 3 for each) yielded an average

2r uncertainty of 0.089% for both 207Pb/206Pb and208Pb/206Pb. These uncertainties are small relative to

the magnitude of natural variation observed in the

sample set.

2.3. Isotope composition of end-member sources of

lead

Here, we provide our best estimates of the207Pb/206Pb and 208Pb/206Pb isotopic ratios of the

parent rock, natural dust, and anthropogenic sources.

In Hawaii, each source has a reasonably well-defined

isotopic signature, but there are uncertainties, partic-

ularly in the case of the anthropogenic sources.

2.3.1. Bedrock values

The isotopic compositions chosen to represent the

basaltic bedrock for an individual soil site were based

on data from related fresh materials found at the

younger sites (Table 1). At the two oldest sites, local

Table 1

Isotopic composition of lead in end members for Hawaiian soils

Site Type of end member 207Pb/206Pb 208Pb/206Pb Reference

Kauai Bedrock 0.8457 2.076 Holcomb et al., 1997

Molokai Bedrock 0.8407 2.062 Stille et al., (1986)

Kohala Bedrock 0.8436 2.056 This study

Laupahoehoe Bedrock 0.8436 2.068 West et al., 1988

Ola’a Bedrock 0.8357 2.058 Pietruszka and Garcia, 1999

Thurston Bedrock 0.8357 2.058 Pietruszka and Garcia, 1999

Pacific mineral aerosols Natural aerosols 0.832 2.073 Jones et al., (2000)

Asian anthropogenic Anthropogenic 0.8714 2.114 This study

North American

anthropogenic

Anthropogenic 0.8196 2.025 Sturges and Barrie, 1987;

Rosman et al., 1994;

Simonetti et al., 2000

Pololu (Kohala) Local Remote Soils 0.8481 2.085 DeCarlo and Spencer, 1997

Ala Wai Canal (O’ahu) Local Anthropogenic 0.8684 2.104 Spencer et al., 1995


field relationships are unavailable because of deep

weathering, so we used data from compositionally and

temporally similar basalts from other locations on

Kauai and Molokai. The 207Pb/206Pb and 208Pb/206Pb

of the parent material at the Kauai site is estimated

from the average isotopic composition of Kauai

basalts (Holcomb et al., 1997). The Molokai and

Kohala soil sites are both formed from mugearite/

hawaiite. Although there were limited data on Pb

isotopic ratios of mugearite/hawaiite on East Molokai,

we used the average isotopic value of alkali and

hawaiite samples from Stille et al. (1986). Fresh

parent rock samples from Kohala were analyzed in

this study. The parent composition of the Laupahoe-

hoe soil was estimated from isotopic ratios of fresh

alkaline basalt from Kalahea flow of the Laupahoehoe

series of Mauna Kea (West et al., 1988). The Thurston

and Ola’a end members were estimated using data

from Kilauea shield tholeites (Pietruszka and Garcia,

1999).

2.3.2. Mineral aerosol values

The 207Pb/206Pb and 208Pb/207Pb of the natural

mineral aerosol end member was based on the study

by Jones et al. (2000), who measured Pb isotopic

compositions in Holocene and Quaternary sediments

from the North Pacific Ocean (Table 1). These sedi-

ments are derived from eolian deposition and should

represent a good estimate of the isotopic composition

of mineral aerosol delivered to Hawaii. We took an

average of their samples that were (a) near Hawaii and

(b) had neodymium isotope ratios close to eNd =� 10

previously reported by Nakai et al. (1993). An eNdvalue of � 10 appears to be representative of the flux

of Asian mineral aerosols to the North Pacific (Nakai

et al., 1993). Nd isotopic analyses from the LSAG

soils are consistent with an eNd value for aerosol

inputs of � 10 (Kurtz et al., 2000).

2.3.3. Anthropogenic values

Both local and remote sources of anthropogenic

lead likely contribute to Hawaiian soils (Spencer et

al., 1995). Local sources are largely derived from

gasoline, and an estimate of the isotopic composition

of Hawaiian anthropogenic Pb is available from a

study of 20th century sediments in a polluted canal in

urban Honolulu (DeCarlo and Spencer, 1997; Spencer

et al., 1995). In core G8 from the Ala Wai canal, the

maximum Pb concentration occurs at a stratigraphic

level believed to date from approximately 1975, and

this horizon also contains the least radiogenic (highest207Pb/206Pb and 208Pb/206Pb) isotope ratios. We take

the values from this horizon as representative of

anthropogenic Pb of local Hawaiian origin (Table 1).

North America and Asia are likely sources of

anthropogenic Pb to Hawaii, which receives 70% of

its wind from the northeast trade winds and the other

30% predominantly from westerlies that can carry air

masses from Asia. The United States both imports and

produces lead from various sources. An important

source is the Mississippi Valley district with anoma-

lously low 207Pb/206Pb and 208Pb/206Pb, sometimes

referred to as ‘‘J-type’’ lead after Joplin, Missouri

(Russell and Farquhar, 1960). The sources of industrial


Pb have varied over time and consequently caused

variations in the overall isotopic composition of Pb in

U.S. aerosols (Sturges and Barrie, 1987; Rosman et al.,

1994; Simonetti et al., 2000). The average U.S. an-

thropogenic Pb isotopic composition from 1960 to

1990 (Table 1) was used to represent the isotopic

signature of anthropogenic lead derived from the

continental United States (Sturges and Barrie, 1987;

Rosman et al., 1994; Simonetti et al., 2000).

Constraining the Asian anthropogenic signature is

more difficult, as detailed production and import

figures are not uniformly available from industrialized

nations in Asia. A number of Asian countries, includ-

ing Taiwan, Thailand, North Korea, South Korea and

Vietnam, did not develop substantial heavy industry

until later in the 20th century. Consequently, their

impact on the overall Asian anthropogenic signature

Table 2

(a) Origin of Pb consumed in Japan on a decadal basis

1900–1920 1920–1940 1940

Australlia 43 9 5.2

Canada 2 35 29

China 1 0 0

India 2 11 0

Japan 24 8 30.6

Mexico 0 0 0

North Korea 0 0 0

others 4 13 2.8

Peru 0 0 6.2

scrap 0 0 21

South Africa 0 0 0

South Korea 0 0 4.3

Thailand 0 0 0

UK 5 1 0

US 19 23 0.9

Total 100% 100% 100%

Tons of Pb 397,000 1,440,000 4,567

Column figures are percentages from various domestic and imported sour

consumed during the interval indicated.

(b) Weighted average lead isotopic composition for Japanese consumption

Weighted average 1900–1920 1920–1940 1940

207Pb/206Pb

Average Ratio 0.8876 0.9283 0.854

Total Pb (%) 6 6 30

208Pb/206Pb

Average Ratio 2.131 2.208 2.092

Total Pb (%) 6 6 30

Isotopic data from references cited in text. We take these values as our be

could have changed substantially over time. Japan,

however, has been industrialized for most of the 20th

century and records are available for lead imports and

mining activities. We chose to use the average isotopic

Pb signature of Japan to represent the average isotopic

signature of Asia. An investigation of Pb pollution in

major Asian cities in the late 1980s has shown that

Japanese cities and Beijing have similar isotopic

signatures (Mukal et al., 1993). Based on this data,

we assume that Japan and China have had similar

signatures in the past as well. To estimate the average

isotopic signature of industrial Pb used in Japan

during the 20th century, we binned the lead consump-

tion data from Hirao et al. (1986) and Mukal et al.

(1993) into five periods (1900–1920, 1920–1940,

1940–1970, 1970–1980, and 1980–1990) and

weighted the isotopic ratios (Sturges and Barrie,

–1970 1970–1980 1980–1990

7.4 27

27.8 34

0 0

0 0

17.6 15

4.6 0

4.6 0

6.6 0

10.5 12

15.8 0

1.8 12

1.8 0

1.5 0

0 0

0 0

100% 100%

,500 4,930,000 3,697,500

ces (references cited in text). Last row is the total tonnage of Pb

from 1900–1990

–1970 1970–1980 1980–1990

9 0.8632 0.8844 0.8714

33 25 100

2.101 2.129 2.114

33 25 100

st estimate of anthropogenic Pb originating from Asia.


1987; Rosman et al., 1994; Simonetti et al., 2000;

Russell and Farquhar, 1960) of each imported or

domestic Pb source with the percentage that each

source contributes to Japan’s total Pb consumption

(Table 2).

These data were used to calculate a weighted mean

isotopic value for industrial Pb released during the

interval of 1900–1990. We take the mean value for

Japan as an estimate of Asian contributions of anthro-

pogenic lead to Hawaii, which was 0.8714 for207Pb/206Pb and 2.114 for 208Pb/206Pb. Recent studies

by Bollhofer and Rosman (2001) on Pb aerosols

collected from 1994 to 1999 found similar isotopic

ratios to the ones proposed in this paper for Pb in Japan

and United States. However, their studies reflect pres-

ent-day isotopic ratios of Japan and United States and

not an isotopic signature that represents a 100-year

average. The isotope ratios of lead for the Hawaiian

and Japanese industrial sources are similar and not

easy to distinguish, while both contrast substantially

with either mineral aerosol sources or anthropogenic

sources from the continental United States.

3. Results and discussion

3.1. Mass budget of Pb in the soils

Lead addition to soils occurs through basaltic rock

weathering and natural and anthropogenic atmospher-

ic deposition, whereas losses occur through erosion or

leaching (Table 3). The amount each source contrib-

utes to the overall Pb inventory for each soil is a

function of the input flux each source contributes to

the sites over time minus the amount of Pb that has

been removed. Here, we compare the inventory of

lead currently in the soils (Table 4) with estimates of

the individual source contributions.

To account for changes in the bulk composition and

density of the soils, we apply the open-chemical-

system transport function defined by Chadwick et al.

(1990):

sj;h ¼Cj;h

Cj;p

� ��Ci;h

Ci;p

� �� 1 ð1Þ

which normalizes changes in the concentration of an

element j in soil horizon h to changes in the concen-

tration of an ‘‘immobile’’ index element i, where the

subscript p refers to the unweathered parent material,

respectively. Kurtz et al. (2000) have demonstrated

niobium (Nb) to be the least mobile element in the soil

sites, and we used Nb as our index element (Table 3).

Positive values of sh imply net accumulation of Pb

relative to the inventory available in the parent mate-

rial, and sh < 0 implies net loss. All sampled horizons

demonstrate net accumulation (sh>0) of Pbwith respectto potential contribution of the parent rock (Fig. 2).

The excess Pb is derived from atmospheric depo-

sition and we compare the total quantity of Pb

provided from both eolian deposition and basalt

weathering with the present soil inventory to obtain

an estimate of net leaching or deposition relative to

the sum of these two inputs. The present soil Pb

inventory on a mass per unit area basis (MPb) at each

soil site is

MPb ¼Xh

CPbh qhZh ð2Þ

where qh is the dry bulk density of the soil in each

horizon (h), Zh is the thickness of the soil horizon, and

ChPb is the lead concentration in the soil horizon.

The time-integrated input of Pb from atmospheric

deposition is estimated from the average mineral

aerosol mass flux to Hawaii (Kurtz et al., 2001).

Based on the Pacific pelagic sediment record, the

chemical composition of eolian material delivered to

the Hawaiian region has not changed significantly

over the past 4 million years (Kyte et al., 1993); it

closely resembles the average upper continental crust

as proposed by Taylor and McLennan (1995). The

natural eolian input of lead into the soil sites is

estimated as:

Pbdust ¼ FdustCPbdustt ð3Þ

where CdustPb is the average lead concentration (20 mg

kg� 1) found in upper continental crust (Taylor and

McLennan, 1995); Fdust is the average long-term dust

deposition rate for Hawaii (0.125 mg cm� 2 year� 1

for soils = 20,000 years or 0.030 mg cm� 2 year� 1 for

soils < 20,000 years; Kurtz et al., 2000; Rea, 1994);

and t is the age in years of the soil site.

The maximum quantity of Pb that basaltic weath-

ering could contribute to the soil profile (Pbwea)

is estimated from the original Pb content of the

Table 3

Lead and niobium concentration data and lead isotopic composition for individual soil horizons

Sample Depth (cm) Pb (ppm) Nb (ppm) 207Pb/206Pb 208Pb/206Pb

Kauai (4100 ka)

Kai4 0–6 0–6 8.71 66 0.8328 2.060

Kai4 6–13 6–13 8.17 158 0.8276 2.049

Kai4 13–20 13–20 28.99 293 0.8389 2.073

Kai4 20–33 20–33 15.51 149 0.8433 2.079

Kai4 33–52 33–52 8.53 131 0.8410 2.070

Kai4 52–65 52–65 7.48 135 0.8405 2.065

Kai4 65–80 65–80 7.18 133 0.8416 2.066

Kai3 0–8 0–8 21.08 276 0.8341 2.063

Kai3 8–16 8–16 27.59 267 0.8345 2.067

Kai3 16–22 16–22 26.78 207 0.8381 2.073

Kai3 22–34 22–34 19.49 194 0.8407 2.077

Kai3 34–60 34–60 10.20 129 0.8411 2.072

Kai3 60–86 60–86 6.63 122 0.8407 2.070

Kai3 86–105 68–105 5.60 125 0.8408 2.064

Kai3 105–125 105–125 7.22 140 0.8429 2.068

Molokai (1400 ka)

Mk2 0–8 0–8 17.11 18 0.8604 2.102

Mk2 8–13 8–13 42.25 56 0.8429 2.081

Mk2 13–28 13–28 19.23 166 0.8366 2.069

Mk2 28–40 28–40 9.17 76 0.8387 2.066

Mk2 40–60 40–60 5.57 42 0.8393 2.062

Mk2 60–90 60–90 4.60 68 0.8393 2.060

Kohala (150 ka)

KO4 0–5 0–5 10.25 105 0.8398 2.070

KO4 5–15 5–15 30.84 247 0.8405 2.076

KO4 15–24 15–24 40.63 367 0.8405 2.078

KO4 24–30 24–30 48.21 395 0.8411 2.079

KO4 30–42 30–42 40.45 376 0.8401 2.076

KO4 42–60 42–60 18.78 211 0.8415 2.075

Laupahoehoe (20 ka)

LA3 0–5 0–5 8.34 40 0.8415 2.075

LA3 5–12 5–12 12.21 180 0.8406 2.075

LA3 12–28 12–28 6.84 94 0.8402 2.073

LA3 28–38 28–38 5.75 80 0.8406 2.070

LA3 38–54 38–54 5.14 62 0.8401 2.070

LA3 54–80 54–80 7.15 62 0.8395 2.071

LA3 80–100 80–100 8.10 52 0.8366 2.075

Ola’a (2.1 ka)

OL5 0–5 0–5 3.85 6 0.8602 2.101

OL5 5–12 5–12 2.30 13 0.8524 2.088

OL5 12–20 12–20 2.05 12 0.8468 2.074

OL5 20–28 20–28 1.32 11 0.8480 2.084

OL5 28–45 28–45 1.17 10 0.8462 2.077

OL5 45–62 45–62 1.06 10 0.8382 2.064

OL5 62–85 62–85 1.11 12 0.8452 2.078

(continued on next page)


Thurston (0.3 ka)

T6 0–3 0–3 9.50 9 0.8585

T6 3–12 3–12 4.74 10 0.8502 2.093

T6 12–26 12–26 1.29 11 0.8384 2.080

T6 26–37 26–37 1.23 13 0.8381 2.066

T6 37–40 37–40 1.44 12 0.8462 2.064

T6 40–49 40–49 1.81 12 0.8510 2.076

Table 3 (continued)

Sample Depth (cm) Pb (ppm) Nb (ppm) 207Pb/206Pb 208Pb/206Pb


weathered substrate, where CbasPb is the concentration

of Pb in the parent rock, Zt is the total soil thickness,

qr is the average density of fresh substrate, and Sc is

the soil collapse factor.

Pbwea ¼ CPbbasZtqrSc ð4aÞ

The soil collapse factor for each soil site was deter-

mined by dividing the inventory of Nb in present in

the soil profile (corrected for dust deposition) by the

mass of Nb expected in a basalt column of thickness

Zt, where MNb is the present inventory of soil Nb after

correction for eolian inputs, and CbasNb is the concen-

tration of Nb in fresh substrate.

Sc ¼ MNb= CNbbasZtqr

� �ð4bÞ

If, after correction for eolian inputs, more Nb is

present in the soil profile than could be provided by a

basalt column of equal thickness and initial density,

we infer mass loss of other elements to have reduced

the total column thickness by a factor Sc. We calcu-

lated an average soil collapse factor of 2 for Kohala,

Molokai, and Kauai and a soil collapse of 1.5, 1.05,

Table 4

The net excess or deficiency of Pb in each LSAG soil site is

compared to the basaltic weathering and natural eolian deposition

Soils

Pb,

mg/cm2

Dust

Pb,

mg/cm2

Basalt

Pb,

mg/cm2

Basalt

and dust

(%yield)

Excess (+)/

deficiency

(�), %

Kai4 0.867 10.250 0.598 10.5 � 89.5

Kai3 1.285 10.250 0.650 11.8 � 88.2

MK2 0.576 3.500 0.765 13.5 � 86.5

KO4 1.085 0.375 0.299 161.1 61.1

LA3 0.315 0.050 0.188 132.6 32.6

Ol5 0.114 0.001 0.089 125.6 25.6

T6 0.075 0.000 0.049 151.7 51.7

and 1 for Laupahoehoe, Ola’a, and Thurston. These

values are consistent with the extensive leaching of

more soluble elements evident at the older sites

(Vitousek and Farrington, 1997; Chadwick et al.,

1999).

Over time, eolian inputs can substantially exceed

the input from substrate weathering alone in Hawaii.

Thus, it is useful to consider the budget of soil lead

relative to the sum of natural inputs, weathering, and

mineral aerosol deposition. The fractional net loss or

gain of Pb from the system relative to both basaltic

and eolian inputs is given by the ratio of the present

lead inventory to the estimate of inputs (Pbdust, Pbwea):

Pbnet ¼ ½MPb=ðPbdust þ PbweaÞ � 1� � 100 ð5Þ

where MPb is the present inventory of Pb in the soils,

Pbdust is the natural eolian input of lead into the soil,

and Pbwea is the maximum quantity of Pb that basaltic

weathering could contribute to the soil profile. Table 4

displays the calculated net excess or deficiency of Pb

in the soil sites compared to basaltic weathering and

natural eolian dust.

Teutsch et al. (1999) conducted a study of metal

concentrations on a transect established by Chadwick

et al. (1994) along the southwest slope of Kohala. The

transect represents a climate gradient over a short

distance. Concentrations of Pb in soils were also

found to be enriched by 59–198% relative to bedrock

at all sites. There was a linear correlation between Pb

enrichment in the soils and the amount of rainfall

indicating that deposition of atmospheric Pb is asso-

ciated with precipitation events through particle scav-

enging processes.

The amount of Pb contributed from precipitation

(dissolved marine aerosols) over time can be esti-

mated from the average Cl concentration in Hawaiian

Fig. 2. Net loss or gain of Pb relative to original soil inventory using mass balance indexed to a ‘‘conservative’’ soil element, in this case,

niobium. sPb calculated from Eq. (1). Large enrichments of Pb relative to fresh basaltic substrate in soil surface horizons indicate atmospheric

deposition from either mineral aerosol or anthropogenic sources.


rainfall at 1200 m height (Eriksson, 1957), the average

yearly rainfall (using present value 2500 mm), the age

of the soils, and the Pb/Cl of Pacific surface seawater.

This latter value is highly perturbed by recent anthro-

pogenic sources and is not well known for preindus-

trial times. We take an Atlantic surface Pb/Cl ratio of

3.89� 10� 10 (mol/mol) from modern data (Sherrell et

al., 1992) as an upper limit. The calculated Pb

contribution from marine aerosols would account for

less than 1% of the total Pb additions to the oldest

soils and much less for younger sites.

3.2. Sources of Pb in young soils

Profiles of Pb concentrations for Thurston ( < 0.3

ka) and Ola’a ( < 2.1 ka) are similar and show distinct

near surface maxima (Fig. 3). Because the soils at

Thurston and Ola’a have been developing for a short

time under interglacial conditions, deposition of min-

eral aerosol from Asia cannot contribute significantly

to the soil lead budgets there. Furthermore, leaching

losses of major elements are sufficiently small that the

surface maxima cannot be the result of residual

enrichment. sPb at both sites show strong surface

maxima, indicating that the lead content of the surface

horizons has increased by 4 to 6 times the initial

inventory of basaltic lead (Fig. 2). Integration of the

entire soil column shows a 26–52% net gain of Pb

relative to combined basalt and aerosol inputs (Table

4). These large Pb excesses in the young soils imply

an anthropogenic source.

The isotopic compositions of Pb in the profiles

parallel the concentration profiles (Fig. 3). Analysis

of the lead concentration (ppm) and 207Pb/206Pb data

yields a correlation coefficient of 0.82 at Thurston

and 0.90 at Ola’a. The middle parts of the profiles

have 207Pb/206Pbc 0.838, close to a likely basaltic

end member (Fig. 4). The upper horizons have207Pb/206Pbc 0.86, indicating input of Pb from an

anthropogenic source. The strong correlation between

lead concentrations and isotopic composition in the

soils confirms that the excess Pb in the upper soil

horizons is not a result of natural weathering of

Hawaiian basalt and displays signatures that are con-

sistent with Hawaiian or Asian anthropogenic sources

of Pb. The most radiogenic Pb here is from the

uppermost horizon at Ola’a, with 208Pb/206Pb = 2.101

and 207Pb/206Pb = 0.8602, approaching our estimates

of anthropogenic end members. These two soil sites

have been significantly impacted by anthropogenic

lead, resulting in several fold increases in lead con-

centrations in the near surface organic horizons. Mass

balance calculations indicate that 70–90% of the Pb in

the upper 3–5 cm is anthropogenic.

At Thurston, a second concentration maximum is

observed at the bottom of the profile, and this is

Fig. 3. Depth profiles for lead concentration and 207Pb/206Pb at the

Ola’a ( < 2.1 ka) site on Kilauea. The close correlation between [Pb]

and 207Pb/206Pb profiles indicates that the excess lead in the upper

part of the Ola’a profile is exotic. High 207Pb/206Pb (and 208Pb/206Pb)

ratios are consistent with an anthropogenic source for the exotic lead.

The Thurston site (0.3 ka) shows similar profiles.


accompanied by a second maximum in isotope ratios.

The presence of these maxima at the bottom of the

relatively shallow soil ( < 50 cm) suggests that a portion

of the recently deposited anthropogenic Pb may be

labile and has passed through the upper soil to accu-

mulate near its base. The isotopic data suggest a similar,

although less pronounced, movement of Pb at the Ola’a

site. The Thurston site is near a popular tourist road

within Volcano National Park, where automobile emis-

sions may have been an important proximal source of

anthropogenic Pb. The Ola’a site is in a more remote

area, withmuch less nearby road traffic. This difference

in local disturbances could account for the higher Pb

levels at Thurston and would be consistent with a

dominantly local (Hawaiian) source of anthropogenic

Pb. The apparent mobility of anthropogenic Pb at

Thurston might be related to the input of acid volatiles

from nearby ongoing volcanic activity. Acid deposition

could substantially lower the effective distribution

coefficient for Pb in soils, making it more labile than

expected in this particular location.

3.3. Sources of Pb in intermediate-age soils

At the two intermediate-aged soils at Laupahoehoe

(20 ka) and at Kohala (150 ka), the estimated depo-

sition flux of Pb from mineral aerosols integrated over

the site history equals or exceeds the Pb flux available

from weathering the basalt substrate and natural

aerosol inputs (Table 4). The concentration of Pb

and the Pb/Nb ratio in North Pacific mineral aerosols

are much higher than in the basaltic substrate; thus,

over long time scales, atmospheric deposition domi-

nates the soil lead budget. If leaching removes basaltic

lead early in the weathering process, this depletion

makes the soils even more susceptible to the impact of

atmospheric sources (e.g., Kurtz et al., 2001).

The Laupahoehoe site shows slight evidence of

anthropogenic input near the top of the profile, and a

mixture between mineral aerosol and basalt through-

out a majority of the profile (Fig. 5). The middle zone

has ‘‘basaltic’’ isotope ratios, consistent with the

presence of a buried ash horizon identified previously

(Kennedy et al., 1998). The remote location of the

Laupahoehoe site has apparently strongly limited the

input of anthropogenic Pb of local origin and as a

result of a longer history of mineral aerosol deposi-

tion, the site is less enriched in anthropogenic Pb than

are the younger Ola’a and Thurston sites.

The Kohala soil has very high Pb concentrations of

40–50 ppm at mid-depth (15–42 cm), an interval that

has previously been identified as strongly affected by

dust inputs based on mineralogy and neodymium

isotope systematics (Kurtz et al., 2001). 208Pb/206Pb

ratios are highly correlated with Pb concentrations

(correlation coefficient = 0.94), again identifying the

additional lead as exotic to the original basalt. The

combined Pb isotopic data from this interval show

that mineral aerosols heavily influence Kohala soil,

with additional inputs from anthropogenic sources

(Fig. 6).

While mineral aerosol and anthropogenic inputs

both tend to increase the 208Pb/206Pb ratio over basalt

Fig. 4. Three-isotope plot for soils developed on young ( < 2.1 ka) substrates. The end members are from Table 1, and the tick marks indicate the

percentage contribution from a given end member. The filled symbols are from the tops of each profile, while the lines connect samples in order

of increasing depth. Symbols are approximately the size of a 2r error. The filled square (‘‘Ali Wai canal’’) is from polluted sediment in Honolulu

(Spencer et al., 1995) and is our best estimate of the isotopic composition of lead from Hawaiian anthropogenic sources, primarily gasoline.

Fig. 5. Three-isotope plot for the Laupahoehoe site (20 ka). Symbols as in Fig. 4. Symbols are approximately the size of a 2r error. The lowest

horizons show indications of mineral aerosol input, while at mid-depths, a buried ash layer is indicated by a return toward basaltic values. The

upper part of the profile shows some influence from anthropogenic sources despite its remote location in a forest preserve.


Fig. 6. Depth profile for lead concentration and 208Pb/206Pb the

Kohala site (150 ka). Symbols are approximately the size of a 2rerror. The 207Pb/206Pb value for Pacific mineral aerosols differs only

slightly from that of the local basalt substrate, unlike the large

differences in anthropogenic sources. The slight change in207Pb/206Pb from basaltic values and poor correlation of this ratio

with [Pb] are consistent with a dominantly natural eolian source

rather than anthropogenic sources for most of the excess Pb.


values, they have opposite effects on 207Pb/206Pb.

The result in this case is elevated 208Pb/206Pb but

little variation in 207Pb/206Pb in both the Laupahoe-

hoe and Kohala soil sites. Kohala and Laupahoehoe

profiles contain the least anthropogenic Pb because

they are located in remote areas. Most of the anthro-

pogenic component here is probably derived from

long-distance transport from Asian sources. Asian

anthropogenic input of Pb was proposed by DeCarlo

and Spencer (1997) for a soil from Pololu Valley, also

located in a remote area in the Kohala region but at

lower elevation. However, the dominant atmospheric

component at the Laupahoehoe and Kohala sites is

mineral aerosol. Both sites experienced f 60% of

their soil development under cooler and dryer con-

ditions glacial conditions that were characterized by a

higher natural dust influx (Hotchkiss et al., 2000).

This history contrasts sharply with that of the younger

sites, which in addition to having much less time to

accumulate dust have developed under interglacial

conditions with much lower atmospheric aerosol

loadings.

3.4. Sources of Pb in the oldest soils

The present-day soil Pb inventories at the two

oldest sites (1.4 and 4.1 Ma) only comprise 10–

14% of the total natural Pb fluxes received from both

weathering and aerosol deposition, indicating that

leaching has removed most of the Pb that could have

accumulated in these soils (Table 4). Although a large

fraction of the atmospherically deposited Pb is

leached, there still is a net increase in Pb over the

original basaltic inventory. Both the Molokai and

Kauai sites exhibit complex mixtures of basaltic,

eolian, and anthropogenic lead and have experienced

substantial Pb loss and redistribution. Both sites are

also in remote locations, with little or no vehicular

access.

The Molokai soil profile shows a strong subsurface

maximum in lead concentration, exceeding 40 ppm at

8–13 cm. There is a clear trend in isotopic composi-

tion from the bottom to the top of the profile, with

both 208Pb/206Pb and 207Pb/206Pb becoming progres-

sively more influenced first by eolian inputs and then

by anthropogenic inputs near the surface (Fig. 7). The

trend from basaltic to eolian values in lead isotopes is

similar to that found for Sr and Nd (Kurtz et al., 2001)

in the Molokai soil sites. The uppermost horizon has a

very strong signal of anthropogenic Pb, despite being

in a remote area on Molokai. However, Molokai is not

far from the urban areas of Honolulu (Oahu) and

Kahalui (Maui). The high level of anthropogenic Pb in

the Molokai soil may reflect inputs from these areas,

although transport from the large urban area of Hon-

olulu would depend on infrequent westerly air flow.

Kahalui is a much smaller potential Pb source, but

transport from this nearby location may be more

typical.

At Kauai, the profiles display high concentrations

of anthropogenic lead at the top of the profile, high

natural dust input in the middle of the profile, and

high basaltic contributions at the bottom. The two

Kauai soil sites are unusual in that they have low208Pb/206Pb and 207Pb/206Pb values. These values are

Fig. 7. The Molokai site (1400 ka) shows an initial trend from basaltic values near the bottom of the profile toward an eolian component at mid-

depth. The upper soil horizons are strongly impacted by anthropogenic Pb, probably because of the relative proximity of this site to the urban

area of Honolulu. Symbols are approximately the size of a 2r error.


found in the upper part of the soil profiles and are

consistent with input of radiogenic American ‘‘J-

type’’ industrial lead (Fig. 8). They also show

evidence of mineral aerosol signatures, consistent

with data on Nd and Sr isotope ratios from the same

intervals (Kurtz et al., 2001). The lower parts of both

profiles have Pb isotope signatures that appear

‘‘basaltic’’ similar to that observed for Sr and Nd,

despite intense weathering and a long history of atmo-

spheric deposition. The explanation for the resistance

of the deep soil to isotopic change is not certain, but

one possibility is that basaltic Sr, Nd, and Pb are

retained in pedogenic Fe–Ti oxides that are quite

resistant to further weathering (Kurtz et al., 2001).

3.5. Sources of Pb in Hawaiian soils

Locally, large excesses of anthropogenic Pb in

Hawaiian soils appear to be derived from sources

within Hawaii, primarily leaded gasoline. Because

the isotopic signature of industrial lead used in Hawaii

and in Asia are similar, we cannot easily discriminate

between the two sources based on isotope ratios alone.

The geographic and down-profile distribution of Pb

excesses suggests that local sources dominate the

anthropogenic signal, except in remote windward

areas. A clear example of the influence of long-

distance Pb transport occurs at the Kauai site, where

two separate profiles show strong evidence of Pb

input from the western United States. Anthropogenic

Pb at other remote soil sites (such as Kohala) may be

primarily Asian in origin, as suggested by Spencer et

al. (1995). Less frequent air flow patterns associated

with westerlies or selected tropical storms appear to be

responsible for supplying most of the exotic lead to

Hawaii. Mineral aerosols are a major contributor of Pb

at all but the youngest sites we studied, consistent

with available data on Nd, Sr, and Pb from these and

other Hawaiian locations. The strong enrichment of

Pb in Asian aerosols relative to Hawaiian basalts

enhances their impact on the Pb budgets. Highly

weathered soils show evidence of strong leaching of

Fig. 8. Two profiles from the Kauai site (4100 ka) show an initial trend from basaltic toward eolian values. Symbols are approximately the size

of a 2r error. The isotopic composition of the original substrate is somewhat uncertain here. The upper soil horizons are shifted strongly toward

low 207Pb/206Pb and 208Pb/206Pb ratios typical of Mississippi Valley ‘‘J-type’’ lead. The data suggest an input of North American anthropogenic

Pb quite unlike local Hawaiian or Asian anthropogenic sources.


Pb from all sources but surprisingly retain a basaltic

signature at depth in the soil profiles. Anthropogenic

lead appears to be more mobile in the soil environ-

ment and has been quickly distributed throughout the

studied soil profiles.

3.6. Atmospheric circulation and sources of lead

pollution

The dominant airflow onto Hawaii is from the

Northeast trade winds that occur in Hawaii for 70%

of the year (Kodama and Businger, 1998). The North-

east trade winds develop as a result of the southeast

airflow around the North Pacific anticyclone. The

north Pacific anticyclone goes through an annual

migration where the center of the anticyclone in

January is 30jN, 130jW and, by July, its center

moves northward toward 35jN, 155jW. As a result

of this annual migration, during the summer months,

the mean monthly frequency of trade winds over

Hawaiian waters and the percentage of strong trade

wind days are more common than during the winter

months (Hereguchi, 1979). The Northeast trade winds

rarely originate over land and even less often over

highly polluted areas. While the Northeast trade winds

carry some anthropogenic Pb recycled from marine

aerosols, the data and calculations above imply that

the trade winds are not a major source of long-

distance transport of Pb to the soil sites.

All Asian isotopic signatures (both mineral aero-

sol and anthropogenic) observed in the soils result

from wind flow from the west, which is prevalent

only 30% of the year in Hawaii. During the south-

ward migration of the North Pacific anticyclone, the

Northeast trade winds become weak, and strong and

episodic westerlies referred to as West Wind Bursts

develop (Liang and Wang, 1998). Reasons for the

occurrence and the controlling factors of the West

Wind Bursts are not very well characterized; those

that influence Hawaiian wind patterns occur at a


frequency of 30–90 days and are strongest during

the late winter through spring, which coincide with

Asian dust storms (Fasullo and Webster, 2000;

Kessler et al., 1995; Zhang, 1996).

West Wind Bursts are most common from January

until May and this is the season when Hawaii

receives the highest flux of mineral dust. Aerosol

data have been collected at the Mauna Loa Observa-

tory located on the island of Hawaii at 3400 m in

elevation since 1979. Zieman et al. (1995), Perry et

al. (1999) and Harris and Kahl (1990) analyzed

aerosols at the Mauna Loa Observatory varying in

years from 1979 to 1996. All three studies found

increased atmospheric dust transport during the

months of January through May. Back trajectory

analysis demonstrates that air masses carrying high

dust loadings to Hawaii originate from Asia (Zieman

et al., 1995; Harris and Kahl, 1990). Perry et al.

(1999) measured concentrations of Pb in fine aero-

sols on a monthly basis from 1993 to 1996. The

highest concentrations of lead are also found from

January to June.

During the later summer and autumn months,

extratropical and tropical cyclones develop over warm

waters in low-pressure areas. On average, Hawaii

receives about three tropical storms per year (Sander-

son, 1993). There are two different frontal patterns for

the Hawaiian Islands associated with cyclones. The

first is a cold front that extends from an extratropical

cyclone developed to the northeast of the islands. If

the center of the disturbance is close enough, the

winds and storms will blow from the southwest. The

second type of cold front will bring rain and winds

from the northeast if the extratropical cyclone is far

enough away.

The presence of a North American ‘‘J-type’’

component of Pb in the Kauai soils differs from

all of the other sites, in which the anthropogenic lead

is unradiogenic and compatible with either Hawaiian

gasoline or Asian industrial sources. Why the source

of pollutant Pb at the Kauai site should be so

different is unclear, but probably is the result of

meteorological differences. While the Northeast

Trade Winds are the dominant airflow on Kauai as

on the other islands, Kauai is more strongly impact-

ed by easterly storms. In an average wet season the

island of Kauai will receive 15 frontal systems from

the east, where the island of Hawaii will only

receive nine frontal systems (Sanderson, 1993). Fur-

thermore, of the last six major hurricanes to make

contact with the Hawaiian Islands (1950, 1957,

1959, 1982, 1986, and 1992), which originated in

the eastern Pacific Ocean, five have made contact

with Kauai, where only two made contact with the

island of Hawaii. Although the duration of record is

too short to draw statistically robust conclusions,

these data suggest that Kauai is more strongly

influenced by air masses originating in the eastern

Pacific that can entrain air from the west coast of the

United States. Another possibility is that North

American lead is present in higher-latitude air

masses over the north Pacific and some of this is

entrained in the trade winds that transport it to the

northwestern location of Kauai. The remote and

windward location of the Kauai sites strongly limit

local anthropogenic inputs, with the result that

typical mainland United States lead is an important

contributor to soil Pb budgets. These results high-

light the long-distance transport of anthropogenic Pb

over 3000 km from the major urban areas of the

West Coast and the importance of local climatic

conditions that result in the most westerly of the

high islands of the Hawaiian chain receiving the

greatest input of Pb from an easterly source.

Acknowledgements

This work was supported by grants from the A.W.

Mellon Foundation and the National Science Foun-

dation (ATM 9816636 and EAR 0091771). We thank

Eric DeCarlo and an anonymous reviewer for helpful

reviews. [PD]

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