www.elsevier.com/locate/chemgeo
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
V. Monastra et al. / Chemical Geology 209 (2004) 215–231218
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
V. Monastra et al. / Chemical Geology 209 (2004) 215–231 219
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.
V. Monastra et al. / Chemical Geology 209 (2004) 215–231220
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)
V. Monastra et al. / Chemical Geology 209 (2004) 215–231 221
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
V. Monastra et al. / Chemical Geology 209 (2004) 215–231222
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.
V. Monastra et al. / Chemical Geology 209 (2004) 215–231 223
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.
V. Monastra et al. / Chemical Geology 209 (2004) 215–231224
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.
V. Monastra et al. / Chemical Geology 209 (2004) 215–231 225
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.
V. Monastra et al. / Chemical Geology 209 (2004) 215–231226
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.
V. Monastra et al. / Chemical Geology 209 (2004) 215–231 227
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.
V. Monastra et al. / Chemical Geology 209 (2004) 215–231228
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
V. Monastra et al. / Chemical Geology 209 (2004) 215–231 229
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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