Lead Concentrations and Isotopes in Corals and Water
near Bermuda, 1780-2000 A.D.
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Citation
Kelly, Amy E. et al. “Lead Concentrations and Isotopes in Corals
and Water Near Bermuda, 1780-2000.” Earth and Planetary
Science Letters 283.1-4 (2009) : 93-100.
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http://dx.doi.org/10.1016/j.epsl.2009.03.045
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Elsevier B.V.
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Author's final manuscript
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Thu May 26 19:00:21 EDT 2016
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http://hdl.handle.net/1721.1/63109
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1
Lead Concentrations and Isotopes in Corals and Water near Bermuda,
1780-2000 A.D.
Amy E. Kelly1, Matthew K. Reuer1,2, Nathalie F. Goodkin1,2, & Edward A. Boyle1*
1
Department of Earth, Atmospheric, and Planetary Sciences, E25, Massachusetts
Institute of Technology, Cambridge MA, 02142, USA
2
Present address: Department of Environmental Science, Colorado College, Colorado
Springs CO, 80903, USA
3
Present address: Dept. of Earth Sciences, University of Hong Kong, Pokfulam Road,
Hong Kong
*corresponding author: eaboyle@mit.edu
45 Carleton St, E25-619, Cambridge MA 02141
(phone) 617-253-3388 (fax) 617-253-8630
Abstract
The history of the oceanic anthropogenic lead (Pb) transient in the North
Atlantic Ocean for the past 220 years is documented here from measurements of Pb
concentration and isotope ratios from annually-banded corals that grew in coastal
seawaters near Bermuda. Anthropogenic Pb emissions in this area have been dominated
by the industrialization of North America beginning in the 1840’s, the introduction of
leaded gasoline beginning in the 1920’s, and its phase-out that began in the mid-1970’s.
The phase-out of leaded gasoline was largely completed by the late 1990’s. Coral Pb
concentrations occur at a constant low level of about 5 nmol Pb/ mol Ca (~15 pmol/kg
in seawater) from the late 1700’s to ~1850. From ~1850 to ~1900 there is a small
increase rising to a plateau at ~25 nmol Pb/ mol Ca (~80 pmol/kg in seawater) in the
1930’s until the late 1940’s, at which point Pb concentrations rapidly increase to ~60
nmol Pb/ mol Ca (~200 pmol/kg in seawater). In the mid 1970’s, Pb began to decline to
~25 nmol Pb/Ca (40 pmol/kg in seawater) by the end of the 20th century, comparable to
levels occurring in the early 20th century. Pb isotope ratios (Pb I.R.) show maximum
206
Pb/207Pb = 1.21 and 208Pb/207Pb = 2.49 in the middle of the 19th century. We conclude
that this signal is a reflection of the early dominance of Upper Mississippi Valley Pb ore
in the United States, as previously seen in the estuarine sediments of Rhode Island.
After 1900, Pb I.R. decrease only slightly until the 1960’s when there is a significant
2
local maximum in the 1970’s to 206Pb/207Pb = 1.19 and 208Pb/207Pb = 2.45 as low Pb I.R.
sources were phased out. Then, as US leaded gasoline utilization decreased more
rapidly than European Pb gas utilization (which has lower Pb I.R.), western North
Atlantic Pb I.R. decreased to 206Pb/207Pb = 1.17 and 208Pb/207Pb = 2.44, their lowest
values in the past two centuries.
key words: lead, Pb, lead isotopes, Pb isotopes, global anthropogenic pollution
3
1. Introduction
Lead (Pb) occurs naturally in the environment, but the dominant recent sources
are anthropogenic, derived primarily from leaded gasoline and high temperature
industrial activities. Pb concentrations in the western North Atlantic Ocean near
Bermuda have been monitored directly since 1979, but the anthropogenic Pb transient in
the ocean began more than a century earlier. Previous work has shown that the Pb
concentration in the ocean varies in space and time and that measurements of Pb
concentrations, stable isotope ratios, radioactive 210Pb in seawater, and Pb
concentrations and isotope ratios in corals can be used to estimate Pb sources and fluxes
into surface waters (Schaule and Patterson, 1981, 1983; Flegal and Patterson, 1983;
Boyle et al., 1986, Shen and Boyle, 1987; Shen et al., 1987; Patterson and Settle, 1990;
Sherrell et al., 1992; Boyle et al., 1994; Helmers and van der Loeff, 1993; Veron et al.,
1993, 1999; Wu and Boyle, 1997a). Pb data are sparse relative to the observed
variability, and although the existing data are consistent with an anthropogenic Pb
emissions transient (mainly due to high temperature industrial processes and leaded
gasoline utilization), some features in the data are controversial or remain unexplained.
In order to reconcile past anthropogenic Pb concentration and isotope variability with its
modern North Atlantic distribution data, two new high-resolution proxy elemental and
isotopic surface ocean Pb records were developed from corals from North Rock,
Bermuda (32°28’N, 64°46’W) and ~1 km from the shore of John Smith’s Bay on the
southeast edge of the Bermuda platform. Figure 1 shows the location of these two sites
with respect to the Bermudan atoll and the atoll with respect to eastern North America.
We have also extended the seawater time series for Pb concentrations and isotopes to
2000 A.D. using samples from hydro-station S (32º10'N, 64º30'W), Bermuda Atlantic
Time Series (BATS) station (31º50’N, 64º10’W) and the Bermuda Testbed Mooring
(BTM) (31º42’N, 64º11’W).
Bermuda is an appropriate place to study aerosol-borne pollutants, particularly
those that originate in the U.S.A. Prevailing tropospheric westerlies carry Pb on fine
particles of aeolian dust from the U.S.A. towards the Sargasso Sea, where Pb is
deposited into surface ocean waters, dissolves, and is incorporated into coral skeletons
(Shen & Boyle, 1988). Although less frequent, aerosols from Europe also arrive via the
atmosphere (Church & Veron, 2005). The residence time for Pb in oligotrophic surface
4
waters is about 2 years (Bacon et al., 1976; Nozaki et al., 1976) and anthropogenic flux
changes are on decadal scales, so surface Pb concentrations track the incoming Pb flux.
Our use of coralline Pb as a proxy for dissolved Pb in seawater is based on the
assumption that uptake is proportional to the Pb concentration in the waters surrounding
the coral. Because corals are living organisms, their incorporation of minor elements is
not controlled entirely by inorganic crystal chemistry thermodynamics; the coral growth
rate and species-specific factors can be important (Shen & Boyle, 1987; Shen & Boyle,
1988; Goodkin et al. 2005, 2007; Matthews et al., 2008). Once incorporated, Pb is
retained in the crystal lattice structure of the corals and maintains a long-term, annuallyresolved record.
Different ores from around the world have characteristic stable Pb isotope ratios
that were fixed during mineral genesis (Chow & Earl, 1970). Although small (several
permil) heavy isotope mass-dependent process-induced fractionations have been
reported (e.g. Tl, Rehkämper & Halliday, 1999), any mass-dependent Pb fractionation
will be small compared to the isotopic variability associated with different Pb sources
(~30% globally; ~5% in our records). Pb isotopes in environmental samples can help
constrain which ores the Pb may have originated from, and hence different sources of
Pb pollution can be traced far from their sources (e.g., North American versus European
Pb).
Two issues concerning the isotopic evolution of anthropogenic Pb emissions
from North America have arisen recently that inspired us to increase the sample density
and length of the western North Atlantic Pb and Pb I.R. record: (1) what are the preanthropogenic Pb concentration and Pb I.R. of corals and seawater? and (2) how has the
anthropogenic Pb component evolved isotopically through time? Alleman et al. (1999),
assumed that pre-anthropogenic detrital Pb from the island of Bermuda – represented by
the coral Pb occurring in the year 1880 - modified the Pb isotope record of corals, and
“corrected” the Shen and Boyle (1987) isotope data to infer the temporal evolution of
the isotope ratio of the anthropogenic component:
5
Pb I.R.corrected = Pb I.R.(t) * [Pb(t)] – Pb I.R. (1880) * [Pb(1880)]
_________________________________________________________
[Pb(t)] –[Pb(1880)]
One consequence of this method is that 206Pb/207Pb for the “dissolved” Pb inferred coral
samples from the 1950’s would be shifted to lower values (~1.15), producing a record
that more closely resembles the anthropogenic Pb I.R. data from a sediment record in a
subalpine Sierra Nevada Pond (Shirihata et al., 1980) and the Anthropogenic Lead
ArchaeoStratigraphy (ALAS) estimate for the Pb I.R. of U.S. leaded gasoline (Hurst,
2002).
Lima et al. (2005) analyzed the isotopic compositions of Pb during the past 200
years in a laminated sediment core from an anoxic estuary in Rhode Island
(Pettaquamscutt). The annual nature of the laminations was confirmed by observation of
bomb and Chernobyl 137Cs, 210Pb dating, and 14C. This record showed anthropogenic
(i.e., pre-anthropogenic corrected) 206Pb/207Pb =1.19 in the 1950’s that is higher than
either the Sierra Nevada pond (Shirihata et al., 1980) or the ALAS gasoline curve (Hurst,
2002), indicating that anthropogenic Pb in this region – and potentially in the aerosols
departing North America for the North Atlantic – had higher Pb I.R.. Lima et al. (2005)
suggested that the difference was due in part to Pb emitted by coal burning and in part
due to regionally non-uniform Pb I.R. in leaded gasoline. Indeed, Chow and Earl (1970)
reported that aerosols and gasoline from Southern California had 206Pb/207Pb = 1.15
whereas Boston had 206Pb/207Pb = 1.175. Anthropogenic Pb in the Rhode Island estuary
sediments was already substantial by 1880, having risen significantly since ~1840.
Hence taking 1880 coral Pb data as pre-anthropogenic will result in an over-correction
of the Pb I.R. data. Lima et al. also found a pronounced high anthropogenic 206Pb/207Pb
peak in the mid-1800’s (~1.32) in the Rhode Island estuary record. A similarly
anomalous peak had been found in other 19th century samples from the Great Lakes
(Graney et al., 1995) and Chesapeake Bay (Marcantonio et al., 2002). Lima et al. (2005)
traced this peak to a mining district in the Upper Mississippi Valley that was the
6
predominant source of Pb in the U.S at mid-19th century. This evidence suggests that we
should re-examine Alleman et al.’s assumption that anthropogenic derived seawater Pb
in 1880 was negligible. We also expected that the anomalous mid-19th century Upper
Mississippi Valley Pb I.R. may be traceable further downwind of Rhode Island, perhaps
even in the more southerly waters near Bermuda. To test these assumptions, we report a
new detailed record of Pb concentrations and isotopic signatures in the western North
Atlantic based on coral samples from 1780-2000 and water samples from 1984-2000.
2. Methods
The Scleractinian corals Diploria labyrinthiformis and Diploria strigosa were
chosen for this study. Sample preparation and analysis of surface corals and North
Atlantic seawater followed established, calibrated methods (Shen & Boyle, 1988; Wu &
Boyle, 1997b). A complete description of all modifications is provided elsewhere
(Reuer et al., 2003).
2.1 Sample collection, preparation, and analysis
2.11 Coral collection and cleaning for North Rock samples: Two surface coral species,
Diploria strigosa and Diploria labyrinthiformis, were collected in 1983 from North
Rock, Bermuda. The colonies were cored at a water depth of 11 meters, and core
chronologies were based on the annual density band couplets of Diploria (Logan &
Tomascik, 1991) and radiocarbon measurements (Druffel, 1989). The two cores provide
a continuous, overlapping chronology, with age ranges of 1884 to 1955 (D. strigosa)
and 1936 to 1983 (D. labyrinthiformis). Coral samples were cleaned using a
modification of the Shen and Boyle method (Shen & Boyle, 1988), including oxidant
(alkaline H2O2), reductant (hydrazine with citrate), and strong acids (HNO3). Pb/Ca
ratios were determined by isotope dilution ICP-MS (VG PlasmaQuad 2+) for Pb and
flame AAS (Perkin-Elmer 403) for Ca. A 204Pb spike (Oak Ridge National Laboratory)
was utilized for the isotope dilution (calibrated with a gravimetric Pb concentration
standard, J. T. Baker). The mean standard deviation for Pb/Ca replicates is <3%,
including analytical and cleaning uncertainties.
7
2.12 Coral collection and cleaning for John Smith’s Bay sample. To extend the record
further back in time, a Diploria labyrinthiformis coral sample was collected by
Robbertson Smith (Bermuda Institute of Ocean Sciences) and colleagues for Anne
Cohen (WHOI) from about 1 km SE of John Smith’s Bay Research site on the southeast
edge of the Bermuda platform from a depth of about 15 m. This site is within or near the
surface mixed layer for most of the year and should record changes in eolian fluxes.
Three overlapping slabs were cut from this coral, each slab being best suited for a
portion of the chronology but with overlaps that confirm continuity. Further details on
the chronology and Sr/Ca variability of these corals are given by Goodkin et al. (2005,
2008). These samples were then cleaned and prepared using the procedure described for
the North Rock samples. For samples with > 10 nmol Pb/ mol Ca the Pb/Ca relative
standard deviation of replicate samples is ~10%. For samples with < 10 nmol Pb/ mol
Ca the standard deviation is ~1.4 nmol Pb /mol Ca.
2.13 Seawater collection. Surface mixed layer samples were collected from Station S
(32°29'N, 64°48'W), BATS station (31º50’N, 64º10’W) and BTM (31º42’N, 64º11’W)
from 1983 to 1999. Following collection, the unfiltered samples were acidified to pH<2
with triple-distilled 6N HCl. Pb concentrations were determined by small-volume (1200
L) MgOH2 coprecipitation and isotope dilution ICP-MS (Wu & Boyle, 1997b).
2.14 Isotope analysis. Sample preparation for isotopic analysis of corals and seawater
followed the described elemental methods, except that moderate-volume (300-500 mL)
seawater samples were required and both coral and seawater Mg(OH)2 concentrates
were purified by HBr-HCl anion exchange chromatography prior to analysis. All stable
Pb isotope ratios were determined by multiple collector ICP-MS (Micromass/ GV
IsoProbe). The North Rock corals and the seawater samples were analyzed using the
original interface and hard extraction, while the John Smith’s Bay corals were analyzed
using a new interface and soft extraction. Data processing and corrections have been
documented previously, including eliminating the isobaric interference of 204Hg,
exponential (“beta”) mass fractionation correction normalized with a 205Tl/203Tl spike
and NIST SRM-981, and tailing correction using 209Bi (Reuer, 2002). The mean sample
isotope ratio precision for the North Rock coral samples was consistently less than 250
ppm (2) as documented by Reuer et al. 2003.
8
The data for the John Smith’s Bay corals were collected in the same manner
using a slightly different hardware configuration (new interface, plasma shield with soft
extraction). Using the “beta” and Tl tailing corrections alone, the 206Pb/207Pb and
208
Pb/207Pb SRM-981 Pb isotope values from six daily sessions over a 10-month period
came within 300-700 ppm of the TIMS double-spike values reported by Thirlwall (2000)
(with offsets relatively constant within a day but differing from day-to-day). We
adopted daily correction factors based on the Thirlwall TIMS values and applied these
to the beta-corrected values. A measure of the reliability of this method comes from an
independent internal laboratory standard run on the same days. Corrected in this fashion,
the 206Pb/207Pb and 208Pb/207Pb ratios for this secondary standard from the six sessions
(with 208Pb signal intensities of 0.2-1.5 V from ~25 ng of Pb consumed over ~4 min.)
showed 2standard deviations of 340 ppm and 170 ppm, respectively. The average 2
206
Pb/207Pb error for triplicate measurements on low-Pb coral samples was ~1000 ppm.
This larger replicate variability compared to that on standards is caused by seasonal and
interannual variability between different coral fragments formed over a two year period.
A series of acid blanks were run at the beginning and end of each session and
NBS-981 and an independent Pb standard, both spiked with the Tl solution, were
measured after every ~7 samples to normalize the data. Acid blanks were subtracted
from sample signals using the “on peak zero” assumption. Three procedural column
blanks were also run approximately every 12 samples. These column blanks were
averaged for each day’s run and subtracted from the sample data assuming that the Pb
I.R. of the blank was the average value over the entire column blank dataset. These
column blanks were typically ~50 pg for the John Smith’s Bay samples, and the
correction made little difference to the final data relative to the raw data. During the
course of these measurements, we observed that the on-peak acid blank does not fully
compensate for Pb coming off of the instrumental hardware because ion-laden samples
with 0.2-0.6 V signals of Tl and Pb ablate more Pb off of the hardware (cones, hexapole
tips) than the high purity acid used in the on-peak zero correction. The isotope ratio of
this additional ablated blank depends on which samples had been run recently. Because
we used samples with sufficient Pb that this correction would not be large, we have not
corrected for this ablation blank in this data set, but we suggest that future data sets
9
obtained by plasma ICP-MS could obtain more accurate values on smaller samples if
this ablation blank is taken into account. In particular, we recommend that the low Pb
I.R. NBS-981 standard is run only at the beginning and end of each day, and that a Tlonly solution is aspirated for some time after running these high-Pb standards to
minimize the ablated Pb signal before low-Pb, high Pb I.R. samples are run.
For the purposes of this paper, we will interpret only the 206Pb, 207Pb and 208Pb
data (and not ratios to 204Pb) for the following reasons: (a) 204Pb signals were low for
many samples so the data are not as accurate as we would desire; (b) we report
206
Pb/207Pb because it has been a traditional measure since the early days of
environmental Pb studies by Patterson and coworkers; and (c) we report 208Pb/207Pb
(rather than 208Pb/206Pb) for time series because the temporal variability of 208Pb and
206
Pb in these samples is more similar to each other than either is to 207Pb, so the signal
range is larger for 208Pb/207Pb. But we use 208Pb/206Pb for the isotope/isotope ratio
comparison because this plot has been traditional in the field of anthropogenic Pb
isotope research and researchers are familiar with this diagram. The 206Pb/207Pb,
208
Pb/207Pb and 206Pb/204Pb ratios for the John Smith’s Bay D. labyrinthiformis and the
North Rock D. labyrinthiformis and D. strigosa are given in Table 1A of the
Supplementary Online Material (SOM).
2.2 Estimating seawater [Pb] via coral-seawater partition coefficients
Raw Pb/Ca records from the corals illustrate the century-scale anthropogenic Pb
transient without any fine tuning, although the absolute Pb/Ca ratio from any one coral
can be influenced by biomineralization reactions (see below). Figure 2 presents the
Pb/Ca data for the John Smith’s Bay D. labyrinthiformis, North Rock D.
labyrinthiformis and D. strigosa, and previous North Rock D. strigosa data from Shen
and Boyle (1987). In order to compare these data with modern measurements of
seawater available in this region for the past 30 years, we transform the raw coral Pb/Ca
data using a partition coefficient:
Pbseawater 

[ Pb
Ca
]coral Caseawater
DP
10
where Dp is the empirical partition coefficient and [Ca]seawater is assumed constant (10.3
mmol kg-1). Although thermodynamics of seawater and crystal chemistry are important
in establishing DP, other factors are known to be important: biological differences
between coral species, temperature, coral growth rate, and possibly other factors. As an
example, two species living in the same water show different Pb/Ca (the overlap
between North Rock D. strigosa and D. labyrinthiformis), and two specimens of the
same species show different Dp for the same time period (the two North Rock D.
strigosa specimens). Although we cannot establish partition coefficients with certainty,
we can describe our logic for deriving Dp estimates from our data. In addition to Dp
differences between corals associated with variable growth rates or lattice density, we
must contend with the difficulties of comparing coral Pb data to spatially- and
temporally-limited seawater Pb data. As shown by Boyle et al. (1986) and Wu and
Boyle (1997a), Pb concentrations at an open-ocean site can vary on seasonal and shorter
times scales, yet a sample of coral processed as described here contains the average
response during one or two years of growth. Additionally, Shen and Boyle (1987)
showed that seawater [Pb] near Bermuda in 1984 varied with proximity to the island,
with the distant North Rock (NR) site having [Pb] similar to nearby open-ocean waters
whereas the John Smith’s Bay (JSB) site - less than 1 km from land - had significantly
higher [Pb]. As will be seen below, the JSB sites are also offset in their Pb isotope ratios
(Pb I.R.) compared to the North Rock corals and open-ocean seawater; these differences
must be due to island-based Pb contamination. For this reason, we separate our
discussion of coral-based concentration estimates between NR corals and JSB corals.
This logic is summarized in table 1.
For NR Dp estimates, we aimed to join data from three corals with the seawater
data from Station S, BATS, and BTM (Table 4A) to obtain the most consistent match
between the water data and all of the corals. Unfortunately, our NR coral data
terminates in 1982 whereas the seawater data begins with single samples collected in
the summers 1979 and 1980 with seasonal or higher sampling beginning only in 198384. We obtained a reasonable match for the published 1884-1982 D. strigosa Pb/Ca
record (Shen and Boyle, 1987) by setting DP = 2.8 (Table 2A of SOM, Figure 3). We
then compared our more recent NR D. strigosa Pb/Ca record (1888-1947) with the Shen
11
and Boyle record and observed that the estimates for [Pb] overlapped when DP for the
new NR coral was set to 3.2 (Table 2A of SOM, Figure 3). Pb/Ca data in an NR D.
labyrinthiformis (1938-1978) specimen shows higher Pb/Ca than the two D. strigosa
specimens, and can best be reconciled by setting DP = 3.6 for this specimen (Table 2A
of SOM, Figure 3).
There are two corals from the nearshore JSB sites: a D. strigosa record from
Shen and Boyle (1987) covering 1933-1983, and our new D. labyrinthiformis record
covering 1778-1997. The comparison of these records is complicated by significant site
differences between the two corals: the Shen D. strigosa was taken just inside the
fringing reef crest, whereas the new D. labyrinthiformis was taken just outside the
fringing reef. It is possible and perhaps even likely that the D. strigosa specimen is
more strongly influenced by island Pb than is the new D. labyrinthiformis. Shen and
Boyle (1987) report a dissolved [Pb] concentration of 198 pmol/kg in nearby waters
(April 1984); their D. strigosa record joins this dissolved Pb measurement when DP is
set to 2.8, the same DP value as their NR D. strigosa specimen. If we assume that the
JSB D. labyrinthiformis DP is 3.6 as inferred for the NR D. labyrinthiformis, the
estimated [Pb] for this outside reef crest specimen implies lower [Pb] than the inside
reef crest D. strigosa record for most of the overlap period (Table 2A of SOM, Figure 4).
This difference may make sense given the site differences as noted above. However, we
cannot rule out other values of Dp for either specimen, so this uncertainty must be
considered when interpreting inferred seawater [Pb] differences between the JSB and
NR sites.
3. Results and Discussion
3.1 Pb/Ca data and estimated seawater Pb
Pb/Ca in the coral samples range from 4-80 nmol/mol. The lowest Pb
concentration in the corals is ~4 nmol Pb/ mol Ca for the John Smith’s Bay coral in
1786 (Figure 2). This low concentration remains relatively constant until the mid-1800s.
Averaging 33 subsamples between 1778-1938 (excluding 9 high analyses which we
consider likely to be slightly contaminated for Pb) we estimate 5.0 nmol Pb/ mmol Ca
(1 s.d.= 1.6) as the best estimate for the pre-anthropogenic baseline at this site. Using
12
the Dp value of 3.6 we have selected for this coral, seawater Pb concentrations of 15
pmol/kg are implied as the pre-anthropogenic Pb of western North Atlantic surface
waters. Even if a “detrital” correction were to be considered necessary (we do not), we
can take this value as a confident upper limit for the pre-anthropogenic surface seawater
Pb concentration in this region. In the North Rock corals, estimated seawater [Pb] rises
from 15 to 80 pmol/kg from 1850 to 1925, followed by a 20 year plateau. The major
sources of Pb pollution before 1930 were due to coal combustion and ore smelting, but
after 1930, the use of leaded gasoline eventually dominated all other sources by the
1970’s (Nriagu, 1978, 1979; Nriagu & Pacyna, 1988). There is an abrupt increase in
seawater [Pb] from 1945 to 1955, closely following the consumption of leaded gasoline
in the United States (Wu & Boyle, 1997a). This Pb concentration increase continued
and reached maximum Pb concentrations of ~210 pmol/kg in the 1970s (due to North
American and to some extent European leaded gasoline) followed by a steep twodecade decrease due to the phasing out of leaded gasoline. The rise in estimated [Pb]
concentrations in the John Smith’s Bay samples lagged the rise in the North Rock
samples, probably reflecting a local lag in leaded gasoline utilization on the island
relative to that of the United States. Shen and Boyle (1987) observed that one seawater
sample (1983) and the D. strigosa coral samples (1924-1982) included here from the
John Smith’s Bay area (south of the island, Figure 1) had somewhat higher Pb (~15%)
than seawater and corals from the North Rock area (north of the island, Figure 1). They
attributed this difference to Pb from the island; probably mainly due to Pb gasoline use
as in the U.S., but perhaps also due to other Pb uses (boat bottom paint; Pb anchors and
weights). Peak concentrations and the decline due to the elimination of leaded gasoline
occurred simultaneously at the two sites. We follow Shen and Boyle in their
interpretation, but note that if the D. labyrinthiformis partition coefficient is the same
for the North Rock and John Smith’s Bay corals as we have assumed, the water outside
the fringing reef has lower Pb and a lesser island influence than the coral within the
fringing reef crest.
Despite the uncertainties, we believe that the composite Pb histories in Figure 3
and 4 are a reasonable representation of the anthropogenic Pb transient in western North
Atlantic surface waters for the past two centuries. The interval of biggest discrepancy
between the various records is in the period from ~1880-1920 A.D., where the John
13
Smith’s Bay coral indicates significantly higher Pb than the North Rock corals. We
suspect that the difference is due to significant uses of Pb on the island during that
period, but this must be considered an assumption rather than as an established fact. We
recommend the 1880-1980 North Rock data as the best representation of the western
North Atlantic open-ocean response to anthropogenic Pb emissions for this period.
3.2 Pb Isotope Ratio Data
From the late 1700’s to about 1840 A.D., the 206Pb/207Pb (1.200) and
208
Pb/207Pb
(~2.465) values of our John Smith’s Bay coral are relatively high. These Pb I.R. values
are similar to those of Quaternary marine sediments and ferromanganese nodules in this
basin (the averages of 16 samples excluding one anomalous sample are 206Pb/207Pb =
1.210 and 208Pb/207Pb = 2.509: Chow and Patterson, 1962). Therefore these early coral
samples appear to represent natural Pb inputs to Bermuda surface waters. In the John
Smith’s Bay D. labyrinthiformis coral, maximum Pb I.R. is seen in the middle of the
19th century (Figure 5). We attribute this feature to the distal transport of high Pb I.R.
Upper Mississippi Valley (UMV) emissions that have also been seen in the Great Lakes,
Chesapeake Bay, and Rhode Island estuary sediments (Graney et al. 1995; Marcantonio
et al 2002; Lima et al. 2005). As UMV Pb production declines relative to other Pb
sources, JSB Pb I.R. drop until 1900 (to 1.173 and 2.443), after which 206Pb/207Pb
fluctuates between 1.175-1.185 (until 1965) and 208Pb/207Pb rises to 2.46 in 1939
(before falling again to 2.441 in 1965). Both Pb I.R. rise from 1965 to 1975 (to 1.191
and 2.447), which we suggest is due to the phasing out of some low Pb I.R. sources due
to environmental concerns (e.g. see Ragaini et al. 1977). Then, when Pb gas is phased
out after the late 1970’s, with the U.S. shift preceding the European shift, high U.S. Pb
I.R. becomes dominated by lower Pb I.R. European Pb (Veron et al. 1993). This data
feature is seen in the John Smith’s Bay coral data and Station S, BATS, and BTM water
samples (Table 3A, Figures 6 and 7).
A plot of isotope composition vs 1/Pb (figure 8) can be useful in some contexts
because linear mixing between end members of fixed Pb concentration and isotopic
composition fall along linear trends in this projection. Because surface seawater Pb
concentration represents the steady-state balance between the atmospheric deposition
flux and a (relatively) fixed biological scavenging residence time, the meaning of “fixed
14
concentration” in this context is ambiguous; a single atmospheric source of fixed
isotopic composition but varying flux would appear as a horizontal line; two
atmospheric sources of different fixed isotopic compositions but varying combined flux
would appear the trapezoid defined by the two fixed-source Pb. I.R. horizontal lines; a
single source whose fluxes and isotopic compositions vary independently but smoothly
with time would fall as a trajectory within the limits of the axes.
Although the patterns of Pb I.R. variability in John Smith’s Bay and North Rock
corals are similar between 1880 and 2000, there is a significant Pb I.R. offset between
the John Smith’s Bay coral and the North Rock corals, with the JSB coral showing
lower Pb I.R. (~0.02 for both ratios) than the NR corals. Because both coral records are
well-dated by annual X-ray density counting (and the JSB coral additionally by Sr/Ca
cycle counting, Goodkin et al. 2005, 2008), we do not think it likely that the offset is
due to age errors (i.e., that the estimated 1880 sample from the North Rock coral
actually lived at the same time as the estimated 1850 sample in the JSB coral). Goodkin
et al. 2008 suggest that errors on the order of 10 years are possible for the oldest JSB
samples. The NR strigosa/labyrinthiformis and JSB Pb I.R. isotope records were
generated by the same methods and the same MC-ICPMS instrument with strict quality
control to the NBS981 and an internal lab standard, so this difference cannot be due to
analytical errors (the older Pb isotope record of Shen and Boyle was done by a different
instrument and method (TIMS), so that we cannot be as confident that the small
differences between the two NR records are real). We suggest that the lower Pb I.R. for
the JSB coral must be due to local uses of Pb on Bermuda with lower Pb I.R. than the
contemporaneous emissions from North America.
The trends before the late 1970’s are best explained by evolving Pb ore production
patterns in the United States. Evidence for this explanation includes (1) increased Pb
ore production in the southeast Missouri district from 40% (1966) to 93% (1982) total
US production (United States Bureau of Mines, 1983); (2) elevated 206Pb/207Pb ratios for
the southeast Missouri district (1.303) (Brown, 1967) compared to other US or imported
Pb ores; and (3) the higher consumption of leaded gasoline in the United States relative
to European sources, (approximately 85% in 1970, Wu & Boyle, 1997a). There was an
increased relative proportion of European relative to U.S. leaded gasoline consumption
from 1982 to 1990 (41% to 74%, Wu & Boyle, 1997a), which has lower Pb I.R. than
15
North America (Church et al., 1990; Bollhöfer & Rosman, 2001). Seawater and John
Smith’s Bay coral both show a marked decrease in the 206Pb/207Pb ratio in the past two
decades (Figure 9). This trend is also seen in the 208Pb/207Pb data (online Figure 1A).
As observed in the Rhode Island estuary sediment data, a mid-1800s peak in the
206
Pb/207Pb ratio is visible (1.2106 in ~1850). This maximum is also seen in the
208
Pb/207Pb data. Due to the timing (and the “anthropogenic” Pb I.R., see below), we
believe that this Pb I.R. peak is due to emissions from the crude mid-19th century
smelting of Upper Mississippi Valley (UMV) Pb deposits, whose production climaxed
in 1845 (Heyl et al., 1959; USGS, 1998; Lima et al., 2005). There is a small time
difference between the Rhode Island estuary sediment Pb I.R. peak (1.263 in ~1842
A.D.) and the John Smith’s Bay Pb I.R. peak (1850-1854 A.D.), but this difference may
simply be due to a few years error for both chronologies. Also, it may take a few years
for ocean currents to bring the UMV Pb from the more northern air trajectories derived
from the UMV region (~40°N) southwards to Bermuda (32°N). Although it is difficult
to discern the subtle ~1840 A.D. rise in Pb/Ca in the John Smith’s Bay record, if we
assume a baseline concentration of Pb/Ca = 5 nmol/mol with 206Pb/207Pb = 1.198 and
208
Pb/207Pb = 2.464, we estimate the peak anthropogenic Pb I.R. component has an
isotopic composition of 206Pb/207Pb = 1.32 and 208Pb/207Pb = 2.58. These values are
comparable to the Rhode Island estuary sediment estimates for peak 19th century
anthropogenic Pb I.R. (206Pb/207Pb = 1.325 and 208Pb/207Pb = 2.57). This match supports
the premise that both Pb I.R. peaks (RI estuary and Bermuda coral) are due to UMV Pb
emissions. As one would expect, the 206Pb/207Pb maximum value decreases with
distance from the source (as the influence of the high ore values decreases). In more
recent decades, the JSB anthropogenic 206Pb/207Pb is estimated as 1.167 in 1965, rose to
a peak of 1.198 in 1973 as low Pb I.R. ores were phased out in the U.S., and then fell to
1.157 by 1997 as European Pb sources became more important than U.S. Pb sources.
Regarding the isotopic composition of Pb I.R. in the 1950’s and using the same
assumptions as in the previous paragraph, we estimate that anthropogenic Pb in the
western North Atlantic in the 1950’s had 206Pb/207Pb of ~1.17, which is lower than the
Rhode Island estuary (1.19) but significantly higher than inferred from a Sierra Nevada
pond (1.15, Shirihata et al., 1980) and ALAS curve (1.16, Hurst, 2002). Drawing
16
together the results from the Rhode Island estuary and Bermuda coral with the direct
measurements of Chow and Earl (1970) showing that Boston aerosols and gas have
~0.025 higher 206Pb/207Pb than Southern California aerosols and gas, we argue that east
coast aerosols and the anthropogenic Pb delivered to the western North Atlantic ocean
have had consistently higher Pb I.R. than west coast aerosols and gasoline. Alternatively,
it is possible that the southeast U.S. aerosols are somewhat lower than those of the
northeast U.S. [e.g., as Chesapeake Bay (Marcantonio et al., 2002) shows a lower mid19th century Pb I.R. peak than Rhode Island (Lima et al., 2005)], resulting in
intermediate values for the Sargasso Sea.
The temporal evolution of 208Pb/207Pb vs. 206Pb/207Pb (figure 10) demonstrates
several distinct Pb I.R. sources evolving with time. From 1778-1939, both JSB and NR
corals follow a 208Pb/207Pb vs. 206Pb/207Pb trend that falls above that of the later
anthropogenic era. Pb I.R. values climb towards UMV values in the early period (JSB
samples) then fall to low values in the early portion of the anthropogenic era. From
1949-1970, Pb. I.R. rise and fall along a new trend lying below the earlier period.
Finally, during the period of leaded gasoline phase-out, where the U.S. phase-out
precedes that of Europe, Pb I.R. for NR and JSB corals and seawater samples (from
Station S, BATS, and BTM) traverse to lower values on trend lower in
208
Pb/207Pb than
seen previously. This most recent trend falls near the mixing line between Rhode Island
anthropogenic aerosols and European aerosols.
4. Implications
Pb emitted from the Upper Mississippi Valley mining district in the mid-19th
century can be traced past the Great Lakes, Rhode Island, and Chesapeake Bay out into
in the western North Atlantic Ocean. Thus, the isotopic ratios of Pb are useful as tracers
of anthropogenic Pb emissions and as an age control point for mid-19th century Pb
emitted from the U.S. into the North Atlantic Ocean.
Given these new data, we now know that the Pb concentration of western North
Atlantic surface water was lower in 1800 than 1880, so the correction proposed by
17
Alleman et al. (1999) to estimate anthropogenic Pb I.R. over-corrects the late 20th
century Pb isotope ratios. Hence we conclude that anthropogenic Pb I.R. in the late 20th
century western North Atlantic is significantly higher that found in the Sierra Nevada
alpine ponds and ALAS curves because of additional Pb I.R. sources along its path
across the U.S.
Acknowledgements. We thank E. Druffel and S. Griffin who generously provided the
North Rock coral samples and radiocarbon chronology shown here, R. Smith and A.
Cohen who provided the John Smith’s Bay coral samples, F. Dudas for sharing his Pb
isotope database, and R. Kayser and B. Grant for laboratory assistance.
Figure 1. Map of Bermudan atoll, in reference to eastern North America, with
indications of sites of corals analyzed, modified from URL://www.unesco.org/csi/pub/
papers/smith.htm.
18
Pb/Ca (nmol/mol)
100
John Smiths Bay- D. labyrinthiformis
North Rock- D. strigosa
North Rock- D. labyrinthiformis
Shen- North Rock- D. strigosa
80
60
40
20
0
1750
1800
1850
1900
1950
2000
Age
Figure 2. Pb/Ca of different coral species to the N and S of Bermuda. Note that this plot
does not take into account the varying Dp of the corals.
250
[Pb] pM
200
D. strigosa
D. labyrinthiformis
Shen- D. strigosa
Water: annual avg
150
100
50
0
1880
1900
1920
1940
Age
1960
1980
2000
Figure 3. Surface coral from North Rock and seawater from Station S, BATS and BTM.
Inferred Pb concentrations (in pmol kg-1) from surface coral proxy records and observed
mean annual mixed layer Pb concentrations. The inferred concentrations were
calculated from assumed partition coefficients (Dp) as described in the text.
19
300
[Pb] pM
250
D. labyrinthiformis
Shen- D. strigosa
200
150
100
50
0
1750
1800
1850
1900
1950
2000
Age
Figure 4. Surface coral time series from John Smith’s Bay. Inferred Pb concentrations
(in pmol kg-1) from surface coral proxy records. The inferred concentrations were
calculated from assumed partition coefficients (Dp) as described in the text.
1.22
1.21
Pb/207Pb
1.20
206
1.19
1.18
1.17
1.16
1750
1800
1850
1900
1950
2000
Age
Figure 5. 206Pb/207Pb ratios of Bermudan coral from John Smith’s Bay from the late
1700s to 2000.
20
1.22
1.21
Pb/207Pb
1.20
206
1.19
1.18
1.17
North Rock coral
Seawater
1.16
1850
1900
1950
2000
Age
Figure 6. 206Pb/207Pb ratios of Bermudan corals from North Rock and seawater from
Station S, BATS and BTM.
2.49
208
Pb/207Pb
2.48
2.47
2.46
2.45
2.44
2.43
1850
1900
1950
2000
Age
Figure 7. 208Pb/207Pb ratio of Bermudan corals from North Rock and seawater from
Station S, BATS and BTM. Same legend as Figure 7.
21
Figure 8. 206Pb/207Pb vs. 1/[Pb] (expressed as seawater estimated [Pb] for the corals) for
North Rock and John Smith’s Bay corals.
1.22
1.21
Pb/207Pb
1.20
206
1.19
1.18
1.17
No rth Ro ck Co ral
Jo hn Smiths B ay Co ral
Shen- No rth Ro ck Co ral
Seawater
1.16
1850
1900
1950
2000
Age
Figure 9. 206Pb/207Pb ratio of Bermudan corals from North Rock and John Smith’s Bay
and surface seawater from Station S, BATS and BTM. See online figure 1A for
208
Pb/207Pb.
22
Figure
10. Temporal evolution of 208Pb/206Pb vs 206Pb/207Pb in Bermuda corals. Note expanded
range inset at upper right. JSB=John Smith’s Bay Coral; NR = North Rock corals. The
position of some relevant end members are indicated: RI = Rhode Island anthropogenic
Pb in 1999; Eur = European aerosols (average of data reported by Bollhöfer and
Rosman, 2001: 206Pb/207Pb = 1.14, 208Pb/206Pb = 2.12); N. Atl. seds = average of
sediments and nodules from Chow and Patterson,1962: 206Pb/207Pb = 1.21, 208Pb/206Pb =
2.07; UMV = Upper Mississippi Valley, average of several ores by Heyl et al., 1966;
Russell and Farquhar 1960; Millen et al. 1995: 206Pb/207Pb = 1.38, 208Pb/206Pb = 1.89)
23
2.49
2.48
Pb/207Pb
2.47
208
2.46
2.45
2.44
Reuer No rth Ro ck Co ral
2.43
Jo hn Smith B ay Co ral
Shen No rth Ro ck Co ral
Seawater
2.42
1850
1900
1950
2000
Age
Online Figure 1A. 208Pb/207Pb ratio of Bermudan corals from North Rock and John
Smith’s Bay and seawater from Station S, BATS and BTM.
24
John Smith Bay- D. labyrinthiformis
Slab
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Year
1997
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
1973
1965
1961
1955
1949
1939
1935
1929
1922
1920
1914
1910
1904
1900
1916
1892
1886
1884
1882
1878
1872
1870
1866
1864
1860
1858
1856
1854
1852
1850
1848
1846
1844
1842
1838
1832
206
207
Pb/ Pb
1.1665
1.1686
1.1703
1.1729
1.1697
1.1738
1.1831
1.1829
1.1861
1.1901
1.1908
1.1877
1.1867
1.1736
1.1739
1.1755
1.1737
1.1845
1.1816
1.1788
1.1787
1.1752
1.1800
1.1797
1.1765
1.1733
1.1834
1.1876
1.1900
1.1966
1.2015
1.1959
1.1982
1.1999
1.2020
1.2014
1.1983
1.2051
1.2103
1.2115
1.2055
1.2057
1.2041
1.2008
1.1995
1.1991
1.1921
1.1922
208
207
Pb/ Pb
2.4409
2.4353
2.4357
2.4332
2.4339
2.4366
2.4426
2.4400
2.4418
2.4464
2.4468
2.4442
2.4433
2.4406
2.4411
2.4450
2.4431
2.4603
2.4599
2.4563
2.4550
2.4514
2.4529
2.4519
2.4467
2.4430
2.4506
2.4655
2.4652
2.4708
2.4740
2.4697
2.4718
2.4752
2.4715
2.4732
2.4822
2.4756
2.4832
2.4848
2.4712
2.4784
2.4770
2.4656
2.4750
2.4751
2.4653
2.4594
206
Pb/
18.36
18.26
18.46
18.32
18.30
18.32
18.49
18.58
18.67
18.60
18.64
18.57
18.58
18.31
18.32
18.34
18.31
18.50
18.45
18.40
18.51
18.41
10.45
18.43
18.42
18.33
19.19
18.53
18.66
18.82
19.07
18.68
18.73
18.75
18.75
18.80
18.89
18.79
18.96
18.93
18.90
18.83
18.81
18.75
18.73
18.73
19.07
18.61
North Rock Corals
204
Pb
Year
1981
1978
1977
1976
1975
1974
1973
1972
1970
1969
1968
1964
1962
1961
1960
1959
1958
1957
1956
1954
1953
1952
1951
1950
1949
1948
1947
1946
1945
1944
1943
1942
1941
1940
1937
1936
1935
1932
1929
1928
1927
1926
1925
1924
1923
1922
1921
1920
206
207
Pb/ Pb
1.1993
1.2014
1.2053
1.2037
1.2006
1.1976
1.1964
1.1976
1.1905
1.1889
1.1802
1.1827
1.1836
1.1846
1.1810
1.1799
1.1819
1.1791
1.1818
1.1840
1.1842
1.1885
1.1905
1.1879
1.1865
1.1875
1.1866
1.1864
1.1866
1.1848
1.1865
1.1863
1.1855
1.1857
1.1852
1.1826
1.1828
1.1854
1.1863
1.1862
1.1847
1.1836
1.1822
1.1819
1.1825
1.1800
1.1821
1.1835
208
207
Pb/ Pb
2.4545
2.4540
2.4570
2.4578
2.4530
2.4511
2.4488
2.4516
2.4483
2.4496
2.4455
2.4478
2.4482
2.4506
2.4464
2.4501
2.4499
2.4473
2.4482
2.4509
2.4521
2.4548
2.4564
2.4545
2.4540
2.4562
2.4549
2.4545
2.4548
2.4525
2.4530
2.4537
2.4537
2.4528
2.4511
2.4558
2.4559
2.4597
2.4597
2.4604
2.4626
2.4599
2.4578
2.4581
2.4596
2.4571
2.4605
2.4600
206
Pb/
20.27
21.40
21.43
21.38
22.53
21.39
21.39
21.44
20.57
20.76
21.03
18.35
21.34
21.21
19.81
20.83
21.13
18.73
21.47
18.89
20.97
18.61
21.06
19.43
21.51
23.99
19.22
21.96
21.28
22.76
21.32
22.59
21.83
21.08
21.45
21.01
21.69
21.55
21.31
21.48
20.80
21.06
21.08
20.81
20.88
22.51
20.84
21.14
204
Pb
25
2
3
3
3
3
3
3
3
3
3
3
3
3
1822
1836
1832
1826
1818
1810
1802
1798
1794
1786
1782
1780
1778
1.1912
1.2006
1.1964
1.1875
1.1991
1.2025
1.2004
1.1962
1.1893
1.1993
1.2055
1.2015
1.2024
2.4643
2.4735
2.4553
2.4620
2.4665
2.4727
2.4758
2.4605
2.4504
2.4690
2.4725
2.4714
2.4748
19.37
18.78
18.85
18.49
18.68
18.65
18.61
18.80
18.86
18.68
19.28
18.62
18.60
1919
1917
1916
1915
1914
1913
1911
1910
1908
1907
1905
1904
1903
1902
1901
1898
1893
1888
1887
1886
1.1881
1.1876
1.1878
1.1871
1.1886
1.1895
1.1885
1.1888
1.1920
1.1943
1.1976
1.1976
1.1983
1.1985
1.1999
1.2079
1.2112
1.2105
1.2105
1.2111
Table 1A. Pb I.R. data for corals from John Smiths Bay and North Rock
2.4660
2.4645
2.4657
2.4620
2.4671
2.4675
2.4678
2.4685
2.4730
2.4737
2.4774
2.4773
2.4775
2.4815
2.4779
2.4821
2.4845
2.4850
2.4871
2.4877
23.07
22.48
21.09
21.56
23.73
21.01
21.74
18.96
21.83
22.34
20.69
22.23
20.79
21.78
21.41
22.06
21.48
21.18
22.23
20.63
26
John Smiths BayD. labyrinthiformis
Year
1997
1991
1989
1987
1985
1983
1981
1979
1977
1975
1965
1961
1955
1949
1939
1935
1929
1922
1920
1916
1914
1910
1904
1900
1892
1886
1884
1882
1878
1872
1870
1866
1864
1860
1858
1856
1854
1852
1850
1848
1846
1844
1842
1838
Pb/Ca
nmol/mol
13.7
32.16
48.4
47.39
60.16
50.99
51.36
60.7
64.45
55.03
47.22
44.28
39.16
31.89
16.01
20.07
23.21
30.36
29.64
28.42
24.04
26.07
28.07
28.73
15
17.25
10.68
15.06
11.27
9.27
7.02
9.52
6.49
5.49
8.93
6.94
6.36
5.33
6.22
6
5.52
5.36
5.97
5.95
KD =
3.6
North RockD. strigosa
KD =
3.2
[Pb]
pM
40
94
142
139
176
150
151
178
189
161
138
130
115
94
47
59
68
89
87
83
71
76
82
84
44
51
31
44
33
27
21
28
19
16
26
20
19
16
18
18
16
16
18
17
Year
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
[Pb]
pM
21
20
20
21
22
24
24
26
26
27
28
32
30
30
32
31
32
39
36
37
42
46
51
52
48
53
53
56
65
57
53
56
60
60
57
60
66
89
87
71
77
79
83
78
Pb/Ca
6.4
6.1
6.2
6.4
6.6
7.2
7.4
7.8
8.0
8.3
8.6
9.7
9.2
9.0
9.7
9.3
9.6
11.8
11.1
11.3
12.7
13.9
15.5
15.8
14.5
15.9
16.1
17.0
19.8
17.2
16.2
16.9
18.3
18.3
17.4
18.1
20.0
27.1
26.3
21.5
23.2
23.9
25.0
23.7
North RockD. labyrinthiformis
KD =
3.8
Year
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1964
1966
1968
1970
1971
1972
1973
1975
1976
1978
[Pb]
pM
89
82
91
110
102
102
99
90
93
92
111
109
132
135
152
170
170
183
179
165
153
169
180
175
181
185
186
163
185
211
202
209
196
217
223
201
Pb/Ca
31.9
29.7
32.9
39.5
36.6
36.6
35.5
32.3
33.6
33.2
40.0
39.1
47.5
48.5
54.8
61.1
61.2
65.9
64.5
59.2
55.2
60.9
64.9
63.1
65.1
66.7
67.1
58.7
66.4
75.9
72.6
75.2
70.6
78.3
80.2
72.4
27
1836
1832
1826
1822
1818
1810
1802
1798
1794
1786
1782
1780
1778
7.22
6.93
5.81
5.84
10.7
5.91
5.33
6.59
8.05
3.61
5.34
5.01
6.4
21
20
17
17
31
17
16
19
24
11
16
15
19
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
24.0
23.4
25.5
25.6
26.7
24.3
24.0
25.1
23.8
24.8
24.9
22.1
27.5
31.5
30.5
27.9
27.3
26.4
25.5
25.1
79
77
84
84
88
80
79
83
79
82
82
73
91
104
101
92
90
87
84
83
Table 2A. Pb concentration data for corals from John Smiths Bay and North Rock
Seawater
Year
Nov 2000
Jun 1999
Jul 1997
Nov 1996
Dec 1995
Jan 1994
Oct 1990
Oct 1989
Jun 1989
Oct 1988
Sept 1988
Sept 1988
Apr 1987
Jan 1987
Dec 1985
Mar 1985
Dec 1984
Nov 1982
206
207
Pb/ Pb
1.1743
1.1739
1.1749
1.1748
1.1742
1.1786
1.1831
1.1835
1.1869
1.1821
1.1889
1.1855
1.1901
1.1920
1.1925
1.1970
1.1975
1.1994
208
207
Pb/ Pb
2.4436
2.4417
2.4437
2.4430
2.4408
2.4437
2.4452
2.4448
2.4475
2.4457
2.4428
2.4458
2.4472
2.4508
2.4482
2.4507
2.4505
2.4511
Table 3A. Seawater samples collected 1979-2000. Data through 1996 from Wu and
Boyle (1997a) and references therein, supplemented by more recent samples collected
by our laboratory.
Year
[Pb] pM
28
1979
1980
1983
1984
1985
1986
1987
1988
1989
1990
1993
1995
1996
1997
1998
1999
160
163
128
122
102
87
87
87
72
64
55
58
50
39
40
37
Table 4A. Annual average of seawater samples near Bermuda from 1980-2000. This
includes the data given in Wu and Boyle (1997a) and references therein as well as
unpublished data from our laboratory.
29
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