Introduction
Humans have greatly perturbed the biogeochemical cycle of Pb, with the most
dramatic changes during the 1950s–1990s (Schwikowski et al., 2004). This
resulted in large increases in Pb to not only local environments (Harris and
Davidson, 2005), but also to remote areas such as Greenland (Bory et al.,
2014) and Antarctica (Rosman et al., 1994). Because Pb is a potent neurotoxin
(ATSDR, 2007), efforts to reduce anthropogenic Pb emissions were widespread
throughout the 1980s–1990s. Since the phasing out of leaded gasoline by most
northern European and American countries and the passing of other forms of
clean air regulation, atmospheric Pb emissions have declined dramatically in
the past 3 decades (EMEP WebDab, 2017). As a result, far less Pb has been
mobilized into the atmosphere and less deposited in remote places such as the
open ocean.
Map of the cruise transect. GEOVIDE samples are solid black squares
(concentration and isotope data) and circles (concentration data only). The
blue star is GA 03 (2010); the red circles are Atlantis II 123 (1989); the
white squares are EN328 (1999); the white triangles are JGOFS (1989); the red
pentagons are TTO 1981; the blue squares are IOC-2 (1993); the yellow stars
are GA 02 (2010).
Lead pollution in the North Atlantic Ocean has been studied more than the
other ocean basins. The United States consumed the largest quantity of leaded
gasoline of any nation from 1930 to 1980, and carried by the prevailing
westerly winds (30–60∘ N), this produced the most visible oceanic
contamination in the North Atlantic Ocean. Of relevance to this study
(Fig. 1), surface Pb concentrations ([Pb]) were measured in 1981 (TTO, Weiss
et al., 2003), 1989 (Atlantis II 123, this
work; JGOFS, Martin et al., 1993), 1993 (IOC-2, Veron et al., 1999), and 1999
(Endeavor 328, Noble et al., 2015, and this work).
More recent campaigns through the GEOTRACES program occurred in 2010 (GA02,
Schlitzer et al., 2018) and 2010/2011 (GA03, Noble et al., 2015).
In the western North Atlantic, repeat sampling of time series locations has
documented the reduction in oceanic [Pb] and changes in sources with time. At
BATS (Bermuda Atlantic Time Series) in the 1970s and 1980s, concentrations
were 80–160 pmol kg-1 near the surface, but 25 pmol kg-1 at
depth (Boyle et al., 2014, and references therein). As Pb emissions were
reduced and older high-Pb surface waters penetrated the interior, elevated
[Pb] could be seen as a subsurface plume in waters at increasingly deeper
depths over time. At the latest occupation of BATS in 2011, surface water
concentrations were less than 20 pmol kg-1 (Noble et al., 2015)
compared to 160 pmol kg-1 observed in 1979 (Schaule and Patterson,
1983) and ∼ 200 pmol kg-1 inferred from coral data from the
mid-1970s (Kelly et al., 2009). Despite a dramatic reduction in [Pb], it is
still believed that a large fraction of current Pb deposition results from
coal and other combustion, and industrial processes, based on positive matrix
factorization analysis of aerosols (Shelley
et al., 2017, 2018; Noble et al., 2015). In the tropical Atlantic, another
2010–2011 study found that 50 %–70 % of Pb in the surface ocean was
anthropogenic in origin (Bridgestock et al., 2016), with the remaining
fraction from natural North African dust.
Our aim in this study was to evaluate whether the North Atlantic Ocean was
still dominated by anthropogenic sources as in previous expeditions, and if
so which ones. This was motivated by the changes in sources documented by
Noble et al. (2014) in their study of Pb sources to the mid-North Atlantic,
who found a shift from North American to European to industrial sources of Pb
over the past couple of decades. The North Atlantic Ocean has been well
studied for Pb contamination since the 1970s, enabling us to place this work
in the greater context of historical Pb emissions. This study was strongly
enhanced by the partnership of the environmental trace metal GEOTRACES
program with the OVIDE program's long-term studies of physical oceanographic
parameters in the northeastern Atlantic (García-Ibáñez et al.,
2015).
Methods
Sample collection
The GEOVIDE cruise track began in Lisbon, Portugal, on 15 May 2014 and
followed the OVIDE section from the Iberian upwelling system to the subpolar
North Atlantic region up to the Greenland margin before continuing on to the
Labrador Sea at the Canadian margin, finishing on 30 June 2014. One liter
Nalgene HDPE sample storage bottles were acid cleaned and stored, and
double-bagged as previously described (Noble et al., 2015). Trace metal clean
seawater samples were collected using the French GEOTRACES clean rosette
(General Oceanics Inc. Model 1018 Intelligent Rosette), equipped with new,
pre-cleaned 12 L GO-FLO bottles (Cutter and Bruland, 2012). The rosette was
deployed on a 6 mm Kevlar cable with a dedicated custom-designed clean
winch. Immediately after recovery, GO-FLO bottles were individually covered
at each end with plastic bags to minimize contamination. They were then
transferred into a clean container (ventilated by class-100 source air) for
sampling. For Stations 1, 11, 15, 17, 19, 21, 25, 26, 29, and 32, samples
were filtered with 0.2 µm capsule filters
(SARTOBRAN® 300, Sartorius). For all other
stations (13, 34, 36, 38, 40, 42, 44, 49, 60, 64, 68, 69, 71, 77) seawater
was filtered directly through paired filters (Pall Gelman Supor
0.45 µm polystersulfone, and Millipore mixed ester cellulose MF
5 µm) mounted in Swinnex polypropylene filter holders, following
the Planquette and Sherrell (2012) method. All samples were acidified back in
the MIT laboratory with 2 mL trace metal clean 6 M HCl per liter of
seawater (final pH ∼2).
Previously unpublished Pb and Pb isotope data from cruises from 1989
(Atlantis II cruise 123) and 1999 (Endeavor cruise EN328)
are included here for evaluation of the decadal evolution of Pb in the
eastern North Atlantic. We supplement our 1989 data with two published JGOFS
stations (Martin et al., 1993). Our 1989 samples were collected using “vane
bulb” samplers (Boyle et al., 1986) and the 1999 samples were collected
using the MITESS mooring sampler (Bell et al., 2002). Samples were stored in
acid-cleaned 250 mL HDPE bottles.
Pb concentrations
GEOVIDE samples were analyzed for Pb concentrations
at least 1 month after acidification during more than 36 analytical sessions
using the isotope-dilution ICP-MS method described in Lee et al. (2011),
which includes pre-concentration on nitrilotriacetate (NTA) resin and
analysis on a quadrupole ICP-MS (Fisons PQ2+). Method details including all
cleaning protocols are available in the metadata file, along with the data,
in the BCO-DMO repository (see Sect. 2.4).
Briefly, triplicate subsamples (1.3 mL) were spiked with a known 204Pb
spike and the pH was raised to 5.3 using a trace metal clean ammonium acetate
buffer, prepared at a pH of between 7.95 and 7.98. Approximately 2400 beads
of cleaned NTA Superflow resin (Qiagen Inc., Valencia, CA) were added to the
mixture and equilibrated. After equilibration, the resin was rinsed with
distilled water and then Pb was eluted with a 0.1 M solution of trace metal
clean HNO3 before analysis by ICP-MS.
On each day of sample analysis, procedural blanks were determined for 12
replicates of in-house reference seawater with negligible [Pb]. The blanks
analyzed concurrently with these samples
ranged from 2.2 to 9.9 pmol kg-1, averaging 4.6±1.7 pmol kg-1. Within a day, procedure blanks were very reproducible,
with an average standard deviation of 0.7 pmol kg-1, resulting in
detection limits (3× the low-level standard deviation) of
2.1 pmol kg-1. Replicate analyses of three different large-volume
seawater samples (one with ∼11 pmol kg-1, another with
∼24 pmol kg-1, and a third with ∼38 pmol kg-1)
indicated that the precision of the analysis is 4 % or
1.6 pmol kg-1, whichever is larger. Triplicate analyses of an
international reference standard, SAFe D2, were 27.2±1.7 pmol kg-1.
Pb concentration analysis for 1989 samples (Atlantis II 123) was
achieved by 204Pb isotope dilution with Mg(OH)2 coprecipitation
followed by VG PQ2+ quadrupole ICPMS (Wu and Boyle, 1997) (analyzed in
1996) and 1999 (Endeavor 328) Stations 4, 5, 7, 9, 10 and 11
(analyzed between 1999 and 2003). Endeavor 328 Stations 2, 3, 8, and
10 were determined using NTA-extraction ID ICPMS (Lee et al., 2011)
(determined in 2010). Long-term quality control seawater samples were
included in each run, and overlapped with new QC samples when the previous QC
samples were depleted. Endeavor 328 Station 10 was determined twice
by two analysts 8 years apart (in 2002 by Mg(OH)2 coprecipitation
ID-ICPMS, and in 2010 by NTA-extraction ID ICPMS). A regression of the 2010 vs. 2002
data forced through the origin had a slope of 0.945. We suggest that this
small offset provides a reasonable estimate of our inter-decadal analytical
reproducibility. It also demonstrates that Pb is not continuously leached
from well-cleaned HDPE bottles during decadal-scale storage.
Stable Pb isotopes
GEOVIDE samples were analyzed for stable Pb isotopes during 11 mass
spectrometry sessions by the method of Reuer et al. (2003) as modified by
Boyle et al. (2012). In brief, ∼500 mL of seawater was
pre-concentrated using a low-blank double magnesium hydroxide
co-precipitation, induced by minimal addition of high-purity ammonia solution
and mixing (typically 8 µL ammonia per 1 mL seawater sample). The
precipitate was dissolved in a minimal amount of high-purity 6 M HCl before
undergoing another ammonia addition and second Mg(OH)2 coprecipitation.
The final precipitate was dissolved in ∼1 mL of high purity 1.1 M HBr
the day of purification by anion exchange chromatography (Eichrom
AG1x8). Samples were
dried and stored in PTFE vials until isotope ratio analysis on a
GV / Micromass IsoProbe multicollector ICPMS using an APEX / SPIRO
desolvator. Just before analysis, samples were dissolved for several minutes
in 10 µL concentrated ultrapure HNO3 followed by addition of
400 µL of ultrapure water and spiked with an appropriate amount of
Tl for mass fractionation correction. IsoProbe multicollector ICPMS Faraday
cups were used to collect on 202Hg, 203Tl, 205Tl, 206Pb,
207Pb, and 208Pb. An Isotopx Daly detector with a WARP filter was
used to collect on 204Pb + 204Hg. Because the deadtime of the
Daly detector varied from day to day, we calibrated deadtime on each day by
running a standard with known 206Pb / 204Pb at a high 204 count
rate. The counter efficiency drifts during the course of a day, so we
established that drift by running a standard with known
206Pb / 204Pb (and a 204 count rate comparable to the samples)
every five samples. Tailing from one Faraday cup to the next was corrected by
the 209Bi half-mass method as described by Thirlwall (2000).
On each analytical date, we calibrated the instrument by running NBS981 and
normalized measured sample isotope ratios to our measured raw NBS981 isotope
ratios to those established by Baker et al. (2004). Using this method for 22
determinations of an in-house Pb isotope standard solution shows that for
samples near the upper range of the Pb signals shown for samples
(∼ 1 V), 206Pb / 207Pb and 208Pb / 207Pb
were reproduced to ∼200 ppm (2 SE). Low-level samples will be worse
than that but generally better than 1000 ppm (2 SE) in this data set.
Because of the drift uncertainty in the Daly detector,
206Pb / 204Pb for samples in the mid-to-upper range of sample
concentrations will be reproducible at best to ∼500 ppm (2 SE).
We have intercalibrated Pb isotope analyses with two labs as reported in
Boyle et al. (2012). The outcome of that intercalibration suggests that the
accuracy of our measurements approaches the internal analytical
reproducibility we note above.
Pb isotope precision for the complete analytical procedure can be assessed by
duplicate measurements of samples. In most cases, the replicated samples were
chosen because they fell off of the trend of adjacent samples. That could be
due either to contamination of the subsample used for the analysis or to the
contamination of the sample in its primary sample bottle. As shown in Fig. S2
in the Supplement, the replicate analysis usually agreed within
better than 1000 ppm for 206Pb / 207Pb and
208Pb / 207Pb, and 5000 ppm for 206Pb / 204Pb.
Given a fixed sample size and an order of magnitude range of Pb
concentrations in samples, the poorest replicates are at the lower
concentrations. Using the pooled 2 sigma standard deviation of duplicates
(excluding a few outliers), the formal statistics are shown in Table 1.
2σ pooled standard deviation of duplicate Pb isotope
analyses.
[Pb]
206Pb / 207Pb
208Pb / 207Pb
206Pb / 204Pb
pmol kg-1
0–20
0.0077 (n=10)
0.0025 (n=10)
0.09 (n=4)
20–40
0.0030 (n=15)
0.0021 (n=15)
0.08 (n=14)
40–60
0.0007 (n=8)
0.0021 (n=8)
0.08 (n=4)
Near-surface (11–20 m) concentrations of Pb. Plot created in Ocean
Data View (Schlitzer, 2017).
Pb isotope data from the 1999 samples were obtained by IsoProbe
Multicollector ICPMS after Mg(OH)2 preconcentration and anion exchange
purification as described by Reuer et al. (2003). As for the GEOVIDE samples,
the mass spectrometer was calibrated using NBS981.
Data management
All [Pb] and isotope data related to the GEOVIDE data set in this manuscript
have been submitted to BCO-DMO and are available at
http://www.bco-dmo.org/dataset/651880/data (last access: 3 August 2018)
and http://www.bco-dmo.org/dataset/652127/data (last access: 3 August
2018) (Boyle et al., 2016) and from the 2017 BODC
International GEOTRACES Intermediate Data Product v2 (Schlitzer, 2018). All other data are available in Table 2.
Results and discussion
Outliers
In this data set, we did not encounter any samples that did not yield
acceptably reproducible results upon repeated analysis, so we believe that
the data truly represent the concentration and isotope ratio of
Pb in the sample collection bottle. However, there were a few samples with
elevated Pb (based on comparison to adjacent samples) and for which no
obvious hydrographic argument could be made for the anomaly. We observed that
the samples taken from the GOFlo in rosette position 1 (usually the
near-bottom sample) were always higher in [Pb] than the samples taken
immediately above that, and that the excess decreased as the cruise proceeded
(Fig. S1). The Pb isotope ratios of these samples were higher than the
comparison bottles as well. At two stations, where our near-bottom sample was
taken from rosette position 2 rather than 1, there was no Pb excess over the
samples immediately above. We believe that this evidence points to GoFLO
bottle-induced contamination that was being slowly washed out during the
cruise, but never completely. A similar pattern was observed for the samples
taken from rosette positions 5, 20 and 21, when compared to the
depth-interpolated [Pb] from the samples immediately above and below. We do
not believe that these samples should be trusted as reflecting true ocean
[Pb], so all of the samples from these GOFlos are excluded in our discussion
of this work, although they are included and flagged as unreliable within the
data repositories.
Data from the 1989 Atlantis II 123 and 1999 EN328 cruises.
Depth
Pb
T
S
m
pmol kg-1
∘C
pss
Atlantis II cruise 123, Station 4, 22∘ N 36∘ E, 15 Oct 1989
1
44
27.40
19
62
27.20
36.521
39
75
27.20
36.792
58
58
24.46
37.097
77
83
21.38
37.150
97
91
21.14
37.225
116
77
21.30
37.271
135
108
21.29
37.269
154
96
21.00
37.190
174
104
20.12
37.085
212
106
19.75
37.020
232
121
20.21
37.017
251
139
19.07
36.779
270
117
17.38
36.457
361
139
14.54
35.971
425
135
13.39
35.824
477
129
12.37
35.685
574
139
11.07
35.518
594
140
10.78
35.483
622
132
10.37
35.434
815
93
7.68
35.028
844
85
35.004
872
88
34.981
Atlantis II cruise 123, Station 5, 26.33∘ N 33.67∘ E, 16 Oct 1989
1
47
27.20
37.389
19
74
27.20
37.384
39
74
27.06
37.364
77
95
21.53
37.336
97
112
21.35
37.282
116
111
21.37
37.262
135
103
21.21
37.230
154
99
20.95
37.173
174
111
20.46
37.065
193
106
19.33
36.863
212
114
18.70
36.717
359
158
14.98
36.047
476
156
13.19
35.790
575
156
11.78
35.606
595
162
11.56
35.580
624
132
11.26
35.541
879
101
35.173
1018
83
35.092
1244
89
35.063
1278
95
35.069
Continued.
Depth
Pb
T
S
m
pmol kg-1
∘C
pss
Atlantis II cruise 123, Station 7, 31∘ N 31∘ E, 20 Oct 1989
1
95
21.10
19
97
39
100
58
89
22.59
35.334
77
94
19.33
36.795
97
85
18.84
36.697
116
94
18.61
36.655
154
104
18.09
36.553
174
100
17.71
36.488
193
104
17.33
36.429
212
117
16.97
36.363
232
119
16.58
36.293
366
134
14.26
35.928
405
130
13.74
35.862
484
135
12.74
35.730
582
136
11.67
35.592
601
136
11.53
35.575
630
145
11.31
35.547
826
100
9.27
35.404
883
92
35.331
1244
75
35.328
1312
71
6.44
35.303
Atlantis II cruise 123, Station 9, 35∘ N 29∘ E, 22 Oct 1989
1
94
22.50
36.480
19
90
22.50
36.480
58
86
22.20
36.473
97
87
16.88
36.186
194
99
14.77
35.947
253
115
14.33
35.835
272
106
13.97
35.793
389
116
13.19
35.665
409
110
13.09
35.639
488
105
11.97
35.604
587
123
11.52
35.483
607
121
11.28
35.480
836
104
10.43
35.551
865
99
10.00
35.544
1294
88
35.257
1328
91
10.67
35.225
Continued.
Depth
Pb
206Pb / 207Pb
208Pb / 207Pb
T
S
m
pmol kg-1
∘C
permil
Endeavor cruise 328, Station 2, 26.5∘ N 38.5∘ E, 1 Sep 1999
0.5
36.9
26.600
37.590
48
39.1
26.191
37.573
146
37.9
19.896
36.832
196
41.0
18.237
36.552
293
48.3
16.741
36.318
441
60.3
14.536
35.974
589
80.3
12.503
35.689
687
89.9
10.994
35.505
785
79.4
9.246
35.326
931
65.2
7.772
35.195
1076
51.1
6.626
35.142
1273
44.6
5.820
35.150
Endeavor cruise 328, Station 3, 24∘ N 37.5∘ E, 2 Sep 1999
0.5
25.5
21.800
36.120
49
26.9
25.947
37.536
98
30.9
22.853
37.385
147
33.2
21.215
37.161
194
36.6
19.224
36.785
290
45.9
17.000
36.369
429
61.4
14.575
35.979
569
83.2
11.940
35.607
653
83.1
10.978
35.495
744
81.1
9.455
35.343
883
63.9
7.850
35.193
1017
47.7
6.754
35.121
1216
41.3
5.782
35.108
Endeavor cruise 328, Station 4, 22∘ N 36∘ E, 3 Sep 1999
0.5
28.8
26.500
37.430
56
35.0
1.1793
2.4469
24.292
37.463
102
34.9
1.1795
2.4478
22.404
37.386
151
39.5
1.1784
2.4456
21.510
37.276
201
38.5
1.1812
2.4461
19.852
36.923
296
48.5
1.1847
2.4460
17.198
36.412
438
65.0
1.1881
2.4484
14.096
35.931
584
89.4
1.1880
2.4481
11.889
35.618
664
94.4
1.1872
2.4478
10.563
35.456
765
84.6
9.310
35.299
957
49.5
1.1847
2.4485
7.009
35.087
1222
41.7
1.1852
2.4527
5.556
35.057
1244
36.2
5.559
35.058
1473
25.4
1.1872
2.4582
4.812
35.071
1886
24.4
1.1859
2.4581
3.774
35.027
2117
18.8
1.1873
2.4614
3.409
35.000
2442
13.1
1.1881
2.4617
2.929
34.965
2848
17.7
1.1899
2.4599
2.569
34.938
3396
14.1
1.1910
2.4654
2.241
34.911
3858
9.5
2.407
34.894
4472
13.6
2.556
34.928
5293
8.9
2.406
34.876
Continued.
Depth
Pb
206Pb / 207Pb
208Pb / 207Pb
T
S
m
pmol kg-1
∘C
permil
Endeavor cruise 328, Station 5, 26.33∘ N 33.67∘ E, 6 Sep 1999
0.5
37.8
26.300
37.560
50
39.5
26.234
100
38.6
20.685
150
43.6
19.399
200
44.3
17.815
304
49.1
16.130
439
61.4
13.939
585
86.6
12.386
35.684
680
97.9
10.794
35.505
781
87.8
9.629
925
71.6
8.158
1054
64.5
7.118
1283
42.9
6.242
1459
44.4
5.623
35.207
1955
34.4
4.184
35.082
2299
25.2
3.482
35.008
2638
20.3
3.013
34.962
3810
14.4
2.453
34.899
Endeavor cruise 328, Station 6, 27.5∘ N 29.33∘ E, 7 Sep 1999
0.5
40.5
25.600
37.440
52
43.9
25.353
37.274
100
45.7
20.192
36.973
149
43.8
19.566
36.972
198
48.5
18.041
36.640
295
53.0
15.842
36.192
437
66.5
13.819
35.880
590
95.0
10.609
35.438
682
98.6
780
91.8
9.604
35.397
933
75.5
8.225
35.293
1078
59.4
7.464
35.248
1272
52.0
6.447
35.264
Endeavor cruise 328, Station 7, 31∘ N 31∘ E, 9 Sep 1999
0.5
37.8
26.000
37.050
47
38.8
1.1783
2.4452
23.523
36.932
98
32.3
1.1783
2.4458
20.337
36.763
98
34.1
1.1783
2.4445
20.337
36.763
148
42.2
7.620
34.411
197
41.3
1.1829
2.4455
18.186
36.558
295
46.4
1.1842
2.4471
16.676
36.294
436
56.8
1.1866
2.4483
14.879
36.018
586
58.3
1.1888
2.4493
12.734
35.716
680
68.1
1.1865
2.4468
11.259
35.520
782
82.4
1.1867
2.4478
9.903
35.405
937
75.9
1.1846
2.4492
8.310
35.351
1261
62.6
1.1821
2.4507
6.498
35.322
1458
65.1
1.1812
2.4505
5.725
35.264
1745
49.7
1.1806
2.4501
4.606
35.142
2038
43.9
1.1803
2.4506
3.857
35.060
2337
36.1
1.1808
2.4518
3.198
34.990
2681
16.3
2.797
34.956
3528
1.1812
2.4532
2.261
34.912
4016
12.6
2.510
0.000
4311
16.5
2.506
0.000
Continued.
Depth
Pb
206Pb / 207Pb
208Pb / 207Pb
T
S
m
pmol kg-1
∘C
permil
Endeavor cruise 328, Station 8, 35∘ N 29∘ E, 11 Sep 1999
0.5
32.8
25.400
36.390
40
40.8
94
39.8
146
44.1
195
45.2
294
50.3
390
59.1
485
66.7
586
72.9
687
80.7
787
80.0
856
80.6
9.081
35.480
1025
75.2
7.993
35.489
1130
71.4
1273
68.7
5.891
35.241
1518
67.0
4.847
35.107
1764
60.5
3.973
34.989
1960
52.5
3.706
34.979
2152
44.9
3.466
34.965
2352
40.0
3.347
34.967
2943
25.2
2.902
34.940
3091
21.1
2.825
34.935
3242
19.8
2.807
34.933
Endeavor cruise 328, Station 9, 45.52∘ N 21.48∘ E, 15 Sep 1999
0.5
40.1
18.400
35.730
48
39.0
1.1825
2.4481
18.756
35.750
146
45.9
1.1834
2.4469
13.690
35.743
195
50.7
1.1839
2.4465
13.511
35.757
291
1.1871
2.4485
12.676
35.640
392
57.7
1.1870
2.4482
11.909
35.545
446
56.3
1.1863
2.4482
11.697
35.550
616
69.3
641
78.9
1.1843
2.4484
9.872
35.325
660
82.3
1.1842
2.4482
9.474
35.276
841
63.9
1.1861
2.4506
7.508
35.156
1005
59.3
1.1854
2.4510
6.281
35.140
1189
66.6
1.1851
2.4514
5.068
35.038
1353
65.3
1.1839
2.4504
4.264
34.961
1732
62.0
1.1834
2.4489
3.571
34.899
2061
52.2
1.1827
2.4482
3.333
34.896
2321
45.8
1.1822
2.4490
3.282
34.922
2702
32.2
3.050
34.942
2817
38.5
2.958
34.943
2840
25.6
1.1835
2.4507
2.944
34.943
3310
16.3
1.1831
2.4520
2.727
34.929
Continued.
Depth
Pb
206Pb / 207Pb
208Pb / 207Pb
T
S
m
pmol kg-1
∘C
permil
Endeavor cruise 328, Station 10, 42∘ N 17.75∘ E, 16 Sep 1999
0.5
53.5
20.000
35.900
39
50.4
1.1797
2.4438
18.929
35.855
95
51.9
13.555
35.762
147
56.0
1.1794
2.4438
13.025
35.740
197
54.1
1.1808
2.4443
12.589
35.684
294
54.4
1.1830
2.4468
12.014
35.612
441
67.0
1.1811
2.4458
11.464
35.575
588
76.3
1.1830
2.4466
10.641
35.474
688
85.2
1.1814
2.4458
10.464
35.536
780
91.3
1.1821
2.4462
10.459
35.655
931
90.0
1.1804
2.4483
10.430
35.873
1078
88.3
1.1800
2.4479
9.901
35.898
1272
78.8
1.1814
2.4487
7.886
35.602
1440
74.2
1.1813
2.4471
5.519
35.211
1680
69.5
1.1814
2.4465
4.043
34.995
1864
67.5
1906
77.5
1.1819
2.4477
3.494
34.935
2215
45.3
1.1796
2.4467
3.348
34.963
2518
35.9
1.1788
2.4455
2.983
34.961
2974
33.6
3604
16.9
1.1811
2.4508
2.281
34.915
4086
17.6
1.1815
2.4522
2.176
34.905
Endeavor cruise 328, Station 11, 38.58∘ N 22.28∘ E, 19 Sep 1999
0.5
52.3
22.600
36.480
50
50.2
99
48.4
150
95.9
196
63.7
14.829
35.997
296
67.1
13.613
35.838
429
72.8
12.393
35.688
586
91.3
11.207
35.552
659
81.7
784
95.9
10.467
35.628
876
82.4
9.057
35.450
1150
80.9
1270
77.4
7.149
35.442
1635
75.7
4.690
35.088
1925
75.7
2046
62.6
3.707
34.976
2344
51.5
3.337
34.967
2557
37.8
3.128
34.958
2845
42.4
2.872
34.947
3025
31.1
2.767
34.937
3331
28.3
2.652
34.928
3468
28.2
2.622
34.923
3907
30.3
2.586
34.913
4212
25.1
2.568
34.908
In addition, we observed high [Pb] in most of the samples from Station 1 and
very scattered Pb isotope ratios. The majority of these concentrations were
far in excess of those values observed at nearby Station 11, and also the
nearby USGT10-01 (Noble et al., 2015). Discussion among other cruise
participants revealed similarly anomalous data for other trace metals (e.g.,
Hg species; Lars-Eric Heimburger, personal communication, 2016). After discussion at the 2016
GEOVIDE post-cruise workshop, we came to the conclusion that this is evidence
of GoFlo bottles not having sufficient time to “clean up” prior to use, and
that most or all bottles from Station 1 were contaminated. Station 1 data are
not discussed in this work, but as with the suspicious GOFlos throughout the
cruise, the Station 1 data are included and flagged as unreliable in the data
repositories.
We include mention of these outlier data to demonstrate the high quality of
our other data, and to encourage future expeditions to both clean their GOFlo
bottles before the cruise (as was done here) and also test them for
contamination-prone elements prior to embarking on their research expeditions
or onboard (e.g., Fe as in Measures et al., 1995). As demonstrated after
Station 1, although soaking in seawater is often sufficient to “clean” the
bottles, gaskets or other bottle components could remain as persistent
contamination sources, as seen on 4 of the 24 bottles from this expedition.
Section plot of Pb concentrations in the GEOVIDE section. Plot
created in Ocean Data View (Schlitzer, 2017).
Near-surface ocean
Near-surface waters (11–20 m) displayed a moderate range in [Pb] of
11–30 pmol kg-1 across the transect (Fig. 2). The highest
concentration was located near the Portuguese coast (30 pmol kg-1).
Lead concentrations decreased 3-fold with distance from the coast, down to
11.5 pmol kg-1, in the core of the far arm of the North Atlantic
Current. An excellent pictorial representation of the relevant water masses
discussed here can be found in García-Ibáñez et al. (2018).
Near-surface concentrations were higher in the Iceland Basin and Irminger Sea
(Stations 21–60; 18.8–23.5 pmol kg-1), and in Station 64, just past
the tip of Greenland. The remainder of the Labrador Sea (Stations 68–77) had
lower [Pb] (12.1–16.2 pmol kg-1).
Section plot of 206Pb / 207Pb concentrations in
the GEOVIDE section. Plot created in Ocean Data View (Schlitzer, 2017).
The pattern of decreasing [Pb] over the Iberian Abyssal Plain
(Stations 11–19) correlates strongly with increasing distance from the shore
(Pearson's correlation, r=-0.989, p<0.001). This finding
agrees well with atmospheric deposition models that show higher dust inputs
closer to the African continent (Schepanski et al., 2009). Stations located
north of 55∘ in the meandering NAC have higher concentrations than
those in the West European Basin. Although dust deposition to the North
Atlantic Ocean is typically associated with northern African dust from the
Sahara, Prospero et al. (2012) and Bullard et al. (2016) found that
high-latitude dust emissions, specifically volcanic-based soils from Iceland,
could be substantial enough to impact oceanic Fe cycling; therefore we
suggest that the elevated Pb in the near-surface waters of the Iceland Basin
and Irminger Sea may possibly be dust-derived. In the GEOVIDE shipboard
aerosol data (Shelley et al., 2017, 2018), Pb concentrations were high in the
Iceland Basin but low in the Irminger Sea. However, as Pb has a residence
time of ∼1 year in this region, seasonal changes in the flux also
could account for this difference. As the North Atlantic Current becomes the
Irminger Current near Greenland and joins with the East Greenland Current,
they wrap around the southern tip of Greenland and flow toward the Arctic
Circle. This entrains Pb into the northeastern part of the Labrador Sea,
whereas the remainder of the Labrador Sea is influenced by the Labrador
Current, returning from the Arctic, which has low [Pb].
Pb concentration depth profiles. References: GA03 (Noble et al.,
2015); EN328 (this work); JGOFS (Martin et al., 1993); TTO (Weiss et al.,
2003); IOC-2 (Veron et al., 1999); GA02 (The GEOTRACES Group, 2015).
Despite the variations in [Pb] across the Atlantic Ocean, Pb isotope ratios
were relatively homogenous throughout the section, and largely decoupled from
the [Pb] patterns (Figs. 3 and 4). 206Pb / 207Pb isotope ratios
varied from 1.178 to 1.186, with the majority of samples analyzed being
1.180–1.183. 208Pb / 206Pb and 206Pb / 204Pb
isotope ratios showed similar minimal variability. No trend in isotope ratios
was observed in the Iberian Abyssal Plain extending away from the coast. The
low variability of isotope ratios indicates that the majority of Pb in the
North Atlantic Ocean is well mixed in the atmosphere prior to deposition. The
relatively low [Pb] and similar isotope ratios contrast sharply with surface
water measurements from the previous century (Figs. 5 and 6). During the
1970s to early 1990s, the predominant source of Pb to the North Atlantic was
US leaded gasoline (Weiss et al., 2003; Martin et al., 1993; Veron et al.,
1999), which was reflected in the high 206Pb / 207Pb isotope
ratios (∼1.20).
The mixed layer [Pb] nearest the Iberian Peninsula (30 pmol kg-1) is
lower than that measured by the 2010 US GEOTRACES expedition
(42 pmol kg-1), which we attribute to the much closer proximity of the
US GEOTRACES station to the coastline (50 km) than GEOVIDE Station 11
(280 km). As mentioned previously, [Pb] at GEOVIDE Stations 11–19 has a
strong inverse correlation with distance from the shore, and adding USGT10-01
(GA-03) maintains this high correlation (Pearson correlation, r=-0.990,
p<0.001). Isotopically, the USGT10-01 near-surface waters are
similar to GEOVIDE Station 11, indicating similar Pb sources in recent years.
206Pb / 207Pb isotope ratio depth profiles.
References: GA03 (Noble et al., 2015); EN328 (this work); TTO (Weiss et al.,
2003); IOC-2 (Veron et al., 1999).
Iberian Abyssal Plain (S11–S19) and West European Basin (S21–S29)
Overall, [Pb] measured from this cruise was highest in the subsurface waters
of the Iberian Abyssal Plain (Station 13). The core of the elevated
concentrations (∼61 pmol kg-1, Station 13) was ∼1200 m
deep and several hundred kilometers from the coast. This subsurface plume of
Pb (concentrations of 40–50 pmol kg-1) was dispersed throughout the
Iberian Abyssal Plain at depths of 700–2000 m. The Pb plume was less
pronounced in the rest of the West European Basin, with concentrations of
30–40 pmol kg-1. Extended Optimum MultiParameter (eOMP) water mass
analysis shows that this elevated [Pb] coincides with Mediterranean Water
(MW) from 700 to 1500 m and Labrador Sea Water (LSW) from 1500 to 2000 m
(García-Ibáñez et al., 2018). Our finding is in good agreement
with [Pb] in MW measured in 2010–2011 by Noble et al. (2015) and highlights
the high [Pb] previously found in the Mediterranean Sea (Moos and Boyle,
2018). In the lower portion of the plume, the LSW in the Iberian Abyssal
Plain and West European Basin is among the oldest water sampled during
this expedition. According to CFC-11 data, LSW in this region has a combined
age (subduction plus admixed relic age) of ∼25 years (Fine, 2011).
That age and the elevated [Pb] observed are consistent with the atmospheric
Pb emissions by North America and Europe in the 1980s. The isotope ratios
further support this finding, as the ocean interior has similar isotope
ratios throughout (206Pb / 207Pb = 1.1832±0.0025,
1σ; 208Pb / 206Pb = 2.4525±0.0024, 1σ), but these are distinguishably more like US aerosols from the early 1990s
(Bollhöfer and Rosman, 2001) at the core of the Pb maximum (Station 13,
206Pb / 207Pb = 1.1894;
208Pb / 206Pb = 2.4544; Fig. 5).
The offshore profiles (Stations 13–29) showed consistent decreases in [Pb]
in the MW and LSW from 1989 (JGOFS S19) and 1999 (Endeavor 328 S15, 17, 21)
to 2014 (Martin et al., 1993; this work). In the 10–15 years between
sampling events the Pb maxima advected into the ocean interior as the more
shallow waters were ventilated with lower-Pb surface waters, a trend also
seen in the western North Atlantic near Bermuda (Boyle et al., 2012).
Below the broad subsurface plume, water mass analysis indicates depths
greater than 2500 m are predominantly Northeast Atlantic Deep Water (NEADW)
that contains a major component of Antarctic Bottom Water (AABW), as
evidenced by high silica concentrations (García-Ibáñez et al.,
2018). In the NEADW, [Pb] were 10–20 pmol kg-1 and similar to
previous sampling campaigns nearby in 1989 and 1999 (Fig. 5). Isotope ratios
(206Pb / 207Pb = 1.1827±0.0013;
208Pb / 206Pb = 2.4511±0.0013) were also similar
across the 25 years in the West European Basin (Fig. 6). This makes sense
because the estimated age of NEADW is several hundreds of years (Matsumoto,
2007).
Below 1000 m, the [Pb] at Stations 11 and 13 was very similar to the 2010
[Pb] measured on GA03 (USGT10-01; Fig. 5), but the isotope ratios are
dissimilar (Fig. 6). Conversely, the upper 1000 m of the water column had
different [Pb] but similar isotope ratios. In the upper ocean, this
discrepancy can be related to the distance of the stations from the shore, as
calculated in Sect. 3.2, with greater Pb inputs and therefore greater
concentrations at stations closer to the shore. In the deep ocean, the
contrast in isotope ratios between the more coastal GA03 station and offshore
GEOVIDE station, only 4 years apart, supports the eOMP findings that slightly
different mixes of water masses were sampled in the two cruises. Despite the
close proximity of the two stations (∼250 km), the GEOVIDE cruise
sampled waters > 1000 m that had relatively more LSW and less MW
(or MOW per Jenkins) compared to the 2010 GA03 station (Jenkins et al., 2015;
García-Ibáñez et al., 2017).
Iceland Basin (S32–S36) and Reykjanes Ridge (S38)
In the Iceland Basin and above the Reykjanes Ridge, [Pb] throughout the water
column is similar to that found in the West European Basin, with a
subsurface [Pb] maximum (∼30 pmol kg-1) in the core of LSW. In
the deepest samples (2500–3000 m), [Pb] (5–10 pmol kg-1) is lower
than the NEADW observed in the Iberian Abyssal Plain and West European
Basin, and the 206Pb / 207Pb isotope ratios are slightly lower
(206Pb / 207Pb = 1.1812±0.0005) than the overlying
water at 800–2000 m
(206Pb / 207Pb = 1.1845±0.0014). Water mass analysis
indicates very little NEADW was present in the Iceland Basin, and the deeper
samples were strongly influenced by Iceland–Scotland Overflow Water (ISOW),
particularly at Stations 32–36 (García-Ibáñez et al., 2017). The
1993 IOC-2 survey by Veron et al. (1999) found that ISOW
(206Pb / 207Pb = 1.173–1.176) was isotopically distinct
from LSW (206Pb / 207Pb = 1.190–1.20) and that ISOW
reflected atmospheric emissions from Europe at that time. The differences in
Pb isotopes (and 2- to 3-fold reduction in concentrations) between sampling
campaigns highlight the young age of ISOW, which reflected large source
changes over a 21-year time period (Figs. 5 and 6).
Triple isotope plot of (a) the surface GEOVIDE samples
compared to possible sources, (b) all GEOVIDE data and (c)
spatiotemporal trends from 1999 (EN328) and 2014* (dark grey circles, GEOVIDE samples at all depths from
Stations 11 to 26 and depths > 800 m from Stations 29 to 77) and
2014** (light grey circles, GEOVIDE samples at depths < 800 m
from Stations 29 to 77). References: pre-Holocene sediments (Hamelin et al.,
1990); corals (Kelly et al., 2009); North African dust (Bridgestock et al.,
2016); US and EU aerosols, 1990s (Bollhöfer and Rosman, 2001); US
aerosols, 2011 (Noble et al., 2015); 1981 seawater (Weiss et al., 2003);
EN328 and GEOVIDE seawater (this work).
In addition, we note that the present-day Norwegian Sea waters must have low
[Pb], and that their Pb isotope ratios reflect a greater contribution from
European sources than North American sources. ISOW is formed as a mixture of
LSW and Norwegian Sea water that overflows the Iceland–Scotland sills.
Because LSW has higher [Pb] and higher 206Pb / 207Pb isotope
ratios than ISOW, we hypothesize that Norwegian Sea water must have a lower
206Pb / 207Pb isotope ratio and much lower [Pb] because Pb is
scavenged only on a decadal–century scale in deep water and retains it
source signatures during decadal penetration into the deep ocean. By this, we
mean that in the open ocean with relatively low particulate concentrations
and minimal sediment interactions we expect Pb to behave in a
quasi-conservative manner over short mixing timescales. Although other work
has demonstrated that isotopic exchange with particles can influence the
dissolved Pb isotope composition (Wu et al., 2010; Chen et al., 2016), this
occurs over much longer timescales than the mixing of Norwegian Sea water and
LSW to form ISOW. Using our Pb data for Station 32 and the eOMP analysis that
the deepest samples are 100 % ISOW and ∼20 % LSW
(García-Ibáñez et al., 2017), we back-calculate a Norwegian Sea
water that is ∼7 pmol kg-1 and 206Pb / 207Pb
∼1.180. The relatively lower 206Pb / 207Pb isotope
ratios of the Norwegian Sea are consistent with what Veron et al. (1999)
observed in 1993 (1.169), and are indicative of atmospheric Pb from a more
European provenance than a North American one (Fig. 7).
North–south [Pb] sections in the eastern Atlantic Ocean,
1989–2014. Plot created in Ocean Data View (Schlitzer, 2017).
Irminger Sea (S40–S60)
In the Irminger Sea, a broad Pb maximum with little concentration variability
was observed between the near surface and 1800 m (Fig. 3). The diffuse
elevation in [Pb] throughout the upper 1800 m is attributed to both Irminger
Subpolar Mode Water (0–1000 m) and LSW (500–2500 m)
(García-Ibáñez et al., 2017). As in the Iceland Basin, ISOW is
observed in the Irminger Sea deep water, but in a lower proportion
(40 %–60 %) than in the Iceland Basin (80 %–100 %). At
Stations 42 and 44 ISOW is distinguished by its low [Pb]
(5–8 pmol kg-1) and a low 206Pb / 207Pb ratio (1.1798).
Further north in the Irminger Sea along the Greenland continental slope, the
near-bottom samples at Stations 49 and 60 are Denmark Straight Overflow Water
(DSOW). The DSOW has a slightly higher [Pb] (10–18 pmol kg-1) and a
higher 206Pb / 207Pb ratio (1.1854) than ISOW, consistent with
the 1993 data of Veron et al. (1999;
206Pb / 207Pb = 1.179–1.182). DSOW is a mix of the Nordic
Sea waters overflowing the Greenland–Iceland sill and mixing with LSW; DSOW
is also reported to have inputs from dense Greenland shelf water and
cascading Polar Intermediate Water (García-Ibáñez et al., 2015;
this study). The resulting DSOW isotope composition is very similar to LSW,
which could indicate shelf water has very little Pb, and so its signal is
dominated by the LSW signal, although we cannot rule out the possibility that
the shelf water entrained Pb with a similar isotope composition to LSW.
The Irminger Sea was previously sampled for Pb during the 1993 IOC-2
expedition (Veron et al., 1999) and the 2010 GA02 expedition near GEOVIDE
Stations 42 and 44 (analyses by Middag and Bruland as reported by The
GEOTRACES Group, 2015) (Fig. 5). There is a large decrease in [Pb] at all
depths from 1993 to 2010, and a surprisingly large decrease between 2010 and
2014. We suspect that the difference between 2010 and 2014 could also be a
result of the 2012 deep winter convection event (∼1200 m) as reported
by Fröb et al. (2016). The 206Pb / 207Pb values between
1993 and 2014 do not appear to have changed significantly (perhaps in view of
limited 1993 water column coverage) (Fig. 6).
Labrador Sea (S64–S77)
In the Labrador Sea, the [Pb] maximum coincides with LSW (0–2500 m) and is
very broad (Fig. 3). Similar concentrations (∼25 pmol kg-1) are
found from 100 m to nearly 2000 m. At depths greater than 2000 m, the [Pb]
decreases to ∼8 pmol kg-1 and water mass analysis indicates
this is primarily ISOW. Throughout the entire Labrador Sea water column Pb
isotope ratios are homogenous, in contrast to the Icelandic and Irminger
basins, which are isotopically distinctive from overlying LSW. The similarity
of the Pb throughout the Labrador Sea can be attributed to deep winter
convection that annually varies from 1000 to 2000 m deep (Lazier et al.,
2002; Lilly et al., 1999; Vage et al., 2009). Hydrographic observations and
Argo floats indicate winter 2014 convection was ∼1700 m deep (Kieke
and Yashayaev, 2015). Fine (2011) assigns a combined age of 17–19 years to
these waters. The similar Pb profiles throughout the entire water column
indicate there were minimal changes in magnitude of Pb sources to the LSW
over the 2 decades preceding sampling, and the isotopically indistinguishable
ISOW suggests it is also relatively well mixed with LSW in this basin
(Figs. 4 and 6).
The Labrador Sea also confirms the continued changes to oceanic Pb since the
phase-out of leaded gasoline usage by North America and Europe. Lead
concentrations in the upper 2000 m of the water column were 3 to 4 times
lower in 2014 and in 2010 than those measured in 1993 (2010 analyses by
Middag and Bruland as reported by the GEOTRACES Group, 2015; Veron et al.,
1999) (Fig. 5). Surface water Pb isotope ratios in 2014 were also much lower
(206Pb / 207Pb = 1.186) than during the early 1990s
(206Pb / 207Pb = 1.209) (Fig. 6), in agreement with the
rest of the North Atlantic Ocean surface Pb changes.
North–south Pb isotope sections in the eastern Atlantic Ocean, 1999
and 2010–2014. Plot created in Ocean Data View (Schlitzer, 2017).
Sources of Pb in 1999 and 2014
Overall, Pb isotope ratios throughout the GEOVIDE expedition were relatively
uniform, in both the upper and deep ocean, and in the eastern and western
basins. This finding is similar to that of Noble et al. (2015)
from the US GEOTRACES expeditions in the mid-Atlantic in 2010 and 2011, but
differs from the expeditions of the 1980s and 1990s when Pb isotope ratios
ranged much more broadly (206Pb / 207Pb = 1.165–1.201)
(Veron et al., 1999). Compared to the dramatic differences in isotope ratios
of 25+ years before, it would appear there is a decoupling of variable Pb
concentrations vs. uniform Pb isotopic composition in the North Atlantic.
However, examining the Pb isotope ratios using a triple isotope plot
(208Pb / 206Pb vs. 206Pb / 207Pb), it is clear
that there have been small spatiotemporal Pb source changes between the 1999
EN328 cruise and regions of the 2014 GEOVIDE cruise that have the most recent
atmospheric Pb inputs (Fig. 7c). Most of the 1999 data (except for the oldest
deep waters) fall on the lower branch of the European–US mixing trend
(yellow squares). The GEOVIDE data from Stations 11 to 26 at all depths and
the > 800 m samples from Stations 29 to 77 fall on an
intermediate trend, while the < 800 m samples from GEOVIDE
Stations 29 to 77 (most recent Pb inputs) fall on the high side of the trend.
We do not have enough source isotope information to explain these changes,
but they clearly indicate spatiotemporal evolution of the evolving
anthropogenic Pb transient in the northern North Atlantic Ocean. This trend
is likely not as dramatic as the changes in both concentration and isotope
ratios of previous decades because the magnitude of the total atmospheric
flux of Pb into the North Atlantic has changed far more than the
proportions of Pb emitted between the various sources.
In the industrial sector, emissions by European and Canadian/US sources have
been relatively constant over the last 15 years. Pb emissions estimates were
evaluated using the EMEP (European Monitoring Evaluation Program) database.
Atmospheric Pb emissions for European countries along with the USA and Canada
were evaluated from 1990 to 2014 (Fig. S3). Cumulative atmospheric Pb
emissions have reduced by a factor of 10 in Europe and by a factor of 5 in
North America over that time period. The ratio of Pb emissions from US and
Canadian vs. European sources was 1:7 in 1990, but that ratio steadily
increased to 1:3 by 1999 and has remained about the same since then, due to
the much larger reductions in emissions by Europe (following upon earlier US
emission reductions). The similarity of emissions for ∼15 years
contrasted with the spatiotemporal trend in the isotope ratios could be a
result of several phenomena. First, despite maintaining similar overall
emissions, the sources of atmospheric Pb from each nation could have changed
in characteristic but not quantity. Second, the evolution of Pb isotope
ratios could be a result of uncounted emissions from non-point sources.
Finally, natural mineral dust could be playing an influential role in
seawater Pb isotope composition.
Atmospheric deposition is the main source of Pb to the ocean, with trace
metals in anthropogenic-sourced carbonaceous aerosols known to be far more
seawater soluble than silicate-bound metals in naturally derived aerosols
(Desboeufs et al., 2005). Trace metal enrichment factors of dry aerosols and
wet deposition were collected during the GEOVIDE cruise (Shelley et al.,
2017, 2018). Results for Pb enrichment indicated atmospheric Pb was
predominantly anthropogenic in origin (20–120, median 30). Using positive
matrix factorization of the aerosol concentration data, Shelley et al. (2017,
2018) estimated that ∼60 % of the Pb was from a mineral dust
source and only 40 % was of anthropogenic origin. This finding parallels
the 2010 study of Pb in the tropical North Atlantic by Bridgestock et
al. (2016) that found 30 %–50 % of total dissolvable Pb in seawater
was from natural mineral dust from the North African dust plume. Despite a
large fraction of the total atmospheric flux of Pb being natural in origin,
the seawater isotope ratios are skewed towards the anthropogenic ratios due
to the higher solubilization of anthropogenic Pb compared to mineral Pb. In
the North Atlantic GEOVIDE, a larger contribution of mineral dust Pb could be
obscured in the dissolved Pb signal because of the differing solubility.
Lead isotope ratios are a useful tool in resolving possible sources because
they are not fractionated significantly by scavenging or other natural
processes (compared to the large differences due to radiogenic sources). In
the open ocean water column there is minimal opportunity for exchange of Pb
between particles and water (unlike the sediment–water interface), so we
would expect dissolved Pb isotopes to be representative of atmospheric
inputs. Triple isotope plots of the waters from this cruise (Fig. 7a, b)
compare the possible sources of Pb to the North Atlantic. Pre-Holocene
sediments and corals from the North Atlantic (Hamelin et al., 1990; Kelly et
al., 2009) are representative of the pre-industrial Pb background ratio we
would expect to find in seawater if there were no anthropogenic inputs;
unfortunately the Icelandic dust end-member is not known, but we suspect it
is similar to North African dust. Because there is significant overlap in
historic USA Pb emissions and modern North African dust with these isotope
ratios, it is difficult to fully resolve the different sources. The aerosol
signatures of anthropogenic sources in the USA and Europe fall along a linear
mixing line, while the more natural Pb sources deviate from this line. The
spatiotemporal trend (Fig. 7c) supports the hypothesis that an increasing
amount of the most recent Pb inputs to the ocean is increasingly natural in
origin. However, the seawater Pb isotope ratios will lag in reflecting the
atmospheric Pb changes due to the preferential solubility of anthropogenic
Pb, so the ocean will never fully reflect natural atmospheric Pb sources
until all anthropogenic Pb sources are eliminated.
Evolution of Pb and Pb isotopes in the Eastern Atlantic Water
Column, 1989–2014
Data for [Pb] from the 1989 (Atlantis II 123), 1999
(Endeavor 328), and 2010–2014 (GA03, and GA01 GEOVIDE) cruises, and
Pb isotopes from 1999 and 2010–2014, are plotted as north–south sections in
Figs. 8 and 9. It is evident that Pb is strongly decreasing in the upper
ocean during this period, a fact that can be attributed to the phasing out of
tetraethyl Pb gasoline in North America and Europe. All three periods show a
Pb maximum in the deep thermocline, and this maximum deepens from decade to
decade, as it has also done in the western North Atlantic water column near
Bermuda (Boyle et al., 2014; Noble et al., 2015). As Noble et
al. (2015) demonstrated for the 2010/2011 GA-03
trans-North Atlantic section, this maximum is located in waters with SF6
ventilation dates from the 1970s, when leaded gasoline Pb utilization was at
its maximum. A similar result can be seen in the 1989 data based on
3He–3H dating (Jenkins, 1987). Hence the location of the maximum
is dominantly a reflection of Pb emissions at the ventilation age of the
water rather than an association with a particular water mass. When
considered in this light – as a snapshot of an evolving three-dimensional
transient tracer experiment – some of the features in these sections require
an interpretation that differs substantially from that usually placed on
quasi-steady-state tracers such as salinity, oxygen, and nutrients. For
example, the [Pb] maximum seen at ∼25∘ N is not the source of
a northward-spreading plume, it is the southern extent of high-[Pb] waters
that were subducted into the thermocline in the 1970s and advected
southwestwards by the dynamics of the ventilated thermocline (Luyten et al.,
1983). In addition to the general ventilation of the North Atlantic water
column, some [Pb] features are due to specific hydrographic features. The
1999 [Pb] maximum near 1000 m was enhanced by a strong “meddy”, a coherent
mesoscale feature created by pulses of dense salty water from out of the
Mediterranean Sea (Armi et al., 1989), as demonstrated by the salinity data
from that profile (Fig. 10). It is also evident that the ∼ 1800 m
Labrador Sea water has had consistently higher Pb than the denser
Greenland–Scotland overflow water.
CTD data from EN328 Station 10 (42∘ N, 17∘45′ W)
showing a strong salinity maximum due to the Mediterranean outflow eddy.
It is likely that the evident decline in the Pb inventory of the eastern
North Atlantic is decreasing not only because of advective–diffusive
spreading of the water out of the basin, but also because of scavenging.
Radiochemical studies (Bacon et al., 1976) have shown that deep water column
210Pb activities are lower than 226Ra activities, signifying
removal of 210Pb from the deep water column. Some of this scavenging is
due to sinking particles, but in near-bottom waters, “boundary scavenging”
accounts for a higher fraction (Bacon, 1988).
The evolution of the Pb isotope data between 1999 and 2010–2014 is striking
in that the deepest waters in the tropical eastern Atlantic are significantly
changed between these periods. Near the surface, recent changes are mainly
due to a greater reduction of the relative North American high
206Pb / 207Pb sources relative to the European low
206Pb / 207Pb sources, and possibly some influence of natural
dust. But in the deep water, this change probably represents the “conveyor
belt” motion of deep high 206Pb / 207Pb introduced from the
surface in the early 1900s being replaced by lower
206Pb / 207Pb from the 1920s and later (as seen in historical
Pb isotope ratios in Bermuda corals – Kelly et al., 2009).