1
Supporting Information for
2
Six centuries of changing oceanic mercury
3
Yanxu Zhang, Lyatt Jaeglé, LuAnne Thompson, David Streets
4
5
1 Model comparison to modern atmospheric mercury observations
6
We compare our simulation of present-day (2008) atmospheric Hg to observations
7
collected in the atmospheric marine boundary layer and at terrestrial sites, as summarized
8
in previous GEOS-Chem model studies [Selin et al., 2007; Holms et al., 2010; Soerensen
9
et al., 2010; Soerensen et al., 2012]. These ground-based observations were collected
10
between 2004 and 2008 at 42 ground-sites. Observations in the marine boundary layer are
11
from 4 cruises over the same period.
12
Figure S1 (top panel) shows the modeled spatial distribution of surface
13
atmospheric total gaseous mercury (TGM = Hg0+HgII) concentrations, while Figure S2
14
shows the latitudinal distribution of TGM concentrations. Our present-day simulation
15
predicts a global mean atmospheric TGM concentration of 1.2 ng/m3. The present-day
16
distribution of TGM shows large spatial variability, with concentrations ranging from 0.5
17
to 3.5 ng/m3. The highest concentrations are modeled over regions with strong
18
anthropogenic emissions including East Asia, Western Europe, and North America
19
(Figure S1 top panel). The mean observed concentrations of TGM are 2.0±0.88 ng/m3,
20
compared to 1.4±0.50 ng/m3 in the model sampled at the location of observations. The
21
spatial distribution of TGM is similar in both model and observations, with higher
1
1
concentrations near strong anthropogenic regions. The spatial correlation coefficients
2
between model and observation is high (r2 = 0.90). For terrestrial sites, the modeled TGM
3
concentrations (1.5±0.5 ng/m3) are 25% lower than observations (1.9±1.0 ng/m3). The
4
model also under-predicts observed TGM concentrations over the North Atlantic (obs.:
5
1.9±0.16 ng/m3; mod.: 1.3±0.02 ng/m3) and North Pacific Oceans (obs.: 2.5±0.4 ng/m3;
6
mod.: 1.3±0.07 ng/m3).
7
Anthropogenic emissions lead to a small interhemispheric gradient, with 1.2-1.5
8
ng/m3 in the northern hemisphere and 0.9-1.0 ng/m3 in the southern hemisphere (Figure
9
S2 top panel). The modeled latitudinal gradient falls at the low edge of the envelope of
10
observations.
11
12
13
2 Model comparison to concentrations of Hg in the surface ocean
Figure S1 (bottom panel) shows the modeled present-day total aqueous Hg
14
concentrations in the surface ocean. The latitudinal distribution of zonal average
15
concentrations is shown in Figure S2 (bottom panel). The modeled global mean total Hg
16
concentration over the surface ocean is 0.72 pM, with no significant latitudinal and inter-
17
hemispheric gradients. Overall, the large-scale horizontal spatial pattern of surface ocean
18
Hg concentrations has not changed much at present-day compared with that for natural
19
conditions, indicating similar controlling factors (e.g. ocean circulation and particle
20
sinking flux) [Zhang et al., 2014].
21
We compare our model results of surface ocean Hg concentrations to observations
22
spanning the last two decades, as summarized by Soerensen et al. [2010]. Observations
23
display a large variability, with values ranging from less than 0.1 pM to more than 7 pM
2
1
(Figure S1 bottom panel). Observed median total Hg concentrations are 1.0 pM. The
2
model sampled at the same locations as the observations displays median values that are
3
33% lower (0.67 pM). The modeled concentrations fall within the center of envelope for
4
the observations with most density data points (Figure S2 bottom panel), indicating a
5
reasonable agreement between them. For the North Atlantic Ocean (obs: 1.0±0.48 pM;
6
mod: 0.68±0.12 pM), North Pacific Ocean (obs: 0.88±0.31 pM; mod.: 0.70±0.11 pM) and
7
Southern Ocean (obs: 1.0±0.44 pM; mod: 0.69±0.12 pM), we find that the model
8
captures observed surface ocean total Hg concentrations reasonably well.
9
Most of the measurements near coastal and shelf regions are significantly higher
10
(> 2 pM) than the corresponding modeled values (~0.5 pM), such as the measurements
11
over the Hudson Bay region and the coastal Canadian Arctic Ocean Archipelago. This
12
could be due to an underestimate of the riverine input Hg flux, especially because we
13
only considered the erosion of Hg from background soils in our model and do not take
14
into account the historical influence of wastewater discharges from commercial sources
15
[Soerensen et al., 2010; Streets et al., 2011; Horowitz et al., 2014] or the impact of
16
snowpack melt water [Fisher et al., 2012]. Furthermore, the global model does not have
17
adequate resolution to capture advection and mixing of Hg near the coasts.
18
3
1
2
3
Figure S1. Comparison of model results with present-day observations. Top panel:
4
Modeled present-day surface concentrations of atmospheric total gaseous mercury (TGM,
5
in ng/m3). Observed concentrations (2004-2008) are shown as color-coded circles
6
(cruises) and diamonds (ground-based sites). Bottom: Total Hg concentrations (in pM) in
7
the surface ocean. Observed concentrations (1984-2010) are shown as color-coded
8
circles.
4
1
2
3
Figure S2. Zonal mean of Hg concentrations in the atmosphere and surface ocean.
4
Zonal average of top) near-surface atmospheric TGM concentrations and bottom) total
5
Hg concentrations at ocean surface. Observations are shown as open circles. The solid
6
lines correspond to the present-day (orange) and pre-anthropogenic (blue) simulations.
7
3 Uncertainty Analysis
8
9
The uncertainty associated with our estimate of anthropogenic Hg in the ocean is
primarily from the historical emission of Hg and our parameterization of the
10
biogeochemistry of Hg in the ocean. With and without reducing the Hg mining emissions
11
by a factor of 3 result in the total anthropogenic Hg emissions during 1450-2008 as 950
12
and 1750 Mmol, respectively. Adopting the original inventory causes a 50% increase in
13
the anthropogenic Hg in the present-day ocean. This could be even higher if we were to
14
consider the release of commercial Hg to the environment [Horowitz et al., 2014]. As a
15
lower end, reducing the historical Hg emissions in Au/Ag mining to zero results in the
16
total anthropogenic Hg emission as 550 Mmol, which would cause a 20-30% decrease in
5
1
the anthropogenic Hg load in the present-day ocean. The redox chemistry between Hg0
2
and HgII and the partitioning coefficient (Kd) of dissolved HgII onto the particulate phase
3
also play important roles, because they affect the fraction of total Hg as HgII and the
4
strength of its particle scavenging. Observed fractions of HgII range from 50%-80% in the
5
subsurface waters [Lamborg et al., 2014], leading to an uncertainty range of 50%.
6
Although the Kd value could vary for an order of magnitude based on available
7
observations [Soerensen et al., 2010], sensitivity runs suggest the plausible range for this
8
parameter is within a factor of 2 of our current used value, which leads to an additional
9
40% variation in the amount of anthropogenic Hg in the ocean. Another source of
10
uncertainty comes from the model’s representation of the biological pump itself,
11
especially the remineralization depth of sinking organic particles. Choosing different
12
remineralization depths results in a variation of 30% for the amount of anthropogenic Hg
13
in the ocean. Overall, these factors cause an uncertainty range of 190 – 530 Mmol.
14
6
1
Additional information for observed lake sediment profiles
2
3
Table S1. Detailed information for the source of lake sediment observations shown
in Figure 1
Category
Remote
Eastern
U.S.
East Asia
4
a
Region
Greenland
Chile
West Greenland
California-Nevada, U.S.
Svalbard
Upper Midwest, U.S.
Alaska
Arctic Ny-Ålesund
Brazil
New Zealand
Western U.S.
High-latitude Canada
Santa Barbara Basin
Western U.S.
Washington, U.S.
Uganda
Upper Midwest, U.S.
Vermont and New Hampshire, U.S.
Nova Scotia
Adirondack Lake, New York, U.S.
Maine, U.S.
Midcontinental North America
Qinghai Lake, Tibetan Plateau
Taihu Lake, China
Tibetan Plateau
na
1
2
3
2
4
3
15
1
4
1
14
4
1
9
1
3
12
10
3
8
1
7
1
2
8
Reference
Asmund and Nielsen, 2000
Biester et al., 2002
Bindler et al. 2001
Drevnick et al., 2010
Drevnick et al., 2012
Engstrom and Swain, 1997
Fitzgerald et al., 2005
Jiang et al., 2011
Lacerda et al., 1999
Lamborg et al., 2002
Landers et al., 2008
Lockhart et al., 1995
Young et al., 1973
Mast et al., 2010
Paulson and Norton, 2008
Yang et al., 2010
Engstrom and Swain, 1997
Kamman and Engstrom, 2002
Lamborg et al., 2002
Lorey and Driscoll, 1999
Roos-Barraclough et al., 2006
Swain et al., 1992
Wang et al., 2010
Wu et al., 2008
Yang et al., 2010
Number of lake sediment cores used for each location
5
7
1
References
2
Asmund, G., and S. P. Nielsen (2000), Mercury in dated Greenland marine sediments,
3
Sci. Total Environ., 245(1-3), 61-72.
4
Biester, H., R. Kilian, C. Franzen, C. Woda, A. Mangini, and H. F. Scholer (2002),
5
Elevated mercury accumulation in a peat bog of the Magellanic Moorlands, Chile (53
6
degrees S) - an anthropogenic signal from the Southern Hemisphere, Earth Planet. Sc.
7
Lett., 201, 609-620.
8
Bindler, R., I. Renberg, P. G. Appleby, N. J. Anderson, and N. L. Rose (2001), Mercury
9
accumulation rates and spatial patterns in lake sediments from west Greenland: A coast to
10
ice margin transect, Environ. Sci. Technol., 35, 1736-1741.
11
Drevnick, P. E., A. L. C. Shinneman, C. H. Lamborg, D. R. Engstrom, M. H. Bothner,
12
and J. T. Oris (2010), Mercury Flux to Sediments of Lake Tahoe, California–Nevada,
13
Water Air Soil Poll., 210, 399-407.
14
Drevnick, P. E., H. Yang, C. H. Lamborg, and N. L. Rose (2012), Net atmospheric
15
mercury deposition to Svalbard: Estimates from lacustrine sediments, Atmos. Environ.,
16
59, 509-513.
17
Engstrom, D. R., and E. B. Swain (1997), Recent declines in atmospheric mercury
18
deposition in the upper Midwest, Environ. Sci. Technol., 31, 960-967.
19
Fitzgerald, W. F., D. R. Engstrom, C. H. Lamborg, C. M. Tseng, P. H. Balcom, and C. R.
20
Hammerschmidt (2005), Modern and historic atmospheric mercury fluxes in northern
21
Alaska: Global sources and Arctic depletion, Environ. Sci. Technol., 39, 557-568.
8
1
Fisher, J.A., D.J. Jacob, A.L. Soerensen, H.M. Amos, A. Steffen, and E.M. Sunderland
2
(2012), Riverine source of Arctic Ocean mercury inferred from atmospheric
3
observations, Nature Geosci., 5, 499-504.
4
Holmes, C. D., D. J. Jacob, E. S. Corbitt, J. Mao, X. Yang, R. Talbot, and F. Slemr
5
(2010), Global atmospheric model for mercury including oxidation by bromine atoms,
6
Atmos. Chem. Phys., 10(24), 12037-12057.
7
Horowitz, H. M., D. J. Jacob, H. M, Amos, D. G. Streets, and E. M. Sunderland
8
(2014), Historical mercury releases from commercial products: global environmental
9
implications, Env. Sci. Tech., 48(17), 10242-10250.
10
Jiang, S., X. Liu, and Q. Chen (2011), Distribution of total mercury and methylmercury
11
in lake sediments in Arctic Ny-Alesund, Chemosphere, 83, 1108-1116.
12
Kamman, N. C., and D. R. Engstrom (2002), Historical and present fluxes of mercury to
13
Vermont and New Hampshire lakes inferred from Pb-210 dated sediment cores, Atmos.
14
Environ., 36, 1599-1609.
15
Lacerda, L. D., M. G. Ribeiro, R. C. Cordeiro, A. Sifeddine, and B. Turcq (1999),
16
Atmospheric mercury deposition over Brazil during the past 30,000 years, Environ.
17
Biodivers., 51, 363-371.
18
Lockhart, W. L., P. Wilkinson, B. N. Billeck, R. V. Hunt, R. Wagemann, and G. J.
19
Brunskill (1995), Current and historical inputs of mercury to high-latitude lakes in canada
20
and to hudson-bay, Water Air Soil Pollut., 80, 603-610.
21
Lamborg, C. H., W. F. Fitzgerald, A. W. H. Damman, J. M. Benoit, P. H. Balcom, and D.
22
R. Engstrom (2002), Modern and historic atmospheric mercury fluxes in both
9
1
hemispheres: Global and regional mercury cycling implications, Global Biogeochem.
2
Cy., 16, 1104, doi:10.1029/2001GB001847.
3
Lamborg, C., K. Bowman, C. Hammerschmidt, C. Gilmour, K. Munson, N. Selin, and
4
C.-M. Tseng (2014), Mercury in the Anthropocene Ocean, Oceanography, 27(1), 76-87.
5
Landers, D.H., H. E. Tayler, S. Simonich, K. Hageman, D. Jaffe, S. Usenko, L. Geiser, L.
6
Ackerman, D. H. Campbell, J. Schrlau, A. Schwindt, N. Rose, C. Schreck, T. Blett, M.
7
kent, M. M. Erway, W. Hafner (2008), The fate, transport, and ecological impacts of
8
airborne contaminants in western national parks (USA). In: Christie, S. (Ed.), Western
9
Airborne Contaminants Assessment Project Final Report, vol. 1 EPA/600/R-07/138.
10
Lorey, P., and C. T. Driscoll (1999), Historical trends of mercury deposition in
11
Adirondack lakes, Environ. Sci. Technol., 33, 718-722.
12
Mast, M. A., D. J. Manthorne, and D. A. Roth (2010), Historical deposition of mercury
13
and selected trace elements to high-elevation National Parks in the Western U.S. inferred
14
from lake-sediment cores, Atmos. Environ., 44, 2577-2586.
15
Paulson, A. J., and D. Norton (2008), Mercury Sedimentation in Lakes in Western
16
Whatcom County, Washington, USA and its Relation to Local Industrial and Municipal
17
Atmospheric Sources, Water Air Soil Pollut. 189, 5-19.
18
Roos-Barraclough, F., N. Givelet, A. K. Cheburkin, W. Shotyk, and S. A. Norton (2006),
19
Use of Br and Se in peat to reconstruct the natural and anthropogenic fluxes of
20
atmospheric Hg: A 10000-year record from Caribou Bog, Maine, Environ. Sci. Technol.
21
40, 3188-3194.
10
1
Selin, N. E., D. J. Jacob, R. J. Park, R. M. Yantosca, S. Strode, L. Jaegle, and D. Jaffe
2
(2007), Chemical cycling and deposition of atmospheric mercury: Global constraints
3
from observations, J. Geophys. Res., 112(D2), D02308, doi: 10.1029/2006JD007450.
4
Soerensen, A. L., E. M. Sunderland, C. D. Holmes, D. J. Jacob, R. M. Yantosca, H. Skov,
5
J. H. Christensen, S. A. Strode, and R. P. Mason (2010), An Improved Global Model for
6
Air-Sea Exchange of Mercury: High Concentrations over the North Atlantic, Environ.
7
Sci. Technol., 44(22), 8574-8580.
8
Soerensen, A. L., D. J. Jacob, D. G. Streets, M. L. I. Witt, R. Ebinghaus, R. P. Mason, M.
9
Andersson, and E. M. Sunderland (2012), Multi-decadal decline of mercury in the North
10
Atlantic atmosphere explained by changing subsurface seawater concentrations, Geophys.
11
Res. Lett., 39, L21810, doi:10.1029/2012GL053736.
12
Streets, D. G., Devane, M. K., Lu, Z., Bond, T. C., Sunderland, E. M., Jacob, D. J.
13
(2011), All-time releases of mercury to the atmosphere from human activities, Environ.
14
Sci. Technol., 45, 10485-91.
15
Swain, E. B., D. R. Engstrom, M. E. Brigham, T. A. Henning, and P. L. Brezonik (1992),
16
Increasing rates of atmospheric mercury deposition in midcontinental North America,
17
Science, 257, 784-787.
18
Wang, X., H. Yang, P. Gong, X. Zhao, G. Wu, S. Turner, and T. Yao (2010), One
19
century sedimentary records of polycyclic aromatic hydrocarbons, mercury and trace
20
elements in the Qinghai Lake, Tibetan Plateau, Environ. Pollut., 158, 3065-3070.
11
1
Wu, Y. H., X. Jiang, E. Liu, S. Yao, Y. Zhu, and Z. Sun (2008), The enrichment
2
characteristics of mercury in the sediments of Dongjiu and Xijiu, Taihu lake catchment,
3
in the past century, Sci. China D, 51, 848-854.
4
Yang, H. D., R. W. Battarbee, S. D. Turner, N. L. Rose, R. G. Derwent, G. Wu, and R.
5
Yang (2010), Historical reconstruction of mercury pollution across the Tibetan Plateau
6
using lake sediments, Environ. Sci. Technol., 44, 2918-2924.
7
Yang, H. D., D. R. Engstrom, and N. L. Rose (2010), Recent Changes in Atmospheric
8
Mercury Deposition Recorded in the Sediments of Remote Equatorial Lakes in the
9
Rwenzori Mountains, Uganda, Environ. Sci. Technol., 44, 6570-6575.
10
Young, D. R., J. N. Johnson, A. Soutar, and J. D. Isaacs (1973), Mercury concentrations
11
in dated varied marine sediments collected off Southern California, Nature, 244, 273-
12
275.
13
Zhang, Y., L. Jaeglé, A. van Donkelaar, R. V. Martin, C. D. Holmes, H. M. Amos, Q.
14
Wang, R. Talbot, R. Artz, S. Brooks, W. Luke, T. M. Holsen, D. Felton, E. K. Miller, K.
15
D. Perry, D. Schmeltz, A. Steffen, R. Tordon, P. Weiss-Penzias, and R. Zsolway (2012),
16
Nested-grid simulation of mercury over North America, Atmos. Chem. Phys., 12, 6095-
17
6111.
12