Critical Review pubs.acs.org/est

Atmospheric Hg Emissions from Preindustrial Gold and Silver Extraction in the Americas: A Reevaluation from Lake-Sediment Archives Daniel R. Engstrom,*,† William F. Fitzgerald,‡ Colin A. Cooke,§,¶ Carl H. Lamborg,∥ Paul E. Drevnick,⊥,○ Edward B. Swain,# Steven J. Balogh,∇ and Prentiss H. Balcom‡ †

St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, Minnesota 55047, United States Department of Marine Sciences, University of Connecticut, Groton, Connecticut 06340, United States § Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, United States ∥ Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, United States ⊥ INRS-ETE, Université du Québec, Québec City, Québec G1K9A9, Canada # Minnesota Pollution Control Agency, St. Paul, Minnesota 55155, United States ∇ Metropolitan Council Environmental Services, St. Paul, Minnesota 55106, United States ‡

S Supporting Information *

ABSTRACT: Human activities over the last several centuries have transferred vast quantities of mercury (Hg) from deep geologic stores to actively cycling earth-surface reservoirs, increasing atmospheric Hg deposition worldwide. Understanding the magnitude and fate of these releases is critical to predicting how rates of atmospheric Hg deposition will respond to future emission reductions. The most recently compiled global inventories of integrated (alltime) anthropogenic Hg releases are dominated by atmospheric emissions from preindustrial gold/silver mining in the Americas. However, the geophysical evidence for such large early emissions is equivocal, because most reconstructions of past Hg-deposition have been based on lake-sediment records that cover only the industrial period (1850-present). Here we evaluate historical changes in atmospheric Hg deposition over the last millennium from a suite of lake-sediment cores collected from remote regions of the globe. Along with recent measurements of Hg in the deep ocean, these archives indicate that atmospheric Hg emissions from early mining were modest as compared to more recent industrial-era emissions. Although large quantities of Hg were used to extract New World gold and silver beginning in the 16th century, a reevaluation of historical metallurgical methods indicates that most of the Hg employed was not volatilized, but rather was immobilized in mining waste.

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INTRODUCTION Mercury (Hg) is a global pollutant that is released to the biosphere by a variety of natural and anthropogenic processes. Present-day Hg emissions to the atmosphere from primary anthropogenic sources are estimated at 1900−2900 Mg yr−1, exceeding natural geogenic emissions of 80−600 Mg yr−1.1,2 Most of these emissions occur as Hg0, which has an atmospheric lifetime of 0.5−1 years and hence is widely distributed, impacting even the most remote areas of the globe.1 Human-health concerns arising from dietary exposure to methylmercury (CH3Hg+) have led to national and, more recently, international efforts to reduce atmospheric Hg emissions and ultimately deposition to aquatic ecosystems where Hg methylation and biotic uptake occur. Because atmospheric Hg is actively cycled through large earth-surface reservoirs (ocean and terrestrial ecosystems), the anticipated © XXXX American Chemical Society

benefits of emission-reduction agreements, such as the 2013 Minamata Convention, depend greatly on the magnitude and timing of prior anthropogenic enrichment of those reservoirs relative to natural background conditions.3,4 The prevailing view of the global Hg cycle involves a 2- to 5fold increase in atmospheric Hg deposition to remote aquatic ecosystems since ∼1850, the result of increased usage and inadvertent emissions of Hg by human activities through time.5,6 This increase is based on a compilation of historical archives of past Hg deposition−principally lake-sediment cores−and assumes little or no anthropogenic emissions prior Received: December 12, 2013 Revised: May 8, 2014 Accepted: May 12, 2014

A

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Table 1. Geographic Characteristics of the Study Lakes and Rates of Hg Accumulation in Their Sediments lake

latitude

Minnesota August 47° 45.8′ N Bear 47° 17.1′ N Locator 48° 32.5′ N Loiten 48° 31.6′ N Newfoundland Topsail 49° 08.0′ N Frank 49° 10.7′ N Clever 49° 10.2′ N Tomtit 49° 10.3′ N Southeast Alaska Goldeneye 58° 14.8′ N

longitude

depth (m)

area (km2)

AC/ALa

modern Hg fluxb (μg m−2 yr−1)

preanthropogenic Hg fluxb (μg m−2 yr−1)

Hg flux ratioc (m/p)

91° 91° 93° 92°

36.4′ 20.6′ 00.3′ 55.6′

W W W W

6 9 16 15

0.76 0.18 0.54 0.39

4.9 3.3 8.2 6.2

70.9 19.5 19.3 17.1

13.2 4.9 5.4 5.3

5.4 4.0 3.6 3.3

56° 57° 57° 57°

00.8′ 38.9′ 46.1′ 47.4′

W W W W

8 10 14 11

0.053 0.022 0.020 0.025

4.0 6.7 2.3 8.2

20.5 19.6 12.4 21.1

7.4 4.3 4.7 6.5

3.0 4.5 2.7 3.3

50.0′

11

0.030

15.1

17.8

6.6

2.7

52.6′

12

0.030

8.7

16.0

3.9

3.9

49.4′

6

0.025

4.3

9.5

3.7

2.6

50.5′

14

0.020

11.6

17.1

5.1

3.4

Arctic Alaska Perfect 68° 38.9′ N

46.6′

5

0.040

1.2

9.2

3.3

2.8

Efficient

42.2′

13

0.040

4.5

3.1

1.4

2.2

02.3′

12

0.060

2.9

3.5

1.2

3.0

39.0′

6

0.025

3.0

16.7

3.2

5.2

Rectangle Sapsucker Cliff

Relaxing Surprise

135° W 58° 14.2′ N 135° W 58° 14.0′ N 135° W 58° 14.4′ N 135° W 149° W 68° 42.1′ N 149° W 68° 44.4′ N 150° W 68° 32.7′ N 149° W

East Africa Challa 03° 18.9′ S South America Negrilla 13° 09.0′ S El Junco 00° 53.7′ S Sierra Nevada Fallen Leaf 38° 53.8′ N Tahoe-1 Tahoe-2

37° 41.8′ E

94

4.2

0.3

12.7

4.1

3.1

72° 58.0′ W 89° 28.8′ W

33 6

0.06 0.06

5.3 2.1

39.9 31.2

6.5 6.0

6.2 5.2

126

5.7

5.4

19.3

1.0

19.9

1.7

15.2

1.0

14.7

19.1

1.4

13.3

120° 03.8′ W 39° 07.2′ N 120° 04.7′ W 39° 06.2′ N 120° 00.8′ W

501

490

a Catchment area (excluding lake): lake-surface area. bCore-specific fluxes, uncorrected for sediment focusing or watershed inputs. cRatio of modern Hg accumulation rate (1990-present) to preanthropogenic rate (pre-1800 for Minnesota lakes; pre-1500 for all others).

gold and silver from low-grade ores. Recent compilations suggest all-time anthropogenic Hg emissions total about 350 Gg, of which 39% resulted from intensive preindustrial (pre1850) Hg mining and amalgamation.15 Emissions of Hg to the atmosphere from early cinnabar mining and Hg amalgamation have also been incorporated into global Hg models.15−18 These models suggest Hg deposition to, and accumulation within, fast-cycling surface reservoirs should have spiked upward in the mid-1500s (South American Spanish mining era) and peaked in the late 1800s (North American “gold and silver rush”). As a consequence of these large early emissions, the new models infer that earth-surface pools are more highly enriched in anthropogenic Hg than represented in earlier projections (220−260 Gg in the global ocean), and that re-emission of legacy Hg dominates presentday Hg deposition (60% of total), while natural sources comprise only 13%.17 Most critically from a policy perspective, these models predict that proposed emission reductions, such as envisioned in the 2013 Minamata Convention, would

the industrial period. Global Hg-cycling models that impose this secular change in atmospheric Hg attribute roughly onethird of present-day deposition to primary anthropogenic emissions, one-third to natural geogenic emissions (e.g., oceans, soils, volcanism), and one-third to re-emission of prior anthropogenic deposition (legacy Hg).7−11 These models also predict a pool of 40−60 Gg of anthropogenic Hg in the global ocean9,10 and correspondingly rapid response (years to decades) of fish-mercury levels to changes in atmospheric Hg inputs.12 However, recent assessments of anthropogenic Hg releases from geologic stores suggest an earlier and much larger human footprint on the global Hg cycle owing to assumed preindustrial emissions from Hg-mining and precious-metal extraction. Deposits of Hg (mainly as cinnabar; HgS) have been known and exploited for millennia, but large-scale Hg mining and the production of metallic Hg0 did not begin until the mid-16th century, following the development of the patio Hg amalgamation process,13,14 which facilitated the extraction of B

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Voyageurs National Park. Sediment-Hg records from these lakes have been reported previously, but with a focus solely on the industrial era.31 In addition to these sites, core data are presented for two lakes proximal to the Spanish mining region of South America and two lakes near the epicenter of the North American gold/ silver rush. Of the South American sites, Laguna Negrilla is located near the Hg-mining district of Huancavalica, Peru, and the other, El Junco Lake, in the Galápagos Islands off the coast of Ecuador and farther downwind. Hg-records from these same cores have been published previously23,32 and are reproduced here to provide context for the remote North American sites. Similarly, Hg-core data from Lake Tahoe and Fallen Leaf Lake, high elevation sites in the Sierra Nevada, are presented to illustrate near-source Hg deposition trends for the North American Au/Ag mining district. Core results for Tahoe have also been published previously,33 but only for the industrial period, while Fallen Leaf is a new record. Finally, we present a new and important record from Lake Challa, an equatorial crater lake on the lower eastern slopes of Mt. Kilimanjaro (on the border between Kenya and Tanzania). With effectively no surficial watershed, finely laminated sediment, and a near-annually resolved chronology (see below), Lake Challa is a near-ideal recorder of global trends in atmospheric Hg deposition. All of the lakes included in this study are situated in small and relatively undisturbed catchments. Both are important site characteristics that minimize watershed influence on Hg accumulation as well as changes in sediment flux, which can otherwise confound interpretation of Hg-core trends. The remote North American lakes range in size from 0.02 to 0.76 km2, depth from 5 to 16 m, and catchment/lake area ratios from 1.2 to 15.1; Lake Challa and the Sierra Nevada lakes are considerably larger and deeper (Table 1). Core Collection and Dating. The cores presented here were collected between 1996 and 2010 as part of several separate studies. Most were obtained by piston coring methods, with the exception of those from Newfoundland, Southeast Alaska, and the Sierra Nevada (gravity corer), Lake Challa (gravity, mini-Kullenberg, and UWITEC hammer-driven corers), and Laguna Negrilla (Aquatic Instruments percussion corer). Cores were sliced at 0.5−1.0 cm increments (2 cm for deeper strata in some cases). Six widely spaced cores were collected from each of the lakes in Southeast Alaska and Newfoundland, and 11 cores were obtained from each of the lakes in Arctic Alaska. While the cores from within each lake showed effectively the same Hg trends, those illustrated here were either analyzed in greater stratigraphic detail or extended further into the preindustrial past. All sediment cores from the North American lakes, including those from Tahoe and Fallen Leaf, were dated by 210Pb using isotope-dilution, alpha spectrometry methods and the constant rate of supply (c.r.s.) dating model.34,35 The resulting activity profiles for excess 210Pb were effectively exponential with respect to cumulative dry mass (Supporting Information (SI) Figure S1), indicating near-constant sediment accumulation. Hg concentrations were also near-constant in the older (pre1800) sections of all cores, thereby providing independent validation for the assumption of constant sediment accumulation (see Results, below). This allowed reliable extrapolation of dates prior to the oldest explicit 210Pb date (typically 1800− 1850) based on cumulative dry mass (g cm−2) and the mean mass accumulation rate (g cm−2 yr−1) for each core. Dating

produce slower declines in global Hg deposition than would otherwise occur if the legacy pool were smaller.17 This new paradigm, which envisions a 7-fold global increase in present-day Hg deposition over preanthropogenic rates, has been incorporated in at least six new works,1,3,4,19−21 yet the evidence for such a large increase has received little scrutiny. The magnitude of preindustrial Hg emission is based largely on an inventory of historical Hg mining and estimated emission factors.14−16,22 However, the geophysical evidence (from lake, peat, and ice cores) in support of these estimates is decidedly mixed. To date few published lake-sediment records extend far enough into the past to fully capture the impact of new-world Spanish mining, and those that do are located very proximal to major mining districts and hence may reflect mostly local contamination, for example, from cinnabar dust.23 Other types of records (certain peat and ice cores24−26 report measurable upticks in preindustrial Hg deposition. However, a critical evaluation of peat-core chronology and diagenesis has raised doubts about the reliability of such claims.27 The more recent North American mining signal (late 1800s) does overlap with many published lake-core records, but very few show evidence of a large pulse of Hg deposition during that time. This apparent contradiction includes lake-sediment cores collected in close proximity to the Upper Fremont Glacier ice core, which appears to contain such a signal.24,28,29 In an effort to place this debate on a firmer geophysical footing, we have compiled a large suite of lake-sediment records with chronologies extending back at least 500 years to upward of 2000 years. These records, obtained largely from remote regions of North America, are compared with those from sites proximal to the mining districts of South America and western North America. A final comparison is made with a crater lake core from equatorial East Africa. In all cases, the remote sediment cores show near-constant Hg accumulation during the preindustrial past (pre-1850) followed by an exponential rise beginning with the onset of the industrial period. We go on to offer an alternative assessment of the likely fate of the large amounts of Hg mined and used in the amalgamation of precious metals and why most of this Hg did not make it into the global atmosphere or oceans. Thus, this critical review represents an examination, not of the latest generation of global Hg-cycling models, which are greatly advanced over initial attempts to understand the system, but rather of the historical data that are currently being used to drive them.

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MATERIALS AND METHODS Study Sites. The lake-sediment cores reported in this assessment were collected from undisturbed wilderness lakes located in four widely spaced regions of North America (four lakes per region): Arctic and southeastern Alaska, western Newfoundland, and northern Minnesota (Table 1). The Arctic lakes are situated on the north slope of the Brooks Range in the vicinity of the Toolik Lake LTER (Long-Term Ecological Research) field station and have been reported on previously, primarily in the context of industrial-era Hg cycling.30 Those from southeastern Alaska are situated along the north Pacific coast in the temperate rainforests of north Chichigof Island in the Alexander Archipelago. The Newfoundland lakes are located immediately to the southeast of Gros Morne National Park along the Gulf of St. Lawrence; local vegetation is a mosaic of spruce-fir forest and coastal tundra. And the Minnesota lakes are scattered between the north shore highlands of Lake Superior and the U.S./Canadian border and include two sites in C

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uncertainty for an extrapolated date of 1500−propagated from the measured variation in sediment accumulation (1800−1950) − averaged +82/-51 years (±1 σ) for the 16 remote-region cores plus those from Tahoe and Fallen Leaf (SI Table S1). This error is comparable to the uncertainty of AMS 14C dates of similar age and less than that with the required calendrical calibration of radiocarbon ages. The cores from the South American sites and Lake Challa were dated by spliced chronologies for 210Pb and 14C.23,32,36 Similar to the North American sites, the Lake Challa chronology and Hg concentration profile indicate relatively constant sediment accumulation over the last 1000+ years. Mercury Analysis. Total Hg was analyzed on freeze-dried sediment samples either by cold vapor atomic fluorescence spectrometry with gold trap amalgamation37−39 (Minnesota and Arctic Alaska cores) or DMA-80 direct mercury analyzer; EPA Method 7473 (all others). Certified reference materials (BCR-320, PACS-2, or MESS-3) were analyzed with each core, along with matrix spikes and replicate analyses (at least 10% of samples). For the cores from Southeast Alaska and Newfoundland, average CRM recoveries (NRCC MESS-3) were within 2% of certified values, matrix-spike recoveries averaged 101%, and the mean relative percent difference (RPD) of replicated samples was 4%. Similar results were obtained for Fallen Leaf and Lake Challa. Quality assurance details are provided elsewhere for the cores from Arctic Alaska,30 Minnesota,31 Lake Tahoe,33 and the South American sites, Laguna Negrilla and El Junco.23,32

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RESULTS Remote North American Lakes. The sediment-cores from the 16 remote North American lakes show a highly convergent pattern in Hg accumulation over time (μg m−2 yr−1). Rates are effectively constant in preindustrial times, showing only small fluctuations with little or no secular trend until c. 1850 and the onset of the industrial period (Figure 1). Hg accumulation then rises exponentially, peaking in the modern era at 2−5× the preindustrial rates. Absolute fluxes differ among regions and lakes within regions (Table 1) owing to geographic differences in atmospheric Hg deposition (low in the Arctic (1.5 μg m−2yr−1),30 higher at midlatitudes (SE Alaska 3.0 μg m−2yr−1, Newfoundland 4.7 μg m−2yr−1, Minnesota 7.2 μg m−2yr−1, wet deposition40) as well as the degree of sediment focusing within individual lake basins and contributions from catchment runoff. There is no obvious peak in Hg accumulation associated with either the Spanish Au/Ag mining era in South American (16th−18th centuries) or its North American counterpart in the late 19th century. However, a number of the cores, especially those from Newfoundland and Southeast Alaska, exhibit a subtle uptickat most a 20% increasein baseline Hg accumulation beginning in the mid-1500s, which could be associated with Hg mining and amalgamation in the distant Andes. The coherence in Hg trends among lakes and regions is all that more significant when one considers the range in sediment dry-mass accumulation rates (DMAR) represented by the cores (15−123 g m−2 yr−1, SI Table 1). For example, the initial rise in industrial-era Hg (c. 1850) occurs at 12−20 cm depth in the Minnesota cores, but only 5−10 cm in those from southeast Alaska. Moreover, the same Hg profiles are replicated among multiple cores collected from the lakes in Newfoundland and Southeast Alaska (six cores/lake) and Arctic Alaska (11 cores/ lake) and among single cores from an additional set of 16 lakes

Figure 1. Hg accumulation trends in sediment cores from remote North American lakes. Fluxes scaled by 0.5× for August, Surprise, and Relaxing lakes. Insets show detail for most recent 200 years.

from northern Minnesota.31 Were the upcore increase in Hg concentrations merely the result of diagenetic remobilization (as frequently seen in other metals such as Fe), the increase would be depth, and not time, dependent. As the increase is temporally coherent across so many lakes/cores, we may conclude that lakes are faithfully recording changes in Hg inputs. Because sediment accumulation rates are relatively constant in these cores, the observed patterns in Hg flux are also reflected in Hg concentration. This is particularly important when considering the preindustrial portions of the records D

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which are beyond the range of 210Pb dating. Here sediment chronology and Hg accumulation are derived by assuming a mean DMAR based on the dated sections of the cores (c. 1800 to present). The observation that Hg concentration is relatively uniform in the preindustrial past strongly supports our contention that both DMAR and Hg inputs to the lakes were also relatively constant. If that were not the case (both DMAR and Hg flux constant), inputs of both Hg and the sediment matrix would have to have changed simultaneously and in equal proportion (to maintain constant Hg concentration), an unlikely coincidence given that they are derived from such distinctly different sources−the atmosphere for Hg vs catchment erosion and in-lake production for the sediment matrix.41 The singular core record from Lake Challa in equatorial East Africa provides further evidence that the Hg core-trends replicated across remote regions of North American are truly global in scope (Figure 2a). Here Hg accumulation rates show no detectable increase throughout the first 800 years of record including the period of Spanish Hg mining and amalgamation− at similar latitudes−in the New World. The Hg flux then rises 3x during the course of the industrial era. Because the core exhibits somewhat greater variability than the North American records, it is difficult to discern precisely where the increase begins (1750−1850). This variability is a consequence of interannual variations in sediment accumulation caused in part by diatom blooms, which are perfectly preserved in Challa’s finely laminated sediments.36 These variations are also evident in the activity profile for 210Pb, which is otherwise exponential (SI Figure S1). Lake Challa represents a near-ideal atmospheric collector because it has almost no surficial watershed (watershed: lake area = 0.33) and thus receives all of its Hg inputs by direct deposition to the lake surface. Mining-Region Lakes. The two lake cores from South America, Laguna Negrilla and El Junco Lake, provide strong evidence for regional increases in atmospheric Hg deposition associated with New World Hg mining and amalgamation during the Spanish colonial period (Figure 2a). These previously published records show large increases in Hg accumulation beginning sometime between 1400 and 1600 AD. In Laguna Negrilla, the site nearest the Huancavelica mining district, Hg accumulation spiked upward about 10-fold during the early colonial period and then decreased gradually over the next 250 years in concert with declining production of metallic Hg0. At the more distant site, El Junco Lake in the Galápagos Islands, the rise in Hg accumulation during the Spanish mining era was smaller and more gradual, increasing about 3× between 1600 and 1800 and then doubling again over the course of the industrial period (1850-present). The two lakes proximal to the Au/Ag mining regions of western North America, Tahoe and Fallen Leaf, show little evidence of increased Hg accumulation during the Spanishcolonial mining era in South America (Figure 2a). In two widely spaced cores (5.8 km apart) from Lake Tahoe, Hg accumulation rates are relatively constant until the mid-1800s, while in Fallen Leaf there is a 2× increase in baseline between 1600 and 1800, followed by a marked rise around 1850. At both sites (and in both cores from Tahoe), Hg accumulation increases to present-day values that are 13−20 times greater than preindustrial background. A smaller peak or plateau in Hg accumulation is also evident in all cores around the turn of the last century. This secondary peak is coincident with gold mining operations in the Sierra Nevada foothills immediately west of the two lakes and with mining of the Comstock silver

Figure 2. (a) Hg accumulation trends in sediment cores from Lake Challa (Kenya/Tanzania), El Junco and Negrilla (S. America), and Tahoe and Fallen Leaf (Sierra Nevada). Insets show detail for most recent 200 years. The two cores shown for Lake Challa were collected in 1999 and 2005 from nearby locations,36 while the two cores from Lake Tahoe were collected from different parts of the basin.33 (b) Primary Hg emissions as estimated by Streets et al.15

load a few kilometers to the east. No mining occurred within the watershed of either lake. The magnitude of increase from preindustrial to preset-day is substantially greater in these cores than the 3−5-fold rise reported from other remote North American lakes, and may be at least partly attributable to Hg emissions associated the nearby Au/Ag mining. However, other arguments have been given for this large increase including decreased Hg0 evasion from the lake caused by recent increases E

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nated mining sites might also contribute to this more recent rise, it is difficult to explain why present-day rates should be so much higher than those c. 1890, when mining use of Hg was at its zenith. Similar to more remote sites, the pattern of Hg accumulation in these near-source cores does not match the emission trajectory proposed by Streets and others.15 Because lake-sediment cores integrate Hg inputs from their watersheds as well as direct atmospheric deposition, it is possible that the North American mining signal could be partially obscured in these records by washout of legacy Hg originally deposited to catchment soils during the peak in mining activity. Moreover, lake-sediment records can be temporally smoothed by sediment mixing (bioturbation), making it difficult to resolve a mining-related peak from the general increase in industrial Hg emissions. The first of these problems would be more likely to occur in lakes with large watersheds (relative to lake surface area).42 However, most lakes in this study have relatively small watersheds (median AC/ AL = 4.5, Table 1), and the crater lakes, Challa and El Junco, have virtually no surficial watershed, so that all Hg inputs are from direct atmospheric deposition. Nonetheless, watershed inputs can be problematic for lakes at high latitudes where atmospheric Hg deposition rates are very low and temporal resolution is limited because of slow sedimentation rates.43 Yet selection of lakes with small watersheds will tend to overcome such problems, as illustrated by our core records from northern Alaska which show recent trends consistent with atmospheric data, global emissions, and model predictions for the North American Arctic.30,43 Regarding the second problem, most cores show little evidence of sediment mixing in their 210Pb profiles (SI Figure S1), while the core from Lake Challa, is finely laminated, indicating a total absence of sediment mixing by benthic organisms. We therefore conclude that the lakesediment cores compiled here represent a comprehensive and reliable record of atmospheric Hg deposition during the peak periods of human exploitation and release of Hg to the global environment. Other Archives. The vast majority of published lakesediment records of Hg pollution are consistent with the observations reported here (Table 2). These records include lakes in West Greenland,44 Svalbard,45 Sweden, and Finland46−48 northeastern U.S.,42,49,50 central and Arctic Canada,51−53 the Canadian Rockies,54 Western U.S.,28,29 New Zealand,55 East Africa,56,57 the Tibetan plateau,58 southern Patagonia59 and the western Great Lakes region of North America.31,60,61 Counting the records reported here, this tally includes more than 200 lakes. All extend back to 1850 or earlier and cover the period of North American Au/Ag mining in the late 1800s, and only five show an Hg accumulation peak preceding and distinct from the general rise in industrial-era Hg. These lake-core results stand in contrast to the singular icecore record from the Upper Fremont Glacier in Wyoming, which has been cited as evidence for large preindustrial Hg emissions.15,17 The ice-core contains several abrupt Hg pulses, which are attributed to historic volcanic eruptions, as well as a sustained peak coincident with North American Au/Ag mining in the late 1800s.24 It seems reasonable that lake cores might act as low-pass filters, smoothing out abrupt volcanic events because of lower accumulation rates, sediment mixing, or lagged Hg inputs from their watersheds. However, it is unlikely that 200 lake-sediment records would collectively miss such a large, sustained load as that ascribed by Streets and others15 to

in algal production and enhanced regional oxidation of gaseous Hg0 at these high elevation sites.33 In either case, present-day rates are substantially higher than those occurring during the peak of the North American gold rush and would thus indicate a more dominant role for global industrial emissions, as compared to Hg from local mining use.

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DISCUSSION In their synthesis of historical Hg releases from human activities, Streets et al.15 estimate cumulative Hg air emissions of 137 Gg prior to 1850, about 90% of which was associated with Spanish Ag extraction in South and Central America during the 16th−18th centuries (Figure 2b). An additional 98 Gg of Hg was subsequently released to the global atmosphere by Ag and Au mining activities between 1850 and present, with the vast majority of these emissions occurring during the North American gold/silver rush of the late 1800s. These estimated emissions exceed all other anthropogenic releases during the industrial period including those from coal combustion, metal smelting, and Hg mining itself. They also suggest that actively cycling earth-surface reservoirs contain a substantial historical burden of anthropogenic Hg which today dominates the global Hg cycle.17 Our lake-sediment records do not support this Hg emission scenario. The 16 cores from remote North American lakes instead suggest near-constant Hg inputs prior to 1850 and an exponential increase thereafter coinciding with the onset of the industrial era. There are no obvious peaks associated with either period of new-world Au/Ag extraction. While several of the cores show a small increase in base-level Hg accumulation between 1400 and 1600possibly associated with mining in Spanish Americathe rise is much smaller than the 5× emission increase suggested for this period by Streets et al.15 and pales in comparison to that occurring during the industrial period. Likewise, the large peak predicted for North American mining emissions (Figure 2b) is not resolvable against the sustained increase in Hg accumulation from 1850 onward (Figures 1 and 2a). While it is possible that the initial rise of Hg accumulation after 1850 includes Hg emissions from 19th century mining activity, the rise is small relative to subsequent events and there is no actual peak as would be expected if mining emissions were as large as proposed (2600 Mg yr−1 in 1890). In contrast to these remote-region records, the cores from near-source sites in South America show clear evidence of regional Hg pollution from Spanish Hg mining and Ag amalgamation. However, similar signals are not evident elsewhere in the world (Arctic Alaska to equatorial Africa), and this strongly suggests that the Hg emissions did not participate in the global cycle. Cooke et al.23 arrived at a similar conclusion based on the Hg isotopic signatures in these same cores, which indicate little differentiation of atmospheric Hg sources until about 1900 when a positive excursion in massindependent fractionation (MIF) signals the large-scale industrial release of gaseous Hg0 into the global atmosphere. Similarly, the sediment cores from sites near the center of the North American gold/silver rush show a small penultimate peak in Hg accumulation around 1900, suggesting an increase in Hg deposition during peak mining activities (Figure 2a). However, these increases are coincident with rising emissions from other industrial sources and are ultimately surpassed by them with the cessation Hg amalgamation in the early 20th century. Although legacy Hg emissions from nearby contamiF

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At present it is difficult to reconcile the Upper Fremont ice core with this global network of lakes. It is quite possible that they are recording different deposition signals−high-elevation winter snowfall for the glacier and both summer and winter precipitation for the lakes.29 As a consequence Hg inputs may be dominated by different Hg species and different source regions.64 For example, the mining signal in the ice core could represent dust-borne Hg rather than globally sourced Hg0. Alternatively, other factors could be responsible for the miningera Hg peak in the ice core. These include climate-driven changes in snowfall amount and source, Hg volatilization from the snowpack, or seasonal melting and solute redistribution.28,65,66 Notable in this regard is a major shift to warmer temperatures at the termination of the Little Ice Age c. 1845, as inferred from oxygen isotope and electrical conductivity measurements of the ice core.66 This climate shift is also coincident with the sharp rise in Hg concentrations which is attributed to the onset of precious-metal mining in western North America. The point here is that no single record of Hg deposition−lake or ice core−is infallible. Validation from multiple sites is needed to be confident that the reconstructed changes in Hg deposition are global. Few previous lake-core studies extend back to the period of Spanish-colonial mining (1500−1700), although a few that do have commented on the absence of a preindustrial Hg peak at this time.30 On the other hand, several published peat-core records do cover this period (and thousands of years prior), and most suggest much larger increases in Hg deposition than those inferred from lake-sediment cores.25,26,67 These records have been cited as evidence for significant anthropogenic Hg emissions in preindustrial times.15,17 However, a recent critical review of natural archives of Hg deposition has documented a serious bias with peat records relating to both dating errors (mobility of 210Pb) and nonquantitative retention and loss of Hg during peat diagenesis−both factors contributing to an overestimation of secular increase.27 In comparison, lakesediment cores have been shown to be largely immune to these problems. Repeat coring of both remote and contaminated sites decades after initial core collection68−70 together with shorterterm experiments with isotopically spiked cores71 have amply demonstrated the stability of historical Hg profiles in organicrich lake sediments. However, interpretation of Hg deposition from all such archives requires careful consideration of sediment dating and the processes controlling Hg delivery and burial. As a general precaution, multiple cores from multiple lakes are needed to establish a reliable record of atmospheric Hg deposition.41 Anthropogenic Hg in the Ocean. One powerful constraint on whether preindustrial mining impacted Hg cycling on a global scale would be an independent estimate of the total amount of anthropogenic Hg in ocean waters as well as its vertical and horizontal distribution. We have recently developed such a data set by noting a correlation between Hg and P delivered to subsurface ocean waters via the biological pump.72 This correlation was found in deep waters (>1000 m depth) outside of the North Atlantic Ocean, indicating a relatively unperturbed Hg cycle prior to about 200 years ago. Furthermore, the Hg/P benchmark allowed estimates for the anthropogenic Hg found in the deep North Atlantic and waters shallower than 1000 m worldwide to be made, and this amounted to 60 Gg. This estimate is considerably less than the 262 Gg of Streets et al.,15 but similar to modeled estimates

Table 2. Compilation of Dated Lake-Sediment Records of Hg Accumulation from Remote Regionsa Hg flux ratio (m/p)b region United States Southeast Alaska Arctic Alaskad Northern Minnesotad N. Minnesota, Wisconsin Isle Royale (Lake Superior) Vermont, New Hampshire New York (Adirondacks) Mainee Wyoming Western US Canada & Greenland Newfoundland Nova Scotia Northern Quebec Arctic Canada Subarctic Canada Midlatitude Canada Central & Northern Canada Western Canada West Greenland South America Southeastern Peru Southern Patagonia (Chile) Scandinavia Finland Sweden Sweden Svalbard (Norway) Africa, Asia, New Zealand Uganda Lake Tanganyika Lake Challa (Kenya) Tibetan Plateau New Zealand total

mean

s.d.

no. lakes

this study 30 31 61

3.1

0.6

4

3.2 3.5 3.7

1.2 1.4 0.6

5 20 7

60

2.7

1.0

4

42

3.9

1.8

10

49

3.5

1.7

8

50 28 29

5.6 3.2

1.3 0.3

8 1 9

reference

this study 55 53 52 52 52 51

3.4

0.8

4

5.3 2.3 2.2 2.5 3.6 4.4

0.6 0.6 0.6 0.8 1.0 3.0

3 10 14 12 16 9

54 44

1.8 2.8

0.8 0.5

9 3

82 59

3.3 2.2

1.5 0.3

4 3

46

4.4

3.0

9

47 48 45

3.0 4.9 3.7

2.1 5.0 1.6

11 6 4

57 56 this study 58 55

2.8 4.9 3.1

0.2 3.3

3 1 1

7.8 3.0 3.6

4.6 0.5 1.2

8 2 208

c. 1890 peakc

Richie

Big Moose

Cli Batchawana

Valkea Kotinen

a

All extend back to 1850 or earlier and cover the period of North American Au/Ag mining in the late 1800s. Only five show a Hg accumulation peak preceding and distinct from the general rise in industrial-era Hg. bRatio of modern Hg flux (1980-present, varies with study) to preindustrial flux (pre-1850). cLakes showing a modest peak in Hg accumulation in the late 1800s. dIncludes core records also shown in this study. eHg flux ratios not available.

19th-century New World mining. Lake cores have been shown to faithfully track recent declines in Hg deposition in industrialized regions of North America and Europe,31,48,62,63 so there is little reason to think that they would not also respond to historical increases of even greater magnitude. G

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cinnabar mining, the latter of which occurred between 1200 BC and 1400 AD. Prior to ∼1450 BC, Hg fluxes were relatively constant, and were roughly one-third the modern Hg flux, again suggesting an all-time, 3-fold increase. This is despite the presence of a clear preindustrial increase in Hg beginning ∼1450 BC and attributed to regional mining activities. In addition, Hg fluxes in these cores were spatially variable and at least an order of magnitude lower than lakes located directly downwind of the cinnabar mines at Huancavelica. A similar spatial gradient was also documented in soils in and around the historic mining center of Potosi,́ 84 which likewise implies Hg emissions with a high deposition velocityparticulates or Hg(II)not Hg0. Thus, preindustrial Hg emissions in South America appear to have occurred primarily as cinnabar dust or reactive Hg, which would have significantly limited the geographic footprint. Implications for Global Hg Models. Ours is not the first study to question the magnitude or importance of preindustrial anthropogenic emissions to global Hg deposition. Others have noted the disparity between Hg emission estimates for North American Ag/Au mining and lake-sediment records,30,55,85 including papers specifically cited in support of such estimates, for example, refs 16 and 86. However, with the recent incorporation of these large preindustrial emissions into global models, the mining narrative is potentially driving a paradigm shift in our understanding of the global Hg cycle as well as policy developments aimed at curbing future emissions.3 This revised world-view includes a much larger anthropogenic enhancement of present-day Hg deposition over natural (preanthropogenic) levels (7−10× vs 3−5× in earlier assessments) with a far larger component of re-emitted Hg from prior releases (60% vs 30%). Additional consequences include larger Hg pools in the deep ocean, atmosphere, and fast terrestrial reservoirs, and a longer response time to emission reduction scenarios.15,17,18 We do not argue that preindustrial mining releases of Hg were small; indeed they match or exceed those from most present-day industrial sectors−only that the portion volatilized to the global atmosphere must have been much smaller than that first advanced by Nriagu and others.13,14,22,74 For example, Strode et al.16 report a far better fit between lake-sediment records and mining inventories if global mining emissions for the late 1800s are cut in half. The latter would be accomplished by reducing the fraction of the mining inventory volatilized to the atmosphere from 60% to 30%, which is more in line with the calculations of Guerrero.73 Such calculations also imply even greater local contamination at former mine sites and the potential for continued volatilization of Hg from those mining wastes.87 In light of the results reported here from a large suite of lakesediment records, we suggest that the newest global Hg models should be rerun using emission scenarios more in-line with a 3−5× present-day enhancement over natural (preanthropogenic) levels and more modest increases associated with Spanish and North American Ag/Au extraction. We thus argue that industrial-era emissions of Hg are largely responsible for much of the current contamination in both marine and freshwater fish stocks, and that legacy Hg from preindustrial emissions is a relatively minor component of that anthropogenic burden. This re-evaluation of the Hg cycle does not imply that there is less need for action to reduce global Hg emissions, but rather should give us hope that such actions will

based on emission inventories that did not include putative preindustrial Hg emissions.9,11,12 Fate of Hg from Historical New World Mining. The geophysical evidence reported here from lake-sediment cores and oceanographic measurements appears to be in direct conflict with estimates of global Hg emissions from preindustrial mining in the Americas, yet there is general agreement that large amounts of Hg were mined and lost in the amalgamation of precious metals, particularly Ag. Reliable historical accounts place these losses at more than 121 000 Mg for Mexico and South America (1560−1810) and 61 000 Mg for North America (1860−1910).13,14,73 It has been widely assumed that somewhere in the vicinity of 60−75% of this unrecovered Hg was volatilized to the atmosphere as gaseous elemental Hg with the remainder lost (as liquid elemental Hg) during transport, washing, or in solid waste.14,22,74 However, these volatilization factors are inconsistent with historical observations and have been recently called into question for Spanish-era mining.73 Based on historical correspondencia (the ratio of Hg lost to Ag produced) and the chemical reactions known to occur during Ag amalgamation, Guerrero73 calculated that 66−93% of total Hg losses would have occurred in the formation of solid calomel (Hg2Cl2) with the remaining 7−34% lost to physical processes (volatilization, but also spills, washings, and other solid detritus). These calculations are based on the observation that Hg served two roles in the extraction of Ag−reduction of AgCl, which was either present in the original ore or was the product of the reaction between magistral (copper sulfate) and silver sulfide in the presence of chloride ions75as well as amalgamation of elemental Ag. (High-grade ores containing native silver were smelted with lead, not amalgamated.) The carefully recorded correspondencia, which averaged 1.8 ± 0.3, quantify stoichiometrically that upward of 90% of the Hg was consumed in reducing Ag sulfides and chlorides to elemental Ag, with the resulting solid calomel washed downstream or buried in landfills. All main 19th century works on Hg amalgamation of silver ores in Mexico mention that losses of mercury during the heating stage of the amalgam were very low to insignificant.76,77 Even within the limits of the theoretical knowledge of chemistry at the time, they also recognized that the formation of a chloride salt of Hg led to the unavoidable consumption of mercury during the process.77 As similar conditions and processing prevailed in North American Ag mining,78−80 it is reasonable to assume that published volatilization estimates for this period are also overestimated and that much of the Hg consumed in the extraction of new-world Ag lays buried at former mine-sites or in depositional basins to which they drained. Losses of Hg during amalgamation appear to have occurred largely as calomel. However, the mining of cinnabar was a significant source of preindustrial Hg emissions,81 as both the Laguna Negrilla and El Junco sediment records attest (Figure 2a). Although both cores record a synchronous rise in regional Hg deposition, the subsequent trends and magnitude of change are dissimilar, suggesting that the deposition was highly localized, and related at least in part to atmospheric transport of cinnabar dust, as opposed to more-widely dispersed gaseous Hg0.23 A similar conclusion was reached recently by Beal et al.82,83 using four new sediment-Hg records, one of which spans the Holocene,83 recovered from remote lakes in southeast Peru. Beal et al.83 note that Hg fluxes were actually lower during Colonial times than during preceding periods of intensive H

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yield reductions in fish Hg concentrations more quickly than the view portrayed in the current Minamata Convention.88

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(4) Sunderland, E. M.; Selin, N. E. Future trends in environmental mercury concentrations: Implications for prevention strategies. Environ. Health 2013, 12, 2−5. (5) Lindberg, S.; Bullock, R.; Ebinghaus, R.; Engstrom, D.; Feng, X.; Fitzgerald, W.; Pirrone, N.; Prestbo, E.; Seigneur, C. A synthesis of progress and uncertainties in attributing the sources of mercury in deposition. Ambio 2007, 36 (1), 19−32. (6) Selin, N. E. Global biogeochemical cycling of mercury: A review. Ann. Rev. Environ. Resour. 2009, 34, 43−63. (7) Mason, R. P.; Fitzgerald, W. F.; Morel, F. M. M. The biogeochemical cycling of elemental mercury: Anthropogenic influences. Geochim. Cosmochim. Acta 1994, 58 (15), 3191−3198. (8) Mason, R. P.; Sheu, G.-R. Role of the ocean in the global mercury cycle. Glob. Biogeochem. Cycles 2002, 16 (4), 1093 DOI: 10.1029/ 2001GB001440. (9) Sunderland, E. M.; Mason, R. P. Human impacts on open ocean mercury concentrations. Glob. Biogeochem. Cycles 2007, 21, GB4022. (10) Selin, N. E.; Jacob, D. J.; Yantosca, R. M.; Strode, S.; Jaeglé, L.; Sunderland, E. M. Global 3-D land-ocean-atmosphere model for mercury: Present-day versus preindustrial cycles and anthropogenic enrichment factors for deposition. Glob. Biogeochem. Cycles 2008, 22, GB2011. (11) Lamborg, C. H.; Fitzgerald, W. F.; O’Donnell, J.; Torgersen, T. A non-steady state compartmental model of global-scale mercury biogeochemistry with interhemispheric atmospheric gradients. Geochim. Cosmochim. Acta 2002, 66 (7), 1105−1118. (12) Mason, R. P.; Choi, A. L.; Fitzgerald, W. F.; Hammerschmidt, C. R.; Lamborg, C. H.; Soerensen, A. L.; Sunderland, E. M. Mercury biogeochemical cycling in the ocean and policy implications. Environ. Res. 2012, 119, 101−117. (13) Nriagu, J. O. Legacy of mercury pollution. Nature 1993, 363, 589. (14) Nriagu, J. O. Mercury pollution from the past mining of gold and silver in the Americas. Sci. Total Environ. 1994, 149, 167−181. (15) Streets, D. G.; Devane, M. K.; Lu, Z.; Bond, T. C.; Sunderland, E. M.; Jacob, D. J. All-time releases of mercury to the atmosphere from human activities. Environ. Sci. Technol. 2011, 45 (24), 10485−10491. (16) Strode, S.; Jaegle, L.; Selin, N. E. Impact of mercury emissions from historic gold and silver mining: Global modeling. Atmos. Environ. 2009, 43, 2012−2017. (17) Amos, H. M.; Jacob, D. J.; Streets, D. G.; Sunderland, E. M. Legacy impacts of all-time anthropogenic emissions on the global mercury cycle. Glob. Biogeochem. Cycles 2013, 27, 1−12. (18) Selin, N. E. Global change and mercury cycling: Challenges for implementing a global mercury treaty. Environ. Toxicol. Chem. 2013, DOI: 10.1002etc.237. (19) Chen, L.; Wang, H.-H.; Liu, J.-F.; Zhang, W.; Hu, D.; Chen, C.; Wang, X.-J. Intercontinental transport and deposition patterns of atmospheric mercury from anthropogenic emissions. Atmos. Chem. Phys. Discuss. 2013, 13, 25185−25218. (20) Sonke, J. E.; Heimbürger, L.-E.; Dommergue, A. Mercury biogeochemistry: Paradigm shifts, outstanding issues and research needs. C.R. Geosci. 2013, 345, 213−224. (21) Wiener, J. G. Mercury exposed: Advances in environmental analysis and ecotoxicology of a highly toxic metal. Environ. Toxicol. Chem. 2013, 32, 2175−2178. (22) Camargo, J. A. Contribution of Spanish−American silver mines (1570−1820) to the present high mercury concentrations in the global environment: A review. Chemosphere 2002, 48, 51−57. (23) Cooke, C. A.; Hintelmann, H.; Ague, J. J.; Burger, R.; Biester, H.; Sachs, J. P.; Engstrom, D. R. Use and legacy of mercury in the Andes. Environ. Sci. Technol. 2013, 47, 4181−4188. (24) Schuster, P. F.; Krabbenhoft, D. P.; Naftz, D. L.; Cecil, L. D.; Olson, M. L.; Dewild, J. F.; Susong, D. D.; Green, J. R.; Abbott, M. L. Atmospheric mercury deposition during the last 270 years: A glacial ice core record of natural and anthropogenic sources. Environ. Sci. Technol. 2002, 36, 2303−2310. (25) Givelet, N.; Roos-Barraclough, F.; Shotyk, W. Predominant anthropogenic sources and rates of atmospheric mercury accumulation

ASSOCIATED CONTENT

* Supporting Information S

A table of 210Pb dating results including sediment accumulation rates, age estimates and their uncertainty, and plots of total 210 Pb activity vs sediment depth for each of the study lakes. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: 651-433-5953 ext. 11; e-mail: [email protected]. Present Addresses ¶

Department of Environment and Sustainable Resource Development, Government of Alberta, Edmonton, Alberta T6B2X3, Canada. ○ University of Michigan Biological Station and School of Natural Resources and Environment, Ann Arbor, Michigan 48109, United States. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank our many colleagues who assisted in the collection of sediment cores used in this study: Rob Tordon (Environment Canada), Chun-Mao Tseng (National Taiwan University), Chad Hammerschmidt (Wright State University), Avery Shinneman (University of Washington), and Bruce Monson (Minnesota Pollution Control Agency). We also gratefully acknowledge those who provided samples from their own cores: Dirk Verschuren (Ghent University) for Lake Challa (collected with permission of the Kenya government; MOEST permit 13/001/11C), Paula Noble (University of Nevada) for Fallen Leaf Lake, and Julian Sachs (University of Washington) for El Junco Lake, as well as those who helped with Hg analyses: Allan Hutchins, Larissa Graham, and Ming Chung (University of Connecticut), and Ben Barst and René Rodrigue (Université du Québec), and 210Pb dating: Erin Mortenson (St. Croix Watershed Research Station). We especially thank Saúl Guerrero (McGill University) for analysis of historical texts and review of our interpretations of silver mining in New Spain. Funding for the various studies embodied in this work was provided by the U.S.EPA-STAR (Science to Achieve Results) program (Grants R829796 and 91643401), the NSF Office of Polar Programs (Grant 9908895), NSERC-Canada (Grant 371567-2009), Yale University, the National Geographic Society (Grant 8922-11), and the Legislative and Citizens Commission on Minnesota Resources (LCCMR).

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