Correspondence/Rebuttal pubs.acs.org/est
Comment on “Ice Core Perspective on Mercury Pollution during the Past 600 Years”
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ecently, Beal et al.1 reported a 600 yr ice core mercury (Hg) record based on stringent ultraclean ice core pretreatment and measurement protocols. However, there is a major limitation in interpreting the implications for the past Hg emissions. The authors estimated that the registered anthropogenic Hg depositions for the Colonial Period and North American “Gold Rush” contributed minor fractions of the total anthropogenic Hg deposition in the record. The Gold Rushrelated Hg peaks for both the Mount Logan (ML) and Upper Fremont Glacier (UFG) ice cores2 are substantially smaller than their respective 20th century peaks. Based on this estimation and comparison with estimates3,4 that showed primary anthropogenic Hg emissions from concurrent periods, the authors claimed that1 “current estimates of Hg emissions from Colonial and Gold Rush mining are erroneously high,” and “global Hg models forced with these high emissions likely overestimate the influence of early mining on anthropogenic Hg in the modern global environment.” We could not accept these arguments and have proposed the reasons and concerns below.
export of Hg from increasing anthropogenic Asian Hg emission.1 These facts indicated that the geophysical location influences ice core records. It is inappropriate to assess global Hg emission using a single record of Hg deposition.
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HISTORICAL ANTHROPOGENIC HG: A PERSISTENT, CUMULATIVE IMPACT ON THE ENVIRONMENT The unique physicochemical properties of Hg allow it to exist as a gas in the atmosphere and actively cycle among environmental reservoirs once liberated from the lithosphere. Since antiquity, humans have increasingly driven the release of Hg to the environment.9 The pre-1900 anthropogenic Hg emission is primarily associated with Hg mining and use of Hg in silver and gold mining.4,10,11 The historical anthropogenic Hg emissions developed by Streets et al.4 considered only the direct releases of Hg into the atmosphere. Horowitz et al.3 then included Hg released from commercial products, and further fractionized the Hg release to various environmental reservoirs, including air, soil, water, and landfill. Both trends revealed a major spike associated with the Gold Rush, comparable to the 20th century peak. The proportion of pre-1900 Hg emissions to the atmosphere might be biased due to the undefined proportion of Hg used in mining that was lost to the atmosphere as Hg0.4,12−14 Despite this ongoing debate, the Hg released by early mining has been actively cycling among environmental reservoirs and has contributed substantially to the Hg accumulation in present environment.9,15,16 Even if the current estimates contained unrecognized bias in Hg emissions, direct comparison of the relative magnitudes of environmental archival records with estimated Hg emission trends is irrelevant, as the estimated Hg emission at a specific time is of “firstorder”, whereas glacier-archived Hg at a specific time represents the cumulative enrichment of prior Hg emissions registered by the glacier. Specifically, although estimates of anthropogenic Hg have shown two comparative surges for the Gold Rush and the 20th century maximum, archival records should register higher peak for the latter one. Figure 1 revealed that the ice core Hg curve agrees better with the simulated historical atmospheric Hg levels and depositions in terms of phases and magnitudes. Note that the late 19th century peak for the ML ice core curve remains less prominent as compared with the simulation. Currently, we cannot discount the validity of either curve in constraining the historical Hg pollution, yet such comparison is more realistic for understanding the impact of anthropogenic Hg emissions on the environment.
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GAPS IN LINKING ICE CORE-REGISTERED HG DEPOSITION TO HISTORICAL HG EMISSION Mercury is very mobile, undergoing complicated transformation within the snowpack and bidirectional exchange at the snow-air interface.5 Therefore, Hg retained in the ML ice core, as noted by the authors, “reflect past changes in midtropospheric atmospheric Hg2+ deposition.” This point has also been proposed that early mining signals in ice cores could represent dust-borne Hg rather than globally sourced Hg0.6 Although greater release of anthropogenic Hg likely induces higher atmospheric Hg levels and subsequent enhanced Hg deposition, Hg retained in glaciers is variable and is often influenced by multiple factors.7,8 The ice core Hg time series is not readily linked to a direct assessment of historical Hg emission unless gaps in Hg emission, transport, deposition and storage in glaciers are carefully considered.
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REPRESENTATIVE ENVIRONMENTAL ARCHIVAL RECORDS IN TERMS OF GEOGRAPHICAL LOCATIONS Historical Hg accumulation can be drawn from multiple natural archives and is limited to local, regional, and global scale in terms of the geographical sampling sites. The ML glacier is located at the high altitude and high latitude, upwind from major mining districts in North America before 1900. It is reasonable that the impact of Gold Rush Hg emission on the ML is smaller than that nearby intensively mined districts, reflected by differences in Hg fluxes in ML and UFG ice cores. In addition, the Gold Rush-related Hg peaks are different in phases for UFG (1840−1860) and ML (1880−1920) ice cores. Note also that the ML ice core recorded an observable increase in Hg deposition in the late 20th century that is ascribed to the © XXXX American Chemical Society
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THE NEED FOR MORE GLACIAL HG RECORDS The relatively high snow accumulation and proximity to the anthroposphere make high-altitude glaciers one of the best indicators of past anthropogenic activities,17 allowing alter-
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DOI: 10.1021/acs.est.5b04320 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Figure 1. Mount Logan ice core HgT flux records (blue points) and LOESS smoother (red line) redrawn from Beal et al.,1 compared with simulated atmospheric Hg levels (sky blue line) and deposition (orange line), modified from Horowitz et al.3 and Amos et al.9 ice core record of natural and anthropogenic sources. Environ. Sci. Technol. 2002, 36 (11), 2303−2310. (3) Horowitz, H. M.; Jacob, D. J.; Amos, H. M.; Streets, D. G.; Sunderland, E. M. Historical Mercury Releases from Commercial Products: Global Environmental Implications. Environ. Sci. Technol. 2014, 48 (17), 10242−10250. (4) 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. (5) Durnford, D.; Dastoor, A., The behavior of mercury in the cryosphere: A review of what we know from observations. J. Geophys. Res. 2011, 116.10.1029/2010JD014809 (6) Engstrom, D. R.; Fitzgerald, W. F.; Cooke, C. A.; Lamborg, C. H.; Drevnick, P. E.; Swain, E. B.; Balogh, S. J.; Balcom, P. H. Atmospheric Hg Emissions from Preindustrial Gold and Silver Extraction in the Americas: A Reevaluation from Lake-Sediment Archives. Environ. Sci. Technol. 2014, 48 (12), 6533−6543. (7) Durnford, D. A.; Dastoor, A. P.; Steen, A. O.; Berg, T.; Ryzhkov, A.; Figueras-Nieto, D.; Hole, L. R.; Pfaffhuber, K. A.; Hung, H. How relevant is the deposition of mercury onto snowpacks? - Part 1: A statistical study on the impact of environmental factors. Atmos. Chem. Phys. 2012, 12 (19), 9221−9249. (8) Durnford, D.; Dastoor, A.; Ryzhkov, A.; Poissant, L.; Pilote, M.; Figueras-Nieto, D. How relevant is the deposition of mercury onto snowpacks? - Part 2: A modeling study. Atmos. Chem. Phys. 2012, 12 (19), 9251−9274. (9) Amos, H. M.; Sonke, J. E.; Obrist, D.; Robins, N.; Hagan, N.; Horowitz, H. M.; Mason, R. P.; Witt, M.; Hedgecock, I. M.; Corbitt, E. S.; Sunderland, E. M. Observational and Modeling Constraints on Global Anthropogenic Enrichment of Mercury. Environ. Sci. Technol. 2015, 49 (7), 4036−4047. (10) Nriagu, J. O. Legacy of Mercury Pollution. Nature 1993, 363 (6430), 589−589. (11) Zhang, Y.; Jaegle, L.; Thompson, L.; Streets, D. G. Six centuries of changing oceanic mercury. Glob. Biogeochem. Cycles 2014, 28 (11), 1251−1261. (12) 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 (9), 4181−4188. (13) Strode, S.; Jaegle, L.; Selin, N. E. Impact of mercury emissions from historic gold and silver mining: Global modeling. Atmos. Environ. 2009, 43 (12), 2012−2017. (14) Nriagu, J. O. Mercury pollution from the past mining of gold and silver in the America. Sci. Total Environ. 1994, 149 (3), 167−181. (15) Amos, H. M.; Jacob, D. J.; Kocman, D.; Horowitz, H. M.; Zhang, Y.; Dutkiewicz, S.; Horvat, M.; Corbitt, E. S.; Krabbenhoft, D. P.; Sunderland, E. M. Global Biogeochemical Implications of Mercury Discharges from Rivers and Sediment Burial. Environ. Sci. Technol. 2014, 48 (16), 9514−9522. (16) 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 (2), 410−421.
native, refined interpretations of anthropogenic Hg pollution. Ice core Hg represents the glacier-retained atmospheric Hg deposition that is potentially linked to historical Hg emission, albeit not directly. Sufficient consideration of Hg behavior in snowpack and processes involving Hg emission, transport and deposition should be coupled to interpret ice core Hg records. The ML ice core Hg record,1 along with the previous worldwide glacial Hg records,2,18,19 indicated that glaciers are likely able to record historical changes in Hg pollution subtly and reproducibly. More glacial Hg records at greater temporal resolution and on extended geographical scales are need to provide comparable, collective information to better understand and anticipate impact of anthropogenic Hg on the environment.
Qianggong Zhang*,†,§,∥ Shichang Kang‡,§ †
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Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, P.R. China ‡ State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, 730000, P.R. China § CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, P.R. China ∥ PSI, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the National Natural Science Foundation of China (41371088 and 41225002). Q.G.Z. has been supported by a grant from the China Scholarship Council.
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REFERENCES
(1) Beal, S. A.; Osterberg, E. C.; Zdanowicz, C. M.; Fisher, D. A. Ice Core Perspective on Mercury Pollution during the Past 600 Years. Environ. Sci. Technol. 2015, 49 (13), 7641−7647. (2) 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 B
DOI: 10.1021/acs.est.5b04320 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
(17) Zhang, Q.; Kang, S.; Gabrielli, P.; Loewen, M.; Schwikowski, M. Vanishing High Mountain Glacial Archives: Challenges and Perspectives. Environ. Sci. Technol. 2015, 49 (16), 9499−9500. (18) Zheng, J. Archives of total mercury reconstructed with ice and snow from Greenland and the Canadian High Arctic. Sci. Total Environ. 2015, 509, 133−144. (19) Fain, X.; Ferrari, C. P.; Dommergue, A.; Albert, M. R.; Battle, M.; Severinghaus, J.; Arnaud, L.; Barnola, J. M.; Cairns, W.; Barbante, C.; Boutron, C. Polar firn air reveals large-scale impact of anthropogenic mercury emissions during the 1970s. Proc. Natl. Acad. Sci. U. S. A. 2009, 106 (38), 16114−16119.
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DOI: 10.1021/acs.est.5b04320 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
