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Sep 18, 2017 - a continuous high-resolution ice-core Hg record from the. Belukha glacier in the Siberian Altai, covering the time period. 1680−2001...
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A 320-year ice-core record of atmospheric Hg pollution in the Altai, Central Asia Stella Eyrikh, Anja Eichler, Leonhard Tobler, Natalia Malygina, Tatyana Papina, and Margit Schwikowski Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03140 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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A 320-year ice-core record of atmospheric Hg pollution in the Altai,

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Central Asia

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Stella Eyrikh1, Anja Eichler,2,3,*, Leonhard Tobler,2,3, Natalia Malygina1,

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Tatyana Papina1, and Margit Schwikowski2,3,4 1

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Institute for Water and Environmental Problems, Siberian Branch of the Russian Academy of Sciences,

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Barnaul 656038, Russia 2

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Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland

Department for Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland

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Corresponding author: e-mail: [email protected], phone: +41 56 310 2077, fax: +41 56 310 4435

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KEYWORDS: ice core, mercury, emission, Central Asia, Siberian Altai

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Abstract

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Anthropogenic emissions of the toxic heavy metal mercury (Hg) have substantially increased atmospheric

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Hg levels during the 20th century compared to pre-industrial times. However, on a regional scale, atmospheric

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Hg concentration or deposition trends vary to such an extent during the industrial period that the

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consequences of recent Asian emissions on atmospheric Hg levels are still unclear. Here we present a 320-

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year Hg deposition history for Central Asia, based on a continuous high-resolution ice-core Hg record from

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the Belukha glacier in the Siberian Altai, covering the time period 1680-2001. Hg concentrations and

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deposition fluxes start rising above background levels at the beginning of the 19th century due to emissions

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from gold/silver mining and Hg production. A steep increase occurs after the 1940s culminating during the

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1970s, at the same time as the maximum Hg use in consumer products in Europe and North America. After a

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distinct decrease in the 1980s, Hg levels in the 1990s and beginning of the 2000s return to their maximum

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values, which we attribute to increased Hg emissions from Asia. Thus, rising Hg emissions from coal

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combustion and artisanal and small-scale gold mining (ASGM) in Asian countries determine recent

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atmospheric Hg levels in Central Asia, counteracting emission reductions due to control measures in Europe

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and North America.

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1. Introduction

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Mercury (Hg) is one of the most studied and notorious global environmental pollutants that has been the

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subject of particular attention of scientists and the public worldwide for many years. In October 2013, the

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Minamata Convention on Mercury, aiming to protect human health and the environment from anthropogenic

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emissions of the toxic heavy metal Hg, was established1. To date, over 50 countries have authorized the

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convention and it became active on 16, August, 2017. Hg is released into the environment from natural

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sources (e.g. mineral dust and volcanoes) and anthropogenic activities (e.g. fossil combustion, gold and silver

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mining, see Fig. 1). Due to the long residence time of its elemental form (Hg0, ~1 year), representing as much

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as 95% of the atmospheric Hg burden2, Hg can be distributed globally via the atmosphere or oceans. Thus,

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ecosystems far away from anthropogenic emission sources can be contaminated after long-range transport of

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Hg0, oxidation to Hg2+, and input by wet or dry deposition3.

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Understanding how the legacy of past anthropogenic emissions contributes to present-day Hg enrichment is

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essential for anticipating the effectiveness of future reductions in Hg emissions4. However, large uncertainties

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exist in quantifying the fluxes in the Hg biogeochemical cycle, particularly regarding natural and

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anthropogenic emissions to the atmosphere, deposition and re-emissions5, 6. Hg concentrations in air and wet

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deposition have been measured since 1990 at selected sites mainly in the Northern Hemisphere7, 8, whereas a

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coordinated observing system to monitor Hg on a global scale started only in 20109. Natural archives provide

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the long-term history of atmospheric Hg deposition. Studies of lake and marine sediments10-14, peat bogs15-17

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and ice-cores18-23 resulted in a general consensus that anthropogenic emissions during the 20th century have

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substantially increased atmospheric Hg levels compared to pre-industrial times. Whereas emission estimates

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suggest that Hg emissions from wide-spread coal burning and artisanal, small-scale gold mining (ASGM) in

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Asia have recently outpaced those from commercial Hg use in the last decades24, the corresponding Hg

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profiles from natural archives do not show such a clear picture. In sediment and ice core records from North

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America and Europe, Hg peaks in the mid- to late 1900s and decreases afterwards due to reduced recent

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anthropogenic Hg emissions in these industrialized regions10, 19, 25-29. This is consistent with a 20-38% decline

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in atmospheric Hg concentrations from 1995 to 2009 at different monitoring stations in the Northern and

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Southern Hemisphere7. In contrast, sediment records from remote areas in the Arctic, Antarctic, and North

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America reveal a continuous Hg increase towards the end of the 20th/ beginning of the 21th century12, 30-33.

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This is also the case for most study sites in close proximity of Asian Hg source regions14, 20, but their majority

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is lake sediments of the Tibetan Plateau, representing deposition histories with a very low resolution. The

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latter is caused by low productivity and correspondingly low sedimentation rates at these high altitudes.

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Further complicating is the finding, that such high recent values in lake sediments do not necessarily indicate

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elevated emissions, but may partly be explained by rather local signals and polluted catchment soils34. To our ACS Paragon Plus Environment

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knowledge, the only available long-term Asian high-resolution Hg record is from the Geladaindong ice core

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(central Tibetan Plateau)20. This record shows an increasing trend since the 1940s similar to the lake

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sediments, but ends too early (1982) to capture the recent period.

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Here, we present a continuous high-resolution ice-core based Hg record from the Belukha glacier in the

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Siberian Altai, covering the time period 1680-2001. This new ice-core record fills the data gap for continental

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Central Asia and allows assessing the influence of anthropogenic emissions from industrial Hg use in Europe

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and North America and of recent emissions from coal burning and ASGM in Asia. The obtained Hg results

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are discussed with respect to estimates of historical atmospheric Hg emissions and compared with other ice

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core records in Europe, North America, the Arctic, and Asia.

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2. Experimental section

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2.1. Study site characteristics and ice core dating

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In July 2001 a 139 m ice core was drilled at the saddle between the two summits of the Belukha

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(49°48’26’’N, 86°34’43’’E, 4062 m a.s.l.), the highest mountain in the Altai region (4506 m a.s.l.)35 (Fig. 2).

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Low constant englacial temperatures of -17ºC (below 15 m depth) indicate that the glacier site belongs to the

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cold infiltration recrystallization zone, where meltwater formed under the influence of solar radiation and

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high air temperature refreezes some centimeters below the surface35. The main precipitation season in the

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Altai is summer, with humid air masses from the Atlantic Ocean and recycled moisture from Central Asian

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sources being the major sources of precipitation36. Winter months (December–February) receive less than 5%

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of the annual precipitation, due to the prevailing stable Siberian High and the predominance of cold and dry

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arctic air masses37. The ice core chronology was obtained by: 1) nuclear dating using the radioactive decay of

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210

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reference horizons related to the maximum of nuclear weapons testing (tritium and plutonium horizon in

Pb, 2) annual-layer counting of seasonal varying signals37,

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and 3) a nonlinear regression39 through

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1963)40 and several explosive volcanic eruptions38,

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(93.7 m water equivalent (w.eq.)) of the Belukha core investigated in this work cover the period 1680-2001

(see Supplementary Information). The upper 113 m

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(Fig. S1) with a mean annual accumulation rate of 0.56 m w.eq. The dating uncertainty is ±3 years for the

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period 1815-2001 and ±5 years between 1680 and 1815.

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The Belukha ice core has already been proven to contain well-preserved records of biogenic and biomass

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burning tracers, anthropogenic pollutants, and water stable isotopes allowing to reconstruct temperature

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changes in the Siberian Altai over the past 750 years37, 41, the history of biogenic emissions and forest fires

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from Siberian forests42, 43, past anthropogenic emissions of NH3, NOx, SO2, and heavy metals from Eastern

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Europe, the former Soviet Union and the Rudny Altai (Russian for ore Altai, Fig. 2)38, 44, 45. In the case of

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such aerosol-related air pollutants with short atmospheric life-times of ~7 days source areas are regional as

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indicated by 7-day air mass back-trajectories44. This is different for the long-lived Hg. Due to the atmospheric

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life time of ~1 year, emission sources relevant for the study site are not restricted to European and Soviet

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Union regions, but are assumed to be globally distributed.

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2.2. Sample preparation

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Ice-core sections (up to 0.7 m long, diameter 7.8 cm) were sealed in polyethylene tubes in the field and

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transported frozen to the Paul Scherrer Institute (PSI, Switzerland) for glaciochemical analyses. 671 samples

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of ice and firn were prepared for Hg analyses in the cold room of the PSI at -20°C. Due to annual layer

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thinning with depth, sample resolution of the continuous record varied between one sample per year in the

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deepest core sections (period 1680-1735) and 5-8 samples per year in the upper part of core (period 1940-

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2001). 8% of the core could not be analyzed due to poor ice quality (small chips)44. Details of ice core cutting

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and the decontamination procedure are described elsewhere45, 46. In brief, for the analysis of Hg only pristine

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inner parts of the core (~2x2 cm) were cut out using a band saw. In a second step, contamination from the

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saw blade and handling were removed by rinsing ice samples with ultrapure water (18 MΩ cm quality) and

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by chiseling firn samples with a ceramic knife. The decontaminated samples for Hg analysis were placed in

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Duran® glass bottles, oxidized to Hg(II) with BrCl solution according to US EPA 163147 and melted at room

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temperature under class-100 clean room conditions at least 24 hour before analysis. Laboratory blanks were

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regularly monitored using frozen ultra-pure water subjected to the same preparation steps.

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All materials, equipment, and labware were pre-cleaned46 and tested for Hg contamination and preservation.

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Glass bottles were pre-cleaned with a 0.5% BrCl solution, rinsed three times with ultrapure water and

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subsequently filled with a 0.05 % HCl solution. Chemical reagents were either ultra-pure or Hg-free. All

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analytical works were carried out following the protocol for ultra-clean condition for the determination of

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ultra-low levels of Hg in ice and snow samples48, and according to US EPA 1631 method47.

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2.3. Hg analysis

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Hg concentrations (total Hg) were determined by Atomic Fluorescence Spectrometry (Mercur Analyser,

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Analytik Jena, Germany). Calibration of the instrument was performed using at least 6 ultra-low

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concentrations standards ranging from 0.2 to 100 ng L-1. Analytical accuracy was verified by determining Hg

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concentrations in a certified reference material (Hg in river water, ORMS-2, Natural Research Council

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Canada) (Table S1) and by standard addition to real firn and ice samples. The recovery was 98-102% and the

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reproducibility 1-6%49, 50. The limit of detection (LoD) calculated as 3σ of 10 blanks as well as laboratory

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blanks (including Hg contribution from water, reagent, containers, and air) were determined for each batch of

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samples (28 batches). Concentrations of Hg in the firn and ice samples were blank-corrected by subtracting

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the laboratory blank. The average laboratory blank from all batches was 0.31 ± 0.04 ng L-1and the average

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LoD 0.04 ng L-1.

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2.4. Hg flux calculations

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Annual Hg fluxes were calculated by multiplying the mean annual Hg concentration with the respective

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accumulation rate (in m w.eq.). Since annual layers were identified only for the period 1815-2001, a mean

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accumulation rate of 0.56 m w.eq. was used for the flux calculations for the period 1680-1814 (see

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Supplementary Information).

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3. Results and Discussion

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Hg concentrations of the 671 individual samples range from 0.07 to 8.9 ng L-1 with a median of 0.89 ng L-1

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(Table 1). Concentration levels are in the same order of magnitude as reported for firn and ice cores from the

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European Alps, Canadian Arctic, Tibetan Plateau, and Greenland (Table 1). This finding indicates that Hg

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levels in the Altai are dominated by global Hg emissions and long-range transport, in accordance with the

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long life-time of elemental mercury of ~1 year.

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Hg deposition rates in the most recent samples (years 2000-2001) of 1.1 ± 0.2 µg m-2 a-1 are in reasonable

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agreement with that of precipitation at rural Central Asian sites between 2012 and 2014 (2 ± 1.6 µg m-2 a-1)51.

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Thus, we assume that major parts of the deposited Hg at the study site are preserved and not lost by possible

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postdepositional processes, such as photoreduction, reemission, and meltwater relocation52-54. This is most

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likely related to the formation of ice layers in the firn by refreezing of meltwater formed at the surface in

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summer. These impermeable ice layers of rarely more than 2 cm thickness efficiently seal the firn layers

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below from interacting with the atmosphere. Because of very cold ice temperatures of -16.6°C at 15 m depth,

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melting and refreezing does not affect more than an annual layer and does not result in runoff. Accordingly

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relocation of Hg in the firn layer is limited to within an annual layer, which is not relevant when discussing

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10-year means.

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The concentration record of Hg is presented in Figure 3. Annual averages were calculated from the raw

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data to account for the varying sample resolution. The annual Hg concentration record reveals two distinct

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features; namely short-term maxima with the most pronounced ones in 1843, 1884, and 1894, and a long-

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term trend. Some of the brief Hg events occur synchronously with increases in Ca2+ or exSO42-

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concentrations, indicating that they can be linked to an input of mineral dust aerosols from Central Asia

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deserts or to emissions from volcanic events (see section 3.1., Fig. 3). To extract the long-term trend, annual

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Hg data were averaged over 10-year periods (Fig. 3), after removing the dust and volcanic related spikes

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depicted in Fig. 3. Hg concentrations start rising above background levels at the beginning of the 19th century,

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followed by a steep increase after the 1940s and reach their maximum values during the 1970s. After a ACS Paragon Plus Environment

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distinct decrease in the 1980s, Hg levels in the 1990s and beginning of the 2000s return to their maximum

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values. This trend is not caused by changing accumulation rates, as demonstrated by the good agreement

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between records of Hg concentrations and fluxes (see Fig. 4).

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3.1. Short-term Hg maxima from mineral dust and volcanic events

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Natural sources include Hg released from the Earth’s crust by weathering of Hg-containing rocks and Hg

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emitted by volcanic eruptions and geothermal activity; however this contribution of geogenic sources has

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large uncertainties on a global and regional level5. Vast arid zones in Central and South Asia are key mineral

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dust sources in the Northern Hemisphere55. Numerous studies of Hg in snow and glacier ice from the Tibetan

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Plateau showed that dust storms are a significant source of Hg deposition in this region54, 56, 57. Likewise, 7 of

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the 15 most pronounced Hg maxima of the Belukha record (in the years 1687, 1768, 1811, 1843, 1884, 1894,

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1985) are accompanied by Ca2+ maxima indicative of dust input (see Fig. 3). Emissions from deserts in

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Kazakhstan and China were already shown to have a considerable input on short-term concentration changes

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of other heavy metals as Cd, Cu, Pb, Sb, and Zn44, 45. Additionally to the dust source of the Hg concentration

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peak in 198558 an impact of the accident at the Tengiz oil field in June 1985 on the high Hg concentrations in

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this year was discussed59.

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Volcanic eruptions are known to release a significant amount of Hg to the atmosphere, accounting for 20–

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40% of natural emissions60. Short-term Hg peaks in an ice core from Upper Fremont Glacier in Wyoming

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were associated with explosive volcanic events from the Northern and Southern Hemisphere23. This

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attribution was challenged with the revised chronology of that record published recently61. In the Belukha

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record the explosive volcanic eruptions in 1739 (Shikotsu), 1783 (Laki), 1815 (Tambora), and 1912 (Katmai)

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are documented as the most prominent exSO42- maxima (Fig. 3,

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concentrations. Only the Hg concentration maxima in 1726 and 1884 (Fig. 3) might be related to volcanic

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input following the eruptions of Oraefojokul (1727) and Krakatao (1883). This finding suggests that only

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), but are not accompanied by high Hg

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selected volcanic eruptions leave an observable Hg imprint in remote natural archives, supporting the new

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interpretation of the Upper Freemont ice core record61.

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3.2. Long-time Hg trend 3.2.1. Pre-industrial period (1680-1850)

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The pre-industrial time is characterized by low Hg concentrations and fluxes (median 0.47 ng L-1 and 0.32

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µg m-2 a-1, respectively) (see Table 1), comparable with values reported for remote areas such as Greenland,

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the Arctic and Antarctica21, 22, 62-64. Belukha Hg levels are rather low during the 17th and 18th century and start

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rising at the beginning of the 19th century. Since the record starts only in 1680, it is ambiguous, whether the

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low concentrations during 1680-~1810 represent natural background values, or are already influenced by

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anthropogenic emissions.

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The early 19th century rise in Hg could be from local or global sources. Although smelting of non-ferrous

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metal ores provided a major Hg source already millennia ago65, the application of liquid Hg for silver and

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gold mining, beginning ~1570 in Colonial Spanish South America, marked very likely the first global

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distribution of anthropogenic Hg18,

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European mines Almaden in Spain and Idrija in present-day Slovenia67. The estimated cumulative losses of

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Hg to the environment due to the production of silver and gold in South America have been estimated at

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196,000 t between 1580 and 1900, with an annual average of 612 t, peaking in the beginning of the 19th

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century68 similar to the ice core record. Thus, during the pre-industrial period, silver and gold mining and Hg

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production were worldwide the dominant Hg sources30, 69.

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. The majority of Hg was extracted in Huancavelica (Peru) and the

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Regionally, mining of precious metals and related metallurgical production began to develop in the Altai in

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the first half of the 18th century. In the early 19th centuries 90% of the Russian silver was produced in the

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Altai region as reflected by increased concentrations of Pb and other metals (Sb, Cd, Ag) in the ice core (see

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Fig. 4, 44, 45, 70). In the first half of the 19th century all Russian silver and gold coins were manufactured in the

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Altai. During that time amalgamation has not been used for silver and gold mining, but Hg was released to ACS Paragon Plus Environment

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the atmosphere from high temperature smelting of gold- and silver-containing ores70, 71. The “Gold Rush”

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occurred in Russia in 1828 after the discovery of numerous deposits of placer gold and the resolution of

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private gold mining. Amalgamation was applied for the extraction of ore gold only in the Urals, whereas it

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was not widely used in the Altai, Eastern Siberia and Transbaikal (mountainous region to the east of Lake

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Baikal), where the most intensive extraction of placer gold was carried out72.

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The steady increase in Hg from the beginning of the 19th century on is not due to changes in the deposition

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regime or rising dust or volcanic input, since accumulation rates and frequency and strength of dust and

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volcanic events remained constant (see Supplementary Information, Fig. 3). This interpretation is supported

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by the fact that only Pb, Sb, and Hg show this early increase related to emissions from metallurgical

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activities, whereas most of the trace elements such as Cu and Zn kept background levels45. A general change

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in deposition regime should have affected all air pollutants.

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The similarity in the Pb and Hg trends during the pre-industrial period suggests that the early increase of

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the ice-core Hg levels in the first half of the 19th century is mainly related to regional emissions from the

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metallurgy of precious metals for the production of Russian coins. An influence from early silver and gold

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refining in South America is less likely, due to missing synchronous evidence from other ice core sites (Fig.

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5), but cannot be ruled out.

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3.2.2. Intermediate period (1850-1940)

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During the intermediate period Hg concentrations and fluxes almost doubled compared to the pre-industrial

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period (0.89 ng L-1 and 0.53 µg m-2 a-1, respectively), displaying a broad peak between the 1880s and 1930s.

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This period is characterised by considerable Hg emissions from the gold/silver rush in Northern America.

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Between 1850 and 1900 the mean annual Hg consumption in the United States was 1360 t; 90% were used

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in the recovery of gold and silver68. Emission estimates for anthropogenic Hg by Streets et al.6 and

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Horowitz et al.24 agree in the magnitude of the peak in 1890 (~2.6 kt/year) due to the gold/silver rush, but

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differ for the major part of the 20th century. Streets et al.6 suggests that this first emission phase was

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dominant, whereas according to the more recent, improved inventory by Horowitz et al.24, overall highest

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Hg releases occurred during the 1970s (~3 kt/year) (see Fig. 1). Although our record agrees in general better

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with the Horowitz et al.24 inventory, the 1880-1930 Hg peak is not as pronounced as the emission estimate

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suggest (Fig. 4). This is in agreement with a large number of lake sediment records from remote sites30 and

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the ice core data in Fig. 5. The discrepancy between the Hg emission estimates and the records from natural

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archives is possibly due to an overestimation of gaseous Hg0 emissions from amalgamation. As shown in

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recent studies73,

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assumed to have partly surpassed that of Hg0 during that time. From the good correspondence between

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world Hg production (mainly in North America, Spain, Italy, and Slovenia)67 and the ice core record in the

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intermediate period (Fig. 4) we conclude that the ice core peak ~1880-1930 is not only due to direct Hg

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emissions from gold and silver mining mainly in North America, but also to releases from worldwide Hg

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production.

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Local mining became less important in the intermediate period. At the end of the 1860s the mining and

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metallurgical industry in the Altai entered a period of crisis due to the depletion of rich ore reserves and the

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fall in silver prices. After 1870 the volume of the silver production decreased by a factor of two and until

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1896 the majority of metallurgical plants were closed. At the same time coal mining started in the Kuznetsk

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Basin, southwestern Siberia, in 1851 where some of the world's largest deposits of coal are located. Coal

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production gradually increased from 17,000 tons in 1890 to 290,000 tons in 1904 (construction of the

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Siberian railway), reaching 21,1 million tons in 194075. Generally, coal combustion is a major Hg source,

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but emits also significant amounts of Pb76. Since ice-core Pb concentrations are not elevated after the 1890s

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(Fig. 4), we assume that coal burning did not contribute to the elevated Hg levels in the intermediate period.

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Likewise, the deviation between Hg and Pb records in the intermediate period suggests that the Hg peak

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during ~1880-1930 is not dominated by enhanced emissions from regional metallurgy, but rather global

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sources (see above) as corroborated by a synchronous increase at many sites18, 30, 61.

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, releases of less volatile Hg2+ compounds calomel (HgCl2) and cinnabar (HgS) are

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3.2.3. Industrial period (1940-2001)

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Hg concentrations and fluxes in the industrial period are about three times higher compared to the pre-

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industrial level (1.43 ng L-1 and 0.88 µg m-2 a-1, respectively). This is consistent with peat bog and lake

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sediment studies, suggesting an Hg enhancement factor of 3-4.3 since pre-industrial times77. Throughout the

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20th century, Belukha Hg levels are lowest in the 1930s and rise strongly from the 1940s on until culminating

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in the 1970s. After a distinct decrease in the 1980s, Hg levels in the 1990s and beginning of the 2000s return

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to their maximum values.

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The most recent emission inventory24 suggests that emissions from Hg use in consumer products like paint

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and batteries and in chlor-alkali plants dominated from the 1940s on. Global Hg emissions dropped after the

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1970s as a consequence of reduced global Hg production67 and air pollution control measures such as

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elimination of open-air waste burning (including incineration of batteries) in developed countries and

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capturing of Hg from chlor-alkali plants in sludges, subsequently dumped on land24. This emission estimate is

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supported by the temporal trend in our data during the industrial period until the end of the 1980s (Figs. 1 and

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4).

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With the end of the 20th century coal combustion, artisanal gold mining, and metal production became the

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major Hg sources24, 78 (see also Fig. 1). Whereas between 1990 and 2005 Hg emissions from these sectors

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decreased in most parts of the world, Asian emissions grew significantly and dominate recent Hg levels (Fig.

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1,5). This is mainly caused by increased Asian coal production, doubling between 1983 and

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200079. Additionally, growing emissions from Hg use in artisanal and small-scale gold mining (ASGM)

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mainly in Asia, South America, and Africa contributed to rising levels in these continents at the end of the

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20th century. In China, for instance, 200 small-scale gold mines started operating in the late 1980s80. The

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increase of the Belukha Hg after the 1980s suggests that during the 1990s and beginning of the 2000s Hg

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emissions from coal burning and ASGM mainly in Asia outpaced those from commercial Hg use. Our results

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seem inconsistent with observations of Hg0 at sites in North America and Europe, showing decreases of ~1–

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2% per year from 1990 to recent8 and with a 20-38% decline in atmospheric Hg concentrations from 1995 to ACS Paragon Plus Environment

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2009 at long-term monitoring stations in the Northern and Southern Hemisphere7. However, a closer look

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reveals that the downward trend is caused by declining concentrations after 2000, whereas Hg levels partially

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increase in the period 1995-2000, in agreement with the Belukha record. Furthermore, no Asian sites were

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included in the analysis due to a lack of long-term data from this region. Model calculations of annual surface

298

air Hg0 concentrations in the period 1990-2008 suggests a positive trend in major parts of Asia, but declining

299

levels elsewhere81.

300

3.3. Comparison with other ice-core records

301

Long-term Northern Hemispheric Hg ice core records from Geladaindong (Tibetan Plateau, 20), Mount Logan

302

(Canada, 18), Upper Fremont Glacier (UFG) (USA,

303

22, 62

304

levels is remarkable. Except for the ice core from Upper Fremont Glacier 10-year mean concentrations vary

305

between the respective detection limit and 2-5 ng L-1. These similar levels confirm that the major part of Hg

306

in the atmosphere is gaseous Hg0, transported across long distances and globally distributed due to its long

307

life-time of ~1 year. Unlike Hg, concentration records of aerosol-borne heavy metals having a shorter life-

308

time vary significantly between different regions. Concentrations levels of Pb with a life-time of ~1 week, for

309

example, are at least one order of magnitude higher at Belukha compared to Greenland and Canadian Arctic

310

sites, due to the closer proximity to the source areas44, 82.

311

Ice-core long-term Hg trends are marked by increasing levels until the 1970s, followed by a sharp decrease.

312

Thus, there is growing agreement among ice core records, supported by lake sediment records that emissions

313

from gold/silver mining in the 19th century are minor compared to the 1970s peak, reflecting the height of Hg

314

use in consumer products like paint and batteries and in chlor-alkali plants. This is only different for the

315

Arctic sites, were none of the records shows a pronounced peak in the 1970s. The increase during the end of

316

the 1990s and beginning of the 2000s due to emissions from coal burning and ASGM is captured at the

23, 61

) and Canadian Arctic as well as Greenlandic sites21,

were compared with the Belukha data (Fig. 5). The agreement in the trend and absolute concentration

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Belukha and the Arctic sites. The record from Mount Logan shows rising annual values between 1993 and

318

199818, whereas other ice core records ended too early to capture recent periods.

319

3.4. Comparison with model results

320

The most recent biogeochemical modelling of atmospheric Hg in the period 1850-201024 and the Belukha ice

321

core record shows a notable agreement (Fig. 4), except the magnitude of the 1880-1930 peak and the most

322

recent period. The discrepancy between the modelled Hg and the ice core data in the 19th century is possibly

323

due to an overestimation of gaseous Hg0 emissions from amalgamation as discussed above. During the most

324

recent period model simulations suggest declining atmospheric Hg concentrations in 1970-2000 and

325

increasing levels after 2000. In contrast, in the Belukha record Hg started rising again around 1990, implying

326

that the influence of recent emissions from regional coal burning and ASGM in Asian countries on

327

atmospheric Hg concentrations can be detected about 10 years earlier in Central Asia compared to the global

328

perspective. This discrepancy might also be explained by uncertainties in emission data, used as input for the

329

biogeochemical models or uncertainties in reservoir exchange constants in the models and dependent on the

330

applied emission data set6,

331

emission estimates with observations from natural archives to better document and understand global trends

332

of atmospheric Hg pollution.

83

, recent trends diverge24. This emphasizes the importance of constraining

333 334

Acknowledgements

335

The authors acknowledge the help of Patrick Ginot, Beat Rufibach (deceased), Martin Lüthi, Henrik Rhyn,

336

Dimitrii N. Kozlov, Sergej Derewstschikow, Vladimir Vashenzev, Andrej Jerjomin (deceased), Veronica

337

Morozova, Alexander Chebotkin, and Igor Karakulko during the ice coring expedition on Belukha glacier.

338

We thank the three anonymous reviewers for their valuable and constructive comments, improving the clarity

339

of the manuscript.

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340

Supporting Information

341

Results of the analyses of a certified reference material (Table S1) and information about the Belukha age-

342

depth scale (Figure S1) are shown. This information is available free of charge via the Internet at

343

http://pubs.acs.org.

344 345

References

346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382

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Table 1: Compilation of total Hg concentrations (ng L-1) and fluxes (µg m-2 a-1) from different Northern Hemisphere ice-core sites. Given are minimum – maximum/median values or means* of the three different time periods 1680-1850, 1850-1940, 1940-2001.

577 Sampling site, elevation, m a.s.l.,

Hg concentrations, ng L-1

Hg fluxes, µg m-2 a-1

Preindustrial period, 1680-1850

Intermediate period, 1850-1940

Industrial period, 1940-2001

Preindustrial period, 1680-1850

Intermediate period, 1850-1940

Belukha glacier, Siberian Altai, 4062 m a.s.l., 2001

0.07 – 2.9 0.47

0.2 – 8.9 0.89

0.08 – 6.0 1.43

0.04 – 1.8 0.32

0.11 – 1.9 0.53

Mt. Geladaindong, Tibetan Plateau, 5750 m a.s.l., 2005

0.002 – 5.0 0.47

Mount Logan, Yukon, Canada, 5340 m a.s.l., 2001, 2002