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Atmospheric mercury depositional chronology reconstructed from lake sediment and ice cores in the Himalayas and Tibetan Plateau Shichang Kang, Jie Huang, Feiyue Wang, Qianggong ZHANG, Yulan Zhang, Chaoliu Li, Long Wang, Pengfei Chen, Chhatra MAni Sharma, Qing Li, Mika Sillanpää, Juzhi Hou, Baiqing Xu, and Junming Guo Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04172 • Publication Date (Web): 14 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016
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Atmospheric mercury depositional chronology reconstructed
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from lake sediment and ice core in the Himalayas and Tibetan
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Plateau
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Shichang Kang a,c*, Jie Huang b,c,h, Feiyue Wang d, Qianggong Zhang b,c, Yulan Zhang a,h
, Chaoliu Li b,c,h, Long Wang e, Pengfei Chen b, Chhatra Mani Sharma f, Qing Li
6
g
, Mika Sillanpää h, Juzhi Hou b, Baiqing Xu b,c, Junming Guo b,h,i
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* Corresponding author email:
[email protected]. Tel/Fax: +86-931-4967368
10 a
11
State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and
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Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China b
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Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of
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Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China c
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CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China
Center for Earth Observation Science, and Department of Environment and Geography,
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University of Manitoba, Winnipeg, MB R3T 2N2, Canada e
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School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution
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Control, Tsinghua University, Beijing, 100084, China f
Human and Natural Resources Studies Centre, Kathmandu University, Kathmandu, 6250, Nepal g
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h
Laboratory of Green Chemistry, Lappeenranta University of Technology, Sammonkatu 12
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School of Geography Science, Southwest University, Chongqing, 400715, China
Mikkeli, FI-50130, Finland i
Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
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ABSTRACT
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Alpine lake sediments and glacier ice cores retrieved from high mountain regions can
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provide long-term records of atmospheric deposition of anthropogenic contaminants
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such as mercury (Hg). In this study, eight lake sediment cores and one glacier ice core
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were collected from high elevations across the Himalaya-Tibet region to investigate
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the chronology of atmospheric Hg deposition. Consistent with modeling results, the
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sediment core records showed higher Hg accumulation rates in the southern slopes of
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the Himalayas than those in the northern slopes in the recent decades (post-World
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War II). Despite much lower Hg accumulation rates obtained from the glacier ice core,
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the temporal trend in the Hg accumulation rates matched very well with that observed
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from the sediment cores. The combination of the lake sediments and glacier ice core
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allowed us to reconstruct the longest, high-resolution atmospheric Hg deposition
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chronology in High Asia. The chronology showed that the Hg deposition rate was low
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between the 1500s and early 1800, rising at the onset of the Industrial Revolution,
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followed by a dramatic increase after World War II. The increasing trend continues to
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the present-day in most of the records, reflecting the continuous increase in
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anthropogenic Hg emissions from South Asia.
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1. INTRODUCTION
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Approximately 95% of global atmospheric mercury (Hg) is gaseous elemental
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Hg0.1 The high vapor pressure and low oxidation potential of Hg0 enables a long
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atmospheric residence time of 0.5-2 years, allowing its transport over long distances
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before being oxidized to Hg(II) and scavenged through wet or dry deposition.1−2
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Anthropogenic Hg emissions to the atmosphere have a long history, increasing
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substantially since the onset of the industrialization and resulting in considerable
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atmospheric Hg deposition.3−5 Hg is one of the most problematic contaminants in the
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global environment because of its significant adverse impacts on ecological and
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human health, and as such has been increasingly regulated with the recent signing of 2 ACS Paragon Plus Environment
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the Minamata Convention.6 Once deposited into the surface environment, inorganic
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Hg becomes available for methylation. The resulting methylmercury (MeHg) is the
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most bioaccumulative and neurotoxic form of Hg, posing a potential threat to human
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health after biomagnification through food chains.7 A recent work has shown elevated
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Hg concentrations in fish from the Tibetan Plateau.8
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With a mean elevation of more than 4000 m.a.s.l and known as the "Roof of the
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World", the Himalayas and Tibetan Plateau region (hereafter, the Himalaya-Tibet) is
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remote, isolated, and presumed to be a sensitive region to anthropogenic impact due
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to its unique landform and fragile ecosystems.9 The landscape of the Himalaya-Tibet
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mainly consists of glaciers, lakes, permafrost and snow cover, all of which could
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serve as natural archives for documenting the modern and past changes of the
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atmospheric chemistry. As atmospheric transport is essentially the only transport
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pathway for anthropogenic Hg to the Tibetan Plateau due to the region’s high
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altitudes, sparse human population and minimal to nonexistent local industrial
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activities, 9−10 age-dated lake sediments and glacier ice cores have been shown to have
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recorded variations in the atmospheric contaminant deposition over the Tibetan
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Plateau,11−14 permitting reconstruction of the emission history of these contaminants
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to the atmosphere.
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The Himalaya-Tibet is the most lake and glacier concentrated region at low- and
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mid-latitudes.15 Although the Himalayas is the world’s highest mountain range and
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could act as a natural barrier to atmospheric contamination in the southern frontier of
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the Tibetan Plateau, previous studies have demonstrated that high Himalayan valleys
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can act as a direct channel, capable of transporting atmospheric contaminants up to
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5000 m a.s.l.,16 and those contaminants could successively be transported
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trans-Himalayan and advected onto the Tibetan Plateau.17−19 Of particular interest are
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the transport and accumulation of Hg in high mountain environments. Since World
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War II, the economy of Asian countries has developed rapidly, and the sustained
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economic growth of these developing countries have important consequences in terms 3 ACS Paragon Plus Environment
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of anthropogenic Hg emissions to the environment and its possible ecosystem and
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human health impacts.20 As South Asia has become one of the world’s largest sources
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of anthropogenic Hg emitted into the atmosphere as a result of rapid economic and
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industrial developments,21−23 there is concern that Hg may be contaminating remote
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ecosystems such as those on the Himalaya-Tibet, as well as on the global scale. There
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have been a few recent reports on Hg contamination of some remote lakes in the
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interior of the Tibetan Plateau,12−13 though data for the southern slopes of the
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Himalayas are more scarce.
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In addition to lake sediments, ice cores retrieved from high-elevation alpine
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glaciers are also considered as an excellent archive for documenting the long-term
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variation of chemical composition of the atmosphere because they provide
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high-resolution, well-preserved, multi-parameter archives of the atmospheric
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signature from remote areas worldwide.5,24−29Although Hg in snow and ice can be
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affected by post-depositional processes such as photoreduction30−31 and percolation32,
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we were motivated to investigate whether the historical trends of atmospheric Hg
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deposition across the Himalaya-Tibet reconstructed from glacier ice cores are
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comparable to those from the sediment cores.
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Here we report the historical record and spatial distribution of Hg across the
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Himalaya-Tibet as reconstructed from lake sediments and glacier ice core retrieved
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from high-elevation lakes and glacier. The goals of our study were three-fold: (1) to
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quantify historical changes in Hg accumulation in high mountain lakes and glaciers;
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(2) to assess the extent of Hg contamination across the Himalaya-Tibet and compare
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the results with simulations of Hg deposition from global Hg models; and, (3) to
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compare the Hg records in lake sediments and glacier ice core to determine if the
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historical trends of atmospheric Hg deposition are consistent.
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2. MATERIALS AND METHODS
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2.1. Sediment Coring and Dating
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A total of eight sediment cores were taken from the deep portion of selected
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lakes from 2008 to 2011 from the southern (Phewa, Gokyo, Gosainkunda) and the
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northern (Qiangyong Co, Nam Co, Bangong Co, Lingge Co, and Tanglha Lake)
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slopes of the Himalayas (Figure 1 and Table 1) using a gravity coring system with a
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6-cm inner diameter polycarbonate tube. The cores (ranged 18.5 to 88 cm in length)
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were sectioned in the field using a stainless steel slicer at an interval of 0.5 cm except
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for Bangong Co and Lingge Co which were sectioned at 1 cm interval, stored in
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plastic bags, and kept frozen until analysis. The sediment samples were freeze-dried
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and homogenized before being radiometrically dated and analyzed for Hg. The
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sediment chronology was constructed by measuring radionuclide (210Pb) at the Key
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Laboratory of Tibetan Environment Changes and Land Surface Processes, Chinese
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Academy of Sciences (CAS), Beijing, China, using an ORTEC HPGe GWL series
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well-type coaxial low background intrinsic germanium detector.
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2.2. Ice Core Drilling and Dating
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Mt. Geladaindong is the highest peak of the Tanglha Range in the central Tibetan
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Plateau and is the source region of the Yangtze River. During the Sino-US
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cooperation expedition in November 2005, a 147-m ice core (33.58°N, 91.18°E, 5750
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m a.s.l.) was drilled at the firn basin in the accumulation zone of the Guoqu Glacier
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(6621 m a.s.l.) of Mt. Geladaindong (Figure 1). The ice core was stored below -15°C
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during transportation to Lanzhou and kept in a cold room, where it was sectioned by a
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modified band-saw set to a total of 1419 segments (approximately every 5-10 cm).
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The segments were analyzed for Hg at the State Key Laboratory of Cryospheric
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Sciences (SKLCS), CAS, Lanzhou, China. The ice core was dated using multiple
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parameters including annual layer counting (based on the seasonal cycles of δ18O,
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major ions and trace elements), radioactivity of tritium (3H) and lead (210Pb), as well
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as the flow model as described in details in Grigholm et al 33 and Kang et al..34
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2.3. Analytical Procedures and QA/QC
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After the lake sediments and ice core were dated, samples were selected from
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each core to provide sufficient temporal resolution to show historical trends in Hg
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concentrations. Hg in the sediment samples (~0.2 g) was analyzed in duplicate on a
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Leeman Hydra-IIC Direct Hg Analyzer (Leeman Lab Hydra, Teledyne Leeman
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Laboratories, Hudson, NH), which involves thermal decomposition and detection by
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atomic absorption spectroscopy, following the US EPA Method 7473. The method
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detection limit (MDL) for lake sediments, defined as 3 times the standard deviation of
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10 replicates measurements of a blank solution, was
