Atmospheric mercury deposition recorded in an ombrotrophic peat core from Xiaoxing'an Mountain, Northeast China

Environmental Research 118 (2012) 145–148

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Atmospheric mercury deposition recorded in an ombrotrophic peat core from Xiaoxing’an Mountain, Northeast China Shunlin Tang a,b,n, Zhongwei Huang a, Jun Liu a, Zaichan Yang a, Qinhua Lin b a b

Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo, Henan province, PR China State Key laboratory of Geochemistry, Chinese Academy of Science, Guiyang, PR China

a r t i c l e i n f o

abstract

Article history: Received 30 July 2011 Received in revised form 15 December 2011 Accepted 19 December 2011 Available online 2 June 2012

The historical mercury accumulation rates (Hg AR) resulting from atmospheric deposition to Xiaoxing’an Mountain were determined via analysis of 210Pb- and 14C-dated cores up to 5000 years old. Natural Hg AR background, pre-industrial Hg AR and maximum industrial Hg AR in Northeast China were 2.2 7 1.0 mg/m2/yr for 5100–4500 BP, 5.7 mg/m2/yr and 112.4 mg/m2/yr, respectively. We assumed that the increase in Hg deposition in the Xiaoxing’an mountain area during industrial time was mainly attributed to local anthropogenic emissions around this peat bog. & 2012 Elsevier Inc. All rights reserved.

Keywords: Mercury Ombrotrophic peat Accumulation rates China

1. Introduction Mercury (Hg) is considered as a global pollutant and one of the most toxic trace elements known to man (Eisler, 2006). Mercury can be emitted into the atmosphere from both natural processes and human activities (Lindqvist et al., 1991). Mercury can be transported far beyond the regions of the emission sources. It has been found in Arctic biota (Riget et al., 2000) at concentrations so high that it presents a threat to human health resulting from the long-range Hg transportation and deposition from anthropogenic sources. The modern model of atmosphere Hg circulation reveals that Hg budget had tripled since pre-industrial times. However, it is difficult to determine the true impact of anthropogenic emissions on the global Hg budget without knowledge of the natural budget and its variations. Cores from ombrotrophic peat bogs have been shown to be effective natural archive of atmospheric Hg deposition (MartinezCortizas et al., 1999; Roos-Barraclough and Shotyk, 2003; Shotyk et al., 2005) because they are hydrologically isolated from the influence of ground and surface waters (Clymo, 1987), and fed only by atmospheric deposition. Compared with lake and marine sediments, glacial ice and snow, however, the determination of Hg in ice has proven to be difficult because of low Hg concentrations (Benoit et al., 1998). The interpretation of lake sediment Hg records is complicated by the fact that some Hg is often supplied

n Corresponding author at: Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo, Henan province, PR China. E-mail address: [email protected] (S. Tang).

0013-9351/$ - see front matter & 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.envres.2011.12.009

from the catchments in addition to the amount deposited on the lake surface. Several studies (Martinez-Cortizas et al., 1999; RoosBarraclough and Shotyk, 2003; Farmer et al., 2009) have employed peat cores from ombrotrophic bogs to quantitatively reconstruct chronological variations in atmospheric mercury accumulation rates (Hg AR) in the Northern and Southern Hemispheres. China is regarded as one of the largest anthropogenic mercury emission sources arising from coal combustion. Dastoor and Larocque (2004) estimated that China and Japan contributed 28% to the total anthropogenic mercury emissions in 1990. Pacyna et al. (2001) reported that Chinese emissions from coal combustion contributed 25% to the total global emissions. However, the relative importance of natural versus anthropogenic sources of Hg to China is poorly understood, leading to an urgent need for the analysis of long term records to quantify Hg AR due to both natural and anthropogenic emissions. To address this data gap, we investigate Hg concentrations and historical Hg AR in 14C and 210Pb dated peat cores from an ombrotrophic peat of Xiaoxing’an Mountain, Northeast China. 2. Material and methods The Xiaoxing’an Mountain is the main area of peat distribution in China. The depth of peat ranges from 1 to 3 m. The annual precipitation is approximately 600 mm yr  1. The duration from the first snow and last snow is approximately 6 months and the annual mean temperature is from  1 to þ1 1C. The sampling site (461420 –481400 N, 1291050 –1291550 E) is located in Tanghongling in Hongxing County, Heilongjiang Province, Northeast China and ecosystem types (Fig. 1) are Larix dahurica–Ledum palustre–Sphagnum, Larix dahurica–Vaccinium uliginosum–Sphagnum and the Sphagnum.

146

S. Tang et al. / Environmental Research 118 (2012) 145–148 where Hg AR was the Hg accumulation rate (mg/m2/yr), Hgc was the Hg concentration of the peat samples (mg/kg), BD was the bulk density of the peat samples (g/cm2) and AR was the peat accumulation rate (cm/yr). AR was calculated using the age of the interval section between two dated samples.

3. Results and discussion

Fig. 1. Sampling site in Xiaoxing’an mountain, Northeast China.

Table 1 Age of ombrotrophic peat in Tanghonglin, Heilongjiang Province, Northeast China. Depth (cm)

d13C (%)

14

Revised age (BP)

30 44 56 72 80 91 102

 26.72  26.58  26.48  26.02  26.85  26.88  26.97

14307 60 18907 70 22407 60 26707 60 31907 80 40107 100 44807 120

1310 1852 2222 2770 3430 4486 5103

a

C age (BP7 yr)a

BP is years before present.

Peat samples were collected using a pit dug and a monolith taken in August 2009. The surface vegetation above the peat was also collected and analyzed. This is necessary for accurate 210Pb dating. The peat profile was frozen from 42 cm to the bedrock (105 cm) in nature. The peat core was of the dimensions 8  8  105 cm and was subdivided onsite into 1 cm subsamples, packed into polyethylene bags, sealed and then taken to the laboratory for processing. The wet peat sections were weighed, freeze-dried and reweighed for bulk density. Samples were ground in a ceramic disk mill to sieve to less than 100 mm, then stored at 4 1C. The prepared peat sections were digested with an acid mixture of HCl/HNO3 (3:1) for total Hg analysis using Brooks Model III CVAFS. Quality control for the Hg was addressed with the Chinese national standard material for soil (GBW07405) and blind duplicates. The limit of determination was 0.01 mg kg  1 for total Hg. The average total Hg concentration of the geological standard GBW07405 was 0.30 7 0.01 mg g  1 (n¼ 5) with a range from 0.29 to 0.31 mg g  1, which is comparable with the certified value of 0.29 7 0.04 mg g  1. The relative percent difference was o 8.5% for total Hg obtained from analysis of duplicates of peat samples. Dried milled samples from the uppermost 10 cm were age dated using 210Pb excess activity as described previously (Biester et al., 2002). Plant macrofossils identified in selected samples from the peat core were 14C dated using Accelerator Mass Spectrometry (AMS). The macrofossils were taken from the centers of selected 1 cm slices at State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Science. Within one week of selection they were processed at the AMS 14C Dating Laboratory, University of Beijing, using a standard procedure for plant material (washed, acid–base–acid treatment). Conventional 14C ages were calibrated using CALIB rev4.3 (Table 1). The calculated Hg accumulation rates were based on the average Hg concentrations, the dry mass of peat and the time interval between the two dated samples. Sections for calculating background Hg deposition rates were selected from those parts of the core where the variations in Hg concentrations and density were comparatively low. The Hg accumulation rates (Hg AR) were calculated according to the methodology proposed by Roos-Barraclough and Shotyk (2003): Hg AR ¼ 10Hgc  BD  AR

We observed that the Hg concentrations at the thawed–frozen interface (depth 42–44 cm) were obviously influenced by the ground and surface water, which were higher(2200 mg/kg) 4–5 times than those of vicinal peat samples, therefore, we ignored these irrelevancies when discussing the Hg concentrations and Hg AR of the peat core. The maximum sectional mercury concentration (major Hg peak in Fig. 2a) recorded was 574.4 mg/kg and the onset of the major increase in mercury concentration occurred at a depth of 9 cm, below which the mercury concentrations averaged 111.3764.7 mg/kg from 11 to 104 cm, sub-dividable into VIII sub-mercury peaks, such as 187.0 743.0 mg/kg (11–35 cm), 79.6726.9 mg/kg (36–52 cm), 74.5758.8 mg/kg (53–63 cm), 115.5765.1 mg/kg (64–68 cm), 99.2 749.1 mg/kg (69–73 cm), 99.9757.2 mg/kg (74–83 cm), 95.4 734.8 mg/kg (84–92 cm) and 49.9721.8 mg/kg (93–104 cm). Liu et al. (2002) who studied the THg, MeHg and organic matter in drained and undrained peat in the thawed layer (0–55 cm) in this area, also observed the highest total mercury at a depth of 5 10 m with concentrations of 65.8–186.6 mg/ kg using V2O5–H2SO4–HNO3 digestion and F732-V CAES. However, the study by Liu et al. (2002) does not evaluate atmospheric Hg deposition in the thawed layer (0–55 cm) and frozen layers. The pre-industrial Hg ARs recorded in the peat (10–105 cm, mean 5.7 mg/m2/yr) were similar to those reported for sites in the Northern Hemisphere where values varied between 1.5 and 8.0 mg/m2/yr with a mean of 3.371.1 mg/m2/yr (Mason et al., 1994; Martinez-Cortizas et al., 1999; Farmer et al., 2009). This mean pre-industrial Hg AR (Fig. 2b) seemed to be comparatively high, compared to the Northern Hemisphere. Firstly, the high Hg accumulation in Xiaoxing’an mountain area in pre-industrial time can reflect metal smelting and the use of core in ancient China, which occurred 1700 earlier than in Europe, such as bronze smelting in late Shang Dynasty (3400 BP), iron smelting in Qing Dynasty(2200 BP) and the use of coal as fuel to smelt iron in the Western Han Dynasty around 2200 BP. Martinez-Cortizas et al. (1999) observed an increase in Hg accumulation rates in a Spanish peat bog at about 1500 years BP, which they attributed to the use of Hg in the first metallurgical revolution during the Islamic period. Lacerda et al. (1999) also reported an increase of Hg accumulation in Northern Brazil during the Colonial period due to the use of Hg to extract silver and gold from soils and sediments. Our results showed eight Hg AR peaks in pre-industrial times. Secondly, the high precipitation rates were another important factor controlling Hg deposition at Xiaoxing’an mountain area, and must be important at other sites also (Fitzgerald, 1996; Roos-Barraclough and Shotyk, 2003). The above factors were considered to have been more or less constant throughout the past 5000 years. In addition, the effects of volcanic activity were a main factor controlling Hg deposition at Xiaoxing’an mountain area. According to historical records, Northeast China was the most active volcanic area in the Cenozoic Era, when there were 34 volcano cluster involving 640 volcanoes such as Wudalianchi, a volcanic cluster which recently erupted during 1719– 1721. Mao et al. (2002) reported discovery of volcanic explosion derived materials in Jinchuan peat cores. As a global natural source of atmospheric Hg, similar Hg AR had been estimated for atmospheric Hg emissions from volcanoes (Fitzgerald, 1996). It is difficult to determine the natural Hg AR because of the difficulty of discerning exactly when human activities began to

4480 ± 120

4010 ± 100

3190 ± 80

2670 ± 70

2240 ± 60

2670 ± 70

1430 ± 60

1880AD

120.0 110.0 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0

147

1970AD

Hg AR (µg/m2/y)

S. Tang et al. / Environmental Research 118 (2012) 145–148

0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 Depth (cm)

III II

500.0 400.0

I

Hg (µg/kg)

V IV

VI

VII

600.0

VII

Major

700.0

300.0 200.0 100.0 0.0

0

5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 Depth (cm)

Fig. 2. (a) Hg concentrations in ombrotrophic peat core of Xiaoxingan’ Mountain, Northeast China. It can be sub-divided into a major peak and 8 sub-mercury peaks. (b) Hg accumulation rates in dated ombrotrophic peat of Xiaoxingan’ Mountain, Northeast China. Ages were obtained by 210Pb excess activity and 14C AMS. dating and were expressed as calibrated 14C years (1s-values).

contribute significantly to atmospheric Hg concentrations. Natural Hg accumulation rates of 0.6 70.2 and  1 mg/m2/yr in Swedish peat cores have been reported over the period from 4000 to 500 yr BP (Bindler, 2003), 1.7 71.3 mg/m2/yr for preindustrial times back to 7000 yr BP, 0.3–0.5 mg/m2/yr for A.D. 550–975 in Greenland and 1.2770.38 mg/m2/yr for 1520 B.C.– A.D. 1385 in the Faroe Islands (Shotyk et al., 2005). Farmer et al. (2009) showed 4.5 mg/m2/yr over the past 2000 years in UK. Biester et al. (2007) consider the median natural accumulation rate of mercury to be 1 mg/m2/yr (range 0.6–1.7 mg/m2/yr). It is noteworthy, however, that values close to the 5 mg/m2/yr are regarded as more likely natural Hg ARs, based on lake sediment measurements by Biester et al. (2007). In the context of our results, we took the I sub-Hg peak (93–104 cm) corresponding to 5100–4500 BP to represent the natural Hg AR background in Northeast China, which was 2.2 71.0 mg/m2/yr, and 5.7 mg/m2/ yr for pre-industrial times back to 5000 years age. Another 7 submercury peak might be affected by anthropogenic releases. Highest Hg concentrations occurred in the uppermost 10 cm of the core, which cover the past 130 years of peat accumulation. During this period Hg concentrations increased from 174.4 to 574.4 mg/kg from 1880 A.D. to 1970 A.D. and the mean Hg concentration was 399.17136.7 mg/kg. Accordingly, the Hg AR increased from 33.1 to 112.4 mg/m2/yr and the mean Hg AR was 74.6 713.1 mg/m2/yr. This corresponds to 3.6 times increase of pre-industrial mean Hg concentration and 13.1 times increase of pre-industrial mean Hg AR from levels of 33.1 mg/m2/yr to a maximum rate of 112.4 mg/m2/yr during the past century, respectively. Therefore, a maximum increase of 19.7 times was obtained in 1970s compared to pre-industrial Hg accumulation rates in our studied peat core. After the maximum, the Hg AR decreased from

112.4 mg/m2/yr to 33.9 mg/m2/yr at the surface of the core. This trend of decreasing Hg AR in the upper section may reflect a trend of decreasing atmospheric Hg emissions from anthropogenic sources in China in the recent decades attributable to Hg emission prevention. Similar Hg AR trends were obtained in Chile by Biester et al. (2002) and in UK by Farmer et al. (2009). In southeast Brazil, the highest Hg AR in industrial times occurred in the 1960s and 1970s. There, the maximum accumulation rates of 80–130 mg/m2/yr were followed by a strong decrease to 20–30 mg/m2/yr in the 1990s (Lacerda et al., 1999). Unfortunately, the global atmosphere Hg inventory had tripled since pre-industrial times because of sustainable man-made emissions (Lamborg et al., 2002). The increased extent of atmospheric Hg AR due to anthropogenic emissions to ombrotrophic bogs at various sites varied from none to 10 times since pre-industrial times depending on the distance of the bogs to emission sources and meteorological conditions. Mercury emissions from sources in the Northern Hemisphere account for more than 90% of total global emitted Hg from anthropogenic sources (Pirrone et al., 1996), and about 50% of the anthropogenic emissions appear to enter the global atmospheric Hg cycle (Mason et al., 1994). China has been regarded as one of the largest anthropogenic mercury emission sources due to substantial coal combustion in this county. Mason et al. (1994) estimated an average value of 16 mg/m2/yr for the Northern Hemisphere using data from several locations within North America, Asia and Europe. Pirrone et al. (1996) reported modern Hg fluxes of 2–35 mg/m2/yr for different regions in the Nordic countries. Moreover, they (Mason et al., 1994; Pirrone et al., 1996) calculated that atmospheric Hg emissions during the 19th and 20th centuries have increased by a factor of 4.5 due to

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S. Tang et al. / Environmental Research 118 (2012) 145–148

anthropogenic activity. In view of a maximum increase of 19.7 times Hg AR recorded in the peat core in 1970s, we believe that the increase in Hg deposition in the investigated peat core must be mainly attributed to local anthropogenic emissions.

4. Conclusions The mean pre-industrial Hg concentration and Hg AR recorded in the peat were 111.3764.7 mg/kg and 5.7 mg/m2/yr in Northeast China. We concluded that natural Hg AR background in Northeast China was 2.2 71.0 mg/m2/yr for 5100–4500 BP. The highest Hg AR occurred around the 1970s, which corresponds to 19.7 times of the mean pre-industrial Hg AR. The mean Hg concentration, Hg AR and maximum of Hg AR were 399.17136.7 mg/kg, 74.6713.1 mg/ m2/yr and 112.4 mg/m2/yr in industrial times, respectively. This corresponds to 3.6 times increase of pre-industrial mean Hg concentration, 13.1 times increase of pre-industrial mean Hg AR and 19.7 times increase of pre-industrial mean Hg AR. We suggest that the increase in Hg deposition in the Xiaoxing’an mountain area must be mainly attributed to local anthropogenic emissions.

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