Effect of recent climate change on Arctic Pb pollution: A comparative study of historical records in lake and peat sediments

Environmental Pollution 160 (2012) 161e168

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Effect of recent climate change on Arctic Pb pollution: A comparative study of historical records in lake and peat sediments Xiaodong Liua, *, Shan Jianga, Pengfei Zhangb, Liqiang Xua a b

Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, PR China Department of Earth and Atmospheric Sciences, City College of New York, New York, NY 10031, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 June 2011 Received in revised form 13 September 2011 Accepted 14 September 2011

Historical changes of anthropogenic Pb pollution were reconstructed based on Pb concentrations and isotope ratios in lake and peat sediment profiles from Ny-Ålesund of Arctic. The calculated excess Pb isotope ratios showed that Pb pollution largely came from west Europe and Russia. The peat profile clearly reflected the historical changes of atmospheric deposition of anthropogenic Pb into Ny-Ålesund, and the result showed that anthropogenic Pb peaked at 1960se1970s, and thereafter a significant recovery was observed by a rapid increase of 206Pb/207Pb ratios and a remarkable decrease in anthropogenic Pb contents. In contrast to the peat record, the longer lake record showed relatively high anthropogenic Pb contents and a persistent decrease of 206Pb/207Pb ratios within the uppermost samples, suggesting that climate-sensitive processes such as catchment erosion and meltwater runoff might have influenced the recent change of Pb pollution record in the High Arctic lake sediments. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Ny-Ålesund Sediment Pb pollution Pb isotope Catchment input Climate warming

1. Introduction The Arctic is a seemingly pristine, remote region, yet increasing data reveal that the Arctic environment has been greatly impacted by anthropogenic metals (Candelone et al., 1995; Bindler et al., 2001a; Givelet et al., 2003; Shotyk et al., 2005; Zheng et al., 2007; Michelutti et al., 2009). Pb is a non-mobile and toxic metal whose biogeochemical cycle has been affected by human activities to a great degree (Komárek et al., 2008). The sources of Pb pollution are mainly derived from industrial discharges through atmospheric transport. The isotopic composition of anthropogenic Pb is different from natural background Pb, and hence measurements of Pb isotopic ratios can indicate Pb sources (Sturges and Barrie, 1989). Estimation of Pb emissions derived from human activities indicates that the atmosphere is the major initial recipient, and anthropogenic sources are at least 1e2 orders of magnitude greater than natural sources (Komárek et al., 2008). Signs of atmospheric Pb pollution could be dated back to three or four millennia ago (Hong et al., 1994; Vare, 2007; Zheng et al., 2007), and its significant fallout occurred in the 20th century, particularly after the World War II (Renberg et al., 2000; Cooke et al., 2007). However, recent reduction in Pb deposition since the 1970s is remarkable due to the elimination of Pb emission and the * Corresponding author. E-mail address: [email protected] (X. Liu). 0269-7491/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2011.09.019

fall in local industrial production, which appeared to be controlled by relevant legislations (Weiss et al., 2002). Indeed, analyses of Pb pollution showed obviously decreasing levels in recent decades (Dunlap et al., 1999; Arnaud et al., 2004; Komárek et al., 2008). Nevertheless, studies on the temporal trend of Pb pollution in lacustrine sediments showed somewhat complex results. Generally, the sediment flux in lakes reflects not only the direct deposition of atmospheric Pb on the lake surface, but also Pb transported to the lake from the catchment. Recently, some climate-sensitive processes such as catchment soil erosion were considered to likely impact sedimentary accumulation rates of Pb in sediments. For example, Klaminder et al. (2010) and Yang et al. (2007) found that the modern decrease of Pb pollution did not reach the expected level in Subarctic and Lochnagar lake sediments, and suggested that soil erosion could contribute a substantial portion of the Pb load to lake sediments. A wide range of paleoclimatic evidences and modern meteorological data suggested that the Arctic experienced a pronounced warming that started around 1840 (Thomas and Briner, 2009). The average temperature over the Arctic has risen at twice the rate of the rest of the world over the past few decades because of various positive feedback mechanisms (ACIA, 2005), and climate models predict that global warming will continue to be magnified at high latitudes (Holland et al., 2006). With the pronounced warming, catchment weathering and erosion could be enhanced significantly, thereby stimulating the transport of post-depositional Pb from

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X. Liu et al. / Environmental Pollution 160 (2012) 161e168

catchment soils to surface waters and sediments (Michelutti et al., 2009; Klaminder et al., 2010). In order to verify this mechanism in the Arctic, in this study we reconstructed the temporal trends of Pb contamination from lake and peat sediment profiles in Ny-Ålesund of Svalbard. Sedimentary Pb record from the lake with a relatively large catchment area would subject to the input of eroded soils from the catchment, whereas Pb record from the peat sediment profile would solely reflect the changes in atmospheric Pb (Shotyk, 1996). 2. Materials and methods 2.1. Study area Svalbard Archipelago (74
e81
N, 10
e35
E), is located between Barents Sea and Greenland Sea. The total land area is about 61,200 km2, and 60% of the land is permanently covered by ice and glaciers. The permanent frost soil is w500 m thick, only 2e3 m of which will thaw in summer. Because the North Atlantic Ocean warm current flows through the archipelago, the west part of the island has the polarmaritime climate characteristics. Ny-Ålesund (78
550 N, 11
560 E) is situated on the Brøggerhalvoya peninsula, the west coast of Spitsbergen archipelago (Fig. 1), and is an area sensitive to climate change. Geologically, the rocks consist mainly of Carboniferous-Permian limestone and dolomite that form very stable bedrock foundation, and a small amount of Neogene sandstones and coaly shales crop out in this area. The Holocene climate of Ny-Ålesund is strongly affected by the changes of atmospheric circulation and oceanic cycle in North Atlantic and Barents Sea (Yuan et al., 2009). This island is rich in coal and apatite deposits. Compared to other areas of the same latitude in the Antarctic regions, the study area has flourish vegetation cover. The species of plants are relatively simple and dominated by polar tundra and desert flora, and they mainly grow in the inland bay. Further details of the geology, climate and vegetation of Svalbard are given by Birks et al. (2004). Field photos of the two sampling locations are presented in Fig. 1. 2.2. Sample collection During the 3rd Chinese Arctic Expedition in 2006, two sediment cores were collected. The 57-cm long sediment core named H2 was collected from the

Knudsenheia basin (Fig. 1), a near-coastal and ice free area in front of Vestre Brøggerbreen, about 3 km northwest of the Yellow River Station of China. On-going monitoring of active layer thickness shows maximum values of 75e80 cm (Schwamborn et al., 2008). As seen in the field view (Fig. 1), the sampled H2 Lake (78
560 2900 N, 11
490 1600 E) has a relatively large catchment area in Ny-Ålesund, and the landform of lake catchment is characteristic of low-lying and flat. A large member of scattered glacial tills can be found around the catchment. At present, the catchment surface is covered with vegetation and the drainage is mainly affected by snowmelt. Frost weathering processes are very active on bedrock slopes in the tributary areas. Coring was carried out from a floating platform by vertically pushing a PVC plastic pipe with 12 cm diameter into the soft substrate, and then sectioned at 1-cm intervals in the field. According to rough lithological observation, the top 20 cm is black sapropel, and the sediments below are brown-yellow mud with very few gravels. Another 25-cm long peat or peat-like sediment profile named S2 was collected from a small depression (78
570 34.600 N, 11
380 16.700 E), which is about 7.5 km to the west of the Yellow River Station (Fig. 1). The sampling location is covered with flourish moss. Glaciations left numerous gravels around the depression. As seen in the field view, the sampling depression is not covered by water. During sampling in the field, the growing moss layer was first removed and then a bamboo shovel was used to take sediment samples at an interval of 1 cm in situ. The samples were rich in yellow bryophytes residues. The field sampling reached a hard frozen layer in the bottom. All the samples were frozen in cold storage prior to analysis.

2.3. Trace metal and Pb isotope analyses Subsamples at intervals of 1e2 cm were homogenized with a mortar and pestle after air drying in a clean laboratory, and then sieved through a 200 mesh sieve. After that, the dried samples were precisely weighed, and then digested by multiacids in a Pt crucible with electric heating. Al, Ti, Na, K and Pb were determined by inductively coupled plasma - optical emission spectrometry (ICP-OES). For determination of CaO in silicate phase only, carbonate bearing samples were treated with 1 N cold dilute HCl acid before digestion. Precision and accuracy of our results were monitored by analyzing sediment standard reference materials (SRM) in every batch of analysis, and the results from the analyses were in good agreement with the reference values. After all the samples were done, about 20% of analytical samples were randomly selected for the reproducibility test, and the relative standard deviation (RSD) was <1% for Al, Ti, Na, K and <5% for Pb, respectively. Pb isotope analysis was performed within an ultra-clean room at Yichang Institute of Geology and Mineral Resources following the procedure of Liu et al.

Fig. 1. Location of the Arctic Ny-Ålesund showing the H2 and S2 sampling sites.

X. Liu et al. / Environmental Pollution 160 (2012) 161e168 (2008). Pb was separated from other elements using sequential HCleHBr column chromatography with AG 1  8 anion exchange resin. Isotopic ratios, measured as 208 Pb/206Pb, 207Pb/206Pb and 206Pb/204Pb, were determined by a Finnigan Mat 261 mass spectrometer running in static multi-collection mode. A standard reference material (NBS981 Common Lead Isotope) was measured as the quality control. The long-term external reproducibility based on repeated measurements of this standard is better than 0.05%. 2.4. Age dating H2 sediment chronology was established using both 210Pb and AMS14C dating techniques. For 210Pb analysis, the top 10 consecutive sediment samples (0e10 cm core depth) were processed. Four bulk sediment samples were selected for AMS14C dating at the Carbon Cycle Accelerator Mass Spectrometry Laboratory, University of California Irvine (KCCAMS UCI). The radiocarbon dates were then calibrated into calendar years before present (cal. a BP). Detailed method of H2 sedimentary chronology was reported by Jiang et al. (2011a; b). S2 sediment chronology was established using both 210Pb and 137 Cs dating techniques. The top 20 consecutive sediment samples (0e20 cm core depth) were processed to measure the activity of the excess 210Pb. The analytical instrument is a low background and high purity Germanium (HPGe) Gamma Ray Spectrograph (GWL-DSPEC-PLUS) produced by AMETEK Company, USA. 2.5. Data analysis We adopted the following two formulas to calculate Pb content caused by human activities and the percentage of anthropogenic Pb in total Pb (Shotyk et al., 2005). ½Pblithogenic ¼ ½Tisample ð½Pb=½TiÞbackground

(1)

½Pbanthropogenic ¼ ½Pbtotal ½Pblithogenic

(2)

where the “background” Pb/Ti ratio was determined using the averaged concentration ratio in the bottom sediment samples. Taking into account the relatively low Pb concentrations and the high Pb isotope ratios, the sediment samples below 47 cm and 19 cm were selected to calculate the average Pb/Ti ratio as baseline for the H2 and S2 profiles, respectively. Although all four Pb isotopes were analyzed (204Pb, 206Pb, 207Pb, and 208Pb), the present discussion and figures focus on the 206Pb/207Pb ratios, which is the most commonly used ratio in Pb pollution studies. Changes in the different isotope ratios are strongly correlated, for example, the correlations between the 206Pb/207Pb ratio and the 206Pb/204Pb, 206Pb/208Pb ratios range from 0.91 to 0.99 in the sediment profiles of S2 and H2. By calculating the excess Pb isotope ratio, using a simple mixing model, it is possible to make some inferences as to the source of Pb pollution at a given site (Farmer et al., 1996). The calculation formula is as follows: 206

Pb=207 Pbexcess ¼

"

! Pbsample  206 Pb=207 Pbsample  Pbref  206 Pb=207 Pb

!#, ref

  Pbsample Pbref (3)

206

207

Pb/ Pbsample refers to the isotope ratio and Pbsample refers to the total Pb where concentration of a given sample, and 206Pb/207Pbref and Pbref refer to the mean isotope ratio and concentration, respectively, in the bottom sediments.

163

3. Results and discussion 3.1. Chronology The complete H2 chronology result was reported by Jiang et al. (2011a). Based on the 210Pb and AMS14C dating results, a third order polynomial curve was obtained by fitting the age and depth data (y ¼ 0.0293x3 þ 2.4829x2 þ 4.3011x, where x is the depth and y is the age), and this equation was used to extrapolate sedimentary age at different depth. For the S2 chronology, 210 Pbe137Cs activities in the sediments were determined (Fig. 2a). The results showed that significant levels of unsupported 210Pb were detected in the top 15 cm and the total 210Pb activities were in equilibrium with the 226 Ra activity below 16 cm. In general, the excess 210Pb activity of S2 sediment profile showed a simple exponential relation with depth (Fig. 2b). Both CIC (constant initial concentration) and CRS (constant rate of sedimentation) models (Appleby, 2004, 2008) were used to determine the year of deposition from the measured 210 Pb activity. There is a close agreement between the CRS and CIC model depth-age curves for the S2 profile (Fig. 2c), indicating steady-state sedimentation and a lack of significant mixing. The 137 Cs peaks in 1e2 cm and 3e4 cm depths, which are widely accepted as the result of fallout from human nuclear testing in 1964 and the Chernobyl nuclear accident in 1986, respectively, are generally in consistent with 210Pb dates (Fig. 2c). The average sediment accumulation rate derived from the CRS model was 0.133  0.04 g cm2 a1. This value has no statistically difference from that estimated using the CIC model (0.114 g cm2 a1), at least over the time-scales of the datable portion of the S2 sediment profile, and this can explain why the chronologies obtained using both models were similar (O’Reilly et al., 2011). The extrapolated dates of CRS model for the profile depths beyond 210Pb dating were calculated using the best estimate of the mean sediment accumulation rate appropriate to the profile (Appleby, 2004). 3.2. A 3000-year historical record of Pb pollution in lake sediment core In the H2 sediments, the range of total Pb concentrations (11.44e16.89 mg g1, Fig. 3a) and its isotopic ratio (1.201e1.221) were consistent with those in some lake sediments in Canadian Arctic (Outridge et al., 2002). The relatively stable values of total Pb concentration in the bottom samples suggested that the anthropogenic Pb was likely negligible, or the Pb was absorbed onto the

Fig. 2. Chronology establishment of S2 sediment profile.

X. Liu et al. / Environmental Pollution 160 (2012) 161e168

a

0.3

Ti (%)

164

0.25

b Ti (%)

0.2

18

0

14 10 6

-1

-1

10 10 8 6 4 2 0

2

10 5 0

-1000 1000

BC

-500 500

0

500

1000

Calendar years

1500

2000

1.200 1700

1750

1800

AD

1850

1900

1950

2000

Calendar years (AD)

Fig. 3. Temporal changes of Ti, total Pb, anthropogenic Pb concentrations and

moss and peat around the catchment before it flowed into the lake. The calculated result showed that the lithogenic Pb content varied from 8.25 to 10.23 mg g1 with a mean value of 9.24 mg g1 and a coefficient of variation of about 6% (data not shown), indicating that the portion of lithogenic Pb only had slight change throughout the H2 core. This might be due to the consistent lithological characteristics in the lake sediments. The range of calculated anthropogenic Pb content was 1.72e8.12 mg g1 (Fig. 3a). The long sediment record (ca. 3000-year old) from H2 revealed that anthropogenic Pb concentrations increased and 206Pb/207Pb ratios decreased toward the surface sediments (Fig. 3a). The down-core profile of anthropogenic Pb contents almost followed that of sedimentary concentration, suggesting that the H2 profile was obviously influenced by increased anthropogenic contamination. The records of anthropogenic Pb content and isotope ratio from H2 could be generally divided into three phases (Fig. 3a). In the first phase, there was a stable 206Pb/207Pb ratio around 1.220 before 1400 AD, and the anthropogenic Pb concentrations maintained at the lowest levels from w1000 BC to w1400 AD, indicating that the Pb was mainly derived from the natural catchment rock and soil during this period. In the second phase, from ca. 1400 to 1900 AD, there was an obviously gradual decline in the 206Pb/207Pb ratio, consistent with the gradual increase trend of anthropogenic Pb contents since 1400 AD. Thereafter, in the third phase, the 206 Pb/207Pb ratio steadily declined to 1.206, accompanied by a sharp increase in anthropogenic Pb toward the sediment surface. The Industrial Revolution (1800 AD) caused increased atmospheric Pb pollution, but it was by no means the start of pollution (Cooke et al., 2007; Meriläinen et al., 2011). The Pb isotope records from Switzerland (Shotyk et al., 1998), Scandinavia (Brännvall et al., 1999) and Greenland ice (Rosman et al., 1997) indicated that longrange atmospheric transport of pollutants from ancient cultural centers to remote regions has occurred for at least 3000 years (Borsos et al., 2003). The Medieval period (around 1000 AD), a significant event in the large-scale atmospheric Pb pollution history, was considered to be the real beginning of the contemporary pollution era. This period in Europe was characterized by both economic growth and increasing population. Mining and metal production increased significantly, leading to the remarkable increase in atmospheric Pb pollution (Renberg et al., 2000; Brännvall et al., 2001). Generally, with the onset of the Medieval

Pb

1.200

207

Pb

1.250

1.300

Pb/

1.210 1.205

207

Pb/

1.350

1.215

206

1.400

206

1.225 1.220

15

-1

Anthropogenic Pb (µg·g )

-1

14

0.1

Pb (µg·g )

18

Pb (µg·g )

0.2

Anthropogenic Pb (µg·g )

0.3

206

Pb/207Pb ratios in the H2 and S2 profiles.

metal industry (1000e1200 AD), the sedimentary records of Pb pollution were characterized by increasing Pb concentrations and a nearly continuous decrease in 206Pb/207Pb ratio (Klaminder et al., 2010). As illustrated in Fig. 3a, the temporal trend of 206Pb/207Pb ratios in the H2 sediments showed increased atmospheric deposition of pollution-derived Pb at about 1400 AD, together with a slight increase of anthropogenic Pb contents, indicating that the anthropogenic Pb inputs to Arctic lacustrine sediments might occur at least 600 years ago, although the pollution degree was very weak during that time. This was likely caused by long-range Pb pollution related to lead and sliver mining and evolution of human metallurgy. After the 15th century, technological improvements for smelting PbeAg ores, as well as the widespread usage of lead in fire-arms, lead to the increase of metallurgical activities in Europe and Pb pollution along with it (Molenda, 1976; Brännvall et al., 2001), which is likely recorded in the Artic lake sediments at Ny-Ålesund of Svalbard. From 1800 AD, an increasing trend of anthropogenic Pb contents was present in the sediments, especially after 1900 AD. The anthropogenic Pb concentrations were quite high compared to the background, indicating that a major peak of anthropogenic Pb fallout occurred in the 20th century. Similar and coherent change patterns were exhibited in other circum-arctic sediments (Farmer et al., 1996; Moor et al., 1996; Brännvall et al., 1997). The 206Pb/207Pb ratio after the 1970s in the H2 sediments did not return to the pre-1900 level, and instead displayed a persistent decreasing trend, implying a steady input of Pb from an anthropogenic origin. 3.3. A 200-year record of Pb pollution in peat profile In the S2 sediments, the contribution of anthropogenic Pb was generally stable from 1750 AD to 1900 AD (Fig. 3b). After that, two peaks of anthropogenic Pb contents occurred at about 1920s and between the 1950s and 1970s, and then the concentrations began to steadily decrease toward the sediment surface. The temporal trend of Pb isotope ratios was opposite to that of the anthropogenic Pb contents. As shown in Fig. 3b, the deeper sediment samples had much higher ratios of 206Pb/207Pb than the near-surface samples that had higher anthropogenic Pb contents. Both the lower 206 Pb/207Pb ratios and higher anthropogenic Pb concentrations occurred around the 1910se1920s and the 1960se1970s,

X. Liu et al. / Environmental Pollution 160 (2012) 161e168

3.4. Excess Pb determination and source analysis Compared with H2 samples, S2 sediments had a larger range of Pb/207Pb ratio (from 1.240 to 1.383). The different ranges recorded in 206Pb/207Pb ratios highlighted the importance of local geology in determining Pb isotopic ratios in lake and peat sediments. Here, excess ratios were calculated only for the uppermost 4 cm and 6 cm sediment intervals in the H2 and S2 profiles, respectively, where the Pb concentrations were much greater than the average concentration in deeper samples. Fig. 4 shows the calculated excess 206Pb/207Pb ratios in both profiles. The mean excess 206Pb/207Pb ratios in H2 and S2 were 1.147 and 1.154, respectively, comparable to those measured in aerosols at Ny-Ålesund (1.154  0.006) when air mass trajectories were traced from central and northern Russia (Sturges and Barrie, 1989). Published values of mean pollution 206Pb/207Pb from four potential source regions including Canada (1.15), USA (1.21), Eurasia (1.14), and the UK (1.06) were examined (Sangster et al., 2000). The calculated excess 206Pb/207Pb ratios for both profiles fall within an area bounded by the Pb isotope fields characterized by W. European (w1.13e1.14) and E. European sources (w1.16e1.17), as well as Russia (1.14e1.17), but they were well below the isotope field for US 206

Pollution Pb source regions

Surface sediments excess ratios

1.25

US

Pb/207Pb

1.2

206

Russia W Eur

1.15 1.1 1.05

S2

H2

E Europr

Russia

Canada

USA

Broken Hill

W Eur modern

1

Eur pre-1900

suggesting that during these two periods the S2 sediments were contaminated by human-derived Pb deposition. The increase of Pb isotope ratios and the decline of anthropogenic Pb contents after the 1970s indicated that Pb pollution had significantly reduced. By now the anthropogenic Pb concentrations and 206Pb/207Pb ratios in the uppermost S2 sediments have almost returned to the pre-1900 values. This finding was consistent with the results of Renberg et al. (2000) and Weiss et al. (2002). The increase in the anthropogenic Pb pollution recorded in the sediments at the beginning of 20th century can be found in most European Pb contamination records (Renberg et al., 2001). In the Arctic Ny-Ålesund, it is clearly shown in Fig. 3b that the anthropogenic input of Pb had a peak around the 1920s. This event was also present in Swiss sediments and peat bogs (Kober et al., 1999; Weiss et al., 1999), and was interpreted as the result of the usage of Pb-containing coal as the main source of energy (206Pb/207Pb values about 1.17e1.19) or Pb ore smelting during the initial period of industrialization. Because of the worldwide economic crisis in 1929, this enhanced contamination period ended (Arnaud et al., 2004). This event was not evident in the H2 record, probably due to the fact that the resolution of the H2 profile was not high enough to catch the short time changes in anthropogenic Pb input during that period. The second period of Pb contamination between the 1960s and 1970s was significant, and its onset was parallel to the extensive use of leaded petrol in most of the 20th century. Since the introduction of alkyl-Pb additives to gasoline in 1923, especially following the World War II, atmospheric Pb emission increased sharply. Leaded petrol in Europe is known to be depleted in radiogenic 206Pb (206Pb/207Pb values about 1.04e1.10) (Bindler et al., 2001b). Thus there is no doubt that the significant decrease to the minimum ratios of 206Pb/207Pb during the 1960e1970s was a consequence of the very large rise in the use of leaded gasoline during that period. Since the 1970s, following the worldwide reduction of Pb additives in gasoline and the introduction of unleaded fuels in many countries, as well as improved environmental legislations, atmospheric Pb emissions decreased rapidly, e.g., by >90% between the 1970s and the late 1990s in some regions (Rühling and Tyler, 2001). The deposition rate at the end of the 1990s almost returned back to the preindustrial level (Brännvall et al., 1999). This recent recovery from Pb pollution can be clearly seen in many lake sediments and peat deposits (Renberg et al., 2001).

165

Fig. 4. 206Pb/207Pb ratios of source regions for Pb pollution and the calculated excess ratios for the uppermost samples of H2 and S2 sediment profiles. All the referenced Pb isotope ratios are cited from Bindler et al. (2001a,b).

pollution (Fig. 4). Sturges and Barrie (1989) suggested that the contributions to Pb pollution at Arctic Ny-Ålesund from the U.S. and western Canadian sources can probably be ruled out, as they have significantly higher 206Pb/207Pb ratios. Our findings agreed well with the results of Bindler et al. (2001b), who suggested strongly that West Europe was an important emission source for the Pb pollution to southern Greenland. Furthermore, according to the original study on polluted air masses at Ny-Ålesund (Maenhaut et al., 1989; Bottenheim et al., 2004), very few mass of the tracer elements came from eastern North America, whereas the former U.S.S.R. and Europe contributed to 60% and 40% during winter and 25% and 75% during summer, respectively. Additionally, the Svalbard archipelago is enriched in coal resources, and coal mining is an important economic activity in Svalbard. Thus, the history of local coal burning may also cause Pb pollution in Ny-Ålesund. However, according to composition analysis on Ny-Ålesund coal, Pb concentration was very low, i.e., only about 5 mg g1 (Headley, 1996), and the 206Pb/207Pb isotope ratio was 1.185e1.188 (Vare, 2007), significantly higher than the excess Pb isotope ratios in the H2 and S2 sediments, so it seems unlikely that the local coal mining is the main contributor to Pb pollution in the area of Ny-Ålesund. This supports the general consensus that the vast majority of pollutants transported into Svalbard were derived from long-range atmosphere transport (Rose et al., 2004; Vare, 2007). 3.5. Comparison of Pb pollution records between H2 and S2 profiles The concept of linear arrays depicting binary mixing on a Pb-Pb co-isotopic plot (206Pb/207Pb vs. 206Pb/204Pb ratios) was used to make inferences regarding the most likely source of anthropogenic Pb impacting this region (Michelutti et al., 2009). Ny-Ålesund is far removed from any industrialized activities, and the excess Pb isotope ratios showed that the source of Pb pollution in this region should reflect regional scale patterns of atmospheric deposition. The sampling sites of S2 and H2 are located in the same region with a close distance (about 4 km, Fig. 1), thus they should have received pollution from similar anthropogenic sources. As illustrated in Fig. 5, Pb isotope data in the S2 peat sediments showed the significant recovery from Pb pollution after 1970s, which has been well identified globally in Sweden (Renberg et al., 2000), Switzerland (Moor et al., 1996), Scotland (Cloy et al., 2008), Spain (Martinez-Cortizas et al., 1997, 2002), and even Arctic Greenland (Bindler et al., 2001b) and Antarctica (Planchon et al., 2003), due to the reduction of global Pb emissions to the

166

X. Liu et al. / Environmental Pollution 160 (2012) 161e168 19.10 19.05

1778 1854 1729 1804 1997 1828 1939 1987 1879

22.00

1021 1481

H2

S2 21.50

Pb

545BC

18.95

Pb/

1594

18.85 1787

1926

18.80 18.75

2000

1987 1971

21.00 1915

204

800BC

1950

18.90

Pb/ 206

1697

70

206

204

Pb

19.00

20.50

1968 1946

20.00

1864 19.50

1981

1957 1963

18.70 18.65 1.200

1.205

1.210 206

Pb/

Fig. 5.

208

Pb/207Pb vs.

206

1.215

1.220

1.225

207

Anthropogenic Pb (µg g -1)

1.30

Pb/

1.35

1.40

207

Pb

Pb/204Pb plots of the sediment records from H2 and S2 (arrow indicates the Pb isotope trend after 1970s).

9 3cm sample

7

1.25

206

Pb

atmosphere in recent years (Renberg et al., 2001). In the Arctic, as reported by Boutron et al. (1991), Pb concentrations in Greenland snow have decreased by a factor of 7.5 over the past twenty years as a result of the policy initiatives. Also, Nriagu (1996) suggested a sharp decrease (approximately seven fold) in the Pb content of Arctic snow fields took place since 1970s, which could be attributed to the phase-out of leaded gasoline in North America and Europe. Thus, the Pb pollution record of S2 profile reflected actually the historical change of atmospheric deposition from anthropogenic Pb source in the Ny-Ålesund. In contrast, the recent recovery of Pb pollution has not yet been recorded by the H2 lake sediments significantly. Within the uppermost two samples the relatively high anthropogenic Pb contents and the persistent decrease of Pb isotope ratio were evident (Fig. 5). This phenomenon is not surprising. Generally, the lake with relatively large catchment area would received plentiful materials from eroded soils around the catchment (Yang et al., 2007), and therefore, the anthropogenic Pb in the H2 upper sediments may change with disturbances in the catchment. Although the anthropogenic Pb deposition into the lake has been reduced dramatically in the past decades, Pb eroded into the lake from catchment may not be reduced in the same level as in the atmospheric deposition, therefore, the relative fractions of Pb contributions to the lake from the atmospheric pollution origin and the natural catchment erosion might have been changed. The H2

8

19.00 1.20

1cm sample

2cm sample

6

r = 0.89 (n=5, p<0.05)

5 4 3

lake catchment soils contain a legacy of atmospheric Pb pollutants deposited over the past thousands of years. With global warming, lakes could have received a larger contribution of previously deposited atmospheric Pb contaminants from the catchment export processes, obscuring the expected increased in Pb isotopic ratio during the last decades (Brännvall et al., 2001; Yang et al., 2007; Klaminder et al., 2010). The temperature increase experienced in the Arctic during the 20th century may have impacted the deposition of Pb via a number of synergistic processes. The continuous temperature record in the vicinity of Longyearbyen, which was started in 1911, revealed an ongoing warming from 1960s (Førland et al., 1997). In accordance with the durative warming since 1960, precipitation also has increased by about 2.5% per decade (Humlum, 2002). Future temperature and precipitation is expected to increase in Svalbard (Førland and Hanssen-Bauer, 2003). The rapid warming, increased humidity and loss of permafrost could have enhanced the chemical weathering rate of the surrounding bedrock in the lake basin, resulting in the increased release of nutrients and dissolved solids to lakes (Jiang et al., 2011a). The CIA value can provide a quantitative measure of chemical weathering intensity by determining the loss of labile elements such as Na, Ca, and K relative to stable ones like Al, and now CIA has been widely used in surface geochemical research (Sun et al., 2005). As seen from Fig. 6, the anthropogenic Pb concentration showed a negative correlation with CIA below 6 cm depth (about 1900 AD). However, the correlation turned into strongly positive in the upper 6 cm, suggesting that the anthropogenic Pb in the surface sediments of H2 could increase with the enhanced surface erosion in the catchment due to climatic warming in Ny-Ålesund. The released weathering products including anthropogenic Pb in the watershed can be delivered to the lake via precipitation and meltwater runoff, and this process might cause the anthropogenic Pb in the uppermost lacustrine sediments still kept relatively high level. However, the results indicated here are only based on a single core, and further research is apparently necessary to examine more sediment cores over a larger area in the Arctic.

2 r =-0.78 (n=32, p<0.01)

1

4. Conclusions

Sediments below 6cm

0 66

68

70

72

74

CIA Fig. 6. Correlations between anthropogenic Pb content and CIA in the H2 sediments. CIA is defined as 100  Al2O3/(Al2O3 þ K2O þ Na2O þ CaO*) (molar ratios, with CaO* being the CaO content in the silicate fraction of the sample). The samples at 3 cm is not included in the linear regression analysis for the upper 6 cm sediment samples, since the significantly high Pb concentration is largely derived from the direct atmospheric deposition of anthropogenic origin. The surface 2 cm sediment samples are apparently related to the CIA.

Arctic lead contamination has been attributed to increasing loading through long-range atmospheric transport, as well as local industrial activities. Here, we reconstructed the historical record and the climatic impact on Pb pollution in Arctic lake and peat sediment profiles at Ny-Ǻlesund. The major conclusions are: (1) the contribution of anthropogenic Pb in Ny-Ålesund was largely originated from W. European and Russia, and the local coal mining is not the main contributor to the Pb pollution. (2) In the area of Ny-Ålesund, the anthropogenic Pb pollution might have occurred

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at about 1400 AD, and displayed a large increase after the Industrial Revolution; the atmospheric fallout of Pb from anthropogenic source reached the peak between 1960 and 1970s, reflecting the very large rise in the use of leaded gasoline during that period. (3) Due to the phasing out of leaded gasoline, the input of anthropogenic Pb recorded in the S2 peat profile has declined sharply after the 1970s, and now the level is almost comparable with the pre-1900 background. (4) In contrast, the recovery signal of Pb pollution in last decades is not present in the surface lacustrine sediments of H2 core, and this may be due to the increased occurrence of the climate-sensitive processes (e.g. surface erosion; precipitation and meltwater runoff, etc.) caused by the recently rapid warming in the Arctic. Acknowledgments We would like to thank the Chinese Antarctic and Arctic Administration of National Oceanic Bureau for the logistic support for field sampling. This study was supported by the National Natural Science Foundation (Grant Nos. 40876096 and 41076123), the Fundamental Research Funds for the Central Universities (Nos. WK2080000004 and WK2060190007), an open research fund from State Key Lab of Environmental Chemistry and Ecotoxicology (KF2010-08). We thank two anonymous reviewers for their constructive comments on this manuscript. References ACIA, 2005. Impacts of a Warming Arctic: Arctic Climate Impact Assessment. Cambridge University Press, UK. 144. Appleby, P.G., 2004. Environmental change and atmospheric contamination on Svalbard: sediment chronology. Journal of Paleolimnology 31, 433e443. Appleby, P.G., 2008. Three decades of dating recent sediments by fallout radionuclides: a review. The Holocene 18, 83e93. Arnaud, F., Revel-Rolland, M., Bosch, D., Winiarski, T., Desmet, M., Tribovillard, N., Givelet, N., 2004. A 300 year history of lead contamination in northern French Alps reconstructed from distant lake sediment records. Journal of Environmental Monitoring 6, 448e456. Bindler, R., Anderson, N.J., Renberg, I., Malmquist, C., 2001a. Paleolimnological investigation of atmospheric pollution in the Søndre Strømfjord region, southern West Greenland: accumulation rates and spatial patterns. Geology of Greenland Survey Bulletin 189, 48e53. Bindler, R., Renberg, I., Anderson, N.J., Appleby, P.G., Emteryd, O., Boyle, J., 2001b. Pb isotope ratios of lake sediments in West Greenland: inferences on pollution sources. Atmospheric Environment 35, 4675e4685. Birks, H.J.B., Jones, V.J., Rose, N.L., 2004. Recent environmental change and atmospheric contamination on Svalbard as recorded in lake sediments e synthesis and general conclusions. Journal of Paleolimnology 31, 531e546. Borsos, E., Makra, L., Béczi, R., Vitányi, B., Szentpéteri, M., 2003. Anthropogenic air pollution in the ancient times. Acta Climatological Chorologica 36e37, 5e15. Bottenheim, J.W., Dastoor, A., Gong, S.L., Higuchi, K., Li, Y.F., 2004. Long range transport of air pollution to the Arctic. Air Pollution 4G, 13e39. Boutron, C.F., Görlach, U., Candelone, J.P., Bolshov, M.A., Delmas, R.J., 1991. Decrease in anthropogenic lead, cadmium and zinc in Greenland snows since the late 1960s. Nature 353, 153e156. Brännvall, M.L., Bindler, R., Emteryd, O., Nilsson, M., Renberg, I., 1997. Stable isotope and concentration records of atmospheric lead pollution in peat and lake sediments in Sweden. Water, Air, & Soil Pollution 100, 243e252. Brännvall, M.L., Bindler, R., Renberg, I., Emteryd, O., Bartnicki, J., Billstrom, K., 1999. The Medieval metal industry was the cradle of modern large-scale atmospheric lead pollution in northern Europe. Environmental Science & Technology 33, 4391e4395. Brännvall, M.L., Bindler, R., Emteryd, O., Renberg, I., 2001. Four thousand years of atmospheric lead pollution in northern Europe: a summary from Swedish lake sediments. Journal of Paleolimnology 25, 421e435. Candelone, J.P., Hong, S.M., Pellone, C., Boutron, C.F., 1995. Post-industrial revolution changes in large-scale atmospheric pollution of the northern-hemisphere by heavy metals as documented in central Greenland snow and ice. Journal of Geophysical Research 100, 16605e16616. Cloy, J.M., Farmer, J.G., Graham, M.C., MacKenzie, A.B., Cook, G.T., 2008. Historical records of atmospheric Pb deposition in four Scottish ombrotrophic peat bogs: an isotopic comparison with other records from western Europe and Greenland. Global Biogeochemical Cycles 22, GB2016. Cooke, C.A., Abbott, M.B., Wolfe, A.P., Kittleson, J.L., 2007. A Millennium of metallurgy recorded by lake sediments from Morococha, Peruvian Andes. Environmental Science & Technology 41, 3469e3474.

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