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Enhanced concentrations of dissolved gaseous mercury in the surface waters of the Arctic Ocean
Marine Chemistry 110 (2008) 190–194
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Marine Chemistry j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m
Enhanced concentrations of dissolved gaseous mercury in the surface waters of the Arctic Ocean M.E. Andersson a,⁎, J. Sommar a, K. Gårdfeldt b, O. Lindqvist c a b c
Department of Chemistry, Göteborg University, Göteborg, Sweden Center for environment and sustainability, Chalmers and Göteborg University, Göteborg, Sweden Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 15 January 2008 Received in revised form 1 April 2008 Accepted 1 April 2008 Available online 16 April 2008 Keywords: Mercury In situ measurements Arctic Ocean Dissolved gaseous mercury Gas-exchange Biogeochemistry
a b s t r a c t During an almost three months long expedition in the Arctic Ocean, the Beringia 2005, dissolved gaseous mercury (DGM) was measured continuously in the surface water. The DGM concentration was measured using an equilibrium system, i.e. the DGM in the water phase equilibrated with a stream of gas and the gas was thereafter analysed with respect to its mercury content. The DGM concentrations were calculated using the following equation, DGM = Hgeq / kH' where Hgeq is the equilibrated concentration of elemental mercury in the gas phase and kH' is the dimensionless Henry's law constant at desired temperature and salinity. During the expedition several features were observed. For example, enhanced DGM concentration was measured underneath the ice which may indicate that the sea ice acted as a barrier for evasion of mercury from the Arctic Ocean to the atmosphere. Furthermore, elevated DGM concentrations were observed in water that might have originated from river discharge. The gas-exchange of mercury between the ocean and the atmosphere was calculated in the open water and both deposition and evasion were observed. The measurements showed significantly enhanced DGM concentrations, compared to more southern latitudes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Mercury is known as a global pollutant which is widespread in the environment. The metal is released into the atmosphere by anthropogenic and natural emissions, mainly in its elemental form (Pacyna and Pacyna, 2002). Mason and Sheu (2002) have estimated the anthropogenic emissions to be in the same range as the natural emissions. Elemental mercury can be transported in the atmosphere for approximately one year, before it reacts to more easily deposited species (Schroeder and Munthe, 1998). Due to the long residence time in air mercury is spread to remote location such as the Arctic environment. Once in the Arctic air-mass, events of mercury being depleted may occur during the polar sunrise i.e. the concentration of elemental mercury decreases rapidly to low concentrations (Steffen et al., 2005). The first ⁎ Corresponding author. E-mail address: [email protected] (M.E. Andersson). 0304-4203/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2008.04.002
evidence of such process was observed by Schroeder et al. (1998) at Alert. Moreover, since then several scientists have reported similar concentration depletions occurring both in the Arctic and Antarctic atmospheres (Steffen et al., 2005, Berg et al., 2003, Lindberg et al., 2001, Skov et al., 2004, Poissant and Pilote, 2003, Ebinghaus et al., 2002). During such events the residence time of elemental mercury is only a few hours (Skov et al., 2004), as compared to the global residence time of approximately a year. Elevated concentrations of both reactive mercury and particulate mercury, i.e. inorganic mercury, have been reported to occur simultaneously (Lu et al., 2001, Lu and Schroeder, 2004, Lindberg et al., 2001, Steffen et al., 2002). Biomagnification of the most toxic form of mercury, i.e. monomethyl mercury which is formed in aquatic environments after deposition of inorganic mercury, has been observed in Arctic marine wild life. For example, mercury levels in Arctic ringed seals and Beluga whales have increased by up to four times over the last 25years. Furthermore, the levels in indigenous people living in the Arctic exceed those in
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people living in more temperate, industrial regions (UNEP, 2002; Steffen et al., 2007 and references therein). The fate of deposited mercury in the Arctic environment is not fully understood. However, fractions of oxidised inorganic mercury are reduced to DGM by biotic and abiotic processes (Allard and Arsenie, 1991; Xiao et. al., 1994, 1995; Mason et. al., 1995; Costa and Liss, 1999; Amyot et. al., 1997, 2004). There have been studies observing mercury in the sea ice and at coastal locations and some measurements of speciation of mercury have been performed in the Russian arctic (AMAP, 2005). To our knowledge three investigations comprising DGM measurements have been conducted in the Arctic Ocean. Sommar et al. (2007) perform near shore measurements of DGM in Kongsfjorden, Temme et al. (2005) performed measurements of DGM from 60°N 0°E to around 85°N 0°E and St. Louis et al. (2007) carried out measurements around the Ellesmere Island in the Canadian Archipelago. Measurements of total mercury and methyl mercury have been conducted in the Mackenzie area (Leitch et al., 2007). Measurements of total mercury have also been performed in Laventiya, the Gulf of Ob, the Laptev Sea, at the coast in the Pechora Sea (AMAP, 2005 and references therein) and at coastal sites along the Northern Siberian coast (Coquery et al., 1995). In this paper DGM in water and total gaseous mercury (TGM) in air have been measured continuously and at high resolution during an expedition of three months in the Arctic Ocean. Certain features in the data are described, e.g. when passing the plume from the Mackenzie River and during a 48h sampling cycle from a costal station in the Chukchi Sea north of the Chukotka peninsula. It has been suggested that the river input might be a significant source of mercury to the Arctic Ocean (AMAP, 2005). Because of the diurnal variation observed at some coastal and off shore sites (Amyot et al., 1994, 1997; Lanzilotta and Ferrara, 2001; Gårdfeldt et al., 2001, 2003, Andersson et al., 2007), at least 24h sampling cycles are interesting to perform. In our investigation mercury flux estimations have been carried out for locations which were not ice covered and an assessment have been conducted applying a scenario where the Arctic Ocean is ice free. This work, together with previously published investigations performed in the Mediterranean Sea and the North Atlantic Ocean (Andersson et al., 2007, submitted for publication), are a series of measurements carried out in order to investigate the possibility of accumulation of mercury in the Arctic Ocean. 2. Experimental The measurement was conducted onboard the Icebreaker Oden. The IB Oden is a 108m long and 31m wide icebreaker which has been rebuilt to a research vessel, i.e. it is equipped with meteorological and oceanographic instrumentation. During an expedition of almost three months, July 13th to September 25th, DGM and TGM were measured continuously and at high resolution, i.e. every 10min. Meteorological parameters were obtained from the ship's data base. The wind speed was measured 30m above the sea surface and water temperature was measured in the bow water inlet approximately 5m under the sea surface, depending on the ship load. On the forth deck an inlet for TGM sampling was placed 2m above the deck (20m above the sea surface). Ambient air was led through a heated teflon tubing to a Tekran 2537A (Schroeder et al., 1995). The DGM concentration was measured with a continuous equilibrium system (Andersson et al., submitted for publication), briefly described here.
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The system was connected to the ship's bow water inlet containing teflon pipes, through which an average water flow rate of 20L min− 1 was constantly pumped, and a water pump stabilised and regulated the water flow rate in the DGM system to 16L min− 1. A gas flow (1.2L min− 1) passed the water flow in the opposite direction, and the mercury in the water phase equilibrated readily with the gas phase. The mercury in the out-going gas was detected by a Tekran 2537A. Hence, the DGM concentration was calculated according to: DGM ¼ Hgeq =kH V where Hgeq is the mercury concentration in the out-going gas and kH' is the dimensionless Henry's law constant at desired temperature and salinity (Andersson et al., submitted for publication). The detection limit of the system was less than 7 fM. The degree of saturation was calculated directly from the measurements in the following way: S ¼ Hgeq =TGM where TGM is the measured ambient air concentration. The flux estimations were made using the gas-exchange model developed by Nightingale et al. (2000): Hgflux ¼ k ðDGM TGM=kH VÞ where k is the gas-transfer velocity, calculated according to the equation: k ¼ 0:22 u210 þ 0:333 u10 ScHg =660 u10 is the wind speed normalised to 10m above sea surface according to: u10 ¼ ð10:4uz Þ=ðlnðzÞ þ 8:1Þ where z is the height where the wind speed was measured. The Schmidt number (ScHg) is the ratio between the kinematic viscosity of the water and the aqueous diffusivity of mercury. The ScHg was calculated using the Wilke– Chang method described in Reid et al. (1987), and corrected for sea water according to Wanninkhof (1992) and references therein. The dependence of temperature and salinity on the viscosity and diffusivity was calculated for each case.
3. Result and discussion 3.1. DGM The cruise track and the measured DGM concentration along with the degree of saturation are shown in Fig. 1. The overall average concentration was 220 ± 110 fM, ranging from 25 to 670 fM. The highest concentration was observed north of Alaska, while the lowest concentration was observed within the Canadian archipelago. The first DGM measurements were conducted southwest of Greenland (approximately 75 fM) and were in the same range as the observations made in the North Atlantic Ocean (Andersson et al., submitted for publication). Once the ship entered the ice (#1), the DGM concentration increased drastically, approximately 80%, from 75 fM to around 350 fM. When leaving the ice the DGM concentration decreased (the ice covered area was left for a short period of time, which is not indicated in Fig. 1). The concentration, however, was slightly higher, i.e. around 200 fM, in the open water of the Canadian archipelago compared to the open water on the west side of Greenland, Fig. 1. In the Canadian archipelago both open water and ice covered areas were passed and as shown in Fig. 1, the elevated concentrations indicate ice covered areas. The next specific feature, along the cruise track, was the plume from the Mackenzie River (after #2). In Fig. 2, a detailed plot of the data from the passage outside the Mackenzie River delta is shown, the area around the Mackenzie River delta was ice free. Before entering the river plume the DGM concentration was stable, around 200 fM, however when the ship entered the plume the concentration increased to around 500 fM. This increase may originate from an increased concentration of DGM in the river discharge. However, the increased DGM concentration might also be explained by an increased concentration of dissolved organic carbon (DOC) being transported with the river water, since it has been observed that DOC facilitates the DGM production from oxidised mercury fractions (Xiao et al., 1995; O'Droscoll et al., 2006; Garcia et al., 2006). After this passage the concentration measured decreased to around 130 fM. In the Strait of Bering, where the exchange of
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Fig. 1. The top figure shows the cruise track and the DGM concentration expressed in fM. The bottom figure is a more detailed plot of the DGM concentration and the degree of saturation along the cruise track. The numbers indicate the location on the map and the approximate ice covered areas are shown. (The top figure was made using the software program ODV, Schlitzer, 2004).
water masses between the Pacific Ocean and the Arctic Ocean occurs, the concentrations decreased with approximately 50%, from 150 fM to 70 fM, which are n the same range as measured in the North Atlantic Ocean (Andersson et al., submitted for publication), the concentrations than once again increased as the ship moved north. However, before the ship left the Russian coast a 48h sampling cycle was conducted north of the Chukotka peninsula in the Chukchi Sea, Fig. 3. A variation between 100 and 300 fM, with a peak as high as 450 fM, was observed during the sample time. The variation did not correlate with the solar radiation, as has been seen at other sites (Amyot et al., 1994, 1997;
Lanzilotta and Ferrara, 2001; Gårdfeldt et al., 2001, 2003, Andersson et al., 2007; Sommar et al., 2007). However, a river mouth was situated further west entering the Arctic Ocean via Koluchin Bay, and the plume from the river may have influenced the concentration of mercury at this station. The absence of diurnal variations was observed in all waters sampled, not only at the 48 h sampling station. However, due to the insecurity in determining diurnal variations along a transect, this site was chosen to show how the DGM concentration varied with time of day. Along the transect over the North Pole, first open waters were sampled with concentrations around 130 fM. When the ship entered the ice-covered
M.E. Andersson et al. / Marine Chemistry 110 (2008) 190–194 areas the concentrations increased. Moreover, the concentration of DGM continuously increased as the ship moved closer to the North Pole, and the maximum concentration was reached close to the pole. It should be noted that these concentrations were 10 times higher then the DGM concentrations observed in the North Atlantic Ocean (Andersson et al., submitted for publication). This enhanced mercury concentration may originate from the river runoff. It has been observed by others, i.e. Andersson et al. (2004) that river water passes along the Lomonosov Ridge. However, the pattern of the river water has changed over the years, and further research is needed to conclude if the pattern observed here is due to river runoff and/or atmospheric deposition of mercury. When exiting the ice the concentration decreased and when moving towards Svalbard the concentrations approached the concentration measured in the North Atlantic Ocean, i.e. 70 fM (Andersson et al., submitted for publication). To our knowledge no previous investigations on DGM concentrations have been carried out in the areas investigated in this study. However, three previous investigations are presented in literature that comprises DGM measurements in adjacent areas. One investigation was performed at a near shore station in Kongsfjorden Ny-Ålesund where a strong diurnal variation was observed with DGM concentrations ranging from 60 to 350 fM (Sommar et al., 2007). One other investigation was carried out by Temme et al. (2005) where measurements were conducted from 60°N 0°E to around 85°N 0°E. In that investigation an increased DGM concentration was observed going from southern latitudes towards the north, i.e. from 50 to 170 fM. The measurements presented by St. Louis et al. (2007) comprise measurements of DGM around the Ellesmere Island, an average DGM concentration of 640 fM was determined and it was stated that the open water of the Arctic Ocean may act as a net source of mercury to the atmosphere.
3.2. TGM Measurements of TGM were also conducted during the expedition and the average concentration was 1.7± 0.4 ng m− 3, with a range of 0.9 to 5.2 ng m− 3. The highest concentration of TGM was associated with ice covered areas and with enhanced DGM concentrations underneath the ice. This may indicate that the sea ice acts as a barrier through which the volatile mercury cannot easily be transported, however, when the ship broke the ice evasion became possible. If the equilibrium was established directly when the ship broke the ice a TGM concentration of approximately 10 ng m− 3 would have been obtained, therefore the high TGM concentration observed may be related to mercury evasion occurring when the ice was broken. The result from the TGM measurements is in the same range as previously published data from the Arctic (Aspmo et al., 2006, Steffen et al., 2005, Berg et al., 2003, Lindberg et al., 2001, Skov et al., 2004).
3.3. Estimations of flux and degree of saturation Since the sea ice may have acted as a barrier for mercury evasion, the evasion was probably restricted in large parts of the Arctic Ocean investigated. During the expedition described here, most of the waters passed were ice covered. Flux calculations were, however, carried out for the open waters passed using the
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Fig. 3. The DGM concentration during a 48 h sampling period, north of the Chukotka peninsula in the Chukchi Sea.
model developed by Nightingale et al. (2000). Episodes of deposition of elemental mercury occurred on the west coast of Greenland, −8 pmol m− 2 h− 1. The highest evasion observed was along the Alaskan coast, 490 pmol m− 2 h− 1. The degree of saturation, Fig. 1, of mercury in the Arctic Ocean was observed to be as high as 1800% which explains the high evasion estimated. This degree of saturation was in the same range as the average estimation for a polluted area in the Mediterranean Sea, i.e. the North Adriatic Sea (Andersson et al., 2007). It should be noted that also under-saturated waters (degree of saturation 40%) were passed causing deposition of elemental mercury. However, the average degree of saturation observed during the entire cruise, including ice covered areas, was 410 ±220%. Despite the ice coverage, which might have hindered mercury evasion, a hypothetic average flux was calculated. The following average values for input data were calculated from expedition data, i.e. DGM concentration 220 fM, TGM concentration 1.7 ng m− 3, wind speed 6 m s− 1 and water temperature −1 °C. Using these values a net evasion of 12 pmol m− 2 h− 1 was estimated, which is comparable to the evasion estimated for the winter season in the Mediterranean Sea (Andersson et al., 2007).
4. Conclusion For the first time DGM concentrations in surface waters have been measured continuously at high time resolution in major areas of the Arctic Ocean. The average DGM concentration measured during this investigation was 220 ± 110 fM, ranging from 25 to 670 fM. Elevated DGM concentrations were observed around the North Pole, and were 10 times higher than the concentrations measured in the North Atlantic Ocean (Andersson et al., submitted for publication), suggesting a possible accumulation of DGM in the Arctic Ocean. Enhanced DGM concentrations were measured in the plume from the Mackenzie River, indicating mercury species being discharged into the Arctic Ocean via the river water. Elevated concentrations of DGM in sea water were observed in the ice covered areas of the Arctic Ocean. This may indicate that the sea ice acts as a barrier for mercury exchange between the Arctic Ocean and the atmosphere. Acknowledgement
Fig. 2. The DGM concentration passing the Mackenzie River.
The Swedish Polar Secretariat is acknowledged for organising the BERINGIA 2005 expedition. The Captain and crew of IB Oden are appreciated for the support and help onboard.
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Contents lists available at ScienceDirect
Marine Chemistry j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m
Enhanced concentrations of dissolved gaseous mercury in the surface waters of the Arctic Ocean M.E. Andersson a,⁎, J. Sommar a, K. Gårdfeldt b, O. Lindqvist c a b c
Department of Chemistry, Göteborg University, Göteborg, Sweden Center for environment and sustainability, Chalmers and Göteborg University, Göteborg, Sweden Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 15 January 2008 Received in revised form 1 April 2008 Accepted 1 April 2008 Available online 16 April 2008 Keywords: Mercury In situ measurements Arctic Ocean Dissolved gaseous mercury Gas-exchange Biogeochemistry
a b s t r a c t During an almost three months long expedition in the Arctic Ocean, the Beringia 2005, dissolved gaseous mercury (DGM) was measured continuously in the surface water. The DGM concentration was measured using an equilibrium system, i.e. the DGM in the water phase equilibrated with a stream of gas and the gas was thereafter analysed with respect to its mercury content. The DGM concentrations were calculated using the following equation, DGM = Hgeq / kH' where Hgeq is the equilibrated concentration of elemental mercury in the gas phase and kH' is the dimensionless Henry's law constant at desired temperature and salinity. During the expedition several features were observed. For example, enhanced DGM concentration was measured underneath the ice which may indicate that the sea ice acted as a barrier for evasion of mercury from the Arctic Ocean to the atmosphere. Furthermore, elevated DGM concentrations were observed in water that might have originated from river discharge. The gas-exchange of mercury between the ocean and the atmosphere was calculated in the open water and both deposition and evasion were observed. The measurements showed significantly enhanced DGM concentrations, compared to more southern latitudes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Mercury is known as a global pollutant which is widespread in the environment. The metal is released into the atmosphere by anthropogenic and natural emissions, mainly in its elemental form (Pacyna and Pacyna, 2002). Mason and Sheu (2002) have estimated the anthropogenic emissions to be in the same range as the natural emissions. Elemental mercury can be transported in the atmosphere for approximately one year, before it reacts to more easily deposited species (Schroeder and Munthe, 1998). Due to the long residence time in air mercury is spread to remote location such as the Arctic environment. Once in the Arctic air-mass, events of mercury being depleted may occur during the polar sunrise i.e. the concentration of elemental mercury decreases rapidly to low concentrations (Steffen et al., 2005). The first ⁎ Corresponding author. E-mail address: [email protected] (M.E. Andersson). 0304-4203/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2008.04.002
evidence of such process was observed by Schroeder et al. (1998) at Alert. Moreover, since then several scientists have reported similar concentration depletions occurring both in the Arctic and Antarctic atmospheres (Steffen et al., 2005, Berg et al., 2003, Lindberg et al., 2001, Skov et al., 2004, Poissant and Pilote, 2003, Ebinghaus et al., 2002). During such events the residence time of elemental mercury is only a few hours (Skov et al., 2004), as compared to the global residence time of approximately a year. Elevated concentrations of both reactive mercury and particulate mercury, i.e. inorganic mercury, have been reported to occur simultaneously (Lu et al., 2001, Lu and Schroeder, 2004, Lindberg et al., 2001, Steffen et al., 2002). Biomagnification of the most toxic form of mercury, i.e. monomethyl mercury which is formed in aquatic environments after deposition of inorganic mercury, has been observed in Arctic marine wild life. For example, mercury levels in Arctic ringed seals and Beluga whales have increased by up to four times over the last 25years. Furthermore, the levels in indigenous people living in the Arctic exceed those in
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people living in more temperate, industrial regions (UNEP, 2002; Steffen et al., 2007 and references therein). The fate of deposited mercury in the Arctic environment is not fully understood. However, fractions of oxidised inorganic mercury are reduced to DGM by biotic and abiotic processes (Allard and Arsenie, 1991; Xiao et. al., 1994, 1995; Mason et. al., 1995; Costa and Liss, 1999; Amyot et. al., 1997, 2004). There have been studies observing mercury in the sea ice and at coastal locations and some measurements of speciation of mercury have been performed in the Russian arctic (AMAP, 2005). To our knowledge three investigations comprising DGM measurements have been conducted in the Arctic Ocean. Sommar et al. (2007) perform near shore measurements of DGM in Kongsfjorden, Temme et al. (2005) performed measurements of DGM from 60°N 0°E to around 85°N 0°E and St. Louis et al. (2007) carried out measurements around the Ellesmere Island in the Canadian Archipelago. Measurements of total mercury and methyl mercury have been conducted in the Mackenzie area (Leitch et al., 2007). Measurements of total mercury have also been performed in Laventiya, the Gulf of Ob, the Laptev Sea, at the coast in the Pechora Sea (AMAP, 2005 and references therein) and at coastal sites along the Northern Siberian coast (Coquery et al., 1995). In this paper DGM in water and total gaseous mercury (TGM) in air have been measured continuously and at high resolution during an expedition of three months in the Arctic Ocean. Certain features in the data are described, e.g. when passing the plume from the Mackenzie River and during a 48h sampling cycle from a costal station in the Chukchi Sea north of the Chukotka peninsula. It has been suggested that the river input might be a significant source of mercury to the Arctic Ocean (AMAP, 2005). Because of the diurnal variation observed at some coastal and off shore sites (Amyot et al., 1994, 1997; Lanzilotta and Ferrara, 2001; Gårdfeldt et al., 2001, 2003, Andersson et al., 2007), at least 24h sampling cycles are interesting to perform. In our investigation mercury flux estimations have been carried out for locations which were not ice covered and an assessment have been conducted applying a scenario where the Arctic Ocean is ice free. This work, together with previously published investigations performed in the Mediterranean Sea and the North Atlantic Ocean (Andersson et al., 2007, submitted for publication), are a series of measurements carried out in order to investigate the possibility of accumulation of mercury in the Arctic Ocean. 2. Experimental The measurement was conducted onboard the Icebreaker Oden. The IB Oden is a 108m long and 31m wide icebreaker which has been rebuilt to a research vessel, i.e. it is equipped with meteorological and oceanographic instrumentation. During an expedition of almost three months, July 13th to September 25th, DGM and TGM were measured continuously and at high resolution, i.e. every 10min. Meteorological parameters were obtained from the ship's data base. The wind speed was measured 30m above the sea surface and water temperature was measured in the bow water inlet approximately 5m under the sea surface, depending on the ship load. On the forth deck an inlet for TGM sampling was placed 2m above the deck (20m above the sea surface). Ambient air was led through a heated teflon tubing to a Tekran 2537A (Schroeder et al., 1995). The DGM concentration was measured with a continuous equilibrium system (Andersson et al., submitted for publication), briefly described here.
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The system was connected to the ship's bow water inlet containing teflon pipes, through which an average water flow rate of 20L min− 1 was constantly pumped, and a water pump stabilised and regulated the water flow rate in the DGM system to 16L min− 1. A gas flow (1.2L min− 1) passed the water flow in the opposite direction, and the mercury in the water phase equilibrated readily with the gas phase. The mercury in the out-going gas was detected by a Tekran 2537A. Hence, the DGM concentration was calculated according to: DGM ¼ Hgeq =kH V where Hgeq is the mercury concentration in the out-going gas and kH' is the dimensionless Henry's law constant at desired temperature and salinity (Andersson et al., submitted for publication). The detection limit of the system was less than 7 fM. The degree of saturation was calculated directly from the measurements in the following way: S ¼ Hgeq =TGM where TGM is the measured ambient air concentration. The flux estimations were made using the gas-exchange model developed by Nightingale et al. (2000): Hgflux ¼ k ðDGM TGM=kH VÞ where k is the gas-transfer velocity, calculated according to the equation: k ¼ 0:22 u210 þ 0:333 u10 ScHg =660 u10 is the wind speed normalised to 10m above sea surface according to: u10 ¼ ð10:4uz Þ=ðlnðzÞ þ 8:1Þ where z is the height where the wind speed was measured. The Schmidt number (ScHg) is the ratio between the kinematic viscosity of the water and the aqueous diffusivity of mercury. The ScHg was calculated using the Wilke– Chang method described in Reid et al. (1987), and corrected for sea water according to Wanninkhof (1992) and references therein. The dependence of temperature and salinity on the viscosity and diffusivity was calculated for each case.
3. Result and discussion 3.1. DGM The cruise track and the measured DGM concentration along with the degree of saturation are shown in Fig. 1. The overall average concentration was 220 ± 110 fM, ranging from 25 to 670 fM. The highest concentration was observed north of Alaska, while the lowest concentration was observed within the Canadian archipelago. The first DGM measurements were conducted southwest of Greenland (approximately 75 fM) and were in the same range as the observations made in the North Atlantic Ocean (Andersson et al., submitted for publication). Once the ship entered the ice (#1), the DGM concentration increased drastically, approximately 80%, from 75 fM to around 350 fM. When leaving the ice the DGM concentration decreased (the ice covered area was left for a short period of time, which is not indicated in Fig. 1). The concentration, however, was slightly higher, i.e. around 200 fM, in the open water of the Canadian archipelago compared to the open water on the west side of Greenland, Fig. 1. In the Canadian archipelago both open water and ice covered areas were passed and as shown in Fig. 1, the elevated concentrations indicate ice covered areas. The next specific feature, along the cruise track, was the plume from the Mackenzie River (after #2). In Fig. 2, a detailed plot of the data from the passage outside the Mackenzie River delta is shown, the area around the Mackenzie River delta was ice free. Before entering the river plume the DGM concentration was stable, around 200 fM, however when the ship entered the plume the concentration increased to around 500 fM. This increase may originate from an increased concentration of DGM in the river discharge. However, the increased DGM concentration might also be explained by an increased concentration of dissolved organic carbon (DOC) being transported with the river water, since it has been observed that DOC facilitates the DGM production from oxidised mercury fractions (Xiao et al., 1995; O'Droscoll et al., 2006; Garcia et al., 2006). After this passage the concentration measured decreased to around 130 fM. In the Strait of Bering, where the exchange of
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Fig. 1. The top figure shows the cruise track and the DGM concentration expressed in fM. The bottom figure is a more detailed plot of the DGM concentration and the degree of saturation along the cruise track. The numbers indicate the location on the map and the approximate ice covered areas are shown. (The top figure was made using the software program ODV, Schlitzer, 2004).
water masses between the Pacific Ocean and the Arctic Ocean occurs, the concentrations decreased with approximately 50%, from 150 fM to 70 fM, which are n the same range as measured in the North Atlantic Ocean (Andersson et al., submitted for publication), the concentrations than once again increased as the ship moved north. However, before the ship left the Russian coast a 48h sampling cycle was conducted north of the Chukotka peninsula in the Chukchi Sea, Fig. 3. A variation between 100 and 300 fM, with a peak as high as 450 fM, was observed during the sample time. The variation did not correlate with the solar radiation, as has been seen at other sites (Amyot et al., 1994, 1997;
Lanzilotta and Ferrara, 2001; Gårdfeldt et al., 2001, 2003, Andersson et al., 2007; Sommar et al., 2007). However, a river mouth was situated further west entering the Arctic Ocean via Koluchin Bay, and the plume from the river may have influenced the concentration of mercury at this station. The absence of diurnal variations was observed in all waters sampled, not only at the 48 h sampling station. However, due to the insecurity in determining diurnal variations along a transect, this site was chosen to show how the DGM concentration varied with time of day. Along the transect over the North Pole, first open waters were sampled with concentrations around 130 fM. When the ship entered the ice-covered
M.E. Andersson et al. / Marine Chemistry 110 (2008) 190–194 areas the concentrations increased. Moreover, the concentration of DGM continuously increased as the ship moved closer to the North Pole, and the maximum concentration was reached close to the pole. It should be noted that these concentrations were 10 times higher then the DGM concentrations observed in the North Atlantic Ocean (Andersson et al., submitted for publication). This enhanced mercury concentration may originate from the river runoff. It has been observed by others, i.e. Andersson et al. (2004) that river water passes along the Lomonosov Ridge. However, the pattern of the river water has changed over the years, and further research is needed to conclude if the pattern observed here is due to river runoff and/or atmospheric deposition of mercury. When exiting the ice the concentration decreased and when moving towards Svalbard the concentrations approached the concentration measured in the North Atlantic Ocean, i.e. 70 fM (Andersson et al., submitted for publication). To our knowledge no previous investigations on DGM concentrations have been carried out in the areas investigated in this study. However, three previous investigations are presented in literature that comprises DGM measurements in adjacent areas. One investigation was performed at a near shore station in Kongsfjorden Ny-Ålesund where a strong diurnal variation was observed with DGM concentrations ranging from 60 to 350 fM (Sommar et al., 2007). One other investigation was carried out by Temme et al. (2005) where measurements were conducted from 60°N 0°E to around 85°N 0°E. In that investigation an increased DGM concentration was observed going from southern latitudes towards the north, i.e. from 50 to 170 fM. The measurements presented by St. Louis et al. (2007) comprise measurements of DGM around the Ellesmere Island, an average DGM concentration of 640 fM was determined and it was stated that the open water of the Arctic Ocean may act as a net source of mercury to the atmosphere.
3.2. TGM Measurements of TGM were also conducted during the expedition and the average concentration was 1.7± 0.4 ng m− 3, with a range of 0.9 to 5.2 ng m− 3. The highest concentration of TGM was associated with ice covered areas and with enhanced DGM concentrations underneath the ice. This may indicate that the sea ice acts as a barrier through which the volatile mercury cannot easily be transported, however, when the ship broke the ice evasion became possible. If the equilibrium was established directly when the ship broke the ice a TGM concentration of approximately 10 ng m− 3 would have been obtained, therefore the high TGM concentration observed may be related to mercury evasion occurring when the ice was broken. The result from the TGM measurements is in the same range as previously published data from the Arctic (Aspmo et al., 2006, Steffen et al., 2005, Berg et al., 2003, Lindberg et al., 2001, Skov et al., 2004).
3.3. Estimations of flux and degree of saturation Since the sea ice may have acted as a barrier for mercury evasion, the evasion was probably restricted in large parts of the Arctic Ocean investigated. During the expedition described here, most of the waters passed were ice covered. Flux calculations were, however, carried out for the open waters passed using the
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Fig. 3. The DGM concentration during a 48 h sampling period, north of the Chukotka peninsula in the Chukchi Sea.
model developed by Nightingale et al. (2000). Episodes of deposition of elemental mercury occurred on the west coast of Greenland, −8 pmol m− 2 h− 1. The highest evasion observed was along the Alaskan coast, 490 pmol m− 2 h− 1. The degree of saturation, Fig. 1, of mercury in the Arctic Ocean was observed to be as high as 1800% which explains the high evasion estimated. This degree of saturation was in the same range as the average estimation for a polluted area in the Mediterranean Sea, i.e. the North Adriatic Sea (Andersson et al., 2007). It should be noted that also under-saturated waters (degree of saturation 40%) were passed causing deposition of elemental mercury. However, the average degree of saturation observed during the entire cruise, including ice covered areas, was 410 ±220%. Despite the ice coverage, which might have hindered mercury evasion, a hypothetic average flux was calculated. The following average values for input data were calculated from expedition data, i.e. DGM concentration 220 fM, TGM concentration 1.7 ng m− 3, wind speed 6 m s− 1 and water temperature −1 °C. Using these values a net evasion of 12 pmol m− 2 h− 1 was estimated, which is comparable to the evasion estimated for the winter season in the Mediterranean Sea (Andersson et al., 2007).
4. Conclusion For the first time DGM concentrations in surface waters have been measured continuously at high time resolution in major areas of the Arctic Ocean. The average DGM concentration measured during this investigation was 220 ± 110 fM, ranging from 25 to 670 fM. Elevated DGM concentrations were observed around the North Pole, and were 10 times higher than the concentrations measured in the North Atlantic Ocean (Andersson et al., submitted for publication), suggesting a possible accumulation of DGM in the Arctic Ocean. Enhanced DGM concentrations were measured in the plume from the Mackenzie River, indicating mercury species being discharged into the Arctic Ocean via the river water. Elevated concentrations of DGM in sea water were observed in the ice covered areas of the Arctic Ocean. This may indicate that the sea ice acts as a barrier for mercury exchange between the Arctic Ocean and the atmosphere. Acknowledgement
Fig. 2. The DGM concentration passing the Mackenzie River.
The Swedish Polar Secretariat is acknowledged for organising the BERINGIA 2005 expedition. The Captain and crew of IB Oden are appreciated for the support and help onboard.
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