Natural and Anthropogenic Sources of Lead

Snow data reported in the previous chapter show that even remote Antarctica is affected by atmospheric deposition of Pb and other metals produced by human activities in continents of the Southern Hemisphere. Until two decades ago, most Pb in the atmosphere and in ocean surface waters derived from the combustion of leaded gasoline and other anthropogenic sources (e.g. Nriagu and Pacyna 1988; Volkening et al. 1988). Combustion of leaded, low-leaded and unleaded gasoline continued to be the major source of atmospheric Pb emissions during the mid-1990s (Pacyna and Pacyna 2001). Fluxes of anthropogenic Pb have characteristic, conservative isotopic compositions

(204Pb:206Pb:207Pb:208Pb), and oceanic dispersion of the metal can be traced through the determination of Pb isotopic ratios. The transient input of Pb from industrial sources complements those of other seawater tracers such as freons and tritium, and has been widely used to study circulation in the North Atlantic (e.g. Boyle et al. 1986; Hamelin et al. 1997). Although fewer studies have focused on the Southern Hemisphere, a recent survey of Pb isotopes in the South Atlantic atmosphere and seawater (Alleman et al. 2001) indicates that anthropogenic lead from the Northern Hemisphere does not directly influence the Southern Hemisphere. Antarctic Bottom Water has a relatively radiogenic Pb component (206Pb/207Pb=1.186±0.002) which probably reflects a mixture of natural and anthropogenic Pb sources within the Southern Hemisphere.

During periods of intense algal blooms, Flegal et al. (1993) measured very low concentrations of Pb (Table 11) in unfiltered surface waters from marginal ice zones of the Weddell and Scotia seas. The low Pb concentrations were attributed to efficient scavenging associated with intense primary productivity and to the relative isolation of the study area. The aeolian Pb flux (about 0.01 pmol cm-2) was at least two orders of magnitude lower than that of any other sea. However, concentrations of Pb in Antarctic waters were positively correlated with those of Fe, which are generally not increased by anthropogenic inputs. It was therefore concluded that a significant amount of Pb in surface waters of the Weddell and Scotia seas derived from natural sources such as volcanic emissions, aeolian dust from South America or sediment transport by ice rafting. The isotopic composition of Pb from the Mt. Erebus volcanic province, from lavas and sediments in Marie Byrd Land, and from landmasses in the Southern Ocean, such as the South Sandwich Islands and Chile, nearly matched the radiogenic component of Pb in Antarctic surface waters. In these waters the relative amount of anthropogenic Pb was highly variable at different sites. It was estimated to range from 30 to 70 % based on the isotopic composition of Mt. Erebus volcanics, and from 0 to 60 % based on the isotopic composition of South American crustal sources. According to Flegal et al. (1993), although the variability of Pb concentrations and isotopic compositions may also reflect anthropogenic point source inputs in Antarctica itself (ships, aircraft, research operations), the isotopic composition of industrial Pb in Antarctic surface waters generally reflects that of urban aerosols emitted in Australia, South Africa and other countries of the Southern Hemisphere. The major sources of this Pb were the Broken Hill and Mount Isa ore deposits in Australia (206Pb/207Pb ~1.04).

A recent study on surface (filtered) waters collected in February/March 1991 from the north-west Weddell Sea (Sanudo-Wilhelmy et al. 2002) confirms that Pb concentrations are low (about half those usually measured in other ocean environments; Table 11), and the anthropogenic component of this metal reflects the isotopic composition of aerosols from South America and Palmer Station.

Table 11. Typical concentrations (range and/or mean±SD; pmol l-1) of Pb in Antarctic waters

Location

Depth (m)

n

(range)

(mean)

Notes

Weddell Sea water column

From surface to 3,600 m

35

5.0-47

10

December 1988-February 1989, unfiltered samples (West-erlund and Ohman 1991b)

Weddell and Scotia Sea

Surface waters (marginal ice zone)

1

9.8-103

45±39

March and June 1988; June and August 1988, unfiltered (Fle-gal et al. 1993)

Weddell Sea, northeast

Surface waters<1 m

11

4.3-40

16±13

February 1991, filtered (Sanudo-Wil-helmy et al. 2002)

7

22-87

38±

19 November 1993, filtered (Scarponi et al. 2000)

Wood Bay, Ross Sea

From 0.5 to 350 m

6

7.6-37

20±9

9 January 1994, filtered (Scar-poni et al. 1999)

5

13-27

20±6

10 February 1994, filtered (Scarponi et al. 1999)

Capodaglio et al. (1991) used Differential-Pulse Anodic Stripping Voltam-metry (DPSAV) to study the complexation of Pb by natural organic ligands in surface waters from Terra Nova Bay (Ross Sea). Total Pb concentrations in the January-February 1988 period ranged from 25 to 114 pmol l-1; open-sea water samples showed lower concentrations of total Pb (25-38 pmol l-1) and ligands (about 44 % of total Pb was in the labile fraction). Total Pb concentrations in coastal water ranged from 61 to 114 pmol l-1 and, due to the higher content of ligands, only about 25 % of total Pb was in the labile fraction. As for the possible origin of Pb ligands, a statistically significant relationship was found between ligand and chlorophyll concentrations in open-sea waters. In the same marine region, Scarponi et al. (1999) studied the evolution of Pb profiles in the water column during the 1993-1994 austral summer. The vertical profile of soluble Pb concentrations was nearly uniform (31 ±6 pmol l-1) in November (Table 11), whereas Pb concentrations in the water column decreased progressively in December and during summer to about half the initial value. Lead concentrations in the water column seemed to increase in February, beginning from the deepest layers. The summer depletion of soluble Pb in the water column was attributed to adsorption on suspended matter and interactions with quickly settling particles, especially during the senescent phase of algal blooms.

6.2.5 A Neglected Element: Mercury

High concentrations of Hg were reported in water samples from the Ross Sea during the 1970s (e.g.Williams et al. 1974). The data were probably affected by the lack of suitable procedures for clean sampling and handling of seawater. However, there appear to be no recent data on Hg concentrations in Southern Ocean waters. This is a drawback because there is evidence that monomethyl-mercury (MeHg) biomagnifies in Antarctic marine coastal food webs - its concentrations in feathers of Antarctic skua and in tissues of a Weddell seal from Terra Nova Bay (Ross Sea) were in the same range as those in related species of seabirds and marine mammals from the Northern Hemisphere (Bargagli et al. 1998a, 2000). The recent introduction of automated techniques for accurately measuring gaseous oxidised Hg species at typical atmospheric concentrations is providing very useful data on the biogeochemical cycle of this metal in the environment. As discussed in Chapter 4, these techniques have revealed that in polar regions after the polar dawn, atmospheric concentrations of Hg° diminish over a period of a few days to as low as 10-20 % of their typical value. Hg depletion is due to gas-phase oxidation of Hg, probably by halogen atoms or halogen-containing radicals. Soluble and insoluble forms of oxidised Hg are deposited in polar ecosystems; this is a cause for concern because terrestrial organisms resume biological activity this time of year. Until recently, the conversion of Hg° to Hg2+ was mainly attributed to the reaction of aqueous and gaseous phases with O3. However, recent research (Hedgecock and Pirrone 2001) on the marine boundary layer (i.e. the air directly above the sea surface) suggests that the role of H2O2 in Hg° oxidation is probably as important as that of O3. Concentrations of oxidised forms of Hg, such as HgO, HgCl2 and HgBr2, in the boundary layer of the Mediterranean Sea are as high as in the more industrial areas of northern Europe (Wangberg et al. 2001). Recent modelling studies (Hedgecock et al. 2003) suggest that deliquesced sea-salt aerosol in the marine boundary layer provides not only a scavenging phase for oxidised Hg compounds but also an almost unlimited supply of Cl, with which Hg++ can form aqueous-phase complexes.

Processes in the marine boundary layer reproduce on a lesser scale many of the reactions which determine Hg° depletion events in Arctic and Antarctic terrestrial ecosystems (UNEP 2002b). It seems likely that in spring and summer, when the surface of the Southern Ocean is ice-free, there is constant deposition of oxidised forms of Hg which are probably replenished in the boundary layer by Hg from the sea and free troposphere.

During the 1980s, Fitzgerald and co-workers (e.g. Gill and Fitzgerald 1988) largely contributed to the establishment of sampling and analytical protocols for producing reliable data on Hg concentrations in the ocean environment. More recently, Mason and Fitzgerald (1997) reviewed aspects of the biogeo-chemical cycle of Hg in oceans: typical concentrations in ocean waters are <5 pmol l-1, and the estimated residence time for Hg is about 350 years. Unlike Zn, Cd and other trace metals, Hg does not generally show nutrient-like regeneration in the water column. Average Hg concentrations are lower in the Pacific than in the Atlantic due to differences in external sources and in the scavenging intensity of settling particles (i.e. biological productivity). Methylated Hg species (MeHg and dimethylmercury, DMHg) are found throughout the ocean water column. In contrast to freshwater ecosystems, DMHg has always been found in deep ocean waters; however, DMHg is usually lacking in open-ocean surface waters, perhaps due to decomposition in the presence of light or loss via evaporation. As this compound is relatively unstable, continual production is necessary to sustain measurable concentrations in marine waters (Fitzgerald and Mason 1997). Elemental Hg (Hg° ) is another ubiquitous Hg species in surface and deep waters. Its formation in surface waters appears to be both an incidental result of primary productivity and a result of photochemical reduction (Amyot et al. 1997). The reduction rate depends on the availability of ionic Hg, which may be supplied by the marine boundary layer and processes such as upwelling, which also drive productivity in surface waters. The flux of particulate matter from the euphotic zone to deeper waters is the main source of Hg to sub-thermocline waters where net methy-lation occurs.

Dalziel (1995) measured the vertical distribution of reactive Hg (i.e. the metal volatilised from water after the addition of 10 % acidic stannous chloride) in the eastern North Atlantic and the south-eastern Atlantic. The latter station (Angola Basin) showed higher biological productivity and the lowest concentrations of reactive Hg. The depletion of Hg in surface waters was probably due to enhanced biological activity (i.e. to the biological reduction of Hg++ and release of Hg° to the atmosphere, and to particulate scavenging processes). Higher concentrations of reactive Hg (about 1.4 pmol l-1) in water samples from depths of 35-200 m coincided with an intense nutrient gradient and O2 depletion. The thermocline extended to a depth of about 500 m (average reactive Hg concentration=0.92±0.54 pmol l-1). Below the thermocline, to a depth of 1,200 m,Antarctic Intermediate Water had a lower average concentration of Hg (0.51±0.04 pmol l-1). The deep vertical profile did not show remarkable variations in reactive Hg concentrations. Values measured in North Atlantic Deep Water (from depths of 2,000-2,800 m) and Antarctic Bottom Water (at depths>4,000 m) ranged from 0.45-0.67 pmol l-1. In two other sampling stations, at lower latitudes (over the Cape Verde Abyssal Plain and the Seine Abyssal Plain), concentrations of reactive Hg in Antarctic Bottom Water progressively increased (from 0.67-1.25 pmol l-1). Dalziel (1995) hypothesised that mixing with North Atlantic Deep Water and/or release of Hg from bottom sediments were the main sources of this metal. In contrast to Antarctic water masses, those coming from the Northern Hemisphere, such as North Atlantic Deep Water, showed progressively lower Hg concentrations (from 1.34-0.48 pmol l-1).

Mason and Sullivan (1999) measured concentrations of different species of Hg in surface and deep waters from the South and equatorial Atlantic. Concentrations of DMHg in Antarctic Intermediate Water (i.e. low-salinity surface water sinking at the Antarctic Polar front) decreased northwards (from 0.11 pmol l-1 at 33° S, 40° W to 0.037 pmol l-1 at 7.5° N, 25° W). The same trend was observed in Antarctic Bottom Water, and Mason and Sullivan (1999) concluded that net decomposition of DMHg occurs in these water masses with time. Concentrations of MeHg in all samples were at or below the detection limit (0.05 pmol l-1), and it was hypothesised that in deep waters this compound is decomposed more rapidly than it is produced and/or that it is scavenged by particulate matter. However, total Hg concentrations in intermediate and bottom Antarctic water (total Hg=2 pmol l-1) were higher than those of Hg° (about 1.0 pmol l-1), suggesting the presence of different Hg species (Mason and Sullivan 1999). In conclusion, available data on Hg concentrations and speciation in Antarctic waters, collected in the Atlantic, seem to indicate that concentrations of reactive Hg in waters from the Southern Ocean are approximately less than or equal to 1 pmol l-1. This fraction mostly consists of Hg° (from about 0.4-1.0 pmol l-1), probably a product of MeHg decomposition. Another significant amount of Hg in Antarctic waters probably consists of DMHg.

Was this article helpful?

0 0

Post a comment