Lead in the marine system

Historical atmospheric lead fluxes have directly affected seawater lead concentrations. The first papers describing the dominance of anthropogenic lead in ocean surface waters were published in the early 1980s using lead concentration and isotope ratios measurements along vertical depth profiles and in surface water in the Pacific and North Atlantic ocean [Flegal and Patterson (1983); Flegal et al. (1984); Schaule and Patterson (1981); Schaule and Patterson (1983)]. Consequent monitoring of surface waters near Bermuda and the western North Atlantic showed a threefold decline in lead concentrations from 1971 to 1987, with continuing but notably slower decline in the 1990s [Reuer et al. (2003); Shen and Boyle (1988b); Wu and Boyle (1997)]. This reduction was concurrent with a 20-fold decrease in leaded petrol consumption in the US from 1979 to 1993 and this apparent difference reflects the amount of emitted lead reaching the North Atlantic and its subsequent admixture within the subtropical gyre (Fig. 7). A similar decrease in lead concentrations has been detected in the Mediterranean [Alleman et al. (2000); Nicolas et al. (1994)].

Historically, the US has consumed lead with high 206Pb/207Pb signatures (notably Missouri lead) whereas European nations used low 206Pb/207Pb lead. This dissimilarity has been apparent in North Atlantic surface water measurements: westerly atmospheric transport results in high seawater 206Pb/207Pb near North America and reduced 206Pb/207Pb ratios to the south most likely reflect north-easterly European fluxes [Hamelin et al. (1997); Veron et al. (1994); Weiss et al. (2003)]. Isotopic boundaries observed between the tropics and sub-tropics agree with lead concentration gradients, with the southerly advection of low lead concentration waters from the South Atlantic [Veron et al. (1994); Weiss et al. (2003)]. With respect to North Atlantic surface waters, the presence of American lead has

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1979 1983 1987 1991 1995 1998

Year

Fig. 7 Lead concentrations in surface water near Bermuda, 1979—1997 showing samples analysed at the MIT and Cal Tech laboratories [Schaule and Patterson (1983); Veron et al. (1993); Wu and Boyle (1997)].

1979 1983 1987 1991 1995 1998

Year

Fig. 7 Lead concentrations in surface water near Bermuda, 1979—1997 showing samples analysed at the MIT and Cal Tech laboratories [Schaule and Patterson (1983); Veron et al. (1993); Wu and Boyle (1997)].

been detected over the entire north and central North Atlantic [Véron et al. (1994)] and the subtropical north-eastern Atlantic [Hamelin et al. (1997)]. In two more recent contributions, lead isotopes were further applied to elucidate the role of oceanic circulation on contaminant distribution in the South Atlantic [Alleman et al. (2001a); Alleman et al. (2001b)].

With the passage of time from the 1972 maximum, the lead tracer has been transported into intermediate and deep water given their greater ventilation ages, allowing stable lead isotopic compositions to be employed as tracers of abyssal mixing. For example, vertical concentration profiles in the sub arctic North Atlantic showed spatial gradients in the isotopic signature which are consistent with the thermohaline circulation pattern of the different water masses in that region and their discrete isotopic signatures [Veron et al. (1999)]. Figure 8 shows three distinct 206Pb/207Pb ratios at the southern location in the Faroe Bank Channel at the base of the Norwegian sea. The ratio (1.179) in the surface water is comparable to that of the aerosols (1.176 ± 0.004) collected at the relatively proximate site in Mace Head Ireland. The ratio (1.184-1.185) markedly increases in the subsurface (130-430 m) immediately below with the salinity maximum characteristics of the North Atlantic Drift (NAD) flowing into the region. The ratios (1.174-1.175) decrease with the freshening of deeper (750-810m) waters associated with the formation of Iceland-Scotland Overflow Waters (ISOW). Likewise, it was shown that advective transport of lead into the deep abyssal waters is facilitated through the formation of North Atlantic Deep Water [Alleman et al. (1999)].

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