5.6       Environmental Pb

 

Interest in the isotopic composition of Pb in environmental systems arose from attempts to date the age of the Earth by the Pb/Pb method. In order to determine a Bulk Earth composition for this dating work, Patterson investigated the composition of pelagic sediments, which were thought to provide an average composition of the whole crust. However, the analyses of pelagic sediments led to considerations about the distribution of Pb in the oceanic system.

 

            A primary necessity in attempting to understand the distribution of Pb in the oceans is the accurate measurement of the Pb concentration of seawater. However, the very low levels of Pb in seawater presented a considerable analytical challenge. The first problem was to find an analytical method with detection limits as low as one part per billion (ppb). The only method that can routinely achieve these kinds of detection limits is isotope dilution (section 2.4), which allows the measurement of Pb isotope composition at the same time. The second problem is anthropogenic contamination of the samples during analysis, referred to as ‘blank’ (section 2.1.4). This was to pose a particular problem, because almost all laboratory materials and equipment has higher Pb levels than those in seawater.

 

            The first workers to successfully overcome both of these problems and achieve accurate analysis of the Pb content of seawater were Tatsumoto and Patterson (1963). They went to extreme lengths to minimise Pb contamination during analysis, and demonstrated the effectiveness of these measures by using the same analytical procedure to analyse seawater samples of different sizes. Since the amount of contamination is determined by the procedure, the application of an identical procedure to samples of different sizes should give rise to a constant Pb blank, whereas the total amount of Pb detected is dependant on the sample size. Hence, the two quantities can be separated (Fig. 5.41). This procedure showed that the Pb content of seawater varied from 0.02 - 0.18 ppb (microgram/litre), whereas the analytical blank was about 50 ng (0.05 micrograms). Previous Pb determinations on seawater had been ten to fifty times higher (2 - 8 ppb), which must be attributed to analytical error.

Fig. 5.41. Plot of analytical Pb yield (in micrograms) against volume of seawater (in litres), allowing the Pb content of seawater to be determined. After Tatsumoto and Patterson (1963).

 

            The accurate measurement of Pb concentration from different water depths resulted in some surprising observations (e.g. Fig. 5.41). These showed Pb concentrations in surface ocean water to be nearly an order of magnitude higher than deep ocean water (2000 - 4000 m). This behaviour was the opposite to that observed for many natural tracers, but resembled the distribution of nuclear fallout in the oceans. Hence, Tatsumoto and Patterson (1963) argued that the principal input of Pb to the oceans at the present day is anthropogenic. Following this discovery, the investigation of Pb in the oceans soon turned to focus on the origins and distribution of anthropogenic Pb in different near-surface environments.

 

 

5.6.1 Anthropogenic Pb

 

The first use of Pb isotopes to trace the sources and distribution of anthropogenic Pb was made by Chow and Johnstone (1965). Based on an observation by Tatsumoto and Patterson (1963) that snow from Lassen Peak National Park had (relatively) very elevated Pb contents (1.6 parts per trillion), Chow and Johnstone made Pb isotope measurements on the snow for comparison with possible sources in Californian leaded gasoline. They found that the Pb isotope signature of Lassen snow was almost identical to that of atmospheric particulates recovered from their clean-lab filter in Pasadena (Los Angeles) and also fell within the range of Pb isotope compositions of local gasoline.  This was due to the practice of adding tetra-ethyl lead to gasoline to prevent engine pre-ignition.

 

            Chow (1970) followed up this work with a study of the world-wide compositional variations of leaded gasoline. He found large isotopic variations, and attributed them to the varying geological age of the Pb ores used for making tetra-ethyl lead in different countries. He then compared these Pb signatures with the isotopic compositions of locally collected pollutant leads, either from air filters or soil samples. The results were presented on a graph that has often been used since (with minor variations) to compare the signatures of pollutant leads (Fig. 5.42). The data revealed a very strong correlation between the Pb isotope composition of gasoline Pb and local pollutant Pb, conclusively demonstrating that gasoline additives were the principal source of pollutant lead in the environment.

Fig. 5.42. Correspondence between lead ore compositions ( ! ) and gasolines ( " ) from different countries on a Pb/Pb isotope plot. After Chow (1970).

 

            By the late 1960s, steps were under way to convert American cars to lead-free gasoline. Ironically, this was not to avoid poisoning the human population but to avoid poisoning catalytic converters which were being fitted to car exhaust systems to control pollution haze (Harrison and Laxen, 1981). As a result of this policy, the use of Pb in American gasoline peaked in 1970 and by 1990 had fallen to less than 5% of the peak level (Wu and Boyle, 1997). Therefore, after 1970, Pb isotope tracer studies were devoted to assessing the relative contributions of various different Pb pollution sources to the environment.

 

            In an early example of this kind of work, Chow and Earl (1972) showed that atmospheric Pb pollution derived from the combustion of coal could be distinguished from that of leaded gasoline by the more radiogenic Pb signature in coal. This is because Pb in the sedimentary system (where coal is deposited) is more radiogenic than Pb in basement rocks, which are the sources of most Pb ore deposits. In a later study, Sturges and Barrie (1987) showed that the isotopic composition of atmospheric Pb pollution from Canadian and American sources could be distinguished, allowing the tracing of cross-border air pollution.

 

            Some more recent studies have used the distinct Pb isotope signatures of North American and European Pb pollution to trace their relative contributions to the contamination of environmental systems far from their sources. For example, Rosman et al. (1993) measured the Pb isotope composition of Greenland snows between 1968 and 1988, and found relatively large variations during this period (Fig. 5.43). Since European leaded petrol generally had lower 206Pb/207Pb ratios than American leaded gasoline, a decrease in the 206Pb/207Pb ratio of Greenland snow (accompanied by a seven fold decrease in overall Pb concentration) was attributed to the earlier phase-out of leaded gasoline in North America.

Fig. 5.43. Changing 206Pb/207Pb ratio in a twenty year section of Greenland snow, attributed to the phase-out of American leaded gasoline. After Rosman et al. (1993).

 

            A final example demonstrates the use of Pb isotopes to trace North American and European anthropogenic contributions to the Pb inventory of North Atlantic surface water (Fig. 5.44). In this study, American Pb sources were found to dominate the Pb isotope composition of North Atlantic surface waters off the North African coast in 1990-1992, despite the earlier phase-out of American leaded gasoline. This Pb was carried across the Atlantic, via the Sargasso Sea, by eastward moving surface water currents. Hence, this study demonstrates that the signature of leaded gasoline will remain in environmental systems for many years to come, as anthropogenic Pb changes from a deadly health hazard to a useful marker of 1970s-age components in hydrological and sedimentary systems.

Fig. 5.44. Mixing of anthropogenic Pb components in the North Atlantic, shown on a Pb/Pb isotope diagram. ( " ) = aerosols carried from Europe and North America; ( ! ) = North African surface waters; ( Î )  = uncontaminated deep-sea sediments. After Hamelin et al. (1997).

 

 

5.6.2 Pb as an oceanographic tracer

 

The extent of anthropogenic Pb contamination of ocean water is so great that Pb isotope measurements of ocean water itself cannot give information about natural Pb circulation. Therefore, studies of Pb as a natural oceanic tracer must be based on inventories of past oceanic Pb, recorded in ferromanganese nodules and pelagic sediments, as well as on the behaviour of 210Pb, a short-lived isotope in the U-series decay chain (section 12.1).

 

            The use of Pb isotope analysis in oceanography was pioneered by the studies of Chow and Patterson (1959) on manganese nodules, and Chow and Patterson (1962) on pelagic sediments. These studies revealed a general distinction between the Pb isotope signatures of Atlantic and Pacific samples, but within each ocean basin, manganese nodules and pelagic sediments gave relatively consistent results. Based on these observations, Chow and Patterson concluded that Pb had a relatively short residence time in seawater, and that the distinct Pacific and Atlantic Ocean signatures reflected the average Pb isotope composition of the continents surrounding each ocean basin.

 

            Based on these inter-oceanic variations, Chow and Patterson estimated a seawater residence time for dissolved Pb of ca. 10 kyr. However, Craig et al. (1973) showed that 210Pb can be used to determine a much more accurate Pb residence time, based on comparison with the relatively long-lived isotope 226Ra, which acts as the parent of 210Pb in seawater. They showed that 210Pb was severely depleted relative to 226Ra in deep ocean water, and hence that Pb must be very rapidly scavenged from seawater by adsorption onto particulate matter. Hence, based on the degree of 210Pb depletion in a vertical section through the North Pacific off Guadalupe, Craig et al. calculated a deep water Pb residence time of only 50 yr (Figure 5.45). This figure was confirmed as the average oceanic Pb residence time by a recent compilation of oceanic 210Pb data, coupled with a general ocean circulation model (Henderson and Maier-Reimer, 2002). However, variation in residence time of up to an order of magnitude were observed between areas of high and low biological productivity.

Fig. 5.45. Profiles of 210Pb and 226Ra activity against depth in the central Pacific Ocean, showing good agreement between measured 210Pb activity ( ! ) and a water column model with a Pb residence time of 54 yr. After Craig et al. (1973).

 

            In the 1970s, interest in environmental Pb isotope analysis switched from studies of natural to anthropogenic Pb, and relatively few studies were made of Pb isotopes as oceanic tracers until the 1990s. However, a study by Reynolds and Dasch (1971) led to a better understanding of the sources of dissolved oceanic Pb. Reynolds and Dasch were able to obtain more accurate Pb isotope data than Chow and Patterson because of advances in mass spectrometry, including the use of a double spiking technique to correct for instrumental mass fractionation (section 2.4.2). They showed that Mn nodules from the Atlantic Ocean had Pb signatures consistent with a source from continental erosion, whereas Mn nodules from the Pacific Ocean appeared to contain a mixture of continental Pb and a component of Pb from submarine volcanic activity.

 

            The importance of submarine hydrothermal activity as a source of Pb in the oceans was confirmed by studies of metal-rich sediments near the East Pacific Rise (Dasch et al., 1971). These sediments have very large Pb contents (ca. 200 ppm) and Pb isotope signatures that overlap the composition of Pacific MORB. On the other hand, Mn nodules not in the immediate vicinity of the ocean ridge were shown to have more radiogenic Pb signatures, indicative of mixing with continental sources (Reynolds and Dasch, 1971). This interplay between continental and hydrothermal Pb fluxes as sources for oceanic Pb was confirmed by O’Nions et al. (1978). They argued that because the oceanic residence of Pb is so short, the isotopic composition of seawater Pb at any one point is essentially a dynamic equilibrium between these competing fluxes.

 

            Despite its very short oceanic residence time, the behaviour of oceanic Pb somewhat resembles Sr (section 3.6.2) in resulting from competing continental and hydrothermal fluxes. The importance of ocean floor hydrothermal activity as a source of Pb can be attributed to the solubility of Pb as high temperature chloride complexes, despite its strong adsorption into particulate matter at low temperatures. In contrast, the solubility of Nd is low at both high and low temperatures. Therefore Nd is not mobilised by sea-floor hydrothermal activity, and oceanic Nd is derived entirely by mixing of riverine and wind-blown components (section 4.5.1).

 

            Much oceanographic evidence shows that Pacific and Atlantic water masses communicate via the circum-polar (Antarctic) Ocean (e.g. section 14.1.7). Therefore, a good understanding of present day oceanic Pb can be obtained from circum-polar Mn nodules.  These were studied by Abouchami and Goldstein (1995), who found evidence for major mixing of Pb between different water masses, as shown on a Pb–Pb isotope diagram (Fig. 5.46). In this plot, the composition of East Pacific Rise metalliferous sediments overlaps the MORB field, while the Pb array in Mn nodules spans the range from the metalliferous sediments to radiogenic crustal sources.

Fig. 5.46. Thorogenic–uranogenic Pb isotope diagram showing circum-polar Mn nodules  ( ! ) resulting from mixing of Antarctic water with Pacific and Atlantic-Indian ocean water. Pacific MORB and metalliferous sediments ( " ) are shown for reference. After Abouchami and Goldstein (1995).

 

            The mixing relationships in Fig. 5.46 can be seen in a dynamic fashion by plotting the Pb isotope ratio of circum-polar Mn nodules against longitude (Fig. 5.47). This plot reveals two principal trends, involving progressive reduction in the Pb isotope ratio of Antarctic water across the south Pacific, and a progressive increase across the southern Atlantic—Indian oceans. These variations occur in response to mixing of these water masses with the eastward-moving circumpolar current, which has a circulation time of ca. 30 yr.

 

            The Atlantic—Indian Pb trend in Fig. 5.47 is correlated with e Nd, which Abouchami and Goldstein attributed to the southerly current that carries North Atlantic Deep Water (NADW) into the Antarctic Ocean. Here, NADW mixes with the Antarctic Bottom Water in which the Mn nodules grow. In contrast, the Pacific Pb trend does not correlate with e Nd, which Abouchami and Goldstein attributed to many isolated mixing events between Pacific and Antarctic water as the circumpolar current moves across the southernmost Pacific. However, when this circum-polar water (CPW) reaches the Drake Passage between South America and Antarctica, the circumpolar current carries this water, with its distinct isotopic signature, half way across the south Atlantic, until it suddenly meets the southward moving NADW. A similar sudden transition occurs SE of Australia, where Pacific water first meets circum-polar water in the Tasman Straits (Fig. 5.47).

Fig. 5.47. Plot of Pb isotope ratio against longitude, showing the changing composition of circum- polar water due to mixing with water masses of the Pacific Ocean ( ! ) and Atlantic-Indian Ocean ( " ). Modified after Abouchami and Goldstein (1995).

 

            Within the Atlantic, Indian and Pacific oceans, the Pb isotope compositions of Mn nodules are more homogeneous than the circumpolar ocean (von Blanckenburg et al., 1996). However, recent work has revealed isotopic provinciality in the Mn nodules from these oceans, with complex mixing relationships between Pb from different water bodies. For example, the  southern Indian Ocean is influenced by NADW and CPW, as discussed above, but the northern Indian Ocean carries a Pb signature from Himalayan erosion (Vlastelic et al., 2001). Similarly, the south-central Pacific is dominated by hydrothermal input from the East Pacific Rise, but the north-central Pacific carries a Pb component derived from wind-blown sediment, largely composed of Chinese loess (Jones et al., 2000). Finally, the marginal north Pacific bears Pb signatures from the erosion of circum-Pacific volcanic arcs.

 

 

5.6.3 Paleo-seawater Pb

 

Because of their slow growth over millions of years, ferromanganese nodules and crusts preserve a record of past variations of seawater Pb isotope composition, as well as geographical variations at the present time. The reliable dating of such material, which is critical in order to make accurate paleo-oceanographic reconstructions, is discussed in section 4.5.3.

 

            In view of the very short residence time of Pb in the oceans, and the multitude of Pb sources discussed above, it might be expected that the records of past oceanic Pb isotope composition carried in ferromanganese crusts would show rapid changes. Rapid changes were indeed observed in the Northwest Atlantic over the past 2 Myr (Fig. 5.48). These were originally attributed to the closure of the Panama gateway  (Burton et al., 1997). However, the observation of similar changes in the Arctic Ocean suggested that the changes were probably due to the input of very radiogenic Pb from the Canadian and Greenland shields as a result of intensified glacial erosion beginning about 3 Myr ago (von Blanckenburg and O’Nions, 1999).

 

            In contrast to these rapid changes, the earlier evolution of North Atlantic Pb was somewhat less variable, while the central Pacific has maintained a practically constant Pb isotope composition over the past 30 Myr (Ling et al., 1977). On the other hand, the Northern Indian Ocean has demonstrated moderately large long-term Pb isotope variations over the past 25 Myr, attributed by Frank and O’Nions (1998) to the effects of Himalayan uplift and erosion. The sensitivity of Pb isotopes to these regional oceanographic processes, coupled with the power of MC-ICP-MS for producing large numbers of accurate measurements (section 2.5.4), means that Pb isotopes promise to be an excellent tracer for detailed paleo-oceanography.

Fig. 5.48. Pb isotope variations in ferromanganese crusts from the major oceans over the past 30 Myr. After Frank and O’Nions (1998).

 

 

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