Natural changes in atmospheric deposition

Only a small amount of lead is found in the atmosphere 1010 g) but this reservoir is the major transport pathway of lead within the environment and consequently atmospheric circulation patterns are of prime importance to understand atmospheric deposition. Natural particles of lead are generally associated with larger particle size, rapid deposition rates and short atmospheric residence time. Primary natural lead sources include windblown dust, volcanic emission and biogenic particulates. Estimated total emission volumes range between 0.9and23.5 x 109 gyr-1 [Nriagu (1989a)] (see Table 2).

The present day atmosphere is dominated by anthropogenic lead, and information regarding the pattern, controls and characteristics of natural atmospheric deposition can only be derived from geochemical archives including ice cores, peats and sediments [Boutron (1995); Last and Smol (2001); Mackay et al. (2004); Shotyk (1996)]. The longest records of atmospheric deposition have been derived form ice cores giving fascinating insights into the glacial-interglacial variations in lead concentrations and lead isotopes, mineral dust fluxes and volcanogenic lead to ice cores.

Table 2 Estimated natural global annual lead emission (in 106 kg y_1) (taken from [Nriagu (1989a)]). The biogenic estimate includes terrestrial and marine sources.

Table 2 Estimated natural global annual lead emission (in 106 kg y_1) (taken from [Nriagu (1989a)]). The biogenic estimate includes terrestrial and marine sources.

Windblown soil

0.3

7.5

Volcanic emission

0.5

-6.0

Wild forest fires

0.1

3.8

Biogenic particulates

0.0

3.3

Sea Salt spray

0.0

2.8

Total

0.9-

-23.5

3.4. Glacial-interglacial variations in lead concentrations and lead isotopes

The Vostok core from Antarctica assessed atmospheric lead deposition from 65 000 to 240000 yrs BP reaching back to the beginning of the penultimate ice age and the preceding interglacial (isotope stage 7.5) [Hong et al. (2003)]. Lead concentrations were highly variable with low values during warm climatic stages and much higher values during cold stages, especially during isotope stage 4.2 and 6.2 to 6.6. Additional work reported from the EPICA Dome C ice core dating back to 220kyrBP showed that also lead isotopic compositions in Antarctic ice varied with changing climate [Vallelonga et al. (2005)]. Figure 4 shows a 206Pb/207Pb ratios decrease during glacial periods, with the lowest values occurring during colder climatic periods (stages 2, 4 and 6) and the Holocene. Low lead concentrations were found during the Holocene and the last interglacial (climate stage 5.5) while higher lead concentrations were found during cold climatic periods.

3.5. Mineral dust fluxes and volcanogenic lead to ice cores

The Vostok core suggests that virtually 100% of natural lead deposition during cold climate can be accounted for using soil dust and rock, while contributions from volcanoes might have been significant during warm stages [Hong et al. (2003)]. The EPICA Dome C core shows also a dominance of soil and rock derived lead for the pre-industrial period [Vallelonga et al. (2005)]. This source apportionment, however, contrasts with findings from other Antarctic locations. Matsumoto and Hinkley [2001] suggested that the deposition rate in the pre-industrial ice from coastal west Antarctica

100 150

Fig. 4 A 220-kyr record of lead concentration and 206Pb/207Pb from the EPICA Dome C ice core. Marine isotope stage numbers are also shown (taken from [Vallelonga et al. (2005)]).

100 150

Fig. 4 A 220-kyr record of lead concentration and 206Pb/207Pb from the EPICA Dome C ice core. Marine isotope stage numbers are also shown (taken from [Vallelonga et al. (2005)]).

was approximately matched by the output rate to the atmosphere by quiescent (non-explosive) degassing of volcanoes worldwide. This conclusion is supported by the isotopic compositions of lead, which are similar to those of a suite of ocean island volcanoes, mostly in the Southern Hemisphere [Matsumoto and Hinkley (2001)].

For Greenland, an ice record from the GRIP core covers the time period between 8250 to 149 100 and it is suggested that lead in the Northern Hemisphere was derived mainly from soil dust during both glacial and interglacial periods [Hong et al. (1996)].

Peat bogs are the other terrestrial archive that testifies only atmospheric deposition and work on long term records in Switzerland [Shotyk et al. (1998)], Sweden [Klaminder et al. (2003)] and Spain [Kylander et al. (2005)] showed a similar control of climate on the lead cycling, for example, the Younger Dryas and Saharan Drying increased atmospheric lead fluxes during the Holocene.

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