Natural lead isotope variations in the atmosphere

摘要

Stable lead (Pb) isotope data from pre-contamination peat sections has the potential to contribute to our understanding of earth system processes (e.g., atmospheric circulation, erosion, and volcanic activity) in the past. Key questions arise however. Can the Pb isotopes archived in peat records be used for assessing aerosol dynamics on a hemispheric scale or do they mainly reflect inputs from local soils? What natural Pb sources are important and do contributions vary over time? In order to answer these questions we have synthesized all available Pb isotope data from pre-contamination peat sections in Europe, North America, South America and Australia. We specifically examine the spatial and temporal variability of the Pb isotope records and identify regionally important trends and Pb sources. A pooling of all available pre-contamination peat data generated an average natural Pb-206/Pb-207 background ratio of 1.21 +/- 0.05 (2 sigma, n=300) (Pb-206/Pb-204 = 18.90 +/- 0.86, Pb-207/Pb-204 = 15.66 +/- 0.10 and Pb-208/ Pb-204 = 38.74 +/- 0.57, n = 207). The majority of the records showed limited temporal and compositional agreement, suggesting that the peat record receives mainly inputs from local (<10 km) and/or regional (10-500 km) sources. Three-isotope plots also support local and regional control and evidence a wide natural spread at some sites, particularly those located in radiogenic geological settings. A temporally synchronous isotope excursion to values between 1.16 and 1.18 at sites across Europe ca 4000-3000 B.C. was detected, however. While usually associated with anthropogenic sources. there are indeed natural aerosols having 206Pb/207Pb signatures as low as 1.16 as evidenced in several peat and ice core records globally. Three-isotope plots suggest that this unlikely to be a signal of mineral dust contributions, which tend to have Pb-206/Pb-207 ratios >= 1.19, but rather can be sourced to volcanic emissions. These results stress caution when using estimates of the upper continental crust to constrain natural Pb sources in, e.g., mixing equations. Considering the strong influence from local and regional sources on Pb-containing aerosols in the peat record, the assessment of aerosol dynamics at lower latitudes is likely best achieved using a compliment of archives.