Stable lead isotopes and lake sediments—a useful combination for the study of atmospheric lead pollution history
Introduction
The lead found in lake sediments is derived from various sources. Naturally, lead is transported to the sediments following weathering of catchment soils and bedrock or transported more directly within mineral matter eroded from the catchment. Airborne lead is another source. Prior to the advent of lead pollution, atmospheric deposition contributed an insignificant fraction of the lead accumulated in lake sediments relative to the supply from the catchment (Bindler et al., 2001). However, since ancient times, lead derived from a variety of human activities has been transported atmospherically and deposited on lakes and their catchments. In modern times, this pollution lead accounts for approximately 95% of the global lead cycle (Settle and Patterson, 1980).
Lead consists of four stable isotopes, 204[Pb], 206[Pb], 207[Pb] and 208[Pb], and the mixture of these isotopes varies in different regions of the earth for geological reasons. The isotopic composition of lead ores is usually different from that of bedrock (Brown, 1962, Faure, 1986), which makes it possible to infer the provenance of pollution lead in the environment based on the lead isotope composition. Analysis of stable lead isotopes, with the 206[Pb]/207[Pb] ratio as the most used isotope signature, has become a useful method in environmental studies to trace lead from different sources. Recent examples include the study of aerosols (Deboudt et al., 1999, Chiaradia and Cupelin, 2000), ice and snow (Rosman et al., 1997, Simonetti et al., 2000), peat bogs (MacKenzie et al., 1998, Weiss et al., 1999), sediments (Brännvall et al., 1999, Brännvall et al., 2001a, Monna et al., 1999) and soils (Bindler et al., 1999, Prohaska et al., 2000).
The ICP-MS technique makes it possible to measure lead isotopes at a relatively low cost. Over the last several years, we have analysed more than 1500 samples of lake sediment, peat and soil in a study of the atmospheric lead pollution history in Sweden. This study included samples from 31 lakes, three ombrotrophic peat bogs and several soil profiles (Renberg et al., 1994, Renberg et al., 2000, Brännvall et al., 1997, Brännvall et al., 1999, Brännvall et al., 2001a, Brännvall et al., 2001b, Bindler et al., 1999, Bindler et al., 2001). Based on experiences from these investigations, we address:
- i
The choice of sample preparation method depending on the objective of the study. Rather than choosing a method that dissolves all mineral grains, we have used a strong acid leaching that does not dissolve the most resistant mineral fractions. Here, we present a comparison of dissolution methods and demonstrate that the strong leaching method produces repeatable and consistent results.
- ii
The background 206[Pb]/207[Pb] ratio in Swedish lake sediments and how and why this has changed over time. Surface soils and recent lake sediments in Sweden have a low 206[Pb]/207[Pb] ratio, only slightly lower than 1.2, which is considered to be the mean of the earth crust. A compilation of data from lake sediments older than 3000 years and of time trends from dated lake sediment cores demonstrate that natural background 206[Pb]/207[Pb] ratios in Sweden were considerably higher, and it highlights the fact that natural background lead signatures cannot be found in the contemporary environment due to long-term accumulation of airborne lead pollution.
- iii
How concentrations of pollution and natural lead can be calculated using a combination of isotope and concentration analyses. Detailed data from lead isotope and lead concentration analyses from lakes with annually-laminated sediments are used to demonstrate how powerful this combination of analyses is for distinguishing pollution lead from natural lead derived from the catchment. The data are also used to discuss limitations with this approach.
- iv
Finally, we use some selected results to briefly describe the atmospheric lead pollution history in Sweden.
Section snippets
The choice of sample preparation method for the analyses
In environmental studies, a variety of methods have been applied for preparing samples for lead analyses (Beck and Sneddon, 2000). Frequently applied are either a strong acid digestion, which dissolves all but the most resistant silicate minerals, or a total dissolution using strong acids and HF. In sediments with only a small fraction of mineral grains, the difference in the 206[Pb]/207[Pb] ratio is minute between a strong extraction and a total dissolution. In sediments with a larger mineral
The background 206[Pb]/207[Pb] ratio and changes in the ratio over time
Analyses of the lead composition of the upper continental crust and marine sediments from various regions of the globe suggest an average 206[Pb]/207[Pb] ratio of approximately 1.2 (Chow and Patterson, 1962, Zartman and Doe, 1981, Hamelin et al., 1990), although there is some regional variation. For example, higher ratios are often shown for sediments derived from regions underlain by pre-Cambrian provinces (Chow, 1965). Lead ores also largely deviate from the global mean, having 206[Pb]/207
To differentiate between pollution lead and natural lead using isotopes
When the sources of lead are known and the lead isotope signatures of the sources are well-characterised, it is possible to estimate the relative contribution of each source to the lead composition of a sample by using an isotope mixing model. In its simplest form, the model is written as a binary mixing model [Eq. (1)], where the lead composition of a given sample has only two lead sources: natural lead, derived from soils and the parent rock material, and pollution lead.
A brief summary of the pollution history
Despite the limitations described above, which can make results from individual sites uncertain, a data set of many lakes gives a reliable picture. Fig. 5 shows how pollution lead concentrations have changed in radiocarbon-dated sediment cores from eleven lakes. Pollution lead concentrations are plotted on a logarithmic scale because of the large spatial gradient over Sweden with higher pollution levels in southern Sweden and lower values to the north. This geographic pattern is the result of
Acknowledgements
This work was funded by the Swedish Natural Science Research Council and the Swedish Environment Protection Agency.
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