Soil organic carbon isotopic discrimination

Soil samples were air dried and prepared for lead isotope analysis using 50-100 mg of the <0.2-mm size fraction. Sulfides, Fe-hydroxides, and other secondary minerals in the Penobscot Formation can be used to monitor the composition of labile Pb and provide the means to discriminate labile (anthropogenic) lead from lead inherited from the parent rocks and sulfides (e.g., Ayuso and Foley, 2008). A cool mild acid leach (1,5N HC1 + 3N HNO3) was used to attack the secondary minerals. This solution likely reflects the labile Pb (e.g., Erel et al., 1997) captured in the Fe-hydroxide, carbonate, or organic materials, or other secondary minerals (clays). These minerals can contain lead, arsenic, and other elements derived from outside of the watershed. Mixed solutions of HF—HN03 were used for final dissolution of the residual fractions. Table 12.2 summarizes the Pb-isotopic data for the leach fractions of the soil horizons (together with Pb and As contents) Table 12.3 shows equivalent data for the residues. 

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