Lead and stable Pb-isotope characteristics of tropical soils in north-eastern Brazil

Stable Pb-isotope ratios are widely used as tracers for Pb-sources in the environment. Recently, a few publications have challenged the predominating view of environmental applications of Pb-isotopes. Present applications of Pb-isotopic tracers in soils largely represent the northern hemisphere. This study focuses on tropical soils from Paraíba, north-eastern Brazil. Lead concentrations and Pb-isotopic signatures (both 7N HNO₃) were determined at 30 sites along a 327 km E–W-transect, from the Atlantic coast at João Pessoa to some kilometers west of Patos, to identify possible processes for the observed (and anticipated) distribution pattern. Thirty samples each of litter (ORG) and top mineral soil (TOP) were taken on pasture land at suitable distance from roads or other potential contamination sources. Lead-content was determined by inductively-coupled plasma atomic emission spectrometry (ICP-AES) and the ratios of ²⁰⁶Pb/²⁰⁷Pb, ²⁰⁶Pb/²⁰⁸Pb, and ²⁰⁸Pb/²⁰⁷Pb by ICP-sector field mass spectrometry (ICP-SFMS). Both sample materials show similarly low Pb-concentrations with a lower median in the ORG samples (ORG 3.4 mg kg⁻¹ versus TOP 6.9 mg kg⁻¹). The ²⁰⁶Pb/²⁰⁷Pb ratios revealed a large spread along the transect with median ²⁰⁶Pb/²⁰⁷Pb ratios of 1.160 (ORG) and 1.175 (TOP). The ²⁰⁶Pb/²⁰⁷Pb ratios differ noticeably between sample sites located in the Atlantic Forest biome along the coast and sample sites in the inland Caatinga biome. The “forest” sites were characterised by a significant lower median and a lower spread in the ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁶Pb/²⁰⁸Pb ratios compared to the Caatinga sites. Results indicate a very restricted influence of anthropogenic activities (individual sites only). The main process influencing the spatial variability of Pb-isotope ratios is supposed to be precipitation-dependent bioproductivity and weathering.