Abstract
A new method for the determination of seawater Pb isotope compositions and concentrations was developed, which combines and optimizes previously published protocols for the separation and isotopic analysis of this element. For isotopic analysis, the procedure involves initial separation of Pb from 1 to 2L of seawater by co-precipitation with Mg hydroxide and further purification by a two stage anion exchange procedure. The Pb isotope measurements are subsequently carried out by thermal ionization mass spectrometry using a 207Pb–204Pb double spike for correction of instrumental mass fractionation. These methods are associated with a total procedural Pb blank of 28±21pg (1sd) and typical Pb recoveries of 40–60%. The Pb concentrations are determined by isotope dilution (ID) on 50mL of seawater, using a simplified version of above methods. Analyses of multiple aliquots of six seawater samples yield a reproducibility of about ±1 to ±10% (1sd) for Pb concentrations of between 7 and 50pmol/kg, where precision was primarily limited by the uncertainty of the blank correction (12±4pg; 1sd). For the Pb isotope analyses, typical reproducibilities (±2sd) of 700–1500ppm and 1000–2000ppm were achieved for 207Pb/206Pb, 208Pb/206Pb and 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, respectively. These results are superior to literature data that were obtained using plasma source mass spectrometry and they are at least a factor of five more precise for ratios involving the minor 204Pb isotope. Both Pb concentration and isotope data, furthermore, show good agreement with published results for two seawater intercomparison samples of the GEOTRACES program. Finally, the new methods were applied to a seawater depth profile from the eastern South Atlantic. Both Pb contents and isotope compositions display a smooth evolution with depth, and no obvious outliers. Compared to previous Pb isotope data for seawater, the 206Pb/204Pb ratios are well correlated with 207Pb/206Pb, underlining the significant improvement achieved in the measurement of the minor 204Pb isotope.
Highlights
The global scale impact of human activities on natural Pb budgets was established more than 40 years ago by Chow and co-workers [2,3,4,5] and massive inputs of anthropogenic Pb into the oceans were soon thereafter identified by Patterson and coworkers [6,7,8], based on the first accurate and precise Pb concentration measurements for seawater
Lead concentrations were determined by isotope dilution (ID), and using a chemical separation method similar to that employed for the isotope ratio measurements but optimized for the processing of smaller seawater samples
The total blank for the Pb isotope measurements was determined by carrying out the complete analytical procedure without an initial seawater sample being present
Summary
Investigations that map the distribution of trace metals and their isotopes in seawater and which study their relationship to and potential impacts on global biogeochemical cycles in the oceans are an overarching goal of modern marine geochemistry and a key objective of the international GEOTRACES program [1]. The most precise Pb isotope analyses of seawater apply either single collector high resolution or multiple collector ICPMS (HR-ICP-MS and MC-ICP-MS, respectively), most commonly following low-blank pre-concentration of the element by coprecipitation with Mg(OH)2 [14,15]. These methodological advances are a consequence of the particular importance of seawater Pb isotope data to studies of environmental pollution and marine geochemistry. Very scarce seawater data are available for the latter ratios due to the analytical challenges but measurements of non-radiogenic 204Pb are deemed to be important to fully exploit the potential of the Pb isotope system
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