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Lead isotopes in South Atlantic seawater from the UK GEOTRACES transect along 40°S

Lead isotopes in South Atlantic seawater from the UK GEOTRACES transect along 40°S. Maxence Paul Mark Rehkämper , Tina van de Flierdt, Dominik Weiss. Generalities and aims. Short resistance time : ~ 50 years Strongly influenced by anthropogenic activities. Pb pmol /kg.

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Lead isotopes in South Atlantic seawater from the UK GEOTRACES transect along 40°S

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  1. Lead isotopes in South Atlantic seawater from the UK GEOTRACES transect along 40°S Maxence Paul Mark Rehkämper, Tina van de Flierdt, Dominik Weiss

  2. Generalities and aims • Short resistance time : ~50 years • Strongly influenced by • anthropogenic activities Pbpmol/kg Aims of the study: Monitor anthropogenic and natural inputs Trace the migration of anthropogenic pollutions Use Pb isotopes as a regional water mass tracer Profiles of Pb content in seawaters near Bermuda from 1979 Boyle and Lee (2010)

  3. Method developed Purification Pre -concentration

  4. Method developed • Sensitivity/ Stability • Limited fractionation • No matrix/memory effect Adapted for small samples But... • 3 analysis are needed • time consuming method

  5. Estimation of the external Reproducibility on 2 ng standards • internal reproducibility: ~ 300 ppm • external reproducibility ~ 800 ppm

  6. Estimation of the external reproducibility on seawater samples • 3 different seawater samples analyzed • worst external reproducibility: • ~ 1700 ppm for 206Pb/204Pb and 750 ppm for 206Pb/207Pb - remaining matrix affects only the internal reproducibility

  7. Results Pb concentrations: surface samples (D357) - Lead concentration range between 10 and 30 pmol/kg - Coastal samples are more concentrated • Concentrations decrease with the distance to the coast

  8. Pb isotopic compositions: surface samples D357 - Coastal samples are characterized by high 208Pb/207Pb ratios Coastal - Samples off coast (purple symbols) show significantly lower isotopic ratios Indian ocean Lee et al. 2012  Samples are localized on a preferential track of Agulhas rings

  9. Surface water Pb isotopes and salinity along D357

  10. First completed depth profile: station 3 - Lead concentrations decrease drastically with the depth - Deep waters are characterized by higher isotopic compositions

  11. Depth profile at station 3 and salinity Salinity (psu) NADW Lead isotopes appear to faithfully reflect different water masses

  12. Defining the sources of lead in surface waters Difficult to clearly define the sources of lead Seawaters Limited amount of data, some sources have changed Consider sorting effect and incongruent dissolution of dust

  13. Conclusions • The lead isotopic composition and concentration of South Atlantic surface waters shows clearly resolvable differences from near shore to offshore • Different water masses show different lead isotopic compositions (i.e., first analysed depth profile and Agulhas Rings) • No clear identification of continental/aerosol sources yet (limited variability in seawater values compared to the currently identified sources)

  14. Pre anthropogenic signature recorded in ferromanganese crust • For South Africa the 206Pb/204Pb pre- anthropogenic signature should be around 18.83 • The anthropogenic signature seems to be still present in deep seawaters Anthropogenic inputs

  15. Witt et al. 2006 Lead isotopes in aerosols Large differences between fine and coarse fraction

  16. Pre-anthropogenic sources from ferromanganese crust

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