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 Boyle and Lee (2010) Profiles of Pb content in seawaters near Bermuda from 1979 Monitor anthropogenic and natural inputs Use Pb isotopes as a regional water mass tracer Trace the migration of anthropogenic pollutions Aims of the study: Pb pmol/kg

Method developed Pre -concentration Purification

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

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

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

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

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

Surface water Pb isotopes and salinity along D357

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

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

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

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)

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

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

Pre-anthropogenic sources from ferromanganese crust