Lead isotopes in deep-sea coral skeletons: Ground-truthing and a first deglacial Southern Ocean record
Past changes in seawater lead (Pb) isotopes record the temporal evolution of anthropogenic pollution, continental weathering inputs, and ocean current transport. To advance our ability to reconstruct this signature, we present methodological developments that allow us to make precise and accurate Pb...
| Main Authors: | , , |
|---|---|
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
2017
|
| Subjects: | |
| Online Access: | https://discovery.ucl.ac.uk/id/eprint/10056780/1/Wilson_Lead%20isotopes.pdf https://discovery.ucl.ac.uk/id/eprint/10056780/ |
| Summary: | Past changes in seawater lead (Pb) isotopes record the temporal evolution of anthropogenic pollution, continental weathering inputs, and ocean current transport. To advance our ability to reconstruct this signature, we present methodological developments that allow us to make precise and accurate Pb isotope measurements on deep-sea coral aragonite, and apply our approach to generate the first Pb isotope record for the glacial to deglacial mid-depth Southern Ocean. Our refined methodology includes a two-step anion exchange chemistry procedure and measurement using a207Pb–204Pb double spike on a Thermo Finnigan Triton TIMS instrument. By employing a 1012 Ω resistor (in place of a 1011 Ω resistor) to measure the low-abundance204Pb ion beam, we improve the internal precision on206,207,208Pb/204Pb for a 2 ng load of NIST-SRM-981 Pb from typically ∼420 ppm to ∼230 ppm (2 s.e.), and the long term external reproducibility from ∼950 ppm to ∼550 ppm (2 s.d.). Furthermore, for a typical 500 mg coral sample with low Pb concentrations (∼6–10 ppb yielding ∼3–5 ng Pb for analysis), we obtain a comparable internal precision of ∼150–250 ppm for206,207,208Pb/204Pb, indicating a good sensitivity for tracing natural Pb sources to the oceans. Successful extraction of a seawater signal from deep-sea coral aragonite further relies on careful physical and chemical cleaning steps, which are necessary to remove anthropogenic Pb contaminants and obtain results that are consistent with ferromanganese crusts. Applying our approach to a collection of late glacial and deglacial corals (∼12–40 ka BP) from south of Tasmania at ∼1.4–1.7 km water depth, we generated the first intermediate water Pb isotope record from the Southern Ocean. That record reveals millennial timescale variability, controlled by binary mixing between two Pb sources, but no distinct glacial-interglacial Pb isotope shift. Mixing between natural endmembers is fully consistent with our data and points to a persistence of the same Pb sources through time, although we cannot ... |
|---|