Radiogenic Isotopes As Paleoceanographic Tracers in Deep-Sea Corals: Advances in TIMS Measurements of Pb Isotopes and Application to Southern Ocean Corals

Abstract:

Deep-sea corals have emerged as a valuable archive of deep ocean paleoceanographic change, with uranium-series dating providing absolute ages and the potential for centennial resolution. In combination with measurements of radiocarbon, neodymium isotopes and clumped isotopes, this archive has recently been exploited to reconstruct changes in ventilation, water mass sourcing and temperature in relation to millennial climate change.

Lead (Pb) isotopes in both corals and seawater have also been used to track anthropogenic inputs through space and time and to trace transport pathways within the oceans. Better understanding of the oceanic Pb cycle is emerging from the GEOTRACES programme. However, while Pb isotopes have been widely used in environmental studies, their full potential as a (pre-anthropogenic) paleoceanographic tracer remains to be exploited. In deep-sea corals, challenges exist from low Pb concentrations in aragonite in comparison to secondary coatings, the potential for contamination, and the efficient elemental separation required for measurement by thermal ionisation mass spectrometry (TIMS).

Here we discuss progress in measuring Pb isotopes in coral aragonite using a ²⁰⁷Pb-²⁰⁴Pb double spike on a ThermoFinnigan Triton TIMS. For a 2 ng NIST-981 Pb standard, the long term reproducibility (using 10¹¹ Ω resistors) is ~1000 ppm (2 s.d.) on ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios. We now show that using a new 10¹² Ω resistor to measure the small ²⁰⁴Pb beam improves the internal precision on these ratios from ~500 ppm (2 s.e.) to ~250 ppm (2 s.e.) and we envisage a potential improvement in the long term reproducibility as a consequence. We further assess the internal precision and external reproducibility of our method using a BCR-2 rock standard and an in-house coral standard. Preliminary evidence on the application of this method to natural samples is derived from cleaning experiments and replication tests on deep-sea corals from the Southern Ocean.