Abstract
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 a 207Pb–204Pb double spike on a Thermo Finnigan Triton TIMS instrument. By employing a 1012Ω resistor (in place of a 1011Ω resistor) to measure the low-abundance 204Pb ion beam, we improve the internal precision on 206,207,208Pb/204Pb for a 2ng load of NIST-SRM-981 Pb from typically ∼420ppm to ∼230ppm (2 s.e.), and the long term external reproducibility from ∼950ppm to ∼550ppm (2 s.d.). Furthermore, for a typical 500mg coral sample with low Pb concentrations (∼6–10ppb yielding ∼3–5ngPb for analysis), we obtain a comparable internal precision of ∼150–250ppm for 206,207,208Pb/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 (∼12–40kaBP) from south of Tasmania at ∼1.4–1.7km 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 rule out a minor influence from recent anthropogenic Pb. Whereas neodymium (Nd) isotopes in the Southern Ocean respond to global ocean circulation changes between glacial and interglacial periods, Pb isotopes record more localised mixing within the Antarctic Circumpolar Current, potentially further modulated by climate through changing terrestrial inputs from southern Africa or Australia. Such decoupling between Pb and Nd isotopes in the Southern Ocean highlights their potential to provide complementary insights into past oceanographic variability.Keywords: Southern Ocean; Tasmania; Deglaciation; Climate change; Ocean circulation; Weathering; Isotope tracers; Pb isotopes; Deep-sea corals
Highlights
Studies on ferromanganese crusts have explored such scenarios on million-year timescales, including the Neogene decoupling between Nd and Pb isotopes in the Indian Ocean linked to evolving Himalayan Pb inputs (Frank and O’Nions, 1998), and the decoupling of Nd and Hf isotopes in the North Atlantic attributed to changing erosional and weathering regimes (Piotrowski et al, 2000; van de Flierdt et al, 2002)
The least radiogenic coral sample clearly lies outside the field of all Southern Ocean ferromanganese nodule data (Fig. 9a,b), and a couple of samples are outside the limits of those data when considering 207Pb/204Pb at a given 206Pb/204Pb. Based on their isotopic compositions, we cannot rule out that the deglacial corals with the least radiogenic compositions may be seeing an anthropogenic Pb contaminant that has not been fully removed in these cases (Fig. 8), but we argue against this possibility for three reasons
We suggest that the simplest interpretation would involve an increase in lateral transport of the radiogenic Pb isotope signature carried from the Atlantic sector and/or Agulhas Basin via Kerguelen Plateau (Abouchami and Goldstein, 1995; Vlastelic et al, 2001), thereby reducing the relative contribution of unradiogenic Australian Pb inputs at these times
Summary
The oceanic distribution of radiogenic isotope tracers, such as neodymium (Nd), lead (Pb) and hafnium (Hf), provides valuable evidence on continental weathering sources and ocean circulation in the present and past oceans (see reviews by Frank, 2002; Goldstein and Hemming, 2003). Given differences between these elements in their continental weathering behaviour and/or oceanic residence times, there is significant potential for a multi-isotope approach to provide better constraints on those processes. Considering the short oceanic residence times of these elements and the timescales of ocean mixing, higher resolution reconstructions at hundred-year to thousandyear timescales would be valuable for resolving glacialinterglacial to sub-millennial changes and fully exploiting such a multi-isotope approach
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