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Lead isotopes in deep-sea coral skeletons: ground-truthing and a first deglacial Southern Ocean record

Wilson, David J. and van de Flierdt, Tina and Adkins, Jess F. (2017) Lead isotopes in deep-sea coral skeletons: ground-truthing and a first deglacial Southern Ocean record. Geochimica et Cosmochimica Acta, 204 . pp. 350-374. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20170213-095417473

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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 ^(207)Pb-^(204)Pb double spike on a ThermoFinnigan Triton TIMS instrument. By employing a 10^(12) Ω resistor (in place of a 10^(11) Ω resistor) to measure the low-abundance ^(204)Pb ion beam, we improve the internal precision on ^(206,207,208)Pb/^(204)Pb for a 2 ng load of NIST-SRM-981 Pb from typically ∼420 ppm to ∼260 ppm (2 s.e.), and the long term external reproducibility from ∼960 ppm to ∼580 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 for ^(206,207,208)Pb/^(204)Pb, 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 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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/j.gca.2017.01.052DOIArticle
http://www.sciencedirect.com/science/article/pii/S0016703717300777PublisherArticle
Additional Information:© 2017 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Received 21 August 2016, Revised 24 January 2017, Accepted 31 January 2017, Available online 9 February 2017. This study was supported by Leverhulme Trust grant RPG-398 to TvdF, and NERC grant NE/N001141/1 to TvdF and DJW. We are very grateful to Sophia Hines for assistance with sampling from the Caltech coral collection and for providing published and unpublished U-Th ages; Luke Bridgestock for providing some of the NIST-SRM-981 standard data measured on the 10^(11) Ω resistor; Kirsty Crocket for supplying the coral powder used to prepare the in-house coral Pb standard; and Maxence Paul for establishing the double spike TIMS method at Imperial and for his initial efforts during installation of the 10^(12) Ω resistor. Katharina Kreissig and Barry Coles provided invaluable maintenance of the MAGIC clean labs and mass spectrometers. We further appreciate careful and thoughtful reviews from Wafa Abouchami, Ed Boyle and Marcus Gutjahr, and editorial handling by Claudine Stirling.
Funders:
Funding AgencyGrant Number
Leverhulme TrustRPG-398
Natural Environment Research Council (NERC)NE/N001141/1
Subject Keywords:Southern Ocean; Tasmania; Deglaciation; Climate change; Ocean circulation; Weathering; Isotope tracers; Pb isotopes; Deep-sea corals
Record Number:CaltechAUTHORS:20170213-095417473
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20170213-095417473
Official Citation:David J. Wilson, Tina van de Flierdt, Jess F. Adkins, Lead isotopes in deep-sea coral skeletons: Ground-truthing and a first deglacial Southern Ocean record, Geochimica et Cosmochimica Acta, Volume 204, 1 May 2017, Pages 350-374, ISSN 0016-7037, http://doi.org/10.1016/j.gca.2017.01.052
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:74232
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:13 Feb 2017 20:06
Last Modified:18 Apr 2017 23:04

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