Elsevier

Chemical Geology

Volume 355, 26 September 2013, Pages 69-87
Chemical Geology

Structural limitations in deriving accurate U-series ages from calcitic cold-water corals contrast with robust coral radiocarbon and Mg/Ca systematics

https://doi.org/10.1016/j.chemgeo.2013.07.002Get rights and content

Highlights

  • Coral radiocarbon contents match local deep water concentrations, ages reliable.

  • Corals older than a few thousand years affected by U-series open-system behaviour

  • Uranium diffusion within and into coral offsetting apparent U-series ages

  • Highly variable initial 232Th/230Th observed in these Amundsen Sea corals

  • Internally homogeneous and elevated Mg/Ca potentially controlled by temperature

Abstract

Radiocarbon and uranium-thorium dating results are presented from a genus of calcitic Antarctic cold-water octocorals (family Coralliidae), which were collected from the Marie Byrd Seamounts in the Amundsen Sea (Pacific sector of the Southern Ocean) and which to date have not been investigated geochemically. The geochronological results are set in context with solution and laser ablation-based element/Ca ratios (Li, B, Mg, Mn, Sr, Ba, U, Th).

Octocoral radiocarbon ages on living corals are in excellent agreement with modern ambient deep-water Δ14C, while multiple samples of individual fossil coral specimens yielded reproducible radiocarbon ages. Provided that local radiocarbon reservoir ages can be derived for a given time, fossil Amundsen Sea octocorals should be reliably dateable by means of radiocarbon.

In contrast to the encouraging radiocarbon findings, the uranium-series data are more difficult to interpret. The uranium concentration of these calcitic octocorals is an order of magnitude lower than in the aragonitic hexacorals that are conventionally used for geochronological investigations. While modern and Late Holocene octocorals yield initial δ234U in good agreement with modern seawater, our results reveal preferential inward diffusion of dissolved alpha-recoiled 234U and its impact on fossil coral δ234U. Besides alpha-recoil related 234U diffusion, high-resolution sampling of two fossil octocorals further demonstrates that diagenetic uranium mobility has offset apparent coral U-series ages. Combined with the preferential alpha-recoil 234U diffusion, this process has prevented fossil octocorals from preserving a closed system U-series calendar age for longer than a few thousand years. Moreover, several corals investigated contain significant initial thorium, which cannot be adequately corrected for because of an apparently variable initial 232Th/230Th. Our results demonstrate that calcitic cold-water corals are unsuitable for reliable U-series dating.

Mg/Ca ratios within single octocoral specimens are internally strikingly homogeneous, and appear promising in terms of their response to ambient temperature. Magnesium/lithium ratios are significantly higher than usually observed in other deep marine calcifiers and for many of our studied corals are remarkably close to seawater compositions.

Although this family of octocorals is unsuitable for glacial deep-water Δ14C reconstructions, our findings highlight some important differences between hexacoral (aragonitic) and octocoral (calcitic) biomineralisation. Calcitic octocorals could still be useful for trace element and some isotopic studies, such as reconstruction of ambient deep water neodymium isotope composition or pH, via boron isotopic measurements.

Introduction

Pleistocene glacial-interglacial fluctuations in atmospheric carbon dioxide (pCO2) concentrations are tightly connected to variations in global overturning circulation and resulting variable ocean–atmosphere net CO2 exchange fluxes (Archer et al., 2000, Brovkin et al., 2007, Anderson et al., 2009). The detection of temporal changes in Dissolved Inorganic Carbon (DIC) concentrations in intermediate and deep water is key in this context, and can, in theory, be tracked by analysing carbon cycle-dependent proxies, such as boron (δ11B) or carbon (δ13C) isotopic compositions, as well as relative past deep water radiocarbon concentrations (Δ14C) in biogenic carbonate. Deep and intermediate ocean Δ14C records are particularly useful for resolving the effect of deep ocean carbon storage (Adkins et al., 1998, Mangini et al., 1998, Sikes et al., 2000, Goldstein et al., 2001, Robinson et al., 2005, Marchitto et al., 2007, Skinner et al., 2010, Burke and Robinson, 2012). Many of these studies used scleractinian cold water corals as palaeoceanographic archives. Yet collection of scleractinian corals is difficult, often serendipitous, and expensive, and as a result large parts of the world's ocean remain unsampled. Given the scarcity of suitable sampling sites in key oceanographic areas, other coral species need to be tested in order to gain better access to, and geographical coverage of, glacial-interglacial deep sea chemistry and carbon cycle parameters.

For any study using fossil cold-water corals the determination of accurate calendar ages (i.e. coral calcification ages) independent of 14C dating is a prerequisite for reconstructing changes in the deep ocean radiocarbon budget. Uranium-thorium (U-Th) disequilibrium ages of scleractinian cold-water corals provide such chronologies because of the long residence time of dissolved uranium in seawater (200–400 ka) and the isotopic enrichment of 234U relative to 238U, resulting in a homogeneous modern seawater δ234U of ~ 146.8 ± 0.1‰ (Andersen et al., 2010). This ambient seawater 234U/238U isotopic composition is incorporated into the coral carbonate and, if behaving as a closed system, the coral 234U/238U and 230Th/238U provide robust age information (Edwards et al., 1987). However, many fossil corals display open system behaviour, suggesting that the U isotopic composition has been modified after growth (Henderson, 2002, Pons-Branchu et al., 2005, Robinson et al., 2005, Scholz and Mangini, 2007). On the other hand, numerous other fossil scleractinian corals do display closed-system behaviour, and thus provide highly relevant palaeoceanographic and palaeoclimatic archives.

The aim of this paper is to evaluate the reliability of a family of calcitic cold-water octocorals, whose specimens were collected from the Marie Byrd Seamounts in the Amundsen Sea (West Antarctica), for deep-sea Δ14C reconstructions by means of coupled U-Th disequilibria and radiocarbon age measurements. Furthermore, an elemental characterisation of these corals is provided to assess differences in biomineralisation compared with scleractinian cold-water corals (e.g., Adkins et al., 1998, Robinson et al., 2005, Mangini et al., 2010). Although a comprehensive calibration cannot be provided with the coral samples investigated in this study, Mg/Ca and Mg/Li from modern and fossil coral specimens are also presented in order to assess the potential of this genus for deep-ocean temperature reconstructions (cf. Case et al., 2010, Raddatz et al., 2013). The specimens investigated here are expected to have recorded the water mass properties of Circumpolar Deep Water (CDW) during growth.

Section snippets

Area description, methods and material

The calcitic cold-water scleraxonian octocorals used in this study were sampled south of the Antarctic Polar Front and are derived from the Marie Byrd Seamounts in the Amundsen Sea in the Pacific sector of the Southern Ocean (~ 123°W, ~ 69°S, 2500 m to 1430 m water depth) (Fig. 1; Table 1). These calcitic cold-water octocorals belong to the family Coralliidae. Corals were dredged during scientific cruises of RV Polarstern in 2006 (ANT-XXIII/4) (Gohl, 2006) and in 2010 (ANT-XXVI/3) (Gohl, 2010).

Radiocarbon dates

The modern Amundsen Sea radiocarbon reservoir effect (i.e., R + ΔR) in intermediate water depth at the Marie Byrd Seamounts, as recorded by cold-water corals, was assessed through analysis of two modern specimens that were alive at the time of dredging. An entire chemically cleaned cross-section of the main stem of PS69/319-1-modern as well as the internal part (ca. 2 mm from the outer rim) of PS75/247-2-modern (see Fig. 2) was sampled and the radiocarbon results are shown in Table 2. The first

Conversion of Amundsen coral conventional radiocarbon- into calendar ages

As shown in Sections 3.2 and 4.2 (below), reliable U-series ages for early Holocene or older octocorals presented in this study cannot be provided. Nevertheless, it is possible to obtain approximate calendar ages based on conventional radiocarbon ages corrected by the likely local radiocarbon reservoir effect for a given time, derived from well-dated fossil corals collected in or close to the Drake Passage (Goldstein et al., 2001, Robinson and van de Flierdt, 2009, Burke and Robinson, 2012).

Conclusions

Calcitic octocorals of the family Coralliidae collected in the Amundsen Sea are unsuitable for U-Th disequilibrium dating, largely because of uranium concentrations below 250 ng/g, uranium open system behaviour and isotopically variable initial Th. The low uranium concentration magnifies diagenetic artefacts caused by alpha-recoiled dissolved (as opposed to lattice-bound) 234U diffusion into the coral, and was clearly resolved by means of micro-sampling of one coral of late deglacial 14C age.

Acknowledgements

The crews and the shipboard scientific parties during RV Polarstern expeditions (ANT-XXIII/4) in 2006 and (ANT-XXVI/3) in 2010, especially chief scientist Karsten Gohl, are thanked for all their help during coral sampling. Alistair Pike is thanked for providing access to the micromill within the Department of Archaeology and Anthropology at Bristol University. The analytical facilities of the Bristol Isotope Group would not work without the dedicated expertise of Chris Coath. Simon Powell

References (73)

  • T.M. Esat et al.

    Coupled uranium isotope and sea-level variations in the oceans

    Geochimica et Cosmochimica Acta

    (2010)
  • R.G. Fairbanks et al.

    Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired Th-230/U-234/U-238 and C-14 dates on pristine corals

    Quaternary Science Reviews

    (2005)
  • G.A. Gaetani et al.

    Rayleigh-based, multi-element coral thermometry: a biomineralization approach to developing climate proxies

    Geochimica et Cosmochimica Acta

    (2011)
  • A.C. Gagnon et al.

    Sr/Ca and Mg/Ca vital effects correlated with skeletal architecture in a scleractinian deep-sea coral and the role of Rayleigh fractionation

    Earth and Planetary Science Letters

    (2007)
  • S.J. Goldstein et al.

    Uranium-series and radiocarbon geochronology of deep-sea corals: implications for Southern Ocean ventilation rates and the oceanic carbon cycle

    Earth and Planetary Science Letters

    (2001)
  • I. Hajdas et al.

    A report on sample preparation at the ETH/PSI AMS facility in Zurich

    Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

    (2004)
  • B.L. Hall et al.

    Constant Holocene Southern-Ocean C-14 reservoir ages and ice-shelf flow rates

    Earth and Planetary Science Letters

    (2010)
  • G.M. Henderson

    Seawater (U-234/U-238) during the last 800 thousand years

    Earth and Planetary Science Letters

    (2002)
  • D.L. Hoffmann et al.

    Procedures for accurate U and Th isotope measurements by high precision MC-ICPMS

    International Journal of Mass Spectrometry

    (2007)
  • D. Langmuir et al.

    The mobility of thorium in natural waters at low temperatures

    Geochimica et Cosmochimica Acta

    (1980)
  • C.H. Lear et al.

    Benthic foraminiferal Mg/Ca-paleothermometry: a revised core-top calibration

    Geochimica et Cosmochimica Acta

    (2002)
  • M. Lomitschka et al.

    Precise Th/U-dating of small and heavily coated samples of deep sea corals

    Earth and Planetary Science Letters

    (1999)
  • X.Z. Luo et al.

    High precision Th-230/Th-232 and U-234/U-238 measurements using energy-filtered ICP magnetic sector multiple collector mass spectrometry

    International Journal of Mass Spectrometry

    (1997)
  • A. Mangini et al.

    Deep sea corals off Brazil verify a poorly ventilated Southern Pacific Ocean during H2, H1 and the Younger Dryas

    Earth and Planetary Science Letters

    (2010)
  • A. Meibom et al.

    Compositional variations at ultra-structure length scales in coral skeleton

    Geochimica et Cosmochimica Acta

    (2008)
  • T. Oomori et al.

    Distribution coefficient of Mg-2 + ions between calcite and solution at 10–50 °C

    Marine Chemistry

    (1987)
  • E. Pons-Branchu et al.

    Early diagenesis impact on precise U-series dating of deep-sea corals: example of a 100–200-year old Lophelia pertusa sample from the Northeast Atlantic

    Geochimica et Cosmochimica Acta

    (2005)
  • J. Raddatz et al.

    Stable Sr-isotope, Sr/Ca, Mg/Ca, Li/Ca and Mg/Li ratios in the scleractinian cold-water coral Lophelia pertusa

    Chemical Geology

    (2013)
  • Y. Rosenthal et al.

    Temperature control on the incorporation of magnesium, strontium, fluorine, and cadmium into benthic foraminiferal shells from Little Bahama Bank: prospects for thermocline paleoceanography

    Geochimica et Cosmochimica Acta

    (1997)
  • D. Scholz et al.

    How precise are U-series coral ages?

    Geochimica et Cosmochimica Acta

    (2007)
  • D. Scholz et al.

    U-series dating of diagenetically altered fossil reef corals

    Earth and Planetary Science Letters

    (2004)
  • E.L. Sikes et al.

    Assessing modern deep-water ages in the New Zealand region using deep-water corals

    Deep-Sea Research Part I

    (2008)
  • C.H. Stirling et al.

    Uranium-series dating of fossil coral reefs: extending the sea-level record beyond the last glacial cycle

    Earth and Planetary Science Letters

    (2009)
  • S. Thatje et al.

    On the origin of Antarctic marine benthic community structure

    Trends in Ecology & Evolution

    (2005)
  • J.F. Adkins et al.

    Deep-sea coral evidence for rapid change in ventilation of the deep North Atlantic 15,400 years ago

    Science

    (1998)
  • J.F. Adkins et al.

    The salinity, temperature, and delta O-18 of the glacial deep ocean

    Science

    (2002)
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    1

    Now at: ETH Zürich, Institute of Geochemistry and Petrology, Department of Earth Sciences, NW D81.4, Clausiusstrasse 25, 8092 Zürich, Switzerland.

    2

    Now at Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA.

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