Structural limitations in deriving accurate U-series ages from calcitic cold-water corals contrast with robust coral radiocarbon and Mg/Ca systematics
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)
- et al.
High-precision U-series measurements of more than 500,000 year old fossil corals
Earth and Planetary Science Letters
(2008) - et al.
Lithium content of sea water by atomic absorption spectrometry
Geochimica et Cosmochimica Acta
(1966) - et al.
Environmental and biological controls on Mg and Li in deep-sea scleractinian corals
Earth and Planetary Science Letters
(2010) - et al.
U-238, U-234 and Th-232 in seawater
Earth and Planetary Science Letters
(1986) - et al.
U–Th dating of deep-sea corals
Geochimica et Cosmochimica Acta
(2000) - et al.
The two-step mode of growth in the scleractinian coral skeletons from the micrometre to the overall scale
Journal of Structural Biology
(2005) - et al.
Sodium, potassium, magnesium, calcium and strontium in seawater
Deep Sea Research
(1966) - et al.
Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis
Earth and Planetary Science Letters
(2006) - et al.
A record of bottom water temperature and seawater delta(18)O for the Southern Ocean over the past 440 kyr based on Mg/Ca of benthic foraminiferal Uvigerina spp.
Quaternary Science Reviews
(2010) - et al.
Variability in the uranium isotopic composition of the oceans over glacial–interglacial timescales
Geochimica et Cosmochimica Acta
(2006)
Coupled uranium isotope and sea-level variations in the oceans
Geochimica et Cosmochimica Acta
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
Rayleigh-based, multi-element coral thermometry: a biomineralization approach to developing climate proxies
Geochimica et Cosmochimica Acta
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
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
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
Constant Holocene Southern-Ocean C-14 reservoir ages and ice-shelf flow rates
Earth and Planetary Science Letters
Seawater (U-234/U-238) during the last 800 thousand years
Earth and Planetary Science Letters
Procedures for accurate U and Th isotope measurements by high precision MC-ICPMS
International Journal of Mass Spectrometry
The mobility of thorium in natural waters at low temperatures
Geochimica et Cosmochimica Acta
Benthic foraminiferal Mg/Ca-paleothermometry: a revised core-top calibration
Geochimica et Cosmochimica Acta
Precise Th/U-dating of small and heavily coated samples of deep sea corals
Earth and Planetary Science Letters
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
Deep sea corals off Brazil verify a poorly ventilated Southern Pacific Ocean during H2, H1 and the Younger Dryas
Earth and Planetary Science Letters
Compositional variations at ultra-structure length scales in coral skeleton
Geochimica et Cosmochimica Acta
Distribution coefficient of Mg-2 + ions between calcite and solution at 10–50 °C
Marine Chemistry
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
Stable Sr-isotope, Sr/Ca, Mg/Ca, Li/Ca and Mg/Li ratios in the scleractinian cold-water coral Lophelia pertusa
Chemical Geology
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
How precise are U-series coral ages?
Geochimica et Cosmochimica Acta
U-series dating of diagenetically altered fossil reef corals
Earth and Planetary Science Letters
Assessing modern deep-water ages in the New Zealand region using deep-water corals
Deep-Sea Research Part I
Uranium-series dating of fossil coral reefs: extending the sea-level record beyond the last glacial cycle
Earth and Planetary Science Letters
On the origin of Antarctic marine benthic community structure
Trends in Ecology & Evolution
Deep-sea coral evidence for rapid change in ventilation of the deep North Atlantic 15,400 years ago
Science
The salinity, temperature, and delta O-18 of the glacial deep ocean
Science
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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.