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

Structural limitations in deriving accurate U-series ages from calcitic cold-water corals contrast with robust coral radiocarbon and Mg/Ca systematics
Structural limitations in deriving accurate U-series ages from calcitic cold-water corals contrast with robust coral radiocarbon and Mg/Ca systematics
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 ?14C, 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 ?234U in good agreement with modern seawater, our results reveal preferential inward diffusion of dissolved alpha-recoiled 234U and its impact on fossil coral ?234U. 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 ?14C 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.
Radiocarbon, U-series dating, Octocoral, Mg/Ca, Mg/Li
0009-2541
69-87
Gutjahr, Marcus
5babbbc4-2a1a-48df-a2e3-d87b2483ea9c
Vance, Derek
9c0575d3-caf4-4d57-b08b-b7a81f6c107c
Hoffmann, Dirk L.
c0692e66-6bca-4e4d-adf3-19dc980fa524
Hillenbrand, Claus-Dieter
8dc0c76e-e79b-4c8f-90ed-07035aabab98
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022
Rae, James W.B.
8788a3a1-fec8-46c2-95a4-a59ce25416f5
Kuhn, Gerhard
2edba081-db06-4283-bfe2-3b5fae86fb4b
Gutjahr, Marcus
5babbbc4-2a1a-48df-a2e3-d87b2483ea9c
Vance, Derek
9c0575d3-caf4-4d57-b08b-b7a81f6c107c
Hoffmann, Dirk L.
c0692e66-6bca-4e4d-adf3-19dc980fa524
Hillenbrand, Claus-Dieter
8dc0c76e-e79b-4c8f-90ed-07035aabab98
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022
Rae, James W.B.
8788a3a1-fec8-46c2-95a4-a59ce25416f5
Kuhn, Gerhard
2edba081-db06-4283-bfe2-3b5fae86fb4b

Gutjahr, Marcus, Vance, Derek, Hoffmann, Dirk L., Hillenbrand, Claus-Dieter, Foster, Gavin L., Rae, James W.B. and Kuhn, Gerhard (2013) Structural limitations in deriving accurate U-series ages from calcitic cold-water corals contrast with robust coral radiocarbon and Mg/Ca systematics. Chemical Geology, 355, 69-87. (doi:10.1016/j.chemgeo.2013.07.002).

Record type: Article

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 ?14C, 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 ?234U in good agreement with modern seawater, our results reveal preferential inward diffusion of dissolved alpha-recoiled 234U and its impact on fossil coral ?234U. 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 ?14C 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.

Full text not available from this repository.

More information

Published date: 26 September 2013
Keywords: Radiocarbon, U-series dating, Octocoral, Mg/Ca, Mg/Li
Organisations: Geochemistry

Identifiers

Local EPrints ID: 359559
URI: https://eprints.soton.ac.uk/id/eprint/359559
ISSN: 0009-2541
PURE UUID: 274f1145-0158-4853-9239-76096b2f2d08

Catalogue record

Date deposited: 04 Nov 2013 13:35
Last modified: 18 Jul 2017 03:20

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