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
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从方解石冷水珊瑚推导准确 U 系列年龄的结构限制与强大的珊瑚放射性碳和 Mg/Ca 系统学对比
DOI：
10.1016/j.chemgeo.2013.07.002
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发表时间：
2013
期刊：
影响因子：
3.9
通讯作者：
Gutjahr M
中科院分区：
文献类型：
--
作者：
Gutjahr M
放射性碳和铀-钍定年结果是从一种方解石质南极冷水珊瑚（珊瑚科），这是从玛丽伯德海隆在阿蒙森海（南大洋的太平洋部分），迄今尚未进行地球化学研究。地质年代学结果与基于溶液和激光烧蚀的元素/Ca比值（Li、B、Mg、Mn、Sr、Ba、U、Th）相结合。活珊瑚的珊瑚放射性碳年龄与现代环境深水Δ 14 C非常一致，而单个化石珊瑚标本的多个样品产生了可重复的放射性碳年龄。如果当地的放射性碳储层年龄可以推导出一个给定的时间，阿蒙森海珊瑚化石应该是可靠的年代测定的放射性碳手段。这些方解石珊瑚的铀浓度比通常用于地质年代学调查的文石六角珊瑚低一个数量级。虽然现代和晚全新世珊瑚产生的初始δ 234 U与现代海水一致，但我们的结果揭示了溶解的α-反冲234 U的优先向内扩散及其对化石珊瑚δ 234 U的影响。除了α反冲相关的234 U扩散，高分辨率采样的两个化石珊瑚进一步表明，成岩铀的流动性抵消了明显的珊瑚U系列年龄。结合优先的α-反冲234 U扩散，这一过程阻止了化石珊瑚保存一个封闭的系统U系列日历年龄超过几千年。此外，几个珊瑚调查含有显着的初始钍，这不能得到充分的纠正，因为一个明显的变量initial 232 Th/230 Th。我们的研究结果表明，方解石冷水珊瑚不适合可靠的U-系列dating。Mg/Ca比值内的单珊瑚标本是内部惊人的均匀性，并出现有前途的环境温度的响应方面。镁/锂的比例显着高于通常观察到的其他深海钙化和我们研究的珊瑚是非常接近海水组成。虽然这个家庭的珊瑚是不适合冰川深水Δ 14 C重建，我们的研究结果突出了一些重要的差异六角珊瑚（文石）和六角珊瑚（方解石）生物矿化。方解石珊瑚仍然可以用于微量元素和一些同位素研究，例如通过硼同位素测量重建环境深水钕同位素组成或pH值。
Radiocarbon and uranium-thorium dating results are presented from a genus of calcitic Antarctic cold-water octocorals (familyCoralliidae), 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-recoiled234U and its impact on fossil coral δ234U. Besides alpha-recoil related234U 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-recoil234U 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 initial232Th/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.
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DOI：
--
发表时间：
2005
期刊：
影响因子：
--
作者：
E. Pons‐Branchu;C. Hillaire‐Marcel;P. Deschamps;B. Ghaleb;D. Sinclair
通讯作者：
D. Sinclair
影响因子：
3.9
作者：
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通讯作者：
Dullo, W-Chr
影响因子：
5.3
作者：
Elderfield, H.;Yu, J.;Nyland, B.
通讯作者：
Nyland, B.
影响因子：
56.9
作者：
Adkins, JF;Cheng, H;Edwards, RL
通讯作者：
Edwards, RL
DOI：
--
发表时间：
2010
期刊：
Radiocarbon: An International Journal of Cosmogenic Isotope Research
影响因子：
--
作者：
W. Jenkins;K. Elder;A. McNichol;K. F. Reden
通讯作者：
K. F. Reden