U-series ages from calcitic cold-water corals from the Amundsen Sea

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 D14C, 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 d234U in good agreement with modern seawater, our results reveal preferential inward diffusion of dissolved alpha-recoiled 234U and its impact on fossil coral d234U. 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 D14C 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.

本数据集呈现了一类隶属于红珊瑚科（Coralliidae）的南极冷水方解石质八放珊瑚（octocorals）的放射性碳测年与铀钍测年结果，样本采自南太平洋扇区阿蒙森海的玛丽·伯德海山，迄今尚未开展相关地球化学研究。本次测年研究同时配套了溶液法与激光剥蚀（laser ablation）法测定的元素/钙比值（Li、B、Mg、Mn、Sr、Ba、U、Th）。

现生八放珊瑚的放射性碳年龄与现代周边深层海水的Δ¹⁴C值高度吻合；单个化石珊瑚标本的多份平行样品可获得重现性一致的放射性碳年龄。若能获取特定时间尺度下的局地放射性碳储层年龄，阿蒙森海的化石八放珊瑚即可通过放射性碳法获得可靠的测年结果。

与放射性碳测年的喜人结果形成对比的是，铀系（U-series）数据的解译难度更高。这类方解石质八放珊瑚的铀浓度比常规用于地球年代学研究的文石质六放珊瑚低一个数量级。尽管现代与晚全新世八放珊瑚的初始δ²³⁴U值与现代海水吻合度较好，但本研究结果显示，溶解态α反冲²³⁴U会优先向内扩散，并对化石珊瑚的δ²³⁴U值产生影响。除与α反冲相关的²³⁴U扩散作用外，对两件化石八放珊瑚的高分辨率采样还进一步证实，成岩作用引发的铀迁移会偏移珊瑚的表观铀系测年年龄。结合优先发生的α反冲²³⁴U扩散作用，该过程使得化石八放珊瑚无法在数千年以上的时间尺度上保持铀系封闭体系，进而无法获取准确的日历年龄。此外，本次研究的部分珊瑚样品含有显著的初始钍，由于初始²³²Th/²³⁰Th比值存在明显差异，无法对其进行充分校正。本研究结果表明，方解石质冷水珊瑚不适用于可靠的铀系测年。

单个八放珊瑚标本内部的Mg/Ca比值具有极高的内部均一性，且其比值变化对周边海水温度具有潜在响应，具备应用前景。镁锂比值显著高于其他深海钙化生物的常规观测值，且本次研究的多数珊瑚样品的镁锂比值与海水组成极为接近。

尽管该科八放珊瑚不适用于冰川时期深层海水的Δ¹⁴C重建，但本研究结果揭示了文石质六放珊瑚与方解石质八放珊瑚在生物矿化过程中的若干重要差异。方解石质八放珊瑚仍可用于微量元素与部分同位素研究，例如通过硼同位素测定重建周边深层海水的钕同位素组成或pH值。