87Sr/86Sr age determination by rapidly formed spherical carbonate concretions
87Sr/86Sr age determination by rapidly formed spherical carbonate concretions
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DOI：
10.1038/s41598-019-38593-9
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发表时间：
2019-01-30
影响因子：
4.6
通讯作者：
Metcalfe, Richard
中科院分区：
文献类型：
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作者：
Yoshida, Hidekazu;Asahara, Yoshihiro;Metcalfe, Richard
孤立的球形碳酸盐结核经常出现在地质时代差异很大的细粒海洋沉积物中。最近对各种球状碳酸盐(CaCO3)结核的研究表明，它们在严格约束条件下形成得非常快。然而，孤立的球状碳酸盐结核的形成年龄一直没有确定。在这里，我们使用锶-87/锶-86比值来确定这些球形结核的年龄。所研究的结核形成于日本中部中新世的Yatsuo群。有些是死后在象牙壳(Fissidentalium spp.)周围形成的，而另一些结核内部没有贝壳化石。结核周围沉积层的变形结合地球化学分析表明，在碳酸钙结核的快速形成过程中，锶被并入其中。利用所有结核的锶-87/锶-86比值计算的锶同位素地层学年龄为17.02 Ma+/-0.27 Ma，比利用亚锁群微化石估算的年龄具有更高的精度。结果表明，孤立的球形碳酸盐结核的锶-87/锶-86比值可普遍用于确定沉积后不久形成的海相沉积物的数值年龄。即使在没有任何化石证据的情况下，锶-87/锶-86年龄测定也具有很高的精度。
Isolated spherical carbonate concretions are frequently observed in finer grained marine sediments of widely varying geological age. Recent studies on various kinds of spherical carbonate (CaCO3) concretions revealed that they formed very rapidly under tightly constrained conditions. However, the formation ages of the isolated spherical carbonate concretions have never been determined. Here we use Sr-87/Sr-86 ratios to determine the ages of these spherical concretions. The studied concretions formed in the Yatsuo Group of Miocene age in central Japan. Some formed post- mortem around tuskshells (Fissidentalium spp.), while other concretions have no shell fossils inside. The deformation of sedimentary layers around the concretions, combined with geochemical analyses, reveal that Sr was incorporated into the CaCO3 concretions during their rapid formation. Strontium isotopic stratigraphy using Sr-87/Sr-86 ratios of all concretions indicates an age of 17.02 +/- 0.27 Ma, with higher accuracy than the ages estimated using micro- fossils from the Yatsuo Group. The results imply that the Sr-87/Sr-86 ratio of isolated spherical carbonate concretions can be applied generally to determine the numerical ages of marine sediments, when concretions formed soon after sedimentation. The Sr-87/Sr-86 age determinations have high accuracy, even in cases without any fossils evidence.