Exploring uranium isotopes in shark teeth as a paleo-redox proxy
Exploring uranium isotopes in shark teeth as a paleo-redox proxy
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DOI：
10.1016/j.gca.2023.11.034
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发表时间：
2023-12
影响因子：
5
通讯作者：
Haoyu Li;M. Kipp;Sora L. Kim;E. Kast;J. Eberle;François L. H. Tissot
中科院分区：
文献类型：
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作者：
Haoyu Li;M. Kipp;Sora L. Kim;E. Kast;J. Eberle;François L. H. Tissot
海水铀同位素组成(δ~(238)U)是反映海洋缺氧程度的有力指标。对于古氧化还原重建，碳酸盐是最受欢迎的铀同位素档案，但最近它们受到了越来越多的研究，因为它们的δ238U值在沉积后受到成岩作用的影响。因此，有必要调查其他可能记录和保存原始海水δ238U信号的档案。在这项研究中，我们探索鲨鱼牙齿是否提供了这样的档案。由于结晶氟磷灰石的溶解度较低，与海相碳酸盐相比，由结晶氟磷灰石组成的鲨鱼牙釉质更能抵抗沉积后的蚀变，对同位素交换不那么敏感。为了评估鲨鱼牙齿中的铀同位素是否可以记录海水的特征，我们测量了39颗鲨鱼牙齿的U同位素(包括δ238U和δ234Usec)，这些化石来自不同的地点，包括班克斯岛(北极)、墨西哥湾和皮斯科盆地(秘鲁)，年龄从现代到白垩纪。我们的结果表明，铀在现代鲨鱼牙齿中的浓度可以忽略不计(1 Ppb)，但在化石样本中却升高(高达数百ppm)，这表明在埋葬过程中，铀被整合到鲨鱼牙齿中。δ238U的值范围为−0.72到‰的+0.57%，δ234U的值范围为−162.1到‰的+969.7。结果表明：(1)海水U同位素比值的成岩叠加在鲨鱼牙齿中普遍存在；(2)δ-238U数据受局部沉积环境的影响。尽管如此，在鲨鱼牙齿中观察到的铀同位素变化与在海洋碳酸盐中观察到的U同位素变化相当，这表明成岩变化较少的样品可能会为了解过去海洋缺氧的程度提供有用的洞察。
The uranium isotope composition (δ238U) of seawater is a powerful proxy for the extent of marine anoxia. For paleoredox reconstructions, carbonates are the most popular U isotope archive, but they have recently come under increased scrutiny as their δ238U values are subject to diagenetic alteration after deposition. Therefore, there is a need to investigate other archives that may record and preserve the original seawater δ238U signal. In this study, we explore whether shark teeth provide such an archive. Shark teeth enameloid consisting of crystalline fluorapatite is more resistant to post-depositional alteration and less sensitive to isotopic exchange than marine carbonates due to the lower solubility of the crystalline fluorapatite. Since U is readily incorporated into phosphate, shark teeth could incorporate and preserve the original δ238U signature of seawater.To assess whether U isotopes in shark teeth can record seawater signatures, we measured the U isotopes (both δ238U and δ234Usec) in 39 fossil shark teeth from various locations, including Banks Island (Arctic), the Gulf of Mexico (GOM), and Pisco Basin (Peru), and ranging in age from modern to Cretaceous. Our results show that U concentrations are negligible in modern shark teeth (<1 ppb) but elevated in fossil samples (up to several hundred ppm), indicating that U is incorporated into shark teeth postmortem during burial. The δ238U values range from −0.72 to +0.57 ‰, and the δ234U values from −162.1 to +969.7 ‰. The data indicate that (i) diagenetic overprinting of seawater U isotope ratios is common among shark teeth, and (ii) δ238U data are influenced by local depositional environments. Nonetheless, the U isotope variations observed in shark teeth are comparable to those seen in marine carbonates, indicating that the samples with less diagenetic alterations might offer useful insight into the past extent of ocean anoxia.