Although rare-earth elements (REEs) in authigenic phosphates and carbonates derived from marine sediments are potential proxies (e.g., REE patterns) for reconstructing paleoenvironmental conditions, the REE patterns appear to be inconsistent. Some marine authigenic phosphates show a “bell-shaped” pattern with an enrichment of the middle REEs (MREEs; NdHo), while others exhibit a “modern seawater-like” pattern characterized by a strong depletion in cerium and an enrichment in heavy REEs (ErLu, Y, and Sc). Although the origin of the “bell-shaped” pattern in phosphates has been extensively debated, the evidence is often ambiguous; therefore, the origin of the pattern remains unclear. In this study, we investigate the microscale and nanoscale mineralogical and geochemical characteristics of the REE-enriched Early Cambrian phosphorites. In situ elemental analyses reveal that the codeposited phosphates (“bell-shaped” REE pattern) and carbonates (“modern seawater-like” pattern) have different REE patterns. A microscale investigation shows that phosphate grains contain some dispersed goethite nanophase, and a nanoscale investigation using transmission electron microscopy coupled with electron energy-loss spectroscopy shows that Fe is distributed along the rim of the goethite nanophase, indicating that the goethite were reduced. These findings suggest that the REE pattern and concentration in phosphates could have been influenced by the release of REEs during the reduction of Fe (oxyhydr)oxide in the early diagenetic process before the final deposition. Although additional evidence is required, our results are important for understanding the effects of Fe redox cycling on the REE pattern and enrichment in marine authigenic phosphates. In addition, our results suggest that compared with authigenic phosphates, authigenic carbonates would provide a more reliable reconstruction of paleoenvironmental conditions.

中文翻译：

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针铁矿纳米相对早期成岩过程中稀土元素模式和海相磷酸盐富集的影响

尽管源自海洋沉积物的自生磷酸盐和碳酸盐中的稀土元素 (REE) 是重建古环境条件的潜在代表（例如 REE 模式），但 REE 模式似乎不一致。一些海洋自生磷酸盐表现出“钟形”模式，其中富集了中稀土元素（MREE；NdHo），而另一些则表现出“现代海水状”模式，其特征是铈的严重消耗和重稀土元素的富集。 ErLu、Y 和 Sc）。尽管磷酸盐中“钟形”图案的起源存在广泛争议，但证据往往是模糊的。因此，该模式的起源仍不清楚。在本研究中，我们研究了富含稀土元素的早寒武世磷矿的微米级和纳米级矿物学和地球化学特征。原位元素分析表明，共沉积的磷酸盐（“钟形”稀土元素模式）和碳酸盐（“现代海水状”模式）具有不同的稀土元素模式。微观尺度的研究表明，磷酸盐颗粒含有一些分散的针铁矿纳米相，而使用透射电子显微镜和电子能量损失谱进行的纳米尺度研究表明，Fe沿着针铁矿纳米相的边缘分布，表明针铁矿被还原。这些发现表明，磷酸盐中的稀土元素模式和浓度可能受到最终沉积前早期成岩过程中铁（羟基）氧化物还原过程中稀土元素释放的影响。尽管需要额外的证据，但我们的结果对于理解铁氧化还原循环对 REE 模式和海洋自生磷酸盐富集的影响非常重要。此外，我们的结果表明，与自生磷酸盐相比，自生碳酸盐将为古环境条件提供更可靠的重建。