Abstract Raman spectroscopy is a minimally intrusive and minimally destructive technique that has played an important role in analyzing Precambrian carbonaceous microfossils and microstructures. Previous studies have shown that heterogeneities in structural order of carbonaceous material (CM) as revealed by Raman spectroscopy can be preserved in Proterozoic silicified cyanobacterial fossils. However, such heterogeneities have not been documented in Proterozoic eukaryotic microfossils preserved as carbonaceous compressions. In this study, we used Raman spectroscopy to investigate the structural characteristics of organic-walled microfossils, including eukaryotes preserved as carbonaceous compressions, from the late Paleoproterozoic Ruyang Group and the early Neoproterozoic (Tonian) Liulaobei Formation in the North China Craton. Raman geothermometers indicate that CM experienced low-grade metamorphism with apparent peak metamorphic temperatures of ~200 °C for both the Ruyang and Liulaobei microfossils. Heterogeneities in structural characteristics of CM in eukaryotic microfossils were found among different subcellular structures of Shuiyousphaeridium macroreticulatum and among different taxa from the same stratigraphic horizon. We suggest that these heterogeneities can be attributed to differences in precursor organic compounds; thus, the importance of organic precursors should be considered in CM Raman geothermometry studies, especially in environments where heating events occurred quickly. The Raman data presented here are also consistent with hypotheses that the Ruyang microfossils Dictyosphaera delicata and S. macroreticulatum are biologically conspecific taxa with their vesicle walls having similar CM Raman features, and that the Liulaobei microfossils Leiosphaeridia jacutica and L. tenuissima are distinct taxa despite their similarities in morphologies other than vesicle wall thickness. Although more data are needed to verify the consistency of interspecies, intraspecies, and intraspecimen variability, this study indicates that Raman spectroscopy may have the potential to provide an independent test of early eukaryote taxonomy, which has been traditionally based only on morphological features, and to reveal heterogeneities of CM of early eukaryotic organisms.

中文翻译：

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华北克拉通元古界有机壁微化石中碳质物质的拉曼光谱及结构异质性

摘要 拉曼光谱是一种侵入性最小、破坏性最小的技术，在分析前寒武纪碳质微化石和微观结构方面发挥了重要作用。先前的研究表明，拉曼光谱揭示的碳质材料（CM）结构顺序的异质性可以在元古代硅化蓝藻化石中保留下来。然而，在保存为碳质压缩物的元古代真核微化石中还没有记录到这种异质性。在这项研究中，我们使用拉曼光谱研究了来自华北克拉通的晚古元古代汝阳组和早新元古代（托尼阶）六老北组的有机壁微化石的结构特征，包括以碳质压缩形式保存的真核生物。拉曼地温计表明，CM 经历了低级变质作用，汝阳和六老北微化石的表观峰值变质温度约为 200 °C。真核微体中CM的结构特征在水油球藻的不同亚细胞结构和同一地层层位的不同分类群之间存在异质性。我们认为这些异质性可归因于前体有机化合物的差异；因此，在 CM 拉曼地温测量研究中应考虑有机前体的重要性，尤其是在加热事件快速发生的环境中。这里提供的拉曼数据也与如阳微化石 Dictyosphaera delicata 和 S. macroreticulatum 是生物学上的同种分类群，它们的囊泡壁具有相似的 CM 拉曼特征，并且六老北微化石 Leiosphaeridia jacutica 和 L. tenuissima 是不同的分类群，尽管它们除了囊泡壁厚度之外的形态相似。尽管需要更多数据来验证种间、种内和种内变异的一致性，但这项研究表明，拉曼光谱可能有潜力为早期真核生物分类提供独立测试，传统上仅基于形态特征，并揭示早期真核生物 CM 的异质性。尽管除了囊泡壁厚外，它们在形态上有相似之处，但tenuissima 是不同的分类群。尽管需要更多数据来验证种间、种内和种内变异的一致性，但这项研究表明，拉曼光谱可能有潜力为早期真核生物分类提供独立测试，传统上仅基于形态特征，并揭示早期真核生物 CM 的异质性。尽管除了囊泡壁厚外，它们在形态上有相似之处，但tenuissima 是不同的分类群。尽管需要更多数据来验证种间、种内和种内变异的一致性，但这项研究表明，拉曼光谱可能有潜力为早期真核生物分类提供独立测试，传统上仅基于形态特征，并揭示早期真核生物 CM 的异质性。