Organic microfossils in Meso- and Neoproterozoic rocks are of key importance to track the emergence and evolution of eukaryotic life. An increasing number of studies combine Raman spectroscopy with synchrotron-based methods to characterize these microfossils. A recurring observation is that Raman spectra of organic microfossils show negligible variation on a sample scale and that variation between different samples can be explained by differences in thermal maturation or in the biologic origin of organic precursor material. There is a paucity of work, however, that explores the extent to which the petrographic framework and diagenetic processes might influence the chemical structure of organic materials. We present a detailed Raman spectroscopy-based study of a complex organic microfossil assemblage in the ca. 1 Ga old Angmaat Formation, Baffin Island, Canada. This formation contains abundant early diagenetic chert that preserves silicified microbial mats with numerous, readily identifiable organic microfossils. Individual chert beds show petrographic differences with discrete episodes of cementation and recrystallization. Raman spectroscopy reveals measurable variation of organic maturity between samples and between neighboring organic microfossils of the same taxonomy and taphonomic state. Scanning transmission X-ray microscopy performed on taphonomically similar coccoidal microfossils from the same thin section shows distinct chemical compositions, with varying ratios of aromatic compounds to ketones and phenols. Such observations imply that geochemical variation of organic matter is not necessarily coupled to thermal alteration or organic precursor material. Variation of the Raman signal across single samples is most likely linked to the diagenetic state of analyzed materials and implies an association between organic preservation and access to diagenetic fluids. Variation in the maturity of individual microfossils may be a natural outcome of local diagenetic processes and potentially exceeds differences derived from precursor organic material. These observations stress the importance of detailed in situ characterization by Raman spectroscopy to identify target specimens for further chemical analysis.

中文翻译：

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大约元古代有机微化石的结构和化学异质性。1 Ga old Angmaat Formation, 加拿大巴芬岛

中和新元古代岩石中的有机微化石对于追踪真核生物的出现和演化至关重要。越来越多的研究将拉曼光谱与基于同步加速器的方法相结合来表征这些微化石。一个反复出现的观察结果是，有机微化石的拉曼光谱在样品尺度上的变化可以忽略不计，不同样品之间的变化可以用热成熟的差异或有机前体材料的生物起源来解释。然而，很少有工作探索岩相框架和成岩过程可能在多大程度上影响有机材料的化学结构。我们提出了一项基于拉曼光谱的详细研究，对约 1 Ga old Angmaat Formation, 加拿大巴芬岛。该地层包含丰富的早期成岩燧石，保存有大量易识别的有机微化石的硅化微生物垫。个别燧石床表现出岩相差异，具有离散的胶结和再结晶事件。拉曼光谱揭示了样品之间以及相同分类和埋藏状态的相邻有机微化石之间有机成熟度的可测量变化。扫描透射 X 射线显微镜对来自同一薄片的埋藏学相似的球状微化石显示出不同的化学成分，芳香族化合物与酮和酚的比例不同。这样的观察意味着有机物质的地球化学变化不一定与热蚀变或有机前体物质相关。单个样品中拉曼信号的变化很可能与分析材料的成岩状态有关，并暗示有机保存与成岩流体的获取之间存在关联。个体微化石成熟度的差异可能是当地成岩过程的自然结果，并且可能超过源自前体有机材料的差异。这些观察结果强调了通过拉曼光谱进行详细的原位表征以识别目标样本以进行进一步化学分析的重要性。个体微化石成熟度的差异可能是当地成岩过程的自然结果，并且可能超过源自前体有机材料的差异。这些观察结果强调了通过拉曼光谱进行详细的原位表征以识别目标样本以进行进一步化学分析的重要性。个体微化石成熟度的差异可能是当地成岩过程的自然结果，并且可能超过源自前体有机材料的差异。这些观察结果强调了通过拉曼光谱进行详细的原位表征以识别目标样本以进行进一步化学分析的重要性。