Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France.
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA.
Geobiology. 2021 Nov;19(6):557-584. doi: 10.1111/gbi.12463. Epub 2021 Jul 23.
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-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同步辐射相结合，以对这些微化石进行特征分析。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几乎没有变化，而不同样品之间的变化可以用热成熟度或有机前体材料的生物起源的差异来解释。然而，很少有研究探索岩石学框架和成岩过程对有机材料化学结构的影响程度。我们对加拿大巴芬岛约 10 亿年前的 Angmaat 组中一个复杂的有机微化石组合进行了详细的基于拉曼光谱的研究。该地层含有丰富的早期成岩硅质岩，保存有大量微生物席，其中有许多易于识别的有机微化石。个别硅质岩床在岩石学上存在差异，具有离散的胶结和重结晶阶段。拉曼光谱分析表明，在样品之间以及具有相同分类和埋藏状态的相邻有机微化石之间，有机成熟度存在可测量的变化。对同一薄片中具有相似埋藏学特征的球形微化石进行扫描透射 X 射线显微镜分析，显示出明显不同的化学成分，芳香族化合物与酮和酚的比例不同。这些观察结果表明，有机质的地球化学变化不一定与热蚀变或有机前体物质有关。单个样品中拉曼信号的变化最可能与分析材料的成岩状态有关，并暗示有机保存与获得成岩流体之间存在关联。单个微化石成熟度的变化可能是局部成岩过程的自然结果，并且可能超过了来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原位特征分析以识别进一步化学分析目标标本的重要性。
