Molecular Fossils for Understanding Biodiversity During the Neoproterozoic-Cambrian Transition in China

Neoproterozoic-Cambrian rocks in South China contain an extraordinary fossil record, including exceptionally well preserved animal embryos, acritarchs, and multi-cellular algae. The goal of this study was to evaluate the microbial diversity associated with these remarkably preserved fossil assemblages at the Neoproterozoic-Cambrian transition. Rock samples of 520-632 Ma in age were collected in the Yangtze Gorges area and southern Anhui Province, China. Samples were powdered and extracted for organic biomarkers. The content of bitumen A accounted for 4-16% of the rock material and most of it (49-79%) was asphaltenes. Saturated and aromatic hydrocarbons accounted for 2-6% and 1-3%, respectively. Analysis using GC-MS indicated the predominance of n-alkanes and less abundant isoprenoid alkanes in the saturated fractions. The n-alkanes were characterized by homologues dominated by C15-C17, which is consistent with the result of high thermal-evolution (Brocks et al., 2003. GCA 67:4321-4335). Hopanoids were present in less abundance and ranged from C29 to C32. A smaller amount of heavy-molecular-weight n-alkanes (C23-C39) was also detected, which indicated a source of high plants and must be contamination from younger organic matter. Still, patterns of variation can be detected among these samples. For example, the ratio of pristane to phytane was all greater than 1.0 except for one sample (JLW9.3) from Yangtze Gorges area. The results indicate that sample JLW9.3 might have been deposited in a reducing environment whereas the other samples might have been formed in relatively oxidative environments. The overall results, however, suggest that rock samples from the Neoproterozoic-Cambrian transition in China have gone through significant metamorphism; thus, understanding of microbial communities using molecular biomarkers in such altered rocks needs to be cautiously executed.