Organic geochemical characteristics of highly mature Late Neoproterozoic black shales from South China: Reappraisal of syngeneity and indigeneity of hydrocarbon biomarkers

摘要

During the Late Neoproterozoic to early Cambrian period, the transition from mainly microbial ecosystems to eukaryotic marine primary productivity was one of the most profound ecological revolutions in Earth's history. Abundant biomarkers have been reported from high maturity Late Neoproterozoic sedimentary rocks in South China, including the Cryogenian Datangpo and Ediacaran Doushantuo formations. These biomarkers were suggested to be organic molecular evidence of the survival of photosynthetic eukaryotes in palaeo-oceanic environments during the Snowball Earth era. To advance the understanding of Late Neoproterozoic ecosystems and to re-evaluate the provenance and validity of the biomarkers, fresh drill core was collected from black shales of the Datangpo and Doushantuo formations in South China. In order to ensure the indigeneity of the solvent extracted hydrocarbons, their composition was determined by conducting experiments under strict laboratory procedures, including using slice experiments on a precision saw that removed external surfaces. The distribution of the recovered aliphatic and aromatic hydrocarbons indicates very a high thermal maturity for the analysed black shales, which is consistent with the equivalent vitrinite reflectance (similar to 2.5%) and the lack of specific biomarkers. Both prokaryotic (bacterial) and eukaryotic (algal) organisms are likely producers during the Neoproterozoic. However, no long-chain steranes (>= C-26 steranes) were detected in the interior of any of the samples, and only trace amounts of hopanes and pregnanes in just one sample from the Doushantuo Formation (xs-199-DST). On the contrary, solvent extraction of millimetre-thick slices cut from the external surfaces of the black shales reveals strikingly high level of contamination from organic residues on the sample surfaces. These contaminants include n-alkanes (C-10 similar to C-33), isoprenoids, monomethylalkanes, aromatic hydrocarbons (naphthalene, phenanthrene and their alkyl isomers) and abundant biomarkers such as tricyclic terpanes, hopanes, steranes and diasteranes. The concentrations of these biomarkers in the exterior sub-samples exceed blank concentrations by more than three orders of magnitude due to surficial contamination. These results indicate that previously studied Late Neoproterozoic samples from South China likely contain mixtures of abundant biomarker contaminants and some indigenous over-mature hydrocarbons. Therefore, special care need to be taken with regard the existing biomarker evidence from South China that has been used to illustrate the contribution of eukaryotic photosynthesis to marine primary productivity during the Snowball Earth period. No Late Neoproterozoic rocks from South China within the appropriate thermal maturity window for survival of indigenous biomarkers are currently known, but these may be found during future exploration.