Late Neoproterozoic organic-rich sediments in South China: Insights into records of paleo-ocean chemistry and biogeochemistry
It has been widely hypothesized that a late-Neoproterozoic rise in oxygen paved the way for the diversification of complex life forms. The fossiliferous late Neoproterozoic sedimentary rocks in South China are some of the best preserved sequences of this time, covering the rise of algae and the emergence of animal life. However, biomarker evidence with strict contamination controls are lacking. Firstly, this thesis focuses on the search for indigenous biomarkers using slice experiments, in order to reveal valid information about the late Neoproterozoic biosphere preserved in fresh drill cores of organic-rich shales. Simple hydrocarbons such as low to medium molecular weight n-alkanes, adamantanes and aromatic hydrocarbons are present, consistent with the high thermal maturity of these shales. However, the detected biomarkers were very limited and poorly taxa-oriented, due to the high thermal maturity. This finding rebuts previously published biomarker evidence for samples from South China with similar or even higher thermal maturities.
The evolution from a dominantly bacterial to eukaryotic marine biosystem has been suggested to be potentially related to reorganization of the chemical composition of the oceans, which could have been induced by icehouse-greenhouse climate cycles.
Multiple geochemical proxies, especially high resolution chemical index of alteration profiles, in combination with mineralogical mapping were used to reconstruct climate transitions. Efficient production of pedogenic clay minerals by elevated chemical weathering under warm conditions might have promoted the sequestration and burial of organic carbon in the aftermath of the Sturtian and Marinoan Snowball Earth events. In contrast, substantial burial of organic carbon during the terminal Ediacaran was likely facilitated by enhanced influx of detrital nutrients caused by predominant physical weathering related to assembly of the Gondwana supercontinent under a relatively cool climate.
Subtle redox state changes of the paleo-ocean after the glaciations were also reflected in the geochemical composition of the black shales. Iron speciation data and enrichments of redox sensitive elements and superheavy pyrite in the interglacial Datangpo Formation indicate persistent deep water anoxic conditions and a decreasing degree of euxinia as the Marinoan glaciation approached, due to the suppressed influx of nutrients. Such conditions in turn favored the burial of organic carbon in the post-glacial ocean.
Episodic oxygenation of the deep waters has been identified for the interglacial Datangpo Formation, during which time a manganese carbonate layer hosted in the black shales was deposited. Co-localization of organic carbon and the manganese-rich phase was revealed by time of flight secondary ion mass spectrometry data, which together with the negative δ13C values of the manganese carbonate indicate an essential contribution of organically-derived carbon. Metallogenesis of the black shale-hosted manganese deposit was likely a microbially-mediated process involving Mnoxidizing bacteria and extracellular polymetric substances generated by a bloom of cyanobacteria after the Sturtian glaciation.
This thesis should inspire following researchers to explore for suitable organic-rich samples with mild thermal maturities for further investigation of late Neoproterozoic biomarkers. Further work should use cutting-edge methods with higher sensitivity such as non-traditional isotopes to advance a more comprehensive understanding of the coevolution of life and Earth.
