Biogeochemical evolution in the neoproterozoic amadeus basin, central Australia

Abstract

The diversification of eukaryotes from increasingly complex single celled organisms to organ grade animals is the defining feature of biological evolution in the Neoproterozoic era. However the causes of this diversification are still largely obscure. It has been suggested that deep water oxygenation may be an essential catalyst, as mid-shelf to deep oceans started to become oxygenated at this time. However, marine oxygenation appears to be diachronous in different basins, so global correlations of aquatic redox and eukaryotic evolution remain tentative. The aim of this thesis is to identify whether there are discernible relationships between redox chemistry of the water column and the evolution of eukaryotes in the Amadeus Basin, central Australia. In this thesis, we analyse iron speciation and elemental chemistry to determine the redox state of the Amadeus Basin. In addition, we analyse indigenous biomarkers, the molecular remains of microbial communities including eukaryotes and complement these proxies with published microfossil data. The results from the Amadeus Basin are then finally placed in a global context. The results of this study demonstrate that ferruginous conditions dominated the Neoproterozoic in Australia from the ca. 820 Ma Bitter Springs Formation to the ca. 635 Ma Pioneer Sandstone, similar to contemporaneous global trends. Deep water oxygenation of the Amadeus Basin occurred immediately preceding the Marinoan glaciation in the Pertatataka Formation ca. 635 Ma, at the same time as recorded in the lower Doushantuo formation in South China. This is in contrast with other basins globally, where oxygnenation does not occur for at least 50 million years after the Marinoan. In this study, we could not identify indigenous Ediacaran biomarkers from the ca. 635 Ma Pertatataka Formation due to thermal destruction and pervasive contamination. Although there is no current indigenous Ediacaran biomarker record in the Amadeus Basin, microfossil evidence suggests that eukaryotes were not radiating co-eval with oxygenation, unlike other localities such as South China or the Eastern European Platform. However, these results may be skewed due to preservation bias. Indigenous hydrocarbons were identified from pre- and inter-Snowball periods. In the Loves Creek Member of the Bitter Springs Formation high concentrations of gammacerane indicates the likely presence of heterotrophic eukaryotes, possibly ciliates that presumably inhabited the chemocline and thrived under anaerobic conditions. However, steranes of aerobic eukaryotes were not detectable in this member. In the overlying Johnnys Creek Member, traces of cholestane are detected, indicating a low abundance and diversity of eukaryotes thriving in relatively shallow water or on land. Higher C₂₈ - C₃₀ steranes are below detection limits, indicating a unique sterane assemblage likely not sourced by algae, but possibly aerobic heterotrophs. In the 657 Ma Aralka Formation low concentrations of cholestane indicate abundance and diversity of eukaryotes is still low in the inter-Snowball period. Furthermore, we do not find any evidence of pre-Marinoan sponges. The results of this thesis provide new insights into redox conditions in the Neoproterozoic of Australia, and ancient microbial systems from pre-Sturtian and the inter-Snowball periods in times previously absent in the global record.