Molecular and isotopic signatures of life surrounding the Neoproterozoic Snowball Earth events

Life on Earth transitioned from being mainly dominated by bacteria prior to the Cryogenian Snowball Earth events (717a 635 Ma) to an ecosystem in which biologically more complex eukaryotes (e.g. Archaeplastida and Metazoa) where able to proliferate. Using lipid biomarker analyses combined with inorganic and isotopic geochemical techniques, this dissertation investigates the sedimentary rocks of the Chuar Group ( 750 Ma, Grand Canyon, USA) and Araras Group ( 635 Ma, Amazon Craton, Brazil), to gain a greater understanding about the environments and community compositions before and after the Cryogenian glaciations. This work shows how the ecosystem recovered after the Snowball Earth events and investigates the carbon isotopic relationship between bulk organic matter and individual hydrocarbons in the Proterozoic. Overall this work observes heterotrophic organisms played a significant role in the investigates Precambrian environments, highlighting their importance in ancient ecosystems

Molekulare und isotopische Signaturen des Lebens vor und nach den Neoproterozoischen Snowball Earth events

Life on Earth transitioned from being mainly dominated by bacteria prior to the Cryogenian Snowball Earth events (717a 635 Ma) to an ecosystem in which biologically more complex eukaryotes (e.g. Archaeplastida and Metazoa) where able to proliferate. Using lipid biomarker analyses combined with inorganic and isotopic geochemical techniques, this dissertation investigates the sedimentary rocks of the Chuar Group ( 750 Ma, Grand Canyon, USA) and Araras Group ( 635 Ma, Amazon Craton, Brazil), to gain a greater understanding about the environments and community compositions before and after the Cryogenian glaciations. This work shows how the ecosystem recovered after the Snowball Earth events and investigates the carbon isotopic relationship between bulk organic matter and individual hydrocarbons in the Proterozoic. Overall this work observes heterotrophic organisms played a significant role in the investigates Precambrian environments, highlighting their importance in ancient ecosystems

Biogeochemistry of the Chuar Group, Grand Canyon and the Araras Group, Brazil

in preparatio

Substantial maturity influence on carbon and hydrogen isotopic composition of n-alkanes in sedimentary rocks

Carbon (δ13C) and hydrogen (δ2H) compound-specific isotope analyses on sedimentary hydrocarbons are widely used for ecological reconstructions and oil-source rock or oil-oil correlations. However, the effects of thermal alteration on isotopic composition are not fully understood, potentially imparting a bias on interpretation of older and more mature sedimentary sequences. We measured δ13C and δ2H of n-alkanes in 23 extracted bitumens from the 1.64 Ga Paleoproterozoic Barney Creek Formation in the southern McArthur Basin, Australia. The samples cover a wide range of thermal maturities with calculated vitrinite reflectance (Rc) values from 0.4% to 1.3%. Our results illustrate that while δ13C of kerogen remains relatively constant, the δ13C and δ2H of n-alkanes have a strong positive correlation with thermal maturity. Average δ13Calk increase by 6.8‰ and δ2Halk by 69‰ among the samples in the analysed maturity range. At the same time, the carbon isotopic offset between n-alkanes and kerogen (Δδ13Calk–ker) climbs from 1.3‰ to 8.5‰ with increasing maturity. Therefore, the substantial maturity influence on stable carbon and hydrogen isotopes of n-alkanes must be considered in palaeoecological and petroleum correlation studies. In the initial stages of maturation, n-alkanes from the Barney Creek Formation display increasingly positive “isotope slopes” in plots of δ13Calk against carbon number. However, with further maturation, the slopes became increasingly negative. The isotope slope inversion indicates that the dominant mechanisms for n-alkane generation and degradation changed during the progression from early diagenesis to metagenesis. Numerical models suggest that the formation of positive and negative isotope slopes may be driven by the balance of the formation of n-alkanes from kerogen and their subsequent degradation, and by dependence of the degradation rate constant k on n-alkane chain length.This research was financially supported by Australian Research Council grants DP160100607 and DP170100556 (to JJB). GV acknowledges support from an Australian Government Research Training Program (RTP) Scholarship. AJMJ publishes with permis- sion of the CEO, Geoscience Australi