Biological regulation of Earth's early atmosphere : metal isotope constraints on Neoarchaean methane cycling

Abstract

The Great Oxidation Event (GOE, c.2.43 Ga) marks a fundamental change in the interaction between the biosphere, atmosphere, and Earth’s surface environment. Recent studies suggest that Earth’s anoxic pre-GOE atmosphere was also heavily influenced by biological feedbacks. Notably, recent geochemical records (carbon and sulphur isotopes) suggest that a hydrocarbon-rich haze periodically formed as a result of enhanced methane fluxes, termed periodic haze events, from the biosphere. The extent to which the biosphere mediated these haze events is under-constrained, yet hypothesised to result from enhanced biological methane cycling. Copper and nickel are both bioessential trace metals utilised by aerobic methanotrophic bacteria and methanogenic archaea respectively, and it has been already demonstrated that methanogenic archaea fractionate Ni isotopes during metabolic uptake. This thesis applies these metal isotope systems to two well-preserved Archaean rock cores; the 2.5 Ga core GKF01 from the Griqualand West Basin, South Africa, and the 2.7 Ga core SV1 from the Hamersley Basin, Western Australia. Both cores exhibit carbon isotope evidence consistent with methane cycling, but reflect contrasting Archaean environments. In Chapter 3, a high-resolution copper isotope record measured in a suite of marine shales and carbonates from core GKF01 is presented. The data show a 0.8‰ range in copper isotope composition and a negative excursion that predates the onset of a pre-identified haze event. Examined alongside previously published carbon and sulphur isotope records, these data demonstrate a clear role for aearobic methanotrophy in the incorporation of methane into Late Archaean sediments. In Chapter 4, a similar high-resolution copper isotope record is analysed in sediments from core SV1, and copper isotope cycling in considered within the context of a mildly oxygenated, high pH lacustrine setting. A Copper isotope record reflecting approximate crustal average (c. 0.05‰) is reported, indicating either (1) a system in which no processes fractionate copper isotopes, (2) sedimentary phase homogenisation and/or overprinting of primary copper isotope signals by volcanic detrital input, or (3) complete consumption of copper by a copper-dependent process, however it is most likely that the record reflects process (2). Finally, Chapter 5 presents a high-resolution record of nickel isotopes in core GKF01 which reflects previously reported values of euxinic sediment and black shales (c. 0.28‰). The data presented in this thesis demonstrate the clear potential for these isotope systems to be applied as biomarkers for the investigation of deep time biological methane cycling, and recommendations for future studies are also explored.