A Theory of Atmospheric Oxygen

There is no direct geologic record of the level of free oxygen in the atmosphere over Earth history. Indirect proxy records have led to a canonical view of atmospheric pO2, according to which the atmosphere has passed through three stages. During the first of these periods, corresponding roughly to the Archean eon, pO2 was less than 0.001% present atmospheric levels (PAL). Oxygen levels rose abruptly around 2.4 billion years ago, a transition referred to as the “Great Oxidation Event” (GOE). This event marks the beginning of the second phase in the history of oxygen, corresponding roughly to the Proterozoic eon, during which pO2 was in the range of 1% to 10% PAL. Between the latest Neoproterozoic and the early Phanerozoic eon, oxygen rose again, beginning the final stage in the history of oxygen, characterized by essentially modern levels of pO2. The processes governing this evolution of the atmosphere are poorly understood. The bio geochemical cycles of redox-sensitive species in the ocean and atmosphere, including oxygen, carbon, iron, and sulfur, must somehow stabilize pO2 on billion-year time scales, much longer than the residence time of the individual species, and yet also allow pO2 to achieve equilib- rium at widely divergent levels at different points in time. Only with a clear understanding of these steady-state processes can we understand how pO2 will respond to the changes in biogeochemical cycling that may have driven the two major oxidation events. In this thesis we use a model of biogeochemical cycling and laboratory experiments to explore the processes that stabilize pO2 at different levels over Earth history. We find that a suite of negative feedbacks, including the oxygen-sensitivity of organic carbon burial, allow the stability of oxygen at modern levels. These feedbacks leave pO2 very insensitive to most aspects of the biogeochemical system, such that stable, Proterozoic levels of pO2 can only be explained by a smaller supply of phosphorus to the biosphere at that time. Experimental results show that inorganic scavenging processes, which compete with biology for phosphorus, may be more effective in low-oxygen environments due to differences in iron-redox cycling. We explore redox dynamics in the Archean by coupling our biogeochemical model to a hydrogen escape calculation that incorporates the effects of changing oxygen levels on thermosphere composition and temperature. We find that the Archean was characterized by several different steady states of oxygen, each corresponding to a different stage in the evolution of life. Furthermore, interactions between the cycles of carbon, oxygen, iron and calcium give rise to a previously unrecognized positive feedback. Our model results show that this feedback allows Archean pO2 to increase rapidly to a new steady state at Proterozoic levels, given a large enough perturbation. The high levels of atmospheric carbon dioxide following a Snowball Earth glacial event do act as such a trigger in our simulations, providing a hypothesis for the apparent synchronicity between the GOE and the Paleoproterozoic Snowball glacials.Earth and Planetary Science