Quantitative modeling of the rise in atmospheric oxygen

Abstract

The abrupt rise of molecular oxygen in Earth’s atmosphere approximately 2.4 billion years ago was perhaps the most profound event in Earth’s history after the evolution of life itself. Biogeochemical cycles in Earth’s atmosphere, ocean, and crust were completely reorganized and it also likely marked the first moment when our planet could be deemed “inhabited” across interstellar space via identification of biogenically produced O2 and O3 in a spectrum of Earth’s atmosphere. This dissertation explores the “Great Oxidation Event” via numerical modeling of evolving ancient atmospheres. In creating a self-consistent description of evolving redox fluxes in the Earth system, we reach the following conclusions. After the evolution of oxygenic photosynthesis, the atmosphere has two primary stable states – one is methane-rich and produces mass-independent fractionation of sulfur isotopes (MIF-S), and one is oxygen-rich and does not produce MIF-S. These two stable states are separated by only a few percent in the fluxes of O2 and CH4 needed to sustain them. The atmosphere evolves rapidly from one state to the other when the net flux of reductants drops below the net flux of oxidants into the atmosphere. The transition between the two states - “the rise of oxygen” - is only feasible once methane levels drop below ~50 ppm. We show numerically that hydrogen escape can drive irreversible oxidation of Earth’s crust, leading to decreasing CH4 concentrations over long timescales. We argue that the disappearance of the MIF-S signal is better described as recording a collapse of atmospheric CH4, rather than the appearance of O2. As CH4 levels decrease, a positive feedback between oxidative weathering, oceanic sulfate concentrations, and the anaerobic oxidation of methane further drives atmospheric instability. Once a critical threshold in CH4 concentration is overcome, the atmosphere transitions from an anoxic to oxic state on the timescale of 103 years. The post-transition levels of O2 and CH4 and the global climate are strongly driven by biological forcing. Considering the events of 2.4 Ga as a “Great Collapse of Methane” helps explain the initiation of Snowball Earth, the disappearance of MIF-S, and the rise of oxygen.