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Exploring the Texture of Ocean-Atmosphere Redox Evolution on the Early Earth

  • Author(s): Reinhard, Christopher Thomas
  • Advisor(s): Lyons, Timothy W
  • et al.
Abstract

The evolution of oxygenic photosynthesis has dramatically reshaped the chemistry of the surface Earth, and the presence of significant quantities of O2 in the atmosphere and ocean now drives the fundamental dynamics of nearly all quantitatively significant biogeochemocal cycles (C, S, P, N, Fe). Whether by direct consumption through the metabolic demands of large, complex organisms, or through the recycling of essential substrates within microbial ecosystems, biologically produced O2 provides nearly all of the compounds used in metabolic electron transfer on a global scale. Although it is widely accepted that the partial pressure of O2 in Earth's atmosphere has increased through time (with attendant, although somewhat complex, changes in ocean ventilation), there is still much debate surrounding the timing of the emergence of oxygenic photosynthesis and little is known about the detailed tempo and mode with which this metabolic innovation came to shape early Earth surface chemistry.

This dissertation explores the early oxygenation of Earth's atmosphere and the relationship between atmospheric oxygen levels and ocean ventilation from a variety of perspectives. First, empirical data based on an integrated suite of paleoredox proxies is used to suggest that biological oxygen production emerged and began exerting significant effects on Earth surface chemistry at least 100 million years prior to the initial accumulation of large quantities of O2 in the atmosphere (often referred to as the "Great Oxidation Event", approximately 2.4 billion years ago). The implications of this time lag between metabolic innovation and large-scale biogeochemical reorganization are explored through a series of quantitative models, focusing on the thermodynamics and kinetics of mineral reactions under various Earth surface conditions, regional oceanographic modeling of surface ocean O2 cycling, and a global sulfur isotope mass balance model that explicitly incorporates rare sulfur isotope systematics (33S, (36S) and the dynamics of sedimentary recycling on long timescales. Finally, the dynamics of ocean ventilation following the initial accumulation of oxygen in the atmosphere are explored by combining a large trace metal database with a spatially explicit mass balance model that exploits the differing redox behavior and surface cycling of molybdenum (Mo) and chromium (Cr).

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