Response of the Nitrogen Cycle to Ocean Redox Chemistry During the Great Oxidation Event

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One of the most fundamental questions that drives natural sciences is how life evolved on Earth. Understanding how life evolved on this planet also informs our search for habitable planets in other solar systems. What we now know beyond a reasonable doubt is that simple single-celled organisms evolved in the ocean greater than 3 billion years ago. We also know that the chemistry of the Earth's surface (both the atmosphere and the oceans) has undergone dramatic changes since that first cell division occurred. What we then seek to understand is how the evolution of life has responded to, and in some cases driven, these changes in Earth surface chemistry. Understanding how life responded to geochemical changes on the planet in the past will additionally help us to more clearly predict how life will respond to chemical changes in the future, for example associated with rapidly approaching climate change. One of the major parameters that affects life in the ocean is the availability of dissolved oxygen, which is not only essential for all higher life forms to exist, but also directly controls the distribution and abundance of essential elements that make up the building blocks of life. For the first ~3.5 billion years of Earth history, the ocean was characterized by a lack of dissolved oxygen, a situation known as anoxia. Oxygen first began to build up in the atmosphere and oceans between ~2.4 and 2.3 billion years ago, in perhaps the most dramatic event in the history of Earth surface chemistry, termed the Great Oxidation Event. It was not until ~580 million years ago that oxygen levels in the atmosphere rose sufficiently to completely oxygenate the oceans. In the intervening ~2 billion years, the chemistry of the oceans fluctuated rapidly between various levels of oxygenation. Thus a major focus for scientific research recently has been on identifying and understanding the response of biology and the cycling of essential nutrients to variable amounts of oxygen in the environment. Nitrogen is an essential element in all living organisms, required along with carbon and phosphorus for the formation of proteins, amino acids, DNA and RNA. The distribution of nitrogen in the biosphere is controlled by biological reactions that respond to environmental parameters, including the amount of available oxygen. Despite the importance of nitrogen to life, very little is known about how the nitrogen cycle responded to fluctuations in oxygen concentrations over Earth history. The aim of this project is specifically to interpret the response of the global nitrogen cycle to changes in the oxygen content of the atmosphere and oceans during the Great Oxidation Event. This will be accomplished by laboratory experiments with microorganisms that are important in the modern nitrogen cycle, and application of the results of the experimental studies to the investigation of nitrogen and indicators of oxygenation in sedimentary rocks spanning the Great Oxidation Event.