Co-evolution of Life and Environment across Earth History: Empirical & Modeling Constraints on Nutrient Cycling, the Rise of Oxygen, and Eukaryotic Evolution

Abstract

That life and its environment have co-evolved through Earth’s history, and in doing so have maintained the conditions required for planetary habitability, is one of the great discoveries in the natural sciences during 20th century. This dissertation joins a large body of work that has arisen since this discovery permeated the Earth sciences; the goal of the work now is not only to better understand the processes that enabled Earth to reach its present state, but also to use the Earth through time as an analog for better understanding the diversity of exoplanets that are rapidly being discovered around nearby stars. Specifcally, the work described here aims to place novel constraints on various aspects of Earth’s evolution as a habitable planet. In Part I, the oxygenation history of Earth’s surface is probed using a relatively new proxy - selenium geochemistry. These data reveal substantial atmospheric and marine oxygen accumulation during the Great Oxidation Event, and also implicate a close relationship between oxygenation and the cycling of major nutrients. Building off these findings, Part II aims to elucidate the response of the major nutrient cycles (nitrogen and phosphorus) to the oxygenation of Earth’s surface environment. These chapters demonstrate that major shifts occurred in both the nitrogen and phosphorus cycles across the Great Oxidation Event, with implications for the early evolution of eukaryotes. In Part III, various new tools and approaches for studying ancient life and environments are explored. This includes a review of C/N ratios in marine sediments and their possible relationship to oxygen availability, as well as a novel application of nitrogen isotope ratios to deduce nitrogen fixation in fossil plants. In total, the projects contained in this dissertation provide various pieces of information that deepen our understanding of Earth’s evolution as a habitable planet. Ultimately these findings may serve to guide our interpretation of exoplanetary atmospheric compositions (and their biospheric implications), or even to help us grasp the magnitude of forthcoming environmental change as humans continue to act as a forceful environment- modifying agent.