The co-evolution of life and the nitrogen cycle on the early Earth

Abstract

Nitrogen is an essential element for all life as we know it. Its abundance and speciation in the atmosphere and ocean has been dynamic through Earth’s history. The dynamic nature of nitrogen cycling is invariably linked to the dynamic nature of oxygenation on the early Earth, and the evolution of these two biogeochemical cycles influenced biological evolution and diversification. Exploration of the mode, tempo, and scale of such changes will add to our mechanistic understanding of these components of the inhabited Earth-system and should provide insight into how these cycles might operate on an alien planet that is different from our own. Here, I explore four critical points in Earth’s redox and biological evolution: Chapter 2 – The dominantly anoxic world of the Mesoarchean, when aerobic nitrogen metabolisms did not exist, and bioavailable nitrogen could only be obtained through biological nitrogen fixation or remineralized ammonium. Biological nitrogen fixation by the Mo-nitrogenase dominated. Chapter 3 – Incipient oxygenation ~300 Myr before the great oxidation event. We start to characterize the mode, tempo, and scale of these transient oxygenation events, and provide the oldest evidence of aerobic nitrogen cycling 2.66 Gyr ago. Chapter 4 – The state of nitrogen cycling during the Mesoproterozoic. We find further evidence that the Mesoproterozoic was characterized by an oceanic nitrate minimum, and that this nitrate scarcity could have prevented the radiation of early eukaryotes until the end of the Proterozoic. Chapter 5 – Nitrogen cycling across the Hirnantian glaciation and end Ordovician mass extinction event. We find that oceans during this period were characterized by widespread ocean anoxia that led to a scarcity in nitrate much like in the Mesoproterozoic. Nitrate scarcity was temporarily alleviated in near-shore settings during glaciation. Much like in the Pacific Ocean during the Last Glacial Maximum, the carbonate compensation depth at multiple sites seems to have deepened substantially during peak Hirnantian glaciation. These results highlight the relationship between ecology, evolution, nitrogen speciation/abundance, and surface oxygen.