Early evolution of environments and metabolism: insights from nitrogen, selenium and sulfur isotopes

Abstract

The evolution of planet Earth and its biosphere are tightly linked through global biogeochemical cycles, and this dissertation seeks to explore this linkage during the Precambrian with new strategies and geochemical techniques. The first chapter is the result of a group project in which we propose that multiple environments and processes were involved in prebiotic chemistry. We conclude that the origin of life can be more plausibly explained if the various building blocks of living cells slowly emerged from global geochemical cycles. In the second part of my thesis (Chapters 2-3), I explore how microorganisms exploited and modified these cycles, in particular the global sulfur cycle. Statistical analyses of sulfur concentrations and isotopic ratios in marine mudrocks support an early Archean origin of microbial sulfate reduction and late Archean enhancement of biological pyrite oxidation on land, leading to increasing fluxes of sulfate and other nutrients to the ocean. In the third part (Chapters 4-7), I further test this hypothesis through analyses of selenium isotopes and abundances. After establishing a new analytical technique, I show that selenium was mobilized by oxidative weathering in the late Archean, concurrently with sulfur. Because of selenium's high redox potential, these results confirm the antiquity of oxygenic photosynthesis. A compilation of selenium data through time further indicates oxygenation of the deep ocean in the Neoproterozoic or mid-Paleozoic, which may have facilitated the rise of metazoans. The late Permian mass extinction, however, was apparently not directly linked to an episode of ocean anoxia. The third part of this work (Chapters 8-10) focuses on nitrogen isotopes as a proxy for nitrogen speciation in the water column. Results show that biological nitrogen fixation using Mo-nitrogenase dates back to at least the mid-Archean and was the dominant nitrogen source at that time. Marine nitrate concentrations were also low in the mid-Proterozoic, which may have restricted the radiation of eukaryotic life. Lastly, I use nitrogen isotopes in lacustrine sediments as a proxy for alkalinity and show that alkaline lakes may have been common on Archean continents, making them potentially important environments for the origin and early evolution of life.