REDOX LANDSCAPE OF THE PALEOPROTEROZOIC OCEANS: IMPLICATIONS FOR EARLY EUKARYOTIC EVOLUTION

The Paleoproterozoic Era (2.5-1.6 Ga) was a time of dramatic environmental and biological changes. The first permanent increase in atmospheric and marine oxygen levels – the Great Oxidation Event (GOE) – had begun by ~2.4 Ga, and fairly oxygenated surface environments persisted until ~2.0 Ga. By ~1.7 Ga the fossil record shows compelling evidence of early eukaryotes, though recent work is suggesting that the first eukaryotes may have in fact arisen earlier, perhaps even coincidentally with the onset of the GOE. In contrast, biomarker records indicate that eukaryotes were minor contributors to organic productivity until ~0.7 Ga. Precisely resolving the timeline of Earth’s oxygenation and the initial emergence of eukaryotic life thus remains a fundamental task in geobiology, and furthermore carries implications for the search for life beyond Earth. Here we discuss the environmental state of the Paleoproterozoic and review recent work that has aimed to characterize the redox landscape of the oceans across the GOE, with a particular emphasis on recent insights from selenium and nitrogen isotope geochemistry. These records suggest that redox- and nutrient-based limitations on eukaryotic evolution were lifted across wide swathes of the ocean between ~2.3 and ~2.0 Ga. Whether eukaryotes temporarily filled these new ecological niches, or were in fact delayed in their emergence for intrinsic evolutionary reasons, remains the subject of future work. In either case, answering this question will require a sustained effort to generate detailed chemostratigraphic and paleobiological datasets across this critical transition in our planet’s history.