Protracted oxygenation of the Proterozoic biosphere

Abstract

Our current understanding of Proterozoic (2.5–0.54 Ga) biospheric evolution highlights the critical role played by the progressive oxygenation of Earth's surface environments and its effect on oceanic redox potential, nutrient availability, carbon isotopic change, and the evolution of life. The most obvious changes occurred in the first 200 million years of the Proterozoic aeon, with the irreversible oxidation of surface environments, and in the last 200 million years of the Proterozoic, when oxygen reached levels sufficient for the diversification of Metazoa. Although the impact of these events cannot be denied, it remains uncertain to what extent these events represent abrupt, large-scale changes in Earth oxygenation versus points of critical threshold along a continuum of biospheric evolution. In this article, we review the current understanding of ocean-atmosphere oxygenation and explore how a view of Earth's oxygenation as a protracted event, spanning nearly 2 billion years, suggests that Phanerozoic-style models might be inadequate to describe the Proterozoic carbon and sulphur cycles and their resultant isotopic records. Furthermore, Proterozoic evolutionary patterns may be better interpreted to reflect localized environmental conditions, rather than abrupt global environmental shifts.