In this Chapter an overview is developed of aspects of the Proterozoic atmosphere and oceans based as much as possible on geologic evidence, but supplemented by theory, whenever such evidence is indirect, incomplete, or lacking. Much of the theoretical treatment is rather oversimplified and speculative. Several biologically important aspects of the Proterozoic environment are addressed, namely, the partial pressures of oxygen and carbon dioxide in the atmosphere and possible changes in their partial pressures as a function of Precambrian time. Aspects of the chemistry and evolution of the Proterozoic ocean are discussed as well.

Banded iron-formations (BIFs) are the most abundant chemical sediments found throughout much of Precambrian time. Because they are generally devoid of clastic components, their chemistry, their oxidation state, and their temporal distribution provide important clues about the chemistry and the chemical evolution of the Precambrian ocean and atmosphere. Section 4.2 provides a synopsis of the average major element chemistry of banded iron-formations throughout the Precambrian; all iron-formations older than about 1.9 Ga represent very similar chemical systems. Iron-formations formed between 0.8 and 0.6 Ga are distinctly different and are more highly oxidized. Few iron-formations are younger than about 1.8 Ga; a minor resurgence in BIF deposition occurred between 0.8 and 0.6 Ga. After about 1.85 Ga, the atmosphere and oceans became rather highly oxygenated and the oceans as a whole became depleted in iron.