title: Box models for the evolution of atmospheric oxygen:  an update.
abstract: A simple 3-box model of the atmosphere/ocean system is used to describe the various stages in the evolution of atmospheric oxygen.  In Stage I, which probably lasted until redbeds began to form about 2.0 Ga ago, the Earth's surface environment was generally devoid of free O2, except possibly in localized regions of high productivity in the surface ocean.  In Stage II, which may have lasted for less than 150 Ma, the atmosphere and surface ocean were oxidizing, while the deep ocean remained anoxic.  In Stage III, which commenced with the disappearance of banded iron formations around 1.85 Ga ago and has lasted until the present, all three surface reservoirs contained appreciable amounts of free O2.  Recent and not-so-recent controversies regarding the abundance of oxygen in the Archean atmosphere are identified and discussed.  The rate of O2 increase during the Middle and Late Proterozoic is identified as another outstanding question.
authors.affiliation: Department of Geosciences, The Pennsylvania State University, University Park 16802, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Kasting
mesh_terms: D001272:Atmosphere; D005075:Biological Evolution; D002244:Carbon / Q000737:chemistry; D002625:Chemistry, Organic; D018534:Earth, Planet; D008956:Models, Chemical; D008962:Models, Theoretical; D009792:Oceans and Seas; D055601:Organic Chemistry Phenomena; D000069497:Origin of Life; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*
chemical_list: D002244:Carbon; D010100:Oxygen