How Earth's atmosphere evolved to an oxic state: A status report

The evolution of the Earth's atmosphere is essentially the story of atmospheric oxygen. Virtually every realm of the Earth sciences-biology, geology, geochemistry, oceanography and atmospheric science-is needed to piece together an understanding of the history of oxygen. Over the past decade, new data from these fields has shown that there were two significant increases in atmospheric O-2 levels at around 2.4-2.3 and 0.8-0.6 billion years ago, respectively. Throughout Earth history, oceanic sulfate concentrations appear to have increased in accord with greater O-2 evels, while levels of methane, a strong greenhouse gas, may have inversely mirrored O-2. Both oxic transitions occurred in eras characterized by Snowball Earth events and significant disturbances in the carbon cycle, perhaps associated with increases in O-2 and losses of methane. To understand what controlled the oxygenation of the atmosphere, it is necessary to determine how O-2 is consumed on geologic time scales through reaction with reductants released from the Earth's crust and mantle. There was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere, and a plausible explanation is that excess reductants scavenged photosynthetic O-2 from the early atmosphere. However, a quantitative understanding of how and why O-2 became abundant on our reducing planet is still lacking. Thus, the study of the early atmosphere remains a frontier field with much to be discovered. (c) 2005 Elsevier B.V. All rights reserved.