Geologic and Geochemical Constraints on Earth's Early Atmosphere

In this review, the authors examine the geologic and geochemical evidence for the evolution of the atmosphere in the first two billion years of Earth's history. The authors focus on evidence relevant for placing limits on the evolution of Earth's surface oxidation state, which provides both direct and indirect information about atmospheric oxygen levels. They also explore the constraints that can be placed on the abundance and chemistry of other atmospheric gases, such as carbon dioxide, methane, sulfur dioxide, ozone, and nitrogen, although this evidence and the resulting inferences are not as well developed as those for oxygen.The picture that emerges from this record is of an early atmosphere differing from that of today with respect to its oxidation state and content of greenhouse gases. The early atmosphere appears to have included a mixture of greenhouse gases, thought principally to be carbon dioxide and methane, at sufficient levels to maintain liquid oceans and temperate to warm climates. Questions remain about how stable Earth's climate, atmospheric composition, and chemistry may have been on the hundreds of millions to billion year timescales. The geologic and geochemical record of Earth's first two billion years ends with the Great Oxidation Event (GOE), a dramatic change in the oxidation state of the Earth's atmosphere that occurred in the earliest Proterozoic. The GOE led to the expansion of oxygenated environments and, eventually, allowed for the evolution of multicellular life.