Origin of the Earth's Atmosphere


Early Earth would have been very different and inhospitable compared to the Earth today.

Evolution of the Atmosphere

Atmosphere - Envelope of gases that surrounds the Earth. Used by life as a reservoir of chemical compounds used in living systems. Atmosphere has no outer boundary, just fades into space. Dense part of atmosphere (97% of mass) lies within 30 km of the Earth (so about same thickness as continental crust).
(figure and table from Lutgens and Tarbuck, The Atmosphere, 8th edition)

First Atmosphere

Second Atmosphere

Produced by volcanic out gassing.

Addition of O2 to the Atmosphere

Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels in the atmosphere? Revisit the oxygen cycle: Throughout the Archean there was little to no free oxygen in the atmosphere (<1% of presence levels). What little was produced by cyanobacteria, was probably consumed by the weathering process. Once rocks at the surface were sufficiently oxidized, more oxygen could remain free in the atmosphere.

 During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10 %. Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3 Ga. Present levels of O2 were probably not achieved until ~400 Ma.

Evidence from the Rock Record

Conclusion - amount of O2 in the atmosphere has increased with time.

Biological Evidence

Atmospheric oxygen built up in the early history of the Earth as the waste product of photosynthetic organisms and by burial of organic matter away from surficial decay. This history is documented by the geologic preservation of oxygen-sensitive minerals,
deposition banded iron formations, and development of continental "red beds" or BIFs. Figure from the University of Michigan's Introduction to Global Change web site.

Atmospheric Structure
Not only does the atmosphere have a relatively stable composition, but it also has a structure to it.  Below are two figures from Lutgens and Tarbuck (The Atmosphere, 8th edition) showing vertical profiles of the atmosphere.  Note that the first major break, or boundary in the atmosphere as we ascend is the tropopause.