Early Earth, Page 23: [Previous Page] [Next Page] [Class Home Page] Textbook pages 54 - 71

The Carbon Cycle on Earth

Atmospheric CO₂ comes from Volcanism. It is released in Mid-Oceanic Ridges (upwelling mantle) and in subduction zones. Early on, there was much volcanism, thus a lot of CO₂ was put into the early Earth atmosphere.

CO₂ is removed from the atmosphere via Photosynthesis from plants, but the Ocean and carbonate sediments are the greatest reservoir for CO₂. Moving back and forth between the Atmosphere and in the form of Carbonates is controlled mostly by the current temperature of the Earth.



This cycle has been going on since the beginning. If the Oceans are warm, the CO₂ more readily precipitates out in the form of carbonates. This allows still more to be removed from the Atmosphere. However, if you reduce CO₂, the earth cools due to reduced 'greenhouse' effect. Conversely, cool oceans precipitate out less CO₂ in to carbonates, which keeps atmospheric levels of CO₂ high and strengthens the greenhouse effect!

In this way, the oceans and volcanism kept the Earth at a very similar temperature, allowing for liquid water (not too hot, not too cool), even in the first billion years when the Sun was not as bright. Burning of fossil fuels has changed this balance in recent years, and may be leading to a heighten recent warming.

See the American Geophysical Unions recent position on this issue here.