The cycles of carbon and sulfur as they affect atmospheric oxygen. Prolonged shifts of each element from the reduced to the oxidized form ultimately require an opposite shift in the other element to prevent excessive fluctuations of atmospheric oxygen. Deep burial and thermal decomposition of the various forms of carbon and sulfur can occur, leading to the emission of gases to Earth's surface. This degassing, along with the air oxidation of any accompanying reduced gases, is included here under the term “weathering.” Also, reduced organic sulfur compounds are lumped together with pyrite.