Chemical stability of the atmosphere and ocean

Abstract

The Earth system processes most of the chemical components of its atmosphere and oceans in geologically short periods of time. It does this in a regulated way, one that maintains a remarkably constant surface environment. This we know primarily from the fossil record of uninterrupted, complex life on Earth that extends over the last billion years. We are only beginning to understand the feedbacks that control the chemistry of the oceans and atmosphere. Numerical models of biogeochemical cycles are most stable when there is feedback between the amount of a chemical component in the ocean or atmosphere and its transfer to or from that reservoir. Coupling of subsystem, especially of those operating on different time scales, enhances stability. An example of the role of feedback in stabilizing Earth's chemical environment is the mechanism of control of atmospheric oxygen. There appears to be no strong relationship between oxygen level and oxygen consumption. However, oxygen production may be a function of oxygen level; the burial rate of organic carbon (oxygen production) in marine sediments may be sensitive to bottom water oxygenation levels. Also, combustion may be an effective mechanism of transferring nutrients (namely phosphorus) from efficient, terrestrial ecosystems to less efficient, marine ecosystems. When O 2 rises, fires become more frequent and P is transferred to the ocean, stimulating marine organic carbon burial but depressing global burial rates. Global O 2 production rates decline, as does the O 2 level: a negative feedback. Models of ocean chemical composition are presently incapable of reproducingthe temporal constancy indiated by geological observations. These models do not incorporate ion-exchange equilibria as important processes in marine geochemical cycles. When included, these equilibria significantly damp the fluctuations in ion ratios calculated by the extant models.