Another method involves computer modeling of past oxygen and carbon dioxide levels through time, based on a set of equations and then checking these model values with the mineralogical or paleontological evidence to validate the models. There have been a number of models specifically derived to deduce past oxygen and carbon dioxide levels through time, with the set of equations for calculating levels of carbon dioxide referred to as “GEOCARB” being the most elaborate and oldest. This model and a separate model for calculating oxygen have been developed by Robert Berner and his students at Yale University.

GEOCARBSULF

The GEOCARBSULF model is a recent combination of the much earlier models for carbon dioxide (GEOCARB) and oxygen (isotope mass balance model). It is a computer model that takes account of the many factors thought to influence atmospheric oxygen and carbon dioxide.

A computer model such as GEOCARBSULF must take account of “forcings,” processes that affect the oxygen levels. Chief among these are the rate of metamorphic and volcanic degassing of reduced carbon- and sulfur-containing gases, the rate of mountain uplift, sea level change, burial of organic matter accompanying land plant evolution, and colonization of land by plants. Each of these factors influences the burial rates of reduced carbon and pyrite sulfur (which cause atmospheric oxygen to increase) and the rates of erosion and thermal decomposition to volcanic gases of sedimentary rocks containing significant quantities of reduced carbon and pyrite sulfur (which cause oxygen to decrease).

Understanding the history of oxygen through time thus involves understanding the causes for the rise and fall of oxygen, and thus it is imperative to understand when and if burial and weathering rates of organic carbon and pyrite were either enhanced or inhibited. Carbon dioxide levels are also influenced by these factors. They are also influenced by the enhancement of weathering by land plants; the apportionment of carbonate burial between deep and shallow seas; and the change of insolation (the amount of sunlight hitting the planet through



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