Using the average nitrogen/carbon ratio (N/C) of sedimentary organic matter, coal, and volcanic/metamorphic gases, existing models of the carbon cycle have been used to calculate the fluxes of nitrogen to and from the atmosphere via the weathering, burial, and metamorphic/volcanic decomposition of organic matter. Results indicate no appreciable change, less than one percent of that present, in the mass of atmospheric N2 over Phanerozoic time. This is in sharp contrast to atmospheric O2, which shows notable variations. Variation of the mass of O2, but not that of N2, must have led to variation in total atmospheric pressure with time. Results are not altered by considering N/C ratios ten times higher than the average for sedimentary rocks nor by considering nitrogen release from silicate weathering. The residence time of N2 in the atmosphere, relative to exchange with and storage in crustal rocks, is estimated to be about one billion years.

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