Geological nitrogen cycle and atmospheric N2 over Phanerozoic time

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 N-2 over Phanerozoic time. This is in sharp contrast to atmospheric O-2, which shows notable variations. Variation of the mass of O-2, but not that of N-2, 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 N-2 in the atmosphere, relative to exchange with and storage in crustal rocks, is estimated to be about one billion years.