related to some unusual symmetry properties. The mass-dependent isotope effect occurs in asymmetric species, and arises from the difference in zero-point energy of the two formation channels available (e.g., 18O16O + 16O vs 18O + 16O16O for formation of 18O16O16O.) These mass-dependent zero-point energy effects cancel one another out and do not affect the enrichment in heavy isotopes observed in ozone. The mass-independent enrichment in ozone is still not fully understood, but may be due to isotopically symmetric O3* having a shorter lifetime than asymmetric O3*, thus not allowing a statistical distribution of energy throughout all the degrees of freedom, resulting in a mass-independent distribution of isotopes. Mass-independent carbon dioxide fractionation The mass-independent distribution of isotopes in stratospheric ozone can be transferred to carbon dioxide (CO2). This anomalous isotopic composition in CO2 can be used to quantify gross primary production, the uptake of CO2 by vegetation through photosynthesis. This effect of terrestrial vegetation on the isotopic signature of atmospheric CO2 was simulated with a global model and confirmed experimentally. Mass-independent sulfur fractionation Mass-independent fractionation of sulfur can be observed in ancient sediments, where it preserves a signal of the prevailing environmental conditions. The creation and transfer of the mass-independent signature into minerals would be unlikely in an atmosphere containing abundant oxygen, constraining the Great Oxygenation Event to some time after . Prior to this time, the MIS record implies that sulfate-reducing bacteria did not play a significant role in the global sulfur cycle, and that the MIS signal is due primarily to changes in volcanic activity. See also Equilibrium fractionation Kinetic fractionation Isotope geochemistry References Isotopes Geochemistry Fractionation