Isotope fractionation and atmospheric oxygen: Implications for phanerozoicO-2 evolution

Models describing the evolution of the partial pressure of atmospheric oxyg en over Phanerozoic time are constrained by the mass balances required betw een the inputs and outputs of carbon and sulfur to the oceans. This constra int has Limited the applicability of proposed negative feedback mechanisms for maintaining Levels of atmospheric O-2 at biologically permissable Level s. Here we describe a modeling approach that incorporates O-2-dependent car bon and sulfur isotope fractionation using data obtained from Laboratory ex periments on carbon-13 discrimination by vascular Land plants and marine pl ankton. The model allows us to calculate a Phanerozoic O-2 history that agr ees with independent models and with biological and physical constraints an d supports the hypothesis of a high atmospheric O-2 content during the Carb oniferous (300 million years ago), a time when insect gigantism was widespr ead.