THE PARADOX OF pO2 IN THE MIDDLE AND LATE DEVONIAN
High levels of marine primary productivity and the consequent development a sulfidic water column in Middle and Upper Devonian epeiric basins may have been facilitated by the emergence of a substantial new source of fixed N. Modern lowland tropical forests are generally not N limited and serve as net exporters of N to aquatic systems. It is likely that warm, moist Devonian lowland forests featured a similar N biogeochemistry. Riverine and atmospheric deposition of terrestrial fixed N facilitated an increase in the relative contribution of anoxygenic photosynthesis to total global C fixation. The Upper Devonian expansion of forests into upland and temperate environments, where N limitation is common, reversed the O2 decline by increasing the ratio of terrestrial O2 produced to terrestrial fixed N exported and decreasing the relative contribution of anoxygenic photosynthesis.