THE PARADOX OF pO2 IN THE MIDDLE AND LATE DEVONIAN

Evidence suggests that the atmosphere of the Middle and Late Devonian was characterized by persistently low levels of pO₂ relative to earlier Paleozoic and modern values despite the expansion of forests and the abundant burial of organic carbon in epicontinental basins. This study sought to address the paradox by examining the role of anoxygenic photosynthetic ecosystems in the Late Devonian Appalachian Basin utilizing stable isotopes of organic C, N, and sulfide S and biomarkers diagnostic of photic zone euxinia. By fixing C into biomass without producing O₂ as a byproduct, sulfide-driven anoxygenic photosynthesis can serve to moderate the flux of O₂ to the atmosphere generated by oxygenic photosynthesis.

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.