THE 500 MA DELAY IN THE RISE OF ATMOSPHERIC O2 AND THE EVOLUTION OF COUPLED BIOGEOCHEMICAL CYCLES

There appears to be a 500 Ma time lag between the appearance of oxygenic photoautotrohs the oxidation of Earth's atmosphere. The abundance of reducing equivalents cannot, by itself, account for the lag. Here we examine the lag in the context of the biologically mediated nitrogen cycle. Initially, the rise of water splitting reaction centers and the pandemic spread of oxygenic photoautotrophs under reducing conditions required significant structural alterations in the reaction centers themselves as well as alternative respiratory pathways . The linkage of photosynthetic and respiratory metabolic processes led to drastic changes in the metabolism of nitrogen and in its biogeochemical cycling. Whereas N2 fixation is fundamentally an anaerobic respiratory process, following the evolution of O2, the product of N2 fixation, NH4+, would have been rapidly oxidized to NO2- and subsequently to NO3- by nitrifying chemoautotrophs. The oxidized N-species are utilized by anaerobic microorganisms(denitrifiers) as respiratory electron acceptors, resulting in the production of N2 and its loss to the atmosphere. Under completely anaerobic conditions, the N cycle is stable, while under quasi oxidized conditions, a zone of instability exists, where fixed N can potentially be lost from the oceans. .There is a strong feedback between N2 fixation and O2 evolution that both enhances the former when the latter is low, and reduces N2 fixation as O2 increases above ca. 20% PAL. An oxidized atmosphere further constrains the amount of fixed nitrogen due to irreversible damage of nitrogenase Fe-S clusters by molecular oxygen. This fixed N barrier is examined in the context of box models, in which feedbacks between N2 fixation, Fe availability, and P supply are explored as constraints on C fixation. Finally, we examine how these limiting factors are related to C burial and the potential production of excess oxidizing equivalents, without which Earth's atmosphere could not have accumulated O2.