OXYGENATION OF THE EARLY ATMOSPHERE: WHY THE DELAY?

It is generally agreed that photosynthetic activity by cyanobacteria was the primary source of free oxygen through much of the Precambrian. Although the age of the earliest cyanobacteria (and biogenic stromatolites) is still under debate, it seems likely that they were present by 3.5 BYBP, and almost certainly by 2.8 BYBP. Sulphur isotope fractionation indicates that the atmosphere changed from anoxic to oxic over a very short interval about 2.1 BYBP. Given their autotrophic bacterial nature it is surprising (as has been noted several times) that cyanobacteria did not more quickly convert the hydrosphere and atmosphere to a free-oxygen state. Simple calculations demonstrate that cyanobacteria are capable of fixing sufficient carbon to oxidize the near surface environment in a very short time. The undeniable fact that the process took several hundred million years, perhaps more than a billion, indicates that there were either sinks to absorb the oxygen produced, or that the cyanobacteria were in some way inhibited. The available sinks are not sufficient to compensate for the biological capacities of bacterial reproduction. The only real alternative is some limitation on the oxygen producers. There are three main nutrients that immediately arise for consideration: iron, nitrogen and phosphorus. None of these appear to have been sufficiently restricted in supply to suppress cyanobacteria enough to stretch out oxygen production for a billion years. There must have been some other check on photosynthesis, one that is only “relaxed” over the long time span implicit in the delay between cyanobacterial appearance and atmospheric oxygenation. The only candidate that seems capable of doing this is carbon dioxide. If the early atmosphere was CO2 rich, it should have been an abundant resource for cyanobacteria. However, if the early earth had limited amounts of CO2 available, photosynthesis would be limited by the rate of carbon dioxide production. If CO2 was produced from more reduced forms by interactions between carbon compounds and minerals in the hot upper mantle, then CO2 availability would depend on the rate of plate tectonic processes and photosynthetic oxygen production would be limited by the rate of delivery of CO2 via volcanoes. This is consistent with the delay of oxygenation observed in the geologic record.