The problematic rise of Archean oxygen.

Archean Oxygen In their intricate study, Catling et al. (1) attempted to explain the rise of atmospheric oxygen on the early Earth. The model they presented relies on a unique anaerobic ecosystem in which necessarily complex microbial fermentations interact closely with methanogens to decompose cyanobacteria and control the global preservation rate of organic carbon. The model begins schematically with photosynthesis, under which 1 mol of organic carbon is produced and 1 mol of free oxygen is released for every mol of CO2 and H2O input [equation 2 of (1)]. Most of that net primary productivity is assumed in the model to convert rapidly to methane [equation 3 of (1); net CH2O yields 0.5CH4] to establish a greenhouse atmosphere of 100 to 1000 parts per million (ppm) CH4; the remainder is buried in sediments averaging ;1% kerogen carbon. A steady amount of the methane is assumed to leak to the stratosphere to enhance hydrogen escape, which results in a net gain of ;10 mol O2 year 21 over ;10 years [equations 4a and 6 of (1)]. The fate of the photosynthetic free oxygen produced each year, however, presents a serious problem for this proposal. Accepting the minimum biogenic-methane input of 3 3 10 mol year assumed by Catling et al. [notes 9 and 10 and equation 3 in (1)] requires that twice that amount of net primary productivity, or 6 3 10 mol C, be recycled annually to CH4. Organic carbon escaping recycling and buried in sediments represents net production of another 10 mol C year [table 1 of (1)]. Thus, at least 7 3 10 mol net cyanobacterial C are produced annually under the model, with 7 3 10 mol free O2 being simultaneously released into the global anoxic environment. The oxidation of “graphite” settling into the troposphere following stratospheric photolysis of CH4 [equation 4b and table 1 of (1)] would leave 6.3 to 6.9 3 10 mol free O2 to be consumed each year (2). Given any geologically and biologically plausible constraints for the Archean, no resources are available to scavenge this annual output of free oxygen. Oxidizing methane [note 9 of (1)] is obviously counterproductive; this would recycle what was produced and eliminate the “greenhouse,” the hydrocarbon smog itself, or both (3). It would also simultaneously add isotopically light CO2 to the atmosphere and diminish hydrogen escape. Highly reactive reduced gases (H2S, H2) were either insufficient to accomplish the necessary scavenging or are unsupported altogether by the rock and isotope records (4–8). Scavenging the oxygen by oxidizing the necessarily large amounts of dissolved Fe also yields implausible rocks (6–9). The “back reaction of O2 and CH2O via respiration” [note 10 of (1)], essential to the model, would also use the same carbon twice by oxidizing the photosynthetic carbon used to produce the methane in the first place—and, in any event, is an aerobic process requiring pO2 greater than ;0.002 atm (6–8, 10). There is abundant evidence to support a low-O2 oxic atmosphere (;0.003 atm) for the Archean (6–8, 11, 12)—and it remains debatable whether other evidence or models to the contrary (1) can indeed distinguish with any certainty between pO2 values of either ,0.0008 atm or ;0.003 atm and a “transitional” value of ;0.03 atm [reference 1 of (1)]. Both the amount of oxygen in early Earth’s atmosphere and the rise of oxygen on the early Earth remain problematic.