A POSSIBLE EXPLANATION FOR THE GREAT OXIDATION EVENT BETWEEN CA. 2.3 AND 2.0 GA

Before the development of photosystem II, H₂ and H₂S must have been the major reductants of CO₂ during photosynthesis.  Since the development of photosystem II, CO₂ has been reduced largely by H₂O; O₂ was and is a byproduct of this process.  The isotopic composition of carbon in carbonates and organic carbon in sedimentary rocks indicates that ca. 20% of volcanic CO₂ was reduced to organic carbon both before and after the development of photosystem II.  This is best explained by phosphate limitation of photosynthesis and the organic carbon burial.

As long as the rate of O₂ generation was less than the injection of reduced volcanic gases, O₂ was unable to build up in the atmosphere.  As soon as the rate of O₂ generation exceeded the rate of injection of reduced volcanic gases, O₂ was able to accumulate in the atmosphere.  The composition of average volcanic gases today contains enough reducing capacity to convert 20% of their CO₂ to C°, but only ca. 1/2 of their contained sulfur to a constituent of FeS₂.  The remaining sulfur leaves the ocean-atmosphere system as sulfates, largely as gypsum and anhydrite.

In volcanic gases that contain just enough reducing capacity to convert 20% of their CO₂ to C° and all of their contained sulfur to a constituent of FeS₂, the ratio

 

                                                MH₂  +  3MH₂S  -  0.4MCO₂           =  2.5

                                                            MSO₂  +  MH₂S

 

We propose that before ca. 2.3 Ga the value of this ratio was > 2.5, and that it has decreased since then to its present average value of ca. 0.2.  Although some of this decrease may have been caused by an increase in the O₂ fugacity of the mantle, most of the decrease is probably due to an increase in the fraction of subduction related gases in volcanic gases as a whole.