A POSSIBLE EXPLANATION FOR THE GREAT OXIDATION EVENT BETWEEN CA. 2.3 AND 2.0 GA
HOLLAND, Heinrich D. and YANG, Wenbo, Earth and Planetary Sciences, Harvard Univ, 20 Oxford Street, Cambridge, MA 02138, holland@eps.harvard.edu

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.

Earth System Processes - Global Meeting (June 24-28, 2001)
Session No. T1
Archean Earth and Contemporary Life: The Transition from an Anaerobic to an Aerobic Marine Ecosystem (Sponsored by NASA Astrobiology Institute)
Edinburgh International Conference Centre: Sidlaw
10:00 AM-4:30 PM, Tuesday, June 26, 2001