PHYSICAL AND GEOCHEMICAL CONDITIONS REQUIRED FOR THE INITIATION OF SNOWBALL EARTH

Evidence for low-latitude glaciations during the Precambrian have been interpreted as global glaciations (Snowball Earth hypothesis). It is interesting to note that all three major glaciations during the Proterozoic seems to be Snowball Earth although such an extreme phenomenon has never occurred during the Phanerozoic. This difference may be derived from difference in the solar luminosity between the Proterozoic and the Phanerozoic. I investigated this possibility by using a one-dimensional energy balance climate model combined with a carbon geochemical cycle model. According to the results, during the Paleoproterozoic (2.4 Ga), pCO2 level required for maintaining a warm climate similar to the present one is 180 PAL (=present atmospheric level), although pCO2 level at the critical condition for initiation of Snowball Earth is estimated to be 75 PAL. On the contrary, pCO2 level should be lowered to 1/40 PAL to achieve the critical condition under the present solar luminosity. Therefore it may have been easier to fall into the snowball Earth state during the Proterozoic than today owing to a variation of pCO2 level with an amplitude of about 1/2 relative to the standard pCO2 level. However, a net CO2 flux into the atmosphere-ocean system (=volcanic CO2 degassing + organic carbon weathering – organic carbon burial) at the critical condition is estimated to be nearly zero both for the Paleoproterozoic and today. Therefore, the most likely reason for the difference between the Proterozoic and the Phanerozoic is difference in CO2 fluxes within the carbon cycle system, rather than difference in the solar luminosity. Although the fundamental reason has still been unclear, difference in tectonic environment may have played an important role in the behavior of the carbon cycle system.