Theoretical study on behavior of the Earth system in the snowball Earth phenomena

元古界低纬冰川的证据表明，在这些时期，整个地球表面可能被冰雪覆盖。在这项研究中，我用理论模型对这种“雪球地球现象”进行了研究。根据主导过程及其特征时间尺度，将雪球地球现象划分为四个阶段。我发现，启动雪球地球现象需要一个关键条件。在这种情况下，进入大气-海洋系统的二氧化碳净输入通量(=火山作用导致的二氧化碳脱气率、有机碳的风化速率-有机碳的埋藏率)几乎为零。因此，雪球地球现象的发生是由于碳循环系统内CO2通量的不平衡造成的。过去的微弱太阳可能是元古界雪球地球现象重复的原因。然而，根据气候系统和碳循环系统的分析，元古界土壤生物活性降低导致的化学风化效率降低，应该完全抵消了太阳光度降低的影响。利用新元古代海水碳同位素记录的变化建立的碳同位素物质平衡模型表明，冰期前有机碳埋藏率变得很大。这表明大气中二氧化碳的消耗速率增加，这可能是新元古代雪球地球现象的始作俑者。

Evidence for low-latitude glaciations during the Proterozoic suggests that the entire surface of the Earth may have been covered with ice and snow in these periods. In this study, I investigated such "snowball Earth Phenomena" with theoretical models.The snowball Earth phenomena will be divided into four stages by dominant processes and their characteristic timescale. I found that there is a critical condition required for an initiation of the snowball Earth phenomena. At that condition, a net input flux of CO2 into the atmosphere-ocean system (=CO2 degassing rate due to volcanism + weathering rate of organic carbon - burial rate of organic carbon) becomes nearly zero. Therefore, the snowball Earth phenomena is occurred owing to imbalance of CO2 fluxes within the carbon cycle system.The faint sun in the past may have been responsible for the repetition of snowball Earth phenomena during the Proterozoic. According to the analysis of the climate system and the carbon cycle system, however, an effect of lower luminosity of the sun should have been cancelled out completely by an effect of lower efficiency of chemical weathering rate due to lower soil biological activity during the Proterozoic. Therefore, the faint young sun may not have been the cause for the snowball Earth phenomena.The carbon isotope mass balance model which uses variation of carbon isotope record of seawater during the Neoproterzoic suggests that organic carbon burial rate became very large just before the glaciations. This indicates increases in the consumption rate of atmospheric CO2, which may have initiated the snowball Earth phenomena during the Neoproterozoic.