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

元古代低纬度冰川作用的证据表明，在这些时期，地球的整个表面可能都被冰雪覆盖。在这项研究中，我用理论模型研究了这种“雪球地球现象”。根据主导过程及其特征时间标度，将雪球地球现象分为四个阶段。我发现雪球地球现象的开始需要一个临界条件。在这种情况下，大气-海洋系统的CO2净输入通量（=火山作用导致的CO2脱气速率+有机碳风化速率-有机碳埋藏速率）接近于零。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。过去微弱的太阳可能是元古代雪球地球现象反复出现的原因。然而，根据气候系统和碳循环系统的分析，元古代土壤生物活性较低，化学风化速率效率较低，应完全抵消了太阳亮度较低的影响。因此，微弱的年轻太阳可能不是雪球地球现象的原因。利用新元古代海水碳同位素记录变化的碳同位素质量平衡模型表明，在冰期之前，有机碳埋藏率变得非常大。这表明大气中二氧化碳消耗速率的增加，这可能是新元古代雪球地球现象的起源。

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.