2015-06-03

第３８２回 †

日時　平成２７年０６月０３日　水曜日　１７時より
場所　東京工業大学　石川台２号館　３１８号室　

＜講師＞　Joseph Kirschvink様（東工大地球生命研究所・カリフォルニア工科大学　教授）
＜題名＞　Earth’s first Snowball and the origin of Atmospheric Oxygen

Earth’s global climate has been relatively stable over the past 10,000 years, a fact that has permitted Human civilization to develop and spread across the globe. From the record of climatic signals in rocks, we know that this Recent interval of calm was unusual, and that during the past ~500 million years Earth experienced the buildup and decay of polar ice caps, abrupt heating pulses, and even prolonged greenhouse conditions where the Poles were warm enough for dinosaurs to roam. However, studies of even older rocks reveal that Earth experienced a series of extraordinarily severe glaciations, in which the advancing ice fronts actually marched over the carbonate platforms, and reached the Equator. These rocks give strong hints that floating ice sheets several km thick covered the oceans, sealing them off from exchange with the atmosphere, curtailing photosynthesis, and shutting down the global hydrological cycle nearly completely. I called this condition the “Snowball Earth” back in 1992.

In the following two decades, evidence for the Neoproterozoic snowball Earth glaciations has basically sealed this debate. Two episodes happened, termed the Sturtian and Marionan glaciations. However, a much more interesting episode seems to have happened between 2.4 and 2.2 billion years ago, termed the Makganyene snowball Earth event. The snowball is also connected in some as yet mysterious fashion with the great oxygenation event, through which Earth's atmosphere switched rather suddenly from anaerobic to aerobic. I will argue in my lecture that this first snowball Earth event was caused directly by the evolution of oxygen-committing cyanobacteria, which destroyed a methane/carbon dioxide greenhouse.