Evolution of Earth's climatic system: Evidence from ice ages, isotopes, and impacts

摘要

Multiple glaciations took place near the beginning and end of the Proterozoic Eon. Neoproterozoic (Cryogenian) glacial deposits are more widespread than those of older (Paleoproterozoic) glacial episodes. Paleomagnetic results suggest that most Proterozoic glaciogenic rocks were deposited at low paleolatitudes. Some contain enigmatic evidence of strong seasonal temperature variations, and many formed at sea level. These attributes inspired both the snowball Earth hypothesis and the high obliquity theory, but only the latter explains strong seasonality at low latitudes. The Proterozoic glaciations may have been triggered by drawdown of atmospheric CO2 during enhanced weathering of elevated supercontinents. Multiple glaciations resulted from a negative feedback loop in the weathering system that ended when the supercontinent broke apart. A radical reorganization of the climatic system took place in the Ediacaran Period. In contrast to previous glaciations, these ice sheets developed in high latitudes and many follow mountain building episodes. During the Ediacaran Period, Earth’s climatic zonation and controls appear to have undergone a radical change that persisted throughout the Phanerozoic Eon. The change may coincide with the world’s greatest negative δ13C excursion, the Shuram event, here interpreted as the result of a very large marine impact that decreased the obliquity of the ecliptic, causing the Earth’s climatic system to adopt its present configuration. Attendant unprecedented environmental reorganization may have played a crucial role in the emergence of complex life forms.