A carbon isotope challenge to the snowball Earth

摘要

The snowball Earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the Neoproterozoic era, in a self-enhanced glaciation caused by the high albedo of the ice-covered planet. In a hard-snowball picture, the subsequent rapid unfreezing resulted from an ultra-greenhouse event attributed to the buildup of volcanic carbon dioxide (CO_2) during glaciation. High partial pressures of atmospheric CO_2 (p)(CO_2); from 20,000 to 90,000 p.p.m.v.) in the aftermath of the Marinoan glaciation (～635Myr ago) have been inferred from both boron and triple oxygen isotopes. These p_(CO_2) values are 50 to 225 times higher than present-day levels. Here, we re-evaluate these estimates using paired carbon isotopic data for carbonate layers that cap Neoproterozoic glacial deposits and are considered to record post-glacial sea level rise. The new data reported here for Brazilian cap carbonates, together with previous ones for time-equivalent units, provide p_(CO_2) estimates lower than 3,200 p.p.m.v.-and possibly as low as the current value of ～400 p.p.m.v. Our new constraint, and our re-interpretation of the boron and triple oxygen isotope data, provide a completely different picture of the late Neoproterozoic environment, with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard-snowball Earth.