School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia;
Department of Earth and Planetary Science, McGill University, Montréal, QC, Canada H3A 0E8.
Proc Natl Acad Sci U S A. 2019 Dec 17;116(51):25478-25483. doi: 10.1073/pnas.1909165116. Epub 2019 Dec 2.
The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations.
地球最严重的冰期中断了真核生物进化的一个关键时期，在严寒期（7.2 亿至 6.35 亿年前）期间，广泛的冰层覆盖。好氧真核生物必须在“雪球地球”冰期生存下来，这需要含氧的海洋生境的持续存在，但这些环境的证据却缺乏。我们研究了全球分布的严寒期冰川序列中的铁建造物，以重建同冰期海洋的氧化还原状态。来自一系列冰海环境的铁同位素比值和铈异常表明，在覆盖着冰的海洋中普遍存在缺氧，但随着接近冰架基底线，氧化作用逐渐增强。我们提出，冰川下的融水的溢出将氧气输送到同冰期海洋中，形成了冰海氧气绿洲。富氧融水和富铁海水的汇合可能为化能合成群落提供了足够的能量。这些过程可能为早期动物和其他真核异养生物提供了生存所需的氧气和有机碳源，从而使它们能够在这些极端冰期环境中生存下来。
