pools, which would freeze at sunset. Oxygenated subglacial meltwater, along with iron-rich sediments dissolved in the glacial water, created a meltwater oxygen pump when it entered the ocean, where it provided eukaryotes with some oxygen, and both photosynthetic and chemosynthetic organisms with sufficient nutrients to support an ecosystem. The freshwater would also mix with the hypersaline seawater, which created areas less hostile to eukaryotic life than elsewhere in the ocean. However, organisms and ecosystems, as far as it can be determined by the fossil record, do not appear to have undergone the significant change that would be expected by a mass extinction. With the advent of more precise dating, a phytoplankton extinction event which had been associated with snowball Earth was shown to precede glaciations by 16 million years. Even if life were to cling on in all the ecological refuges listed above, a whole-Earth glaciation would result in a biota with a noticeably different diversity and composition. This change in diversity and composition has not yet been observed—in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth. One rebuttal to this is the fact that in many of these places where an argument is made against a mass extinction caused by snowball Earth, the Cryogenian fossil record is extraordinarily impoverished. Implications A snowball Earth has profound implications in the history of life on Earth. While many refugia have been postulated, global ice cover would certainly have ravaged ecosystems dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials. Because about half of the oceans' water was frozen solid as ice, the remaining water would be twice as salty as it is today, lowering its freezing point. When the ice sheet melted under a hot atmosphere rich in carbon dioxide, it would cover the oceans with a layer of warm (50°C) freshwater up to 2 kilometres thick. Only after the warm surface water mixed with the colder and deeper saltwater did the sea return to a warmer and less salty state. The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the Cryogenian period. Effect on early evolution The Neoproterozoic was a time of remarkable diversification of multicellular organisms,