The Marinoan glaciation was one of the most severe in Earth's history. It is believed that the ice then completely encased the land and all the seas. Increasing, then retreating. This explains how the Earth's population was able to survive the catastrophic climate change in the late Proterozoic. Among the climatic changes in the history of our planet, episodes of global glaciation stand out - Snowball Earth. They were the time of the onset of ice both in the World Ocean and on the continents (which were then concentrated at the equator).
Geologists distinguish at least two episodes of the Earth's transformation into an ice world. The first was the Stertian glaciation approximately 700 million years ago at the beginning of the Cryogenian period. The second global glaciation occurred on the border of the same Cryogenian and Ediacaran - the period of the appearance of the first large life forms. It is possible that it was facilitated by the most radical climate changes.
However, there remains disagreement about what exactly was happening on the planet at that time. It is known that the Neoproterozoic glaciers could retreat and grow again. But which parts of the planet were affected by such changes? How many were there and how long did they last?
The authors of a new article in the journal addressed the issue. Gondwana Research. They described three scenarios for the Earth's transformation into a snowball, in which life could have survived. The first is a "rich Earth": large thaw zones could remain in the warm belt of the planet. The Jormungand hypothesis (named after a huge serpent from Norse mythology) suggests that there was a single narrow wormwood at the equator.
Finally, the third hypothesis is the “thin ice.” It claims that despite the global nature of the glaciation, the ice in the ocean was thin, so it let in light and life under it did not stop. A thick ice cover over the entire Earth would mean darkness in the water, cessation of gas exchange between the atmosphere and the ocean, and the death of all Proterozoic microbes, starting with photosynthetic ones.
The study authors carefully studied rocks from the Nantuo Formation (or succession) in South China, which formed nearly 650 million years ago at shallow depths during the Marinoan glaciation. The scientists conducted lithofacies analysis, which is comparing closely related sedimentary rocks that formed at the same time but have local differences in composition.

They identified six lithological facies of the Nantuo Formation. These are the massive diamictite facies, the coarse-bedded facies of the same rock, the massive sandstone facies, the layered facies of siltstone and mudstone with characteristic inclusions (dropstones), and finally, the simply layered mudstone (siltstone) facies.
Their subsequent vertical change revealed two groups (associations) that indicate the dynamics of ancient glaciers. Together with data from three areas formed at greater depths, they showed that the Marinoan glacier of the Nantuo Formation advanced and retreated at least twice. Apparently, a number of such repeating cycles took place.
Previously, it was known about strong fluctuations in the thickness of ice in the ocean at great depths. According to the new article, the same fluctuations occurred in shallow waters. The authors believe that ice pulsations covered the entire World Ocean in the late Proterozoic, and the dynamics of the "Snowball Earth" were actually more complex. Such a picture helps to better understand how life survived those harsh times and was later able to begin development at a new level.