Having melted after a colossal glaciation, "Snowball Earth" received freshwater oceans that existed for thousands of years.
Reliable traces of glacial sediments dating back to about 600-800 million years old are found even in areas that were then the most tropics of the Earth. An explanation for this is offered by the "Snowball Earth" hypothesis, according to which during this period the entire planet experienced one of the most grandiose glaciations, completely covered with ice and snow. However, it also ended: the accumulation of excess carbon dioxide in the atmosphere forced the Earth to heat up again and melt, returning to life.
Global melting occurred gradually, first leading to the formation of numerous seas with a very salty bottom layer of water, above which a less dense thick freshwater layer floated. They merged, forming ever more extensive seas and oceans, faster than the layers of water were mixed. A new work by University of Chicago geologist Dorian Abbot and his colleagues showed that mixing was extremely slow and for at least 50 thousand years all surface water on the planet was fresh. Scientists write about this in the journal Geology.
Today, complete mixing of water in the World Ocean takes about 1000 years, but about 600 million years ago, this process was hampered by a large difference in temperature and density of layers. During the previous glaciation, about half of the ocean's water was frozen over with a layer of fresh ice - the liquid water preserved under it became much saltier, twice as salty as today. After thawing, the layer of fresh water above it reached a thickness of 2 km.
However, this "planetary lake" could hardly tempt someone with a swim. According to calculations by Abbott and his co-authors, a powerful greenhouse effect has heated the water on the surface to a temperature of about 50 ° C. It cooled only gradually, mixing with the icy salt water below under the influence of currents, winds and tides.
Sergey Vasiliev