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At the peak of planetary glaciation about 700 million years ago, ocean temperatures were likely -15°C – far colder than any sea temperature recorded today or known to have occurred at any other time.1.
During this proposed big chill, part of a theory known as Snowball Earth, glaciers reached the planet’s equator. A layer of ice interrupted the usual patterns of biogeochemical exchange between the sea surface, land, and atmosphere. This has caused major changes in marine ecosystems. For example, iron that would normally be oxidized by photosynthetic organisms was deposited on the ocean floor as iron-rich sediments.
Cai Lu and Lianjun Feng, of the Institute of Geology and Geophysics of the Chinese Academy of Sciences in Beijing, and their colleagues studied iron isotopes in sedimentary rocks dating to the Snowball Earth phase from around the world, and explored possible scenarios for their formation. They concluded that the sediments were deposited at water temperatures between -22 and -8 degrees Celsius – which, so that the water would not freeze, suggests that Snowball’s seas were not only very cold, but were also unpalatably salty.