A new study shows Earth's temperature plunged to Arctic temperatures -- in Florida, Egypt and other lands near the planet's supposedly warmer equatorial regions during a radical climate shift known as "Snowball Earth."
This finding is based on analysis of oxygen isotopes in rocks bathed in the meltwater of ancient glaciers and shows annual mean temperatures of minus-40 degrees Fahrenheit or colder during a planet-wide deep-freeze roughly 2.4 billion years ago.
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The new data indicate Earth's global oceans froze into a blanket of ice and glaciers 1,000 feet thick, challenging theories that open patches of water existed in the equatorial regions during the first of at least two Snowball Earth periods.
"These events are fascinating. We had times where we really had a completely frozen Earth. If you go now to tropical regions and you imagine thick glaciers and all the oceans frozen, it's crazy I think, but it appears to be that this has happened," geoscientist Daniel Herwartz, with the University of Gottingen in Germany, told Discovery News.
Scientists have been divided about whether some of Earth's oceans remained liquid (or slushy). That research has been dependent on climate models, not hard data.
The new study changes that paradigm with a technique to ferret out a third oxygen isotope from 700-million-year-old rocks from China and 2.4-billion-year-old rocks from northwestern Russia. Both lands were then located close to the equator.
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"These Snowball Earth events also happened when most of the continents were forming a supercontinent, which was located at equatorial regions," Herwartz said.
The research shows the 700-million-year-old rocks were exposed to water temperatures similar to what southern Greenland experiences today. The 2.4-billion-year-old samples, however, experienced much colder temperatures.
"This low temperature would actually require that the whole Earth was completely frozen in and that we really had most of the oceans under hundreds-of-meter-thick ice," Herwartz said.
The study has implications for understanding what happened to life on Earth during the deep freeze, namely whether it could still exist via photosynthesis or if it retreated into more extreme niches powered by hydrothermal vents, for example.
"Life on Earth originated and spread in shallow, light-filled, ice-free environments. Organisms in these environments evolved over billions of years, so glaciations, as long as they may be, are temporary interruptions to our biosphere, which is powered largely by photosynthesis," geobiologist Tanja Bosak, with Massachusetts Institute of Technology, told Discovery News.
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Eventually, Earth broke out of its icy shell due to volcanic activity.
The study also provides food for thought for astrobiologists working to understand what conditions might be necessary for life on ice-covered, ocean-bearing moons in the outer solar system.
"On icy worlds, all life would have to have a source of energy below the surface, or some transfer of nutrients from the surface to the subsurface where the liquid water is. The extent of this transfer and any biosphere powered by it is unclear," Bosak said.
The study appears in this week's Proceedings of the National Academy of Sciences.