Was Earth a solid snowball 700 million years ago? Some geologists have been looking for an answer in an ancient, well-preserved glacial formation in the mountains of northern Oman. Their conclusion–that Earth experienced intermittent ice ages like those in Earth’s more recent history–weakens the snowball theory.
The snowball scenario has Earth freezing several times between 750 and 580 million years ago, during the Neoproterozoic period (SN: 8/29/98, p. 137: https://www.sciencenews.org/sn_arc98/8_29_98/bob1.htm). Each time, so the theory goes, ice covered all of Earth for about 10 million years, until carbon dioxide from volcanoes accumulated and created a greenhouse effect that thawed the planet.
To support the snowball theory, geologists have pointed to sedimentary evidence from the thawing periods. Among this evidence are distinct limestone formations found worldwide on top of Neoproterozoic sediments. These so-called cap carbonates, which only form in warm seas and contain few signs of past life from the prior frozen period, represent Earth’s abrupt transition from a snowball to a greenhouse Earth, some scientists have argued.
But now geologists have looked under cap carbonates to examine the glacial sediments directly. They’ve studied sediments in Oman’s Fiq Member, a formation that is 1.5 kilometers thick and exposed across 50 km of sparsely vegetated terrain. “We were able to really work out the anatomy of this particular glacial deposit,” says Philip A. Allen, a geologist at the Swiss Federal Institute of Technology in Zurich.
The Fiq is “not just a boring old block of sediment deposited during one profound glaciation,” says Allen. On the contrary, Allen and his collaborators report in the October Geology, sediments indicate that glaciers moved back and forth at least seven times during the Fiq’s formation. If so, Earth had an active water cycle, which may undermine the snowball theory.
“We do not have a single longtime freeze-up in which the water cycle is shut down completely,” asserts Allen.
“That’s not an accurate characterization of the snowball,” contends Paul F. Hoffman, the geologist at Harvard University who first described snowball Earth. In his current view, the water cycle in low-latitude regions such as Oman would have slowed down, but not completely halted, in the frozen periods. So, formations like the Fiq could have experienced active glaciation, especially as Earth began to warm at the end of each snowball stage (SN: 5/27/00, p. 343).
The snowball theorists are backing down on their earlier view that the water cycle shut down completely in the Neoproterozoic period, Allen argues. In the end, he says, “there’s either going to be a complete meltdown of the [snowball] theory, or it is going to have to be refined.”
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