During a period of intense cold millions of years ago, almost all of the Earth may have been covered in glaciers. Known as the “Snowball Earth,” this time frame cut off oceans from the sun’s light and created conditions far less hospitable than those of the last Ice Age that ended some 10,000 years ago. But now, a new study published last week in the journal Nature Communications is changing the idea of how the planet looked during this frigid era.
Rather than being nearly entirely blanketed in glaciers, the study posits that Earth may have had sections that were wetter than previously thought. By analyzing prehistoric sediment, scientists suggest that open oceans were present significantly north of the equator during the Snowball Earth, and some organisms could have survived within them.
“The key finding of this study is that open-water—ice-free—conditions existed in mid-latitude oceanic regions during the waning stage of the Marinoan Ice Age,” Huyue Song, first author of the study and a geobiologist at China University of Geosciences, tells Reuters’ Will Dunham.
The Marinoan Ice Age, which spanned 635 to 654 million years ago, was one of two “snowball events” to take place during the appropriately named Cryogenian Period. Had the ocean’s surface been globally frozen over, it would have restricted oxygen and nutrients from reaching the depths and made it difficult for some organisms to persist, per the paper.
But some scientists believe that certain life forms did survive: Fossils from this period indicate that tiny organisms such as algae, which need sunlight and open water, lived both before and after the Snowball Earth. “You have to envision some sort of refuge where these algae can survive,” Shuhai Xiao, a co-author of the study and geobiologist at Virginia Tech, tells Science’s Adam Mann.
To explore how life might have eked out an existence during such frozen conditions, the researchers analyzed a prehistoric layer of black shale, a type of sediment rich in organic matter, that would have been under the ocean during the Marinoan Ice Age, according to the Agence France-Presse (AFP). The shale came from Shennongjia National Forest in south China.
In the Marinoan sediment, the team found fossils of algae, as well as nitrogen compounds that suggest the waters were oxygenated, writes Science. This shows that life could have existed underwater at the time, they say.
Some past climate research had suggested that places at low latitudes—just 5 to 15 degrees, or perhaps 20 to 30 degrees, away from the equator—were not frozen for parts of the Snowball Earth period, per the paper. But the shale in the new study might have resided between 30 and 40 degrees north of the equator in the ocean, meaning that open water could have existed at much higher latitudes than previously thought.
“Until now, ice-free areas had been identified only in peri-equatorial regions,” Song tells the AFP. But “patchy, ice-free areas may have existed much more widely.”
Still, Paul Hoffman, a Harvard University geologist who contributed to the Snowball Earth theory and did not participate in the new study, is less convinced by the findings. He tells Science that he thinks it’s more likely the fossilized algae lived in pools atop glaciers, instead of in open waters.