When you buy through links on our articles, Future and its syndication partners may earn a commission.During the ice ages, Earth could turn into an inhospitable snowball. | Credit: MARK GARLICK/SCIENCE PHOTO LIBRARYThis article was originally published at Eos. The publication contributed the article to Space.com’s Expert Voices: Op-Ed & Insights. Earth froze over 717 million years ago. Ice crept down from the poles to the equator, and the dark subglacial seas suffocated without sunlight to power photosynthesis. Earth became an unrecognizable, alien world—a “snowball Earth,” where even the water was colder than freezing.AdvertisementAdvertisementIn Nature Communications, researchers reported the first measured sea temperature from a snowball Earth episode: −15°C ± 7°C. If this figure holds up, it will be the coldest measured sea temperature in Earth’s history.For water to be that cold without freezing, it would have to be very salty. And indeed, the team’s analysis suggests that some pockets of seawater during the Sturtian snowball glaciation, which lasted 57 million years, could have been up to 4 times saltier than modern ocean water.”We’re dealing with salty brines,” said Ross Mitchell, a geologist at the Institute of Geology and Geophysics of the Chinese Academy of Sciences. “That’s exactly what you see in Antarctica today,” he added, except that snowball Earth’s brines were a bit colder than even the −13°C salty slush of Antarctica’s ice-covered Lake Vida today.Past IronThe Sturtian snowball was a runaway climate catastrophe that occurred because ice reflects more sunlight than land or water. Ice reflected sunlight, which cooled the planet, which made more ice, which reflected more sunlight and so on, until the whole world ended up buried under glaciers that could have been up to a kilometer thick.AdvertisementAdvertisementThis unusual time left behind unusual rocks: Rusty red iron formations that accumulated where continental glaciers met the ice-covered seas. To take snowball Earth’s temperature, the team devised a new way to use that iron as a thermometer.Scientists used information about the iron in formations like this one to estimate the temperature of Earth’s ocean 717 million years ago. | Credit: James St. John/Flickr, CC BY 2.0Iron formations accumulate in water that’s rich in dissolved iron. Oxygen transforms the easily dissolved, greenish “ferric” form of iron into rusty red “ferrous” iron that stays solid. That’s why almost all iron formations are ancient, relics of a time before Earth’s atmosphere started filling with oxygen about 2.4 billion years ago, or from the more recent snowball Earth, when the seas were sealed under ice. Unable to soak up oxygen from the air or from photosynthesis, snowball Earth’s dark, ice-covered seawater drained of oxygen.Iron-56 is the most common iron isotope, but lighter iron-54 rusts more easily. So when iron rusts in the ocean, the remaining dissolved iron is enriched in the heavier isotope. Over many cycles of limited, partial rusting—like what happened on the anoxic Archean Earth—this enrichment grows, which is why ancient iron formations contain isotopically very heavy iron compared to iron minerals that formed after Earth’s atmosphere and oceans filled with oxygen.Snowball Earth’s iron is heavy, too, even more so than iron formations from the distant, preoxygen past. The researchers realized that temperature could be the explanation: Iron minerals that form in cold water end up istopically heavier. We don’t know exactly how hot it was when the ancient iron formations accumulated, but it was likely warmer than during snowball Earth, when glaciers reached the equator. Using a previous estimate of 25°C for the temperature of Archean seawater, the te …
