This text was initially printed at Eos. The publication contributed the article to Area.com’s Professional Voices: Op-Ed & Insights.
Earth froze over 717 million years in the past. Ice crept down from the poles to the equator, and the darkish subglacial seas suffocated with out daylight to energy photosynthesis. Earth grew to become an unrecognizable, alien world—a “snowball Earth,” the place even the water was colder than freezing.
In Nature Communications, researchers reported the primary measured sea temperature from a snowball Earth episode: −15°C ± 7°C. If this determine holds up, it will likely be the coldest measured sea temperature in Earth’s historical past.
“We’re coping with salty brines,” mentioned Ross Mitchell, a geologist on the Institute of Geology and Geophysics of the Chinese language Academy of Sciences. “That is precisely what you see in Antarctica in the present day,” he added, besides that snowball Earth’s brines had been a bit colder than even the −13°C salty slush of Antarctica’s ice-covered Lake Vida in the present day.
Previous Iron
The Sturtian snowball was a runaway local weather disaster that occurred as a result of ice displays extra daylight than land or water. Ice mirrored daylight, which cooled the planet, which made extra ice, which mirrored extra daylight and so forth, till the entire world ended up buried below glaciers that would have been as much as a kilometer thick.
This uncommon time left behind uncommon rocks: Rusty purple iron formations that collected the place continental glaciers met the ice-covered seas. To take snowball Earth’s temperature, the staff devised a brand new means to make use of that iron as a thermometer.
Iron 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 team calculated that the waters that formed the snowball Earth iron formations would likely have been 40°C colder.
“It’s a very interesting, novel way of getting something different out of iron isotope data,” said geochemist Andy Heard of the Woods Gap Oceanographic Establishment, who was not concerned within the examine. “It is a humorous, backwards scenario to be in the place you are utilizing even older rocks as your baseline for understanding one thing that shaped 700 million years in the past.”
Partly due to that backward scenario, Heard thinks the examine is greatest interpreted qualitatively as robust proof that seawater was actually chilly, however perhaps not that it was precisely −15°C.
The staff additionally analyzed isotopes of strontium and barium to find out that snowball Earth’s seawater was as much as 4 occasions saltier than the fashionable ocean. Jochen Brocks of the Australian Nationwide College, who wasn’t concerned within the examine, mentioned the researchers’ outcomes align along with his personal salinity evaluation of snowball Earth sediments from Australia based mostly on a distinct technique. These rocks shaped in a brine that Brocks thinks was salty sufficient to succeed in −7°C earlier than freezing. One other group reaching the same conclusion utilizing completely different strategies makes that excessive situation sound much more believable, he mentioned.
“It was very cool to get the extra affirmation it was really very, very chilly,” he mentioned.
Learn the original article at EOS.org.
![AURORA ALERT] G1 (MINOR) GEOMAGNETIC DISTURBANCE IN PROGRESS (21 March, 2026)](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEieo0I8aAhgWoEDd0LpZqBSsaCv8S3BY50ViCy4vh_F9oFe5eGwC_w5y-i3OlE5QnFQgrTX-W7zhwnHwudceXwYtX0z7XdYHlw2hfStCIX5qvKIlctCZZAw38HRbMdIPPyFq_4nMxRhHq2zSqVzZz_7Meyi5TgzxdJHEHRgGkqD6nYhttLUX30/s320/Screenshot%202026-03-21%20170017.png){kind=small}
