New geochemical sleuthing has pushed Earth’s climate record into almost unimaginable territory, revealing seawater that once chilled to about minus 15° C without turning to ice. The finding comes from rocks laid down when the planet was locked in a Snowball Earth state, a time when glaciers reached the tropics and oceans lurked beneath global ice. By decoding those rocks, scientists are reconstructing how such brutally cold, salty seas stayed liquid and what that meant for life clinging on in the dark.
The picture that emerges is of an ancient ocean far more extreme than today’s polar waters, yet still capable of sheltering microbes and perhaps nudging evolution in new directions. I see this work as more than a curiosity about deep time: it is a stress test of how oceans behave under severe cooling, with lessons that echo into modern debates about circulation, habitability and abrupt climate swings.
Reading minus 15° C in stone
The claim that ancient seawater cooled to roughly 5°F, or minus 15°C, rests on chemical fingerprints locked inside rocks that once sat on the seafloor. Scientists examined deposits that formed when iron rich water welled up beneath thick ice, then settled out as distinctive layers, allowing them to back out the temperature of the water that carried those minerals. By analyzing data from these ancient deposits, the team concluded that parts of the Snowball ocean reached about minus 15°C, colder than any liquid seawater measured in Earth’s history.
Those rocks formed when the planet was in a deep freeze roughly 700 m years ago, during one of the most severe glaciations known. At that point, ice sheets extended into low latitudes and the ocean surface was capped by a thick frozen lid, yet liquid water persisted below. A research highlight on Ancient conditions describes how iron rich brines circulated under the ice and then rained down onto the seafloor as sediment, providing the raw archive that now lets scientists reconstruct just how cold those hidden seas became.
How seawater stays liquid in a global deep freeze
To understand how water can remain liquid at minus 15° C, I start with a basic physical trick: salt lowers the freezing point. Modern polar oceans already exploit this, with sea ice forming at the surface while brine drains away and sinks. When ice forms on sea water, salt is excluded from the ice crystals and, after a complicated process of elimination from the ice, it collects in dense, cold brine at the surface of the liquid ocean, a process described in classic work on When wintertime convection in the Arctic Ocean. Under a global ice lid, that brine production would have been relentless, steadily driving salinity, and therefore the ability to stay liquid at lower temperatures, to extremes.
Field analogues in today’s polar deserts help make this less abstract. In Antarctic lakes such as Lake Fryxell, fresh water freezes over briny liquid water, creating a stacked system where ice floats atop dense, supercooled brine. New research argues that Snowball Earth’s seas behaved similarly, only on a planetary scale, with brines even saltier than modern ocean water. The structure of these layered waters, visible in places like Lake Fryxell, gives a tangible picture of how a frigid but fluid ocean could lurk beneath ice that wrapped the globe.
Reconstructing a briny, stratified Snowball ocean
The new temperature estimates are part of a broader effort to map the structure of Snowball Earth’s oceans, and the emerging view is of a world dominated by dense, hypersaline bottom waters. Kai Lu and Lianjun Feng at the Institute of Geology and Geophysics, Chinese Academy of Sciences, in Beijing and their colleagues linked iron rich sediment layers to circulation patterns in these ancient seas, then explored possible scenarios for their formation. Their modeling suggests that extremely salty brines pooled at depth while slightly fresher water sat above, a configuration consistent with the iron deposits now found on the seafloor and summarized in work led by Kai Lu and.
Independent geochemical reconstructions converge on the same picture of extreme cold. By analyzing data from rock deposits that formed under the ice, researchers estimate that sea temperatures were 5°F (minus 15°C), about 22°F colder than the freezing point of normal seawater, implying much harsher conditions than scientists suspected. These reconstructions, based on iron formations and other proxies, show that the Snowball ocean could get extremely cold without freezing solid, a conclusion supported by detailed work on sea temperatures during that interval.
Life in the coldest seas
What makes these findings so striking to me is that life did not just endure such conditions, it may have been reshaped by them. Some scientists propose that these harsh conditions could have triggered evolutionary pressure, encouraging the development of more complex, multicellular organisms once the planet thawed. According to this view, the Snowball episodes fractured habitats into isolated, diverse environments that led to evolutionary bursts, an idea captured in discussions where Some researchers link glaciations to later explosions of diversity.
More recently, Carl Simpson, a macroevolutionary paleobiologist at CU Boulder, has argued that cold seawater could have jump started, rather than suppressed, evolution from single celled to multicellular life forms. In his view, the physical stress of frigid, stratified oceans may have favored organisms that clumped together, shared resources or developed new ways to cope with freezing conditions. That perspective, grounded in macroevolutionary analysis from Carl Simpson, reframes Snowball Earth not just as a catastrophe but as a crucible for complexity.
Modern polar clues and biochemical tricks
To test how life might function in such cold brines, I look to present day polar ecosystems that push the same limits. Ancient microbes discovered under 60 feet of Antarctic ice in East Antarctica’s Lake Vida are thriving in minus 13°C water that is sealed off from the atmosphere and saturated with salt. That discovery, which documents Ancient microbes in such an extreme niche, offers a living analogue for the kinds of microbial communities that could have persisted in Snowball Earth’s subglacial seas.
Animals, too, have evolved remarkable tools to survive in near freezing brine. For decades, scientists had wondered how arctic fish were able to swim around seawater that was minus 1.8 degrees Celsius, given that the freezing point of their blood is higher than that of the surrounding water. Work on antifreeze proteins shows that these molecules act as a kind of biological coolant, preventing ice crystals from growing inside cells even as the external environment dips below 1.8, a mechanism detailed in research on Celsius scale adaptations. These modern strategies hint at the kinds of biochemical innovations that might have emerged, or at least been tested, in the ultra cold seas of Snowball Earth.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
