Scientists have long believed that around 700 million years ago, Earth experienced extreme cold conditions known as "Snowball Earth", when ice stretched from the poles to the equator.

However, there has been uncertainty about the exact degree of those temperatures. Now, an international research team led by Chinese scientists has provided the first quantitative measurements of ocean temperatures from that period.

The study suggests that ocean temperatures at continental margins — where life was most active — ranged from minus 22 C to minus 8 C, with seawater up to four times saltier than modern ocean water. The findings were recently published in the journal Nature Communications.

"As the first measured ocean temperature during the Snowball Earth period, this discovery offers new insights into understanding the mechanisms by which early life survived in such extreme climates and sheds light on Earth's dramatic climate changes," said Lu Kai, the study's first author and a postdoctoral researcher at the Chinese Academy of Sciences' Institute of Geology and Geophysics.

"This is the coldest measured ocean temperature in Earth's history," Lu said, noting that it is even colder than the minus 13 C salty slush of ice-covered Lake Vida in Antarctica today.

The period left behind distinctive geological evidence: rusty red iron formations that accumulated where continental glaciers met ice-covered seas. Iron isotopes — variants of the same element with equal numbers of protons but different numbers of neutrons — vary in atomic abundance, and their isotopic composition changes with temperature. To determine Snowball Earth's temperature, the team developed a new method using iron isotopes as a thermometer.

"The results showed that the surface seawater temperature during that period was significantly lower than today's 17 C, yet it remained unfrozen," Lu said, attributing this to high salinity levels that reached 150 practical salinity units, more than four times the salinity of present-day seawater. The high-salinity environment acted as a natural antifreeze agent, allowing seawater to remain liquid at temperatures well below the normal freezing point.

Lu said the extreme conditions likely formed at the base of massive ice shelves, similar to the "ice pump" circulation observed beneath modern Antarctic ice shelves. During cycles of melting and freezing at the base of ice shelves, ice expels salts that accumulate over time, creating dense brine layers with extremely low temperatures and high salinity.

"This study provides the first quantitative evidence of the marine environment during the Snowball Earth period and serves as a valuable reference for understanding the dramatic climate changes of that era, as well as the tolerance and resilience of early microorganisms," he said.