Scientists have long believed that around 700 million years ago, Earth experienced extreme cold conditions, known as a "snowball Earth", where ice stretched from the poles to the equator. However, the exact degree of this coldness has remained uncertain. Now, an international research team led by Chinese scientists has achieved a significant breakthrough by providing the first quantitative measurements of ocean temperatures from that period.

The study suggested that ocean temperatures at continental margins — the areas where life was most active — were approximately between minus 22 and minus 8 C, with seawater being up to four times saltier than modern ocean water. These findings were published recently 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, first author of the study and a postdoctoral researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences.

"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.

This unusual time left behind unusual rocks: rusty red iron formations that accumulated where continental glaciers met the ice-covered seas. Iron isotopes are variants of the element with the same number of protons but different neutrons, having variations in atomic abundance, and their isotopic composition changes with temperature. To take snowball Earth's temperature, the team devised a new way to use 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 noted, attributing this to the water's high salinity at the time, which reached 150 practical salinity units, more than four times the salinity of present-day seawater. This high-salinity environment functions as a natural antifreeze, enabling seawater to stay liquid at temperatures far below the normal freezing point.

Lu said this extreme environment was likely to have formed at the base of massive ice shelves, akin to the "ice pump" circulation observed beneath today's Antarctic ice shelves. During the melting and freezing cycles of basal ice shelves, ice expels salts, which continuously accumulate, ultimately creating brine regions featuring extremely low temperatures and high salinity.

"This study has provided the first quantitative evidence of the marine environment during the 'snowball Earth' period and has served as a valuable reference for understanding the dramatic climate changes of that era, as well as for examining the tolerance and resilience of early microorganisms," he said.