More than 700 million years ago, our planet experienced a glacial episode of prodigious intensity, to the point that ice sheets extended all the way to the equator, merging together.
For the first time, geologists have managed to determine the temperature of the ocean under the ice during this period, named the "Snowball Earth" epoch. Their observations, published in
Nature Communications, estimate this temperature to be around -15°C, which is the lowest ever documented for seawater.
In order to establish this estimate, scientists examined deposits of oxidized iron dating from that era. Their study of iron isotopes demonstrated that the water was extremely cold at the time. This approach relies on the principle that the transformation of iron occurs in a distinct manner depending on the temperature in an oxygen-deprived environment, similar to the oceans that were stagnant beneath the ice sheet.
Maintaining the water in a liquid state required extraordinary salinity. The data indicate that the sea was up to four times saltier than contemporary oceans. This high salt concentration, comparable to that of some Antarctic lakes, prevented total freezing despite the intense cold, thus allowing for the persistence of reservoirs of liquid water.
Several specialists have shared their impressions of these results. Geochemist Andy Heard described the method as original, while advocating for a qualitative interpretation of the measurements. For his part, Jochen Brocks pointed out that his own work on salinity supports the existence of these extreme conditions, which increases the plausibility of the described scenario.
The Ice-Albedo Feedback Mechanism
Albedo refers to the reflective power of a surface in the face of solar radiation. Light surfaces, such as ice or snow, reflect a large part of this energy back into space, thus contributing to the cooling of the atmosphere.
When Earth's climate cools, the expansion of ice increases the global albedo. This phenomenon causes a further drop in temperatures, which in turn generates more ice, creating a positive feedback loop. This process can experience rapid amplification.
During the "Snowball Earth" episode, this mechanism likely led to the near-complete icing over of the globe. It shows how initial slight modifications are likely to amplify climatic effects over very long durations.
Nowadays, understanding this feedback is useful for anticipating climate evolution, particularly in a scenario where melting ice, by reducing albedo, could contribute to an accelerated rise in temperatures in a similar but reversed mechanism.