Salt Crystals May Have Deepened Earth's Ancient Deep Freeze

Around 700 million years ago, Earth entered periods of extreme glaciation known as Snowball Earth, when ice is thought to have spread across much or potentially all of the planet's surface. Scientists have long known that expanding ice can drive this kind of cooling on its own: bright ice reflects more sunlight than the darker ocean water it replaces, letting the planet cool further and allowing even more ice to form. Aksel Samuelsberg of the Arctic University of Norway (UiT) and colleagues have now identified another feedback that could have amplified this cooling: salt crystals accumulating on the surface of sea ice.

As sea ice forms and grows colder, dissolved salt does not freeze along with it; it grows more concentrated in trapped liquid until, at low enough temperatures, some crystallizes. If the ice surface then sublimates, turning straight to vapor, the salt crystals are left behind in a thin layer. Lab experiments found a salt crust on very cold sea ice can reflect about 93% of sunlight, versus roughly 83% for fresh snow and about 67% for melting bare sea ice — a mechanism the researchers, publishing in the journal Climate of the Past, call a salt-albedo feedback.

Adding salt deposits to a simplified climate model, the researchers found two possible stable Snowball Earth states — one with salt deposits and one without — with the salt-covered state substantially colder. A transition from a warm, ice-free climate tended to lead directly into this colder state. The feedback could begin at temperatures warmer than about -36 degrees Celsius, the point where seawater freezes almost completely, since different salts crystallize at different temperatures, some around -8 degrees Celsius and others around -23 degrees Celsius.

The salt-covered state was also harder to escape, requiring a much greater rise in atmospheric carbon dioxide to melt the ice than a Snowball Earth without salt deposits. The findings do not mean salt caused the planet to freeze in the first place; the researchers propose it as an additional feedback that strengthened cooling once widespread glaciation was already under way. What originally triggered the Snowball Earth remains an open question, and the model does not account for moving sea ice, which over time could dilute the salty ice from which the crystals form.