Salt Crystals May Have Deepened Earth’s Ancient Global Freeze

Earth’s ancient global freeze may have been intensified by an unexpectedly ordinary material: salt. Around 700 million years ago, the planet entered periods of extreme glaciation known as Snowball Earth, when ice is thought to have covered much or potentially all of the surface. Scientists have long examined the ice-albedo feedback, in which bright ice reflects more sunlight than dark ocean water, causing additional cooling and ice formation. A study by Aksel Samuelsberg of the University of Tromsø—The Arctic University of Norway, and colleagues proposes another possible feedback. Their modeling indicates that salt crystals accumulating on exposed sea ice could have made the frozen surface even more reflective. When seawater freezes, its dissolved salts are concentrated in the remaining liquid trapped within the ice. At sufficiently low temperatures, some of those salts crystallize. If surface ice gradually changes directly into water vapor—a process called sublimation—the crystals can remain behind and build up in a thin layer. Laboratory experiments cited in the report found that salt crust on very cold sea ice could reflect about 93% of incoming sunlight. Fresh snow reflects about 83%, while melting bare sea ice reflects roughly 67%. A brighter surface absorbs less solar energy, potentially creating a cycle in which cooling produces more salt crystals, which then cause further cooling. The researchers added salt deposits to a simplified climate model containing tropical regions of bare sea ice, where evaporation was expected to exceed snowfall. The model produced two possible stable Snowball Earth conditions: one with salt deposits and one without. The salt-covered state was substantially colder and required considerably more atmospheric carbon dioxide to begin melting. The modeled feedback could begin above approximately −36°C, before seawater would have frozen almost completely. Some salts may start crystallizing at temperatures as high as about −8°C, while others begin around −23°C. The findings do not establish that salt caused the initial planetary freeze. The researchers also noted that the model did not include sea-ice movement, clouds, dust, or winds, all of which could dilute, hide, cover, or redistribute the crystals. The study, published in Climate of the Past, presents salt precipitation as a possible additional part of the Snowball Earth puzzle rather than a complete explanation.