Salt precipitation in sea ice and its effect on albedo, with application to Snowball Earth

During the initial freezing of the tropical ocean on Snowball Earth, the first ice to form would be sea ice, which contains salt within inclusions of liquid brine. At temperatures below −23°C, significant amounts of the salt begin to crystallize, with the most abundant salt being hydrohalite (NaCl·2H2O). These crystals scatter light, increasing the ice albedo. In this paper, we present field measurements of the albedo of cold sea ice and laboratory measurements of hydrohalite precipitation. Precipitation of salt within brine inclusions was observed on windswept bare ice of McMurdo Sound at the coast of Antarctica (78°S) in early austral spring. Salinity and temperature were measured in ice cores. Spectral albedo was measured on several occasions during September and October. The albedo showed a gradual increase with decreasing temperature, consistent with salt precipitation. Laboratory examination of thin sections from the ice cores showed that the precipitation process exhibits hysteresis, with hydrohalite precipitating over a range of temperatures between −28°C and −35°C but dissolving at about −23°C. The causes of the hysteresis were investigated in experiments on laboratory-grown sea ice with different solute mixtures. All mixtures showed hysteresis, suggesting that it may be an inherent property of hydrohalite precipitation within brine inclusions rather than being due to biological macromolecules or interactions between various salts in seawater.