Measured and modeled albedos of sea-ice surfaces with implications for Snowball Earth
Carns, Regina Catharine
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The Snowball Earth episodes were extensive glaciations that occurred during the Neoproterozoic, between 600 and 800 million years ago, during which ice covered much or all of the oceans. These glaciations were a result of ice-albedo feedback, a process likely to occur on any Earthlike planet with oceans covering most of its surface. Modeling shows that sublimation would exceed precipitation over large regions of the ice-covered ocean on a Snowball planet; during the initial stages of the Snowball episode, these areas would be entirely covered by sea ice containing inclusions of brine, and sea ice could remain in smaller regions through the whole episode. At temperatures likely to prevail in the Snowball climate, sodium chloride precipitates within brine inclusions as the hydrated salt hydrohalite (NaCl*2H2O, also known as sodium chloride dehydrate). This work used field measurements, laboratory experiments and modeling to constrain the albedo of sea ice surfaces relevant to Snowball Earth. Field measurements of cold sea ice in McMurdo Sound show an increase in the albedo of natural sea ice with decreasing temperatures. Laboratory experiments on natural sea ice show that brine pockets can become supersaturated with respect to sodium chloride at low temperatures, creating a hysteresis in hydrohalite precipitation and dissolution. Experiments show this effect in laboratory-grown ice of several different compositions: grown from an NaCl solution, grown from artificial seawater, and grown from artificial seawater with added extracellular polysaccharides. Sufficiently cold sea ice in a region of net sublimation will eventually develop a lag deposit of salt as the ice sublimates away from precipitated hydrohalite in brine pockets. No sea ice on modern Earth stays cold and dry long enough for such a deposit to form, so we developed a method for measuring the albedo of ice surfaces in a cold-room laboratory. The method uses a dome with a diffusely reflecting interior surface to emulate the light from an overcast sky. We created a crust of hydrohalite and used this "albedo dome" method to measure albedo of the crust as it developed and dissolved. Using these measurements along with a radiative transfer code, we inferred the complex refractive index for hydrohalite and developed a parameterization for the albedo of hydrohalite crusts of any thickness. These results have implications for Earthlike exoplanets with sizable oceans, which would also be susceptible to ice-albedo feedback. The formation of hydrohalite in sub-eutectic sea ice and the development of a lag deposit in cold, dry conditions could intensify the positive feedback that leads to Snowball conditions. This work shows that the albedo of hydrohalite is much higher than that of snow in the near-infrared, which could make the formation of hydrohalite crusts particularly important to the climates of planets that orbit M-dwarf stars, which output a large fraction of their energy in the near-infrared.