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Project Progress

A series of laboratory investigations is being carried out by B. Light which is directed at characterizing the properties of ocean surfaces during Snowball Earth. Currently, summer temperatures and solar insolation produce melting on sea ice surfaces, and at least in some regions, ponding. Air temperatures below the temperature of initial precipitation of hydrohalite (-22.9 C) would almost completely prevent melting, and may in fact, dramatically change the heat and mass balance of sea ice. Ice sublimation could lead to the formation of a salt crust at the ice surface. No surrogate crust is known to exist in nature currently. The most significant salt that would accumulate is not NaCl but rather NaCl • 2H₂O, “hydrohalite,” whose properties have not received much investigation. We are using a cold-room laboratory to investigate the physical and chemical properties of hydrohalite crusts, along with their growth and maintenance.

Four principal questions guide this investigation: (1) Under what conditions does a salt crust form on sublimating sea ice? (2) What are the optical properties of such a crust? (3) Can a salt crust suppress ice sublimation? (4) Are the crystals in a crust cohesive with ice, or are they likely to become wind-borne? During the past year, we have observed hydrohalite crusts forming on the surface of sublimating samples in the laboratory. The crusts have loose, powdery texture, along with high optical reflectivity (comparable to fine grained snow crystals). The powdery deposits have been sampled and photomicrographs reveal individual hydrohalite crystals with diameters approximately 5 – 20 microns.

For a salt crust to form on sublimating sea ice, the rate of sublimation must be faster than the rate of salt migration from the relatively cold surface to the warmer ice interior. To investigate this, observations of the migration of inclusions have also been carried out in the laboratory. A temperature gradient stage for an optical microscope has been designed and built. Thin sections of natural and laboratory grown sea ice were prepared and mounted on the stage. Time lapse photographs of the sample show the motion of individual brine inclusions within the sample in response to the temperature gradient. Migration rates estimated from these photographs fall in close agreement with theoretical estimates for liquid inclusions at temperatures near the eutectic. It is believed these are the first measurements of salt migration at such low temperatures. Ongoing studies using this experimental setup are also addressing the migration of solid inclusions at temperatures below the NaCl-H₂O eutectic.