A Geophysical Investigation of the Arctic Sea Ice Surface

Thesis (Ph.D.)--University of Washington, 2016-06

The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: II. Optical modeling

During the Snowball Earth events of the Neoproterozoic, tropical regions of the ocean could have developed a precipitated salt lag deposit left behind by sublimating sea ice. The major salt would have been hydrohalite, NaCl•2H2O. The crystals in such a deposit can be small and highly scattering, resulting in an allwave albedo similar to that of snow. The snow-free sea ice from which such a crust could develop has a lower albedo, around 0.5, so the development of a crust would substantially increase the albedo of tropical regions on Snowball Earth. Hydrohalite crystals are much less absorptive than ice in the near- infrared part of the solar spectrum, so their presence at the surface would increase the overall albedo as well as altering its spectral distribution. In this paper, we use laboratory measurements of the spectral albedo of a hydrohalite lag deposit, in combination with a radiative transfer model, to infer the inherent optical properties of hydrohalite as functions of wavelength. Using this result, we model mixtures of hydrohalite and ice representing both artificially created surfaces in the laboratory and surfaces relevant to Snowball Earth. The model is tested against sequences of laboratory measurements taken during the formation and the dissolution of a lag deposit of hydrohalite. We present a parameterization for the broadband albedo of cold, sublimating sea ice as it forms and evolves a hydrohalite crust, for use in climate models of Snowball Earth.

Hydrohalite in cold sea ice: Laboratory observations of single crystals, surface accumulations, and migration rates under a temperature gradient, with application to “Snowball Earth"

When NaCl precipitates out of a saturated solution, it forms anhydrous crystals of halite at temperatures above +0.11?C, but at temperatures below this threshold it instead precipitates as the dihydrate ‘‘hydrohalite,’’ NaCl * 2H2O. When sea ice is cooled, hydrohalite begins to precipitate within brine inclusions at about -23C. In this work, hydrohalite crystals are examined in laboratory experiments: their formation, their shape, and their response to warming and desiccation. Sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite, which has the character of a fine powder. The precipitation of hydrohalite in brine inclusions raises the albedo of sea ice, and the subsequent formation of a surface accumulation further raises the albedo. Although these processes have limited climatic importance on the modern Earth, they would have been important in determining the surface types present in regions of net sublimation on the tropical ocean in the cold phase of a Snowball Earth event. However, brine inclusions in sea ice migrate downward to warmer ice, so whether salt can accumulate on the surface depends on the relative rates of sublimation and migration. The migration rates are measured in a laboratory experiment at temperatures from -2C to -32C; the migration appears to be too slow to prevent formation of a salt crust on Snowball Earth.