Gas content and transport in cold first year sea ice

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2014, 2014, 2016
Crabeck, Odile
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Journal of Geophysical Research: Oceans RESEARCH
Abstract Past and recent literature have highlighted that sea ice might play a crucial role in controlling and contributing to the exchange of significant climatically active biogases between the ocean and the atmosphere in polar areas. However, the formation of air inclusions and the transport of gases within sea ice cover are still poorly understood. Thanks to development of a new fast and non-destructive X-ray computed tomography (CT) technique to quantify the air volume fraction and the characterization of gas concentration profiles (Ar, O2, N2, CH4 and pCO2) in natural (Kapisillit fjord, Greenland) and artificial (Sea Ice Research Facility, Winnipeg, Canada) cold sea ice, we revealed some key properteies of gas content and transport in sea ice. We concluded that gases are incorporated in dissolved phase in the brine during ice growth. Nucleation further happened in the brine inclusions at rate depending of the gas saturation factor and the brine volume. Nucleation exerts a strong control on gas content and transport because at any given time when bubbles form in the brine, they are segregated from the transport pathway of dissolved salts and dissolved gas. Due to their low density, bubbles will not drain out of the ice by convection. Instead, nucleation leads to the forced buoyant upward transport and an accumulation of gas in sea ice. In addition, we show that the gas content of air bubbles can be exchanged with the brine medium and diffuse within the brine network at a comparable rate (10-5 cm2 s-1) than aqueous diffusivities. In addition, CT-X-ray results showed that air volume fraction was <2% in most of the internal layers and systematically > 5% at the ice-atmosphere interface (top 2 cm). The evidence of air volume fraction over 5% in granular top layers with the presence of macro bubbles introduces new challenges for the interpretation of sea ice atmosphere gas exchange. Substantiation of air porosity in columnar internal layers and granular top layers questions the current sea ice mathematical description in which the air volume fraction is currently neglected.
Sea ice, gas, bubbles, porosity, microstructure, gas exchange
Crabeck, O., Delille, B., Rysgaard, S., Thomas, D. N., Geilfus, N. -X., Else, B., and Tison, J.-L.: First “in situ” determination of gas transport coefficients ( DO2, DAr, and DN2) from bulk gas concentration measurements (O2, N2, Ar) in natural sea ice, Journal of Geophysical Research: Oceans, 119, 6655-6668, doi: 10.1002/2014JC009849, 2014a.
Crabeck, O., Delille, B., Thomas, D., Geilfus, N. -X., Rysgaard, S., and Tison, J.-L.: CO2 and CH4 in sea ice from a subarctic fjord under influence of riverine input, Biogeosciences, 11, 6525-6538, doi: 10.5194/bg-11-6525-2014, 2014b.
Crabeck, O., Galley, R., Delille, B., Else, B., Geilfus, N.-X., Lemes, M., Des Roches, M., Francus, P., Tison, J.-L., and Rysgaard, S.: Imaging air volume fraction in sea ice using non-destructive X-ray tomography, The Cryosphere, 10, 1125-1145, doi:10.5194/tc-10-1125-2016, 2016.