Contribution to Session U7 (invited), Unfrozen water in natural systems at subzero temperatures: Quantifying its effects on physical, chemical, and biological processes, Fall Meeting of the American Geophysical Union, San Francisco, December 1999

Morphology and microphysics of sea-ice brine inclusions and their importance for fluid transport and microbial life

H. Eicken (1), A. Stierle (1), C. Bock (2), H. Miller (2), K. Junge (3), C. Krembs (3), J. Deming (3)

(1) Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775-7320, hajo.eicken@gi.alaska.edu
(2) Alfred Wegener Institute for Polar and Marine Science, D-27515 Bremerhaven, Germany
(3) School of Oceanography, University of Washington, Seattle, WA 98195

Sea ice is an important component of the cryosphere and the global climate system, and it furthermore harbours a diverse and productive community of microorganisms. Its unique character is in large part due the retention of salt (between 2 and 10 ppt) within the ice cover in the form of µm- to cm-sized brine inclusions. Physical properties and fluid transport in the ice are greatly dependent on the morphology and microphysics of these pores. We have examined the evolution of fluid inclusions in natural and artificial sea ice in the temperature range from -2 to -25 C based on epi-fluorescence/transmitted light microscopy and magnetic resonance imaging.

Of particular importance are critical transitions in the connectivity and morphology of the pore space with varying temperature. At temperatures between -5 and -2 C (corresponding to high brine volume fractions), most pores are fully connected on a macroscopic scale, with intrinsic sea-ice permeabilities determined as >10^-11 m². At lower temperatures, the bulk of the brine constricts into isolated pores and the ice permeability decreases substantially. As a result, convective transport subsides and a significant differentiation in the morphology and chemistry of pores ensues. Even at very low temperatures, a system of interconnected microveins is maintained mostly at high-angle grain boundaries as a result of thermodynamic and interfacial processes. Local gradients and fluctuations of the temperature field may induce fluid flow in these veins, with particulates and biogenic material concentrating at grain-boundary junctions. The evolution of brine inclusions is shown to be a key factor in structuring the sea ice habitat. Through adaptation of fluorescent staining techniques, we have begun to assess the role of ice and mineral surfaces in the distribution of bacteria in the ice matrix. These studies may provide insight into the constraints on microbial life not only in sea ice but also in extraterrestrial environments at very low temperatures.