A magnetic resonance study of temperature-dependent microstructural evolution and self-diffusion of water in Arctic first-year sea ice

• Main Author: H. Eicken
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Arctic; Barrow; Sea ice; Alaska
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.546.7961; http://www2.gi.alaska.edu/snowice/sea-lake-ice/papers/05BE.pdf

The microstructural evolution of brine inclusions in granular and columnar sea ice has been investigated through magnetic resonance imaging (MRI) for temperatures between –28 and –3 ˚C. Thin-section and salinity measurements were completed on core samples obtained from winter sea ice near Barrow, Alaska. Sub-samples of granular (2-5 cm depth in core) and columnar sea ice (20-23 cm depth), were investigated with 2morphological spin-echo and diffusion-weighted imaging in a Bruker 4.7 T 400 mm bore MRI system operating at field gradients of 200 mT/m at temperatures of approximately –28, –15, –6 and –3 ˚C. Average linear pore dimensions range between 0.2 and 1 mm and increase with bulk liquid volume fraction as temperatures rise from –15 to –3 ˚C. Granular ice pores are significantly larger than columnar ice pores and exhibit a higher degree of connectivity. No evidence is found of strongly non-linear increases in pore connectivity based on the MRI data. This might be explained by shortcomings in resolution, sensitivity and lack of truly three-dimensional data, differences between laboratory and field conditions or the absence of a percolation transition. Pore connectivity increases between –6 and –3 ˚C. Pore number densities average at 1.4 ± 1.2 mm–2. The pore number density distribution as a function of cross-sectional area conforms with power-law and lognormal distributions previously identified, although significant variations occur as a function of ice type and temperature. At low temperatures (<–26 ˚C), pore sizes were estimated from 1H self-diffusivity measurements, with self-diffusivity lower by up to an order of magnitude than in the free liquid. Analysis of diffusional length scales suggests characteristic pore dimensions of <1 µm at <–26 ˚C. 31.