Magnetic resonance imaging of sea-ice pore fluids: methods and thermal evolution of pore microstructure

Abstract

Microstructure and thermal evolution of sea-ice brine inclusions were investigated with magnetic resonance imaging (MRI) techniques. Ice samples were kept at temperatures between −2°C and −25°C during 1H imaging in a 4.7-T magnet at 200 MHz. Measurements were completed in a 20-cm diameter cylindrical probe and actively shielded gradient coils (max. 50 mT m −1, pixel dimensions >0.2 mm, slice thicknesses >1 mm), and for higher resolution in a mini-imaging unit with a 9-cm diameter probe with gradient coils of 200 mT m −1 (pixel dimensions <0.1 mm, slice thickness <0.4 mm). Absorption of radio-frequency (RF) signals in the dielectrically lossy brine resulted in degraded signals and was alleviated by use of a contrast agent (decane). MRI data and sea-ice thin section images agree very well (<5% deviation for pore microstructural parameters). Analysis of ice grown under different current speeds indicates that pores are smaller and pore number densities larger at higher current speeds. The thermal evolution of fluid inclusions was studied on cold first-year ice samples, maintained at close to in-situ temperatures prior to experiments. Warming from −21°C to −10°C to −6°C is associated with a distinct increase in pore size (from 1.5 to 1.7 to 2.6 mm for the upper 10-percentile in the vertical) and elongation (4.0 to 4.2 to 6.2 for ratio of major to minor pore axes in the vertical) and a decrease in number densities (0.75 to 0.62 to 0.58 mm −3 in the vertical). Aspect ratios increased from 4:2:1 to 6:2:1 (upper 10-percentile), indicating expansion and merging of pores in the vertical, possibly promoted by microscopic residual brine inclusions.