Cryo-SEM and EBSD on air clathrate hydrates in polar ice

In the deeper part of polar ice sheets, air clathrate hydrates (commonly referred to as air hydrates) trap most of the ancient air molecules in their crystal structure, which is the only direct paleo-atmosphere access used for paleoclimatic reconstructions. They form the cubic structure II (sII or CS-II), which consists out of cages formed by water molecules in which the air molecules are enclosed. However, their microstructure and crystallinity are poorly understood.

Studying air hydrates in polar ice is challenging because they are thermodynamically unstable and dissociate under the temperature and pressure conditions in the cold-laboratories. However, the surrounding ice acts as a pressure cell to keep them metastable for a certain amount of time (in the order of years to tens of years).

We use transmitted light microscopy paired with Cryogenic Scanning Electron Microscopy (Cryo-SEM) to investigate air hydrates in polar ice cores. Transmitted light microscopy enables the localization of air hydrates inside the ice sample. Ice grain boundaries and ice relaxation features, such as plate-like inclusions, are useful for orientation during SEM analysis. Air hydrates at or close to the samples surface already dissociate in the cold-laboratory during sample preparation, and form a characteristic structure. Controlled sublimation inside the SEM chamber allows to observe air hydrates previously located inside the ice sample and to investigate their dissociation behavior with sub-micron resolution. For the first time, we perform Electron Backscatter Diffraction (EBSD) analysis on air hydrates in polar ice, which is a powerful method to study the crystallographic structure of materials.