Complex sentence: Air bubbles that have been trapped are pressurised at this depth to 100 bars, and the nitrogen can combine with the cold ice to form a clathrate; however this can be contaminated with dioxygen forming an air clathrate.
Split into simpler sentences: Air bubbles that have been trapped are pressurised at this depth to 100 bars, and the nitrogen can combine with the cold ice to form a clathrate. However this can be contaminated with dioxygen forming an air clathrate.