Atmospheric readings from ice cores?

Question

How do scientists determine the composition of the atmosphere 100,000+ years ago from core samples of glaciers?

Answer 1

Tiny bubbles become trapped in the ice as it forms. Provided that these bubbles don't leak to the outside world, then they can be analysed by gas chromatography to determine the constituent elements. Problems do arise however in that some gas may have escaped from the bubbles over time, so it is impossible to know precisely what the composition was, but you can determine it probabalistically. The ice is usually dated by looking at the pollen grains that are trapped in the ice, and comparing this pollen to ancient plants that have been dated by other means.