They allow us to go back in time and to sample accumulation, air temperature and air chemistry from another time.
Slow ice flow at the centre of these ice sheets (near the ice divide) means that the stratigraphy of the snow and ice is preserved.
Melt layers are formed when the surface snow melts, releasing water to percolate down through the snow pack.
They form bubble-free ice layers, visible in the ice core.
Stable isotopes of oxygen (Oxygen [The figure above shows changes in ice temperature during the last several glacial-interglacial cycles and comparison to changes in global ice volume.