Sliver of Antarctic ice showing trapped bubbles.
Long Range Technology Plan, p.
Radioactive elements, either of natural origin or created by nuclear testingcan be used to date the layers of ice.
Here are two great resources for a little more on some of what I covered:
Description: The last data point from these ice-core studies wasyears ago, however the graphs are mislabeled labeled as current or present. Where there are melt layers, one can expect lower oxygen content.