Right, which is why they probably use potassium or krypton instead of carbon. I'm just not as familiar with their isotopes or halflives which is why I used carbon in my example.

The other reply has some great info but I can add a bit. Another way we know it hasn't melted is radioactive dating. I'll use carbon dating as the example since I'm most familiar with it. Carbon-14 is radioactive (half life 5700 years) and naturally present at a rate of 1 ppt. If a substance has gas exchange with the atmosphere, it will keep that 1ppt amount of carbon-14. If not, that number will fall as the carbon decays. Since the ice sheet has a much lower percentage of C-14 (or other radioactive element they are testing) it means the ice must have been there for a long time.

Yep, and tencent owns riot.