Viewing a single comment thread. View all comments

Pandarmy t1_ja6lpux wrote

Reply to comment by KronoMakina in How is radioactive dating used to determine historical greenhouse gas levels and temperatures? by pog_irl

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.

11

Undercover_in_SF t1_ja8ggxq wrote

And accuracy decreases the farther back you go. Once you get around 50,000 years you’re at 10 half lifes, so any residual C14 is so small it’s not accurate.

3

Pandarmy t1_ja9bocd wrote

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.

5