The data, covering the end of the last ice age, between 20,000 and 10,000 years ago, show that CO2 levels could have lagged behind rising global temperatures by as much as 1,400 years.
But because air diffuses rapidly through the ice pack, those air bubbles are younger than the ice surrounding them.
There are about two dozen decay pairs used for dating.
Uranium 235 decay to lead has a half-life of 713 million years, so it is well suited to dating the universe.
Other methods do not require knowing the initial quantities.
For example, potassium decays into two different isotopes of argon having different half-lives.