[Terr_]

Just because traces are widespread doesn't mean they'll last for 60 million years.

For example, we don't have any ice core data older than ~5 million years. (Much lower, if you limit it to continuous spans.)

[kergonath]

The content of the atmosphere gets washed out by rainfall and does end up in the sedimentary record. Similarly, we can follow the oceans’ acidity from sediments. Large chemical changes would also show up in isotope ratios.