[njarboe]

You are correct. I had 1000000000 as a trillion. Scientific notation is a much better way to calculate things. At 30mm that is a big enough layer that a geologist could see it in the rock record, if it looked different enough.

[thfuran]

I believe that we've spewed enough radiation that no visual distinction would necessary for detection, at least for the next several tens of thousands of years.

[titzer]

It's true that atmospheric nuclear testing has produced trace elements that were basically never on Earth before 1945, like Strontium-90 [https://en.wikipedia.org/wiki/Strontium-90]. Strontium-90 has a half life of about 29 years, so it will be detectable for a few thousand years or so. Some others last longer.