I talked to J.P.K. at the AGU meeting and he gave me a reprint of the Firestone et al paper on the comet impact at 12,900 years ago that lead to extinction of many of the North American megafauna and may have set off the Younger Dryas climate deterioration (1000 years of cold).
I've posted on that before, when the group working on that had a special session at the spring 2007 AGU meeting. What is new here (besides that the paper is now published) is that there is a layer of diamonds (nanodiamonds) across North America and Western Europe in the soot layer of this age. So, a time marker made of diamonds. I asked how to check for diamonds and the answer is that you disolve everything else and thge diamonds are left.
I assume the diamonds were formed by the high pressures of the impact(s).
The link gives access to the paper...easy to read.
Chris

• open access Firestone et al