I tried to find the article in "Nature" and it was not there. It was instead a 1 page article in "Science". It just basically said they had found nanodiamonds in the 12,900 year B.P. layer. Of interest is the analysis of the debate written in the same issue of Science by Richard Kerr, a science journalist (I think). Not all experts are convinced that what they found are indeed diamonds, or, if they are diamonds, there are some who say nano dimanonds can be found in sediments that are not associated with impacts. The proponents of the 12,900 year BP comet impact hypothesis say that they find diamonds in the 12,900 year layer but not above or below. But, it seems people have not looked for nano-diamonds very much in sediments far removed in time from likely impact layers.
Chris