Ancient carbon... and modern artefacts
Science Now links to an article to come out in Geology magazine giving evidence that one class of diamonds is formed from biological residues. The Jagersfontein kimberlite formation, which is in a mine in south africa, originated near the earth's surface and was subsequently subducted by plate tectonics to about 250 km deep. Diamonds isolated from these rocks have a Carbon12/carbon 13 ratio consistent with living things, indicating that the carbon was already there at the time the rocks were laid down. The abstract says that shallower-forming diamonds do not show this skewed ratio.
Basically what goes down must come up, geologically speaking.
Also in last month's Geology was the finding of kerogen streaks in 3.2 billion-year-old slates. Kerogen (wikipedia) is a sort of precursor wax to oil deposits, and to see it in these shales suggests widespread microbial life at that early stage. I have written before about the difficulty in finding proof of the earliest life on earth.
And, as long as we're handing out wet blankets, a review in May's Trends in Microbiology takes Occam's buzzsaw to about 20 reports of very ancient DNA being recovered from amber, salt crystals (ouch!) or stone formations. They don't like any of them. What's the fun of that?