New research shows that 4.1 billion-year-old zircon crystal found in Western Australia contains biologically-produced carbon, suggesting that life on Earth had begun approximately 300 million years earlier than previously thought. Paper: http://www.pnas.org/content/112/47/14518.full.pdf
I could not open the link. Can you check and repost, or is there a title I can search for? I'm intrigued to know how they identify biologically produced carbon. I can't imagine C14 is any good at such great ages.
I did a little editing of the addy and came up with this: http://www.pnas.org/content/112/47 I did not look for the article, but it seems a good place to start.
I tried again and got through this time. What they have done is find a graphite inclusion in crack-free zircons. They have analysed the 12C/13C ratio of this graphite. 13C is a stable isotope of carbon, like 12C but unlike 14C. Apparently, the biochemistry by which carbon is fixed from the atmosphere preferentially selects 12C, rather than 13C. So specimens of carbon with less 13C than mineral sources of carbon can be presumed (they say) to be of biological origin. The zircons involved have previously been dated, apparently using U/Pb ratio measurement; at least, that is what is mentioned in reference (6) in the paper, which the authors seem be relying on to date the zircon. So it does indeed have nothing to do with carbon 14. The age dating has been done by U/Pb ratios of the zircon matrix, and the C12/13 ratio has been used to determine that the graphite is likely to be of biological origin. Interesting. For me, the most intriguing thing is that carbon fixation is isotopically selective. I had no idea this was so. I'll have to look into this, to find out how it arises - unless another reader can shed light on it for me.....
Well, this is as far as I have got on this, on my own. It seems, according to this: http://www.oxfordreference.com/view/10.1093/oi/authority.20110803100013377 that: a) 13C diffuses more slowly than 12C, as one might expect due to its greater atomic weight and hence lower mean speed of the molecules. (From kinetic theory, we have the relation v(rms) = √(3kT/m) where m is mass of the molecule) ; and b) an enzyme called "rubisco"(an acronym, not sure what it stands for) has a preference for 12C. However I have not so far found an explanation for why this is. Any offers?
It's a link to PDF document. I've no problem opening it in Chrome and Firefox. Here's the title: Potentially biogenic carbon preserved in a4.1 billion-year-old zircon Elizabeth A. Bell, Patrick Boehnke, T. Mark Harrison, and Wendy L. Mao
Thanks. I found when I tried your link again later yesterday that I could get through after all - hence my further comments above. Sorry for any inconvenience.
