Interesting publication: http://www.chemie.de/news/e/84187/ In normal geology jargon, this means that they are pretty convinced to be on to something. However: What they found was depletion of heavy 13C atoms reducing the d13C ratio to values comparable to photosynthesis nowadays as this process favours the lighter 12C atoms, breaking the bonds more easily, I assume. However this is a typical affirming the consequent fallacy: "If it snows the fields are white; the fields are white, hence it snows.". Photosynthesis causes depletion of 13C. There is 13C depletion, hence it was photosynthesis. Not that there are many known processes to cause such a depletion. On the other hand there are many parts in the carbon cycles with very slight fractination processes that accumulate in all those billion years which may also have caused the intial apparant depletion. But interesting.