Moreover, even non-organic carbon samples appear consistently to yield 14C levels well above machine threshold. Graphite samples formed under metamorphic and reducing conditions in Precambrian limestone environments commonly display 14C values on the order of 0.05 pmc. A good
question is what possibly could be
the source of the 14C in this material?
We conclude that the possibility
this 14C is primordial is a reasonable
one.