Regarding one form of “contamination”, I developed the following with some pointers from YEC Nuclear Chemist J. Wile, PhD. Corrections welcome.
There are various reaction mechanisms to create C14. The main one involves Nitrogen in the atmosphere.
An alternative involves C13 acquiring a neutron from some source to become C14.
Some claim all we need is a little Uranium, or some other radio active substance. This thread attempts to critically refute some, but not all, of those claim.
But first from this secular peer-reviewed paper (that was referenced in TalkOrigins):
Many (super 14) C dating laboratories have established that coal samples exhibit a finite (super 14) C age, apparently caused by contamination of the specimens before any laboratory preparation is undertaken.
The bacterial or microbe contamination issue that the paper proposes has it’s flaws too! But more on that perhaps another time…
An estimate of the outer limit of the amount of C14 created by Uranium converting C13 to C14 in the geological record can be made on generous assumptions.
The spontaneous fission rate and neutron release rate is reported here:
For Uranium (mostly U238 where U235 rate negligible),
0.0136 Neutrons per gram per second
If we make the absurdly generous assumption that all neutrons released are thermal (aka, slow enough to be captured by C13), and the absurdly generous assumption that all neutrons find a C13 target (rather than just going through the whole carbon sample altogether), we can frame the reaction generation rate.
Approximately 1 gram of carbon has this many atoms. Using Avogadro’s number and the approximate atomic mass of Carbon.
6.021023/ 12 = 5 x 1022
number C14 atoms in typical 1 gram sample:
1.5x10^-12 * 5 x 10^22 = 7.5 * 10^10 atoms / gram
Converting only C13 to C14 at a rate of .0136 atoms per second creates over 5730 years (the half-life of C14):
5730 * 365 days/year * 24 hours/day * 3600 seconds/hour *.0136 neutrons/gram/second = 2.4 x 10^9 C14 atoms /gram
(2.4 x 109) / ( 7.5 * 10^10 ) ~= 3% of modern carbon
But that assumes 1 gram Uranium per 1 gram carbon! Even a generous assumption of 0.1% Uranium and 99.9% carbon would be about 0.003% typical modern carbon. And this doesn’t even factor in the probability of neutrons hitting C13, or the probability the neutrons are thermal (capturable speed).
This figure parallels a claim in the paper in the link above with an alternate radiation source:
One suggestion is that radium, which is present in some coals at the sub pm level, as a decay product of the uranium/thorium series, may produce 4C during an extremely rare decay event (Rose & Jones, 1984). Jull,Barker and Donahue (1987) have detected 14C from this process in uranium/ thorium ores. Blendowski, Fliessbach and Walliser (1987) however, have shown that the 14( decay mode of 226Ra is only of the order of 10-11 of the preferred a decay channel to 222Rn. Thus, the amount of 14C produced by such events derived from radium in coal must be considered as insignificant.