The c14 method uses assumed c14/c12 ratios. The assumption is that the atmosphere was the same when the organic material is being analyised. With radiocarbon being manufactured in the atmosphere by the action of cosmic rays, historically the assumption is that it has not changed. Therein is an "x" factor.
This is from Fairhill and young and I'll quote it so that I do not mangle it. "We note in passing that the total natural (current?)c14 inventory of 2.16 x 1030 atoms corresponds to the c14 decay rate of 1.63 x 104 disintegrations/m2s of the earth, considerably below the estimated production rate of c14 atoms averaged over the last 10 solar cycles (111 years) of 2.5 x 104 (+0.5 x 104) atoms/m2s. The source of discrepancy is unknown unless the present day production rate is indeed significantly higher than the average production rate." (Advances in Chemistry, vol. 93 pg. 402)
The possible influx/outflow rates are where the possible errors are. This is based on the advancements of tree-ring data showing that the issue is a lot more complex than was/is first thought.
You ask about C14/C12 ratios, and the ratios in the atmosphere in the past, and write "historically the assumption is that it has not changed."
That was true in the early days of radiocarbon dating. But, in 1958, de Vries showed that atmospheric fluctuations did occur and suggested means of accounting for them in dating. The fluctuation appears to be on the order of 1%. Since then, a detailed calibration curve has been worked out using dendrochronology and other techniques.
The current calibration curve is in 1 year increments back to about AD 1600, and in 10 year increments back to about 12,600 years ago. Much of this is based on the tree rings found in standing dead bristlecone pines from the White Mountains of southern California. Bristlecone pines have distinct annual rings because of their environment, unlike other species, which can have more than one ring per year. These tree rings are matched with environmental events, such as volcanoes, of known ages, and the method is shown to be accurate as far back as these comparisons are possible.
The tree-ring calibration curve has been confirmed and extended based on other methods. It goes back past 25,000 years last I heard.
Your next question is on the current C14 levels. You cite an article which notes, "The source of discrepancy is unknown unless the present day production rate is indeed significantly higher than the average production rate."
The answer is likely that since the atomic bomb tests beginning in the 1940s, the atmospheric levels of C14 have increased from what they were prior to the tests. Perhaps this is what Fairhill and Young have noted.
Your final comment on tree-rings; I am not sure what you mean by that.
For a large amount of information on radiocarbon dating, see Radiocarbon -- full text of issues, 1959-2003.
45 journals! Jeez, C-man everytime you send me a link it takes me a year to get through them! :o) I think that is why I only get to discuss this with you about that often!
Also, are you AZ? i haven't check your homepage. If you are I have a question about an excavation down there a while back (50's or 60's) that has a "mysterious" theory about it. If I think of her name and that of the dig she was working on I'll send it.
If I don't get back to you tonight it's because Im over at the link you sent.
FReepgards,
K4