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Ok, here's a question just off the top of my head, just to cheese you off.
The classic picture of the extinction of the dinosaurs changed greatly upon the finding of a thin layer of Iridium at the K-T boundary.
Cool, we now have heavy metal contamination of the earth from an external source.
My question is, how external is external?
The validity of radiocarbon dating depends (in an ordinary, laboratory scale sense) on the homogeneity of the sample, and the uniformity of conditions. That is, we assume all of the initial amounts of radionuclei (in whichever decay series we are using) were all formed at the same time. This is important because the initial concentrations of daughter particles will be the same. (When the sample gets contaminated, you can get problems in dating, see the Shroud of Turin re-weave and sampling controversy for a non-evo example.)
OK, so were there any amounts of any radionuclei (wherever in the sequence) introduced at the K-T boundary or by any other impacts?

Secondly, if there were, do we know whether the radioisotopes in the comet were formed at the same time as the ones on Earth?

No problem, I am always happy to answer questions.

The problem with your question, though, is that you are confusing radiocarbon dating with radiometric dating. I do the former, not the latter.

Radiocarbon only extends back about 50,000 years, so it does not tell us anything about the K-T boundary.

Adapting your question to radiocarbon dating, we can determine initial conditions through the tree ring calibration. That curve goes back about 12,500 years and they are working on other materials that have extended past 20,000 years.

In plain language, by being able to count tree rings back, say 10,240 years into the past, and then directly dating that one ring you can see how closely the radiocarbon age matches the calendar age. By doing thousands of these tests you can establish a correction curve that accounts for atmospheric variation.

Hope this helps.

I know the difference between the two--but since, as you (correctly) pointed out, radiocarbon only goes back to 50,000 years, and some of the earlier hominid fossils date much older than that, I jumped to a wishful thinking conclusion and assumed you had done both. Sorry!

Thanks for the info on the tree rings, btw. It's always good to have multiple, independent, concurring evidence.

BTW, here's another question (tee hee).

For radiometric dating, recall that one of the crevo threads recently posted an article about a naturally occurring uranium-based chain reaction in Africa...this by definition would screw up isotope ratios in the immediate vicinity of the reaction, as well as any places to which the fission by-products or matrix got washed downstream.

How common an occurrence is that? Is it enough to worry about dating of any fossils or local geographical strata?

Cheers!