This is absurd. The idea that the current decay can only be extrapolated back 10,000 years is somewhat misleading, even if we didn't know that the polarity reverses itself periodically. Nevertheless, it is well known that the magnetic field reverses polarity sinusoidally, and thus has decayed many times in the past. The geologists on the list can explain how this is recorded in the magnetism of certain rocks. I recall a creationist criticism of a few years back being that there was no theoretical model for this, whereas they had one for their view.
Fossil radioactivity shortens geologic 'ages' to a few years.
I was under the impression that Gentry's data was problematic for some time, but has now been explained. Many years back, when I first came across the YEC arguments, I found many of them interesting, if not totally convincing. The magnetic field, the 14-C in the atmosphere, etc. The evidence for an old earth is extremely good. In thinking about these things, I found it helpful to separate the age of the earth from evolutionary questions.