I'm not so sure about this. If plants did discriminate against carbon-14, than we could test plants alive now for the ratio of carbon-14, see that it was different from the ratio in the air, and debunk this whole carbon-14 "myth". However, plants tested today
do have the same ratio of carbon-14 as the air and all living organisms. So they are obviously not discriminating.
Why would it need to be calculated outside of recorded history if atom bombs and industrialization are what changed the ratio?
I don't pretend to be a scientist, but I'm pretty sure the scientist know the rate at which the earth is demagnetizing, and if it is relevant, they would have calculated it into carbon dating
In response, there was no world-wide flood. It is a physical impossibility. Where did all the water go? Did God suck it up with a giant straw? There is the same amount of water today as there has always been. It is extemely uncommon for a water molecule to be ripped apart in nature. And how many sons did Noah have? Three? So you've got Noah, his three sons, and all their wive. How did eight people repopulate the entire earth without some massive inbreeding?
All this is moot, because scientist do not use carbon dating to determine the age of the earth. The most direct means of determining the Earth's age is a Pb/Pb isochron age, which is done on samples from the Earth and other samples from meteorites