"As the amount of carbon -14 in the atmosphere is not constant, but varies with the strength of Earth's magnetic field, solar activity and ocean radiocarbon ages must be corrected with a calibration curve."I think there is a typo in there, somewhere. I think it is "varies with ... and ocean ?behavior? - radiocarbon ages must..."
Frankly, any carbon-14 date before written history is based on a model, which cannot be verified. This is an update to the model.
Also, if diamonds regularly show up as being 50,000 years old (the reason carbon-14 cannot be used past 50,000 years) - is that really due to contamination? Is there no way to factor that out or be more careful? It seems to me that it is just wrong. Diamonds should contain 0 C14 (if they are millions of years old).