While there is no proof that the rates were different in the past than they are today, there is also no proof that they were the same.
For more on this subject, see the video Bones in Stones i. Ogden III, "Annals of the New York Academy of Science," 288 (1977): 167-173.
Absolute dating is the process of determining an age on a specified chronology in archaeology and geology.
A further example from a lava flow off the coast of Hawaii shows similar discrepancies.
If dated with the carbon-14 method, the flow appears to be less than 17,000 years old, but dating with the potassium argon method gives dates of 160,000 to 43 million years.
This assumption is backed by numerous scientific studies and is relatively sound.