The C14 timeline is calibrated from radiocarbon years to calendar years: https://en.wikipedia.org/wiki/Calibration_of_radiocarbon_dat...
There are additional levels of calibration above that one, but that is the main way the dates are arrived at.
Also, here is what wikipedia says:
>"Although Willard Libby, the inventor of radiocarbon dating, had pointed out as early as 1955 the possibility that the 14C/12C ratio might have varied over time, it was not until discrepancies began to accumulate between measured ages and known historical dates for artefacts that it became clear that a correction would need to be applied to radiocarbon ages to obtain calendar dates." https://en.wikipedia.org/wiki/Calibration_of_radiocarbon_dat...
Can you explain more how this removes dependence on historical dates? Tree rings need to be calibrated too since more than one can form per year or years can be skipped based on the weather (too cold or too little precipitation -> growth stops).
You then use the samples dated with dendrochronology to calibrate your radioisotope dating.
Assuming that the matching of rings is done perfectly (from my looking it up this is not trivial, where is the best place to find the raw data for one of the reference sequences btw?), I don't see how you could trust this without matching some rings to wood from known artifacts to "check it".
I mean can't you get a decade-long drought and so miss 10 years? And here is a source reporting that missing rings become more frequent as you look back further (it grows to 6% missing at 150 years): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4885872/
Of course that all depends on the species, etc so maybe it can be avoiding by only selecting the "right" samples. Can you share a good source where I can see how one of the reference sequences has been arrived at from scratch?