What do these things have in common? The answer is very surprising
First off, pollen. Every allergy sufferer in existence loathes the stuff. But it turns out pollen is incredibly robust stuff - it can last for thousands of years, buried in mud or clay, such as at the bottom of a lake. Northern lakes form what are called varves, layers of alternating light and dark colored soil that operate like clockwork, year after year. This is due to the fact that heavier, lighter-colored silt tends to settle during summer months, while lighter, darker-colored silt settles in winter months when the lakes freeze. Meaning, it's more accurate than a calendar - at least provided that the lakes are frozen during the winter and thawed during the summer.
For example, Lake Suigetsu, in Japan, is an ideal place to take varve measurements, since it was formed by a volcanic eruption and basically has no outlets and very few inlets, meaning that its varve formation is extremely regular. They have taken sediment core samples that have greater than 60,000 layers - meaning more than 60,000 years, exactly like clockwork. This also means that they can use c-14 dating on those samples (such as plant matter, pollen, and algae), and can validate the accuracy of those dates.
Needless to say, this is among the strongest evidence to support radiocarbon dating that anyone has found yet - and varves have been known about for more than a century. It's only been in the past few decades that they've been used in conjunction with other forms of dating. Given that all three major forms of dating (tree rings, varves, and c-14 dating) form an almost direct correlation with each other, it's extremely unlikely that some other explanation for why they fit together in what is for all intents and purposes a straight line would be believable.