Marine and riverine "reservoir effect" on radiocarbon dating. Fish, fish diets and hard water environments can cause skewed aging by up to 2000 years (older).
"Hard water contains less Carbon-14 than the atmosphere, because dissolved carbonates are Carbon-14 free. A fish caught in hard water has thus a higher Carbon-14 age than contemporaneous terrestrial samples. If such a fish is then cooked in a ceramic pot, the radiocarbon age of the food crust will be higher than if a terrestrial animal was cooked in the pot.
This is known as the “reservoir effect” because the fish’s carbon actually comes from another “reservoir” than the carbon in terrestrial animals from the surrounding area. “Reservoir age” is the difference between the true age and the Carbon-14 date."
Via Spencer Carter