Carbon is all around you, and you consume it through the air and your diet.
Carbon will leave a mark, and thousands of years after your demise your skeleton will reveal when you lived.
But carbon is preserved for a longer time in the ocean – hundreds of years, actually – and whatever a fish eats will be Carbon 14 dated as perhaps 200 years older than equivalent food on land.
The same goes for the fish, and the fish eater. So if your diet is primarily fish, mine is pork, and we are burried same day next to eachother, Carbon 14 tests may suggest that you were burried 200 years before me, fucking up the interpretation of the burial ground all together.
This is specifically interesting when it comes to Viking graves in England, where a battlefield filled with bodies from fallen fisheating Vikings out of Norway and Denmark will lay next to Anglo-Saxons who hasn’t seen a fish in their entire life.
Besides archeological interpretations it’s worth keeping in mind that if you are in doubt whether a fish is fresh enough to eat; smell it, don’t Carbon 14 test it!