It is produced in the upper atmosphere by radiation from the sun.
(Specifically, neutrons hit nitrogen-14 atoms and transmute them to carbon.) Land plants, such as trees, get their carbon from carbon dioxide in the air. The same is true of any creature that gets its carbon by eating such plants. Suppose such a creature dies, and the body is preserved.
That causes a dating problem with any animal that eats seafood. After about ten half-lives, there's very little C14 left.
Carbon has unique properties that are essential for life on Earth.
On the window sill of Prior's office sits the Californian personalised number plate CARBN14, which she used before moving to New Zealand in 1997.
It's that radioactive form of carbon – known as C14 – that is the key to discovering whether a carved ivory sculpture is an antiquity or a modern sham feeding poachers' coffers; whether a water bore is sucking dry an age-old aquifer or tapping a renewable store; whether a picture frame predates the painting in it.
Although little-feted, GNS's Rafter Radiocarbon laboratory in Lower Hutt was among the world's first to use radioactive decay to unravel history. Prior runs the half of the laboratory that cleans and distils samples down to pure elemental carbon, or graphite.
Down the hall is her husband Albert Zondervan's domain – the hulking m machine that splits the carbon into its stable and radioactive forms.