5/18 高機率機經聽力：How we use C14/C12 ratio to determine age.

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原始學生回憶：
講考古,怎麽利用 C14 to C12 的 ratio 來鑒定(同位素法研究)一個化石的年齡

補充資料來源：http://www.youtube.com/watch?v=wREonrelyP4 （同學可先訓練聽力喔！）

重點提示：

大氣中C12: C14維持固定比例： 1 trillion :1

For every 1 trillion C12 in the atmosphere, there is one C14. That is constant ratio. That is maintained. They both combine oxygen to produce CO2 and the ratio between these two remain the same.

植物吸收的CO2裡的C12/C14比例也保持固定。  

Plants take in CO2 in the same ratio that exists in the atmosphere.

動物包含人類吃植物所以體內也存在著同比例的C12/C14.

Animals eat plants.

但是一旦動物死亡，碳14會變成氮氣，數量漸漸減少，C12仍保持不變，

根據這比例可以算出化石年齡。（比例越少年代越久遠。）

As animals die, C14 based CO2 begin to change into nitrogen. C12 based CO2 remains stable.  The ratio in a dead organism is no longer the same as in the atmosphere. We can detect this new ratio and determine just how long it took to get this ratio.