NATURE OF THE GEOLOGIC RECORD 139 



atoms. These radioactive carbon atoms lose their radioactivity spontane- 

 ously by a sort of disintegration. As old ones are disintegrating new ones 

 are being formed by the cosmic rays. The result is an equilibrium in the 

 atmosphere. The equilibrium is at about one radiocarbon atom for a trillion 

 "normal" carbon atoms. These carbon atoms are found in the carbon 

 dioxide of the atmosphere. The equilibrium point represents a concentra- 

 tion at which radiocarbon in the carbon dioxide disintegrates at the same 

 rate that new radiocarbon atoms are being formed. As a result the concen- 

 tration of radiocarbon remains constant and has done so for a long period 

 of time. 



As we have seen (Chap. 5), plants use carbon dioxide in the manufac- 

 ture of carbohydrates; animals obtain the latter by eating plants. Thus 

 during their lifetimes plants and animals are constantly acquiring radiocar- 

 bon atoms along with "normal" ones, and the relative numbers of the two 

 kinds are probably the same in the body as in the atmosphere. Radiocar- 

 bon atoms steadily lose their radioactivity in the body, as they do outside 

 of it. But since new radiocarbon atoms are constantly being taken in as 

 part of the food, the concentration of these "heavy carbon" atoms in the 

 body of plant or animal remains about constant as long as the plant or 

 animal lives. After death no more carbon enters the body, and the radio- 

 carbon already present steadily disintegrates. Hence the amount of radio- 

 carbon present diminishes steadily, a diminution accompanied by a 

 concomitant decrease in the rate of atom disintegration in the tissue. The 

 fewer the radiocarbon atoms present, the smaller the number of them 

 undergoing disintegration at any one time. Thus if we take a piece of 

 ancient wood or bone and measure the rate at which carbon 14 is now 

 disintegrating we can estimate the age of the material, since we know the 

 rate at which disintegration occurs in living wood or bone. When this 

 technique has been applied to materials of known age the demonstrated 

 accuracy has been such as to give confidence in determinations made 

 on materials of unknown age. (See Libby, 1956.) 



The carbon 14 method is applicable only to organic materials still con- 

 taining carbon; it cannot be used on fossils in which all organic matter 

 has been replaced by minerals. Since the amount of radiocarbon present 

 decreases steadily with time, the method can probably never be used on 

 material older than about 70,000 years. Accurate dating within that time 

 span will prove most valuable, however. Among the early inhabitants of 

 North America, for example, were makers of a particular type of stone ar- 

 rowhead (probably used on darts rather than arrows), the so-called Fol- 

 som points. Pieces of burned bone found with some of these points give a 



