Anders: Origin of Carbonaceous Chondrites 



527 



terials are generally depleted in C^^ relative to the source material: atmospheric 

 CO2 or oceanic bicarbonate (Craig, 1953). This effect is quite pronounced if the 

 biogenic carbon comprises only a small fraction of the total available carbon 

 (figure 8). 



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Figure 8. Isotopic composition of carbon from various sources. Processes involving a 

 partial loss of carbon in the form of volatile compounds {e.g., the formation of petroleum from 

 the remains of organisms) result in the depletion of C. Such a depletion is also observed in 

 the case of the meteorites (table 6) in which the C" content declines with decreasing total 

 carbon content. [Reproduced from Craig (1953), with permission of the editor.] 



