Carbon has 2 stable isotopes, carbon-12 (C) and carbon-13 (C). The ratio between the two is denoted , and represents a number of factors.
Because organic matter preferentially takes up the lighter C, an increase in productivity increases the  of the rest of the system, and vice versa.  Some carbon reservoirs are very isotopically light: for instance, biogenic methane, produced by bacterial decomposition, has a  of &minus;60‰ – vast, when 1‰ is a large fluctuation!  An injection of carbon from one of these reservoirs could therefore account for the early Cambrian drop in .
Causes often suggested for changes in the ratio of C to C found in rocks include:
* A mass extinction. Chemistry is largely driven by electro-magnetic forces, and lighter isotopes such as C respond to these more quickly than heavier ones such as C. So living organisms generally contain a disproportionate amount of C. A mass extinction would increase the amount of C available to be included in rocks and therefore reduce the ratio of C to C.
* A methane “burp”. In permafrosts and continental shelves methane produced by bacteria gets trapped in “cages” of water molecules, forming a mixture called a clathrate. This methane is very rich in C because it has been produced by organisms. Clathrates may dissociate (break up) suddenly if the temperature rises or the pressure on them drops. Such dissociations release the C-rich methane and thus reduce the ratio of C to C as this carbon is gradually incorporated into rocks (methane in the atmosphere breaks down into carbon dioxide and water; carbon dioxide reacts with minerals to form carbonate rocks).