Carbon has two stable isotopes, C and C, and one radioactive isotope, C.
The stable carbon isotope ratio, δC, is measured against Vienna Pee Dee Belemnite (VPDB). The stable carbon isotopes are fractionated primarily by photosynthesis (Faure, 2004). The C/C ratio is also an indicator of paleoclimate: a change in the ratio in the remains of plants indicates a change in the amount of photosynthetic activity, and thus in how favorable the environment was for the plants. During photosynthesis, organisms using the  C pathway show different enrichments compared to those using the  C pathway, allowing scientists not only to distinguish organic matter from abiotic carbon, but also what type of photosynthetic pathway the organic matter was using. Occasional spikes in the global C/C ratio have also been useful as stratigraphic markers for chemostratigraphy, especially during the Paleozoic.
The C ratio has been used to track ocean circulation, among other things.