Biological sequestration of carbon causes enrichment of carbon-12, so that substances that originate from living organisms have a higher carbon-12 content. Due to the kinetic isotope effect, chemical reactions can happen faster with lighter isotopes, so that photosynthesis fixes lighter carbon-12 faster than carbon-13. Also lighter isotopes diffuse across a biological membrane faster. Enrichment in carbon 13 is measured by delta C(o/oo) = [(C/C)sample/(C/C)standard - 1] * 1000.
The common standard for carbon is Cretaceous Peedee formation belemnite.