δ13C

[1] The measure is also widely used in archaeology for the reconstruction of past diets, particularly to see if marine foods or certain types of plants were consumed.

δ13C varies in time as a function of productivity, the signature of the inorganic source, organic carbon burial, and vegetation type.

[3] The standard established for carbon-13 work was the Pee Dee Belemnite (PDB) and was based on a Cretaceous marine fossil, Belemnitella americana, which was from the Peedee Formation in South Carolina.

Since the original PDB specimen is no longer available, its 13C:12C ratio can be back-calculated from a widely measured carbonate standard NBS-19, which has a δ13C value of +1.95‰.

Note that this value differs from the widely used PDB 13C:12C ratio of 0.0112372 used in isotope forensics[7] and environmental scientists;[8] this discrepancy was previously attributed by a Wikipedia author to a sign error in the interconversion between standards, but no citation was provided.

Other standards calibrated to the same ratio, including one known as VPDB (for "Vienna PDB"), have replaced the original.

The release of large amounts of methane clathrate can affect global δ13C values, as at the Paleocene–Eocene Thermal Maximum.

[14] Positive δ13C excursions are interpreted as an increase in burial of organic carbon in sedimentary rocks following either a spike in primary productivity, a drop in decomposition under anoxic ocean conditions or both.

[15] For example, the evolution of large land plants in the late Devonian led to increased organic carbon burial and consequently a rise in δ13C.

[16] Negative δ13C anomalies, which are thought to represent a decrease in primary productivity and release of plant-based carbon, often mark mass extinctions.