Δ13C

δ13C and delta C-13 is a measure of the isotopic ratio of the stable isotope 13C / 12C between a sample and a standard in geochemistry, paleoclimatology and paleoceanography. It is defined as:

The default is usually Pee Dee Belemnite, PDB or VPDB abbreviated. δ13C varies over time depending on the primary production of organic carbon deposition and the type of vegetation. The determination of this ratio is one of several methods of isotopic analysis.

In 3.5 billion years old rock a shift in the δ13C ratio is detectable, which is regarded as an indication that since that time there is plant life on Earth.

Methane has a very low δ13C - signature: When biogenic methane that is -60 ‰ and -40 ‰ for thermogenic methane. The release of large quantities of methane hydrates affects the global δ13C - signature; this was observed during the Paleocene / Eocene Thermal Maximum.

The δ13C signature can also be used to reconstruct the concentration of carbon dioxide in the atmosphere of yesteryear. This is especially relevant for samples which are older than the oldest available ice cores, thus older than 800,000 years.