delta13C

Delta13C, denoted δ13C, is a stable carbon isotope ratio that expresses the relative abundance of 13C to 12C in a material compared with a standard reference material. It is reported in per mil (‰) and calculated as δ13C = [(Rsample/Rstandard) − 1] × 1000, where R is the ratio 13C/12C. The standard most commonly used for organic and carbonate material is Vienna Pee Dee Belemnite (VPDB).

Measurement is typically done by isotope ratio mass spectrometry (IRMS). Samples are converted to CO2 or another gas, and the isotope ratio is measured; results are normalized to VPDB. Lipids and other components can bias δ13C values; lipid extraction or mathematical normalization may be used.

Biological fractionation yields characteristic ranges: C3 plants typically show δ13C around −28 to −30 ‰, C4 plants around −12 to −14 ‰ due to differing photosynthetic pathways; marine carbonates approximate values near 0 to −2 ‰, atmospheric CO2 around about −8 ‰ in VPDB. These numbers may vary with physiology and environment.

Applications of δ13C include ecological and paleodietary studies that trace carbon flow through food webs, and paleoclimatology and carbon cycle reconstructions. It is also used in terrestrial and marine ecosystem studies and atmospheric science to understand carbon sources and exchanges.

Interpretation caveats: δ13C integrates multiple processes including source isotopic composition and fractionation during metabolism; diagenetic alteration can modify signals; results require calibration against standards and, for tissues, corrections for lipid content or metabolically derived fractions.