Δ13C
In geochemistry, paleoclimatology and paleoceanography δ13C (pronounced "delta thirteen c" or "delta carbon thirteen") is an isotopic signature, a measure of the ratio of stable isotopes 13C : 12C, reported in parts per thousand (per mil, ‰).
The definition is, in per mil:
where the standard is an established reference material.
δ13C varies in time as a function of productivity, organic carbon burial and vegetation type.
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 Pee Dee Formation in South Carolina. This material had an anomalously high 13C:12C ratio (0.0112372), and was established as δ13C value of zero. Use of this standard gives most natural material a negative δ13C. A material with a ratio of 0.010743 for example would have a δ13C value of (0.010743/0.0112372 − 1)*1000 = −44. The standards are used for verifying the accuracy of mass spectroscopy; as isotope studies became more common, the demand for the standard exhausted the supply. Other standards calibrated to the same ratio, including one known as VPDB (for "Vienna PDB"), have replaced the original.
Methane has a very light δ13C signature: biogenic methane of −60‰, thermogenic methane −40‰. The release of large amounts of methane clathrate can impact on global δ13C values, as at the Paleocene–Eocene Thermal Maximum.
More commonly, the ratio is affected by variations in primary productivity and organic burial. Organisms preferentially take up light 12C, and have a δ13C signature of about −25‰, depending on their metabolic pathway. Therefore, higher δ13C in marine fossils is indicative of an increase in the abundance of vegetation.
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