### Δ13c

In geochemistry, paleoclimatology and paleoceanography δ13C is an isotopic signature, a measure of the ratio of stable isotopes 13C:12C, reported in parts per thousand (per mil, ‰).[1]

The definition is, in per mil:

$\mathrm\left\{\delta ^\left\{13\right\}C\right\} = \Biggl\left( \mathrm\left\{\frac\left\{\bigl\left( \frac\left\{^\left\{13\right\}C\right\}\left\{^\left\{12\right\}C\right\} \bigr\right)_\left\{sample\right\}\right\}\left\{\bigl\left( \frac\left\{^\left\{13\right\}C\right\}\left\{^\left\{12\right\}C\right\} \bigr\right)_\left\{standard\right\}\right\}\right\} -1 \Biggr\right) \times 1000\ ^\left\{o\right\}\!/\!_\left\{oo\right\}$

where the standard is an established reference material.

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

## Reference standard

The standard established for carbon-13 work was the Pee Dee Belemnite or (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.[2] 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, including one known as VPDB, for Vienna PDB, have replaced the original.[3]

## What affects δ13C?

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 PETM.[4]

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.

An increase in primary productivity causes a corresponding rise in δ13C values as more 12C is locked up in plants. This signal is also a function of the amount of carbon burial; when organic carbon is buried, more 12C is locked out of the system in sediments than the background ratio (because organic carbon is lighter).

## Geologically significant δ13C excursions

C3 and C4 plants have different signatures, allowing the importance of C4 grasses to be detected through time in the δ13C record.[5] Whereas C4 plants have a δ13C of −16 to −10 ‰, C3 plants have a δ13C of −33 to −24‰.[6]

Mass extinctions are often marked by a negative δ13C anomaly thought to represent a decrease in primary productivity and release of plant-based carbon

The evolution of large land plants in the late Devonian led to increased organic carbon burial and consequently a rise in δ13C.[7]