In geochemistry, paleoclimatology and paleoceanography δ13
C is an isotopic signature, a measure of the ratio of stable isotopes 13C:12C, reported in parts per thousand (per mil, ‰).
where the standard is an established reference, such as ocean water.
C varies in time as a function of productivity, organic carbon burial and vegetation type.
What affects δ13C?
Methane has a very light δ13
C signature: biogenic methane of −60‰ thermogenic methane −40‰. The release of large amounts of clathrate can impact on global δ13
C values, as at the PETM. More commonly, the ratio is affected by variations in primary productivity and organic burial. Organisms preferentially take down light 12
C, and have a δ13
C signature of about −25‰, depending on their metabolic pathway. An increase in primary productivity causes a corresponding rise in δ13
C values as more 12
C is locked up in plants. This signal is also a function of the amount of carbon burial; when organic carbon is buried, more 12
C is locked out of the system in sediments than the background ratio (because organic carbon is lighter).
Geologically significant δ13C excursions
plants have different signatures, allowing the importance of C4
grasses to be detected through time in the δ13
C record. Mass extinctions are often marked by a negative δ13
C anomaly thought to represent a decrease in primary productivity. The evolution of large land plants in the late Devonian also led to increased organic carbon burial and consequently a drop in δ13