Δ13C


In geochemistry, paleoclimatology, archaeology, and paleoceanography δC is a normalized ratio of the two stable isotopes of carbon—C and C—reported in parts per thousand.
The definition is, in per mille:
where the standard is an established Reference materials for [stable isotope analysis|reference material].
The δC of a given compound can vary based on the sources of the precursor material and the biogeochemical processes it has undergone. For example, carbon dioxide derived from ecosystem respiration can be differentiated from carbon dioxide formed from the combustion of fossil fuels using δC, as the precursor materials have different isotopic values—the basis of the "Suess Effect". In the case of photosynthesis, two plants grown adjacently with the same source of carbon dioxide may be isotopically distinguishable due to differing biochemical mechanisms and physiologies preferentially selecting for a given isotope—a process known as "isotopic fractionation".

Causes of ''δ''13C variations

Methane has a very light δC signature: biogenic methane of −60‰, thermogenic methane −40‰. The release of large amounts of methane clathrate can affect global δC 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 C, and have a δC signature of about −25‰, depending on their metabolic pathway. Therefore, an increase in δC in marine fossils is indicative of an increase in the abundance of vegetation.
An increase in primary productivity causes a corresponding rise in δC values as more C is locked up in plants. This signal is also a function of the amount of carbon burial; when organic carbon is buried, more C is locked out of the system in sediments than the background ratio.

Geologic significance

and C plants have different signatures, allowing the abundance of C grasses to be detected through time in the δC record. Whereas plants have a δC of −16 to −10‰, plants have a δC of −33 to −24‰.

Positive and negative excursions

Positive δC excursions are interpreted as an increase in burial of organic carbon in sedimentary rocks following either a spike in primary productivity, a drop in decomposition under anoxic ocean conditions or both. For example, the evolution of large land plants in the late Devonian led to increased organic carbon burial and consequently a rise in δC.

Major excursion events