δ¹³C

In geochemistry, paleoclimatology, archaeology, and paleoceanography δ¹³C (pronounced "delta thirteen c") is a normalized ratio of the two stable isotopes of carbon—¹³C and ¹²C—reported in parts per thousand (per mille, ‰).^[1]

The definition is, in per mille:

${\displaystyle \delta {\ce {^{13}C}}=\left({\frac {({\ce {^{13}C}}/{\ce {^{12}C}})_{\mathrm {sample} }}{({\ce {^{13}C}}/{\ce {^{12}C}})_{\mathrm {standard} }}}-1\right)\times 1000}$

where the standard is an established reference material.

The δ¹³C 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 δ¹³C, as the precursor materials (modern organic matter and petroleum, respectively) have different isotopic values—the basis of the "Seuss 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".

Reference standards are used for verifying the accuracy of isotope ratio measurements, which are typically performed via isotope ratio mass spectrometry, cavity ring down spectroscopy, tunable laser absorption spectroscopy, or nuclear magnetic resonance.

The initial reference material used to standardize carbon isotope ratios "Pee Dee Belemnite" (PDB) —a Cretaceous marine fossil, Belemnitella americana, originating from the Peedee Formation in South Carolina. This material had an anomalously high ¹³C/¹²C ratio (0.0112372^[2]), and was established as δ¹³C value of zero.

Due to the high demand of PDB standard, the supply was ultimately exhausted. Other standards calibrated to the same ratio, including one known as VPDB (for "Vienna PDB"), have replaced the original.^[3] The ¹³C/¹²C ratio for VPDB, which the International Atomic Energy Agency (IAEA) defines as a δ¹³C value of zero is 0.011113.^[4] The use of different primary reference standards will result in isotope ratios that are incomparable due to the difference in scales. To avoid confusion, isotope ratio measurements typically include a subscript denoting the reference material it was corrected to, such as δ¹³C_PDB or δ¹³C_VPDB.

To prevent the depletion of the supply of VPDB, secondary reference materials with isotope ratios determined in direct comparison to VPDB, such as NBS-19 (available from the National Institute of Standards and Technology, δ¹³C_VPDB= 1.95‰),^[5] are commonly used in the laboratory setting for standardizing measurements.

Methane has a very light δ¹³C signature: biogenic methane of −60‰, thermogenic methane −40‰. The release of large amounts of methane clathrate can affect global δ¹³C values, as at the Paleocene–Eocene Thermal Maximum.^[6]

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, an increase in δ¹³C in marine fossils is indicative of an increase in the abundance of vegetation.

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.

C₃ and C₄ plants have different signatures, allowing the abundance of C₄ grasses to be detected through time in the δ¹³C record.^[7] Whereas C₄ plants have a δ¹³C of −16 to −10‰, C₃ plants have a δ¹³C of −33 to −24‰.^[8]

Positive δ¹³C 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.^[9] For example, the evolution of large land plants in the late Devonian led to increased organic carbon burial and consequently a rise in δ¹³C.^[10]

• Lomagundi-Jatuli event (2,300–2,080 Ma) Paleoproterozoic - Positive excursion

• Shunga-Francevillian event (2,080 Ma) Paleoproterozoic - Negative excursion

• Shuram-Wonoka excursion (570–551 Ma) Neoproterozoic - Negative excursion

• Steptoean positive carbon isotope excursion (494.6-492 Ma) Paleozoic - Positive excursion

• Ireviken event (433.4 Ma) Paleozoic - Positive excursion

• Mulde event (427 Ma) Paleozoic - Positive excursion

• Lau event (424 Ma) Paleozoic - Positive excursion

• Cenomanian-Turonian boundary event (93.9 Ma) Mesozoic - Positive excursion

• Paleocene–Eocene Thermal Maximum (55.5 Ma) Cenozoic - Negative excursion

• δ¹⁸O
• δ¹⁵N
• δ³⁴S
• Isotopic signature
• Isotope analysis
• Isotope geochemistry
• Isotopic labeling

• Miller, Charles B.; Patricia A. Miller (2012) [2003]. Biological Oceanography (2nd ed.). Oxford: John Wiley & Sons. ISBN 978-1-4443-3301-5.
• Mook, W. G., & Tan, F. C. (1991). Stable carbon isotopes in rivers and estuaries. Biogeochemistry of major world rivers, 42, 245–264.