Δ13C

In geochemistry, paleoclimatology, and paleoceanography δ¹³C (pronounced "delta thirteen c") is an isotopic signature, a measure of the ratio of the two stable isotopes of carbon—¹³C and ¹²C—reported in parts per thousand (per mil, ‰).^[1] The measure is also widely used in archaeology for the reconstruction of past diets, particularly to see if marine foods or certain types of plants were consumed.^[2]

Foraminifera samples

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.

δ¹³C varies in time as a function of productivity, the signature of the inorganic source, organic carbon burial, and vegetation type. Biological processes preferentially take up the lower mass isotope through kinetic fractionation. However some abiotic processes do the same. For example, methane from hydrothermal vents can be depleted by up to 50‰.^[3]

Reference standard

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 Peedee Formation in South Carolina. This material had an anomalously high ¹³C:¹²C ratio (0.0112372^[4]), and was established as δ¹³C value of zero.

Since the original PDB specimen is no longer available, its ¹³C:¹²C ratio can be back-calculated from a widely measured carbonate standard NBS-19, which has a δ¹³C value of +1.95‰.^[5] The ¹³C:¹²C ratio of NBS-19 was reported as ${\displaystyle 0.011078/0.988922=0.011202}$.^[6] Therefore, one could calculate the ¹³C:¹²C ratio of PDB derived from NBS-19 as ${\displaystyle 0.011202/(1.95/1000+1)=0.011202/1.00195=0.01118}$.

Note that this value differs from the widely used PDB ¹³C:¹²C ratio of 0.0112372 used in isotope forensics^[7] and environmental scientists;^[8] this discrepancy was previously attributed by a Wikipedia author to a sign error in the interconversion between standards, but no citation was provided. Use of the PDB standard gives most natural material a negative δ¹³C.^[9] A material with a ratio of 0.010743 for example would have a δ¹³C value of −44‰ from ${\displaystyle (0.010743\div 0.01124-1)\times 1000}$.

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.^[10] The ¹³C:¹²C ratio for VPDB, which the International Atomic Energy Agency (IAEA) defines as a δ¹³C value of zero is 0.01123720.^[11]

Causes of δ13C variations

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.^[12]

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.^{[citation needed]}

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.

Geologic significance

C₃ and C₄ plants have different signatures, allowing the abundance of C₄ grasses to be detected through time in the δ¹³C record.^[13] Whereas C₄ plants have a δ¹³C of −16 to −10‰, C₃ plants have a δ¹³C of −33 to −24‰.^[14]

Positive and negative excursions

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.^[15] For example, the evolution of large land plants in the late Devonian led to increased organic carbon burial and consequently a rise in δ¹³C.^[16]

Major excursion events

• 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

See also

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