δ13C

Foraminifera samples

In geochemistry, paleoclimatology and paleoceanography δ¹³C (pronounced "delta thirteen c") 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:

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

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 (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] A material with a ratio of 0.010743 for example would have a δ¹³C value of (0.010743/0.0112372 − 1)*1000 = −44. 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.^[3]

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 impact on global δ¹³C values, as at the Paleocene–Eocene Thermal Maximum.^[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. Therefore, higher 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.

Geologically significant δ¹³C excursions

C₃ and C₄ plants have different signatures, allowing the abundance 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]

See also

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

References

1. Libes, Susan M. (1992). Introduction to Marine Biogeochemistry, 1st edition. New York: Wiley.
2. http://www.uga.edu/sisbl/stable.html#calib Overview of Stable Isotope Research – The Stable Isotope/Soil Biology Laboratory of the University of Georgia Institute of Ecology
3. Miller & Wheeler, Biological Oceanography, p. 186.
4. Panchuk, K.; Ridgwell, A.; Kump, L.R. (2008). "Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison". Geology 36 (4): 315–318. doi:10.1130/G24474A.1.
5. Retallack, G.J. (2001). "Cenozoic Expansion of Grasslands and Climatic Cooling". The Journal of Geology 109 (4): 407–426. Bibcode:2001JG....109..407R. doi:10.1086/320791.
6. O'Leary, M. H. (1988). "Carbon Isotopes in Photosynthesis". BioScience 38 (5): 328–336. doi:10.2307/1310735. JSTOR 1310735.
7. http://www.lpi.usra.edu/meetings/impact2000/pdf/3072.pdf

Further reading

• 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. http://www.scopenvironment.org/downloadpubs/scope42/chapter11.html