δ13C

Foraminifera samples.

In geochemistry, paleoclimatology and paleoceanography δ¹³C is an isotopic signature, a measure of the ratio of stable isotopes ¹³C:¹²C, reported in parts per thousand (per mil, ‰).

The definition is, in per mil:

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

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 or (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 ¹³C value of zero. Use of this standard gives most natural material a negative δ¹³C.^[1] The PDB material has been exhausted and the standard replaced by secondary standards.^[1]

What affects δ¹³C?

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 PETM.^[2]

More commonly, the ratio is affected by variations in primary productivity and organic burial. Organisms preferentially take down light 12
C, and have a δ¹³C signature of about −25‰, depending on their metabolic pathway.

An increase in primary productivity causes a corresponding rise in δ¹³C 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

C₃ and C₄ plants have different signatures, allowing the importance of C₄ grasses to be detected through time in the δ¹³C record.^[3] Whereas C₄ plants have a δ¹³C of -16 to -10 ‰, C₃ plants have a δ¹³C of -33 to -24‰.^[4]

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.^[5]

See also

• δ¹⁸O
• Isotopic signature
• Isotope analysis
• Isotope geochemistry

References

1. ^ ^{a b} 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
2. ^ 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. 
3. ^ 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. 
4. ^ O'Leary, M. H. (1988). "Carbon Isotopes in Photosynthesis". BioScience 38 (5): 328–336. doi:10.2307/1310735. JSTOR 1310735.  edit
5. ^ http://www.lpi.usra.edu/meetings/impact2000/pdf/3072.pdf