{{Short description|Measure of relative carbon-13 concentration in a sample}} {{DISPLAYTITLE:''δ''<sup>13</sup>C}}In geochemistry, paleoclimatology, archaeology, and paleoceanography '''''δ''{{sup|13}}C''' (pronounced "delta carbon thirteen") is a normalized ratio of the two stable isotopes of carbon—{{sup|13}}C and {{sup|12}}C—reported in parts per thousand (per mille, ‰).<ref>{{cite book|last=Libes|first=Susan M.|title=Introduction to Marine Biogeochemistry, 1st edition.|year=1992|publisher=Wiley|location=New York}}</ref>
The definition is, in per mille: :<math chem>\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</math>
where the standard is an established reference material.
The ''δ''{{sup|13}}C 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 ''δ''{{sup|13}}C, as the precursor materials (modern organic matter and petroleum, respectively) 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".
==Reference standard== 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 {{sup|13}}C/{{sup|12}}C ratio (0.0112372<ref>{{Cite journal|last=Craig|first=Harmon|date=1957-01-01|title=Isotopic standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide|url=https://dx.doi.org/10.1016/0016-7037%2857%2990024-8|journal=Geochimica et Cosmochimica Acta|language=en|volume=12|issue=1|pages=133–149|doi=10.1016/0016-7037(57)90024-8|bibcode=1957GeCoA..12..133C|issn=0016-7037|url-access=subscription}}</ref>), and was established as ''δ''<sup>13</sup>C 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.<ref name="BO_186">Miller & Wheeler, ''Biological Oceanography'', p. 186.</ref> The {{sup|13}}C/{{sup|12}}C ratio for VPDB, which the International Atomic Energy Agency (IAEA) defines as a ''δ''{{sup|13}}C value of zero is 0.011113.<ref>{{Cite web |last=Meyers |first=Fabienne |date=2024-12-23 |title=The isotopic composition of VPDB |url=https://iupac.org/the-isotopic-composition-of-vpdb/ |access-date=2026-01-06 |website=IUPAC {{!}} International Union of Pure and Applied Chemistry |language=en-US}}</ref> 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 ''δ''{{sup|13}}C<sub>PDB</sub> or ''δ''{{sup|13}}C<sub>VPDB</sub>.
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, ''δ''{{sup|13}}C<sub>VPDB</sub>= 1.95‰),<ref>{{Cite journal |last1=Brand |first1=Willi A. |last2=Coplen |first2=Tyler B. |last3=Vogl |first3=Jochen |last4=Rosner |first4=Martin |last5=Prohaska |first5=Thomas |date=2014-03-20 |title=Assessment of international reference materials for isotope-ratio analysis (IUPAC Technical Report) |journal=Pure and Applied Chemistry |language=en |volume=86 |issue=3 |pages=425–467 |doi=10.1515/pac-2013-1023 |hdl=11858/00-001M-0000-0023-C6D8-8 |issn=1365-3075 |s2cid=98812517 |hdl-access=free}}</ref> are commonly used in the laboratory setting for standardizing measurements.
==Causes of ''δ''<sup>13</sup>C variations== Methane has a very light ''δ''{{sup|13}}C signature: biogenic methane of −60‰, thermogenic methane −40‰. The release of large amounts of methane clathrate can affect global ''δ''{{sup|13}}C values, as at the Paleocene–Eocene Thermal Maximum.<ref name=Panchuk2008> {{cite journal |last1=Panchuk |first1=K. |last2=Ridgwell |first2=A. |last3=Kump |first3=L.R. |title=Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison |journal=Geology |volume=36 |issue=4 |pages=315–318 |year=2008 |doi=10.1130/G24474A.1 |bibcode=2008Geo....36..315P}}</ref>
More commonly, the ratio is affected by variations in primary productivity and organic burial. Organisms preferentially take up light {{sup|12}}C, and have a ''δ''{{sup|13}}C signature of about −25‰, depending on their metabolic pathway. Therefore, an increase in ''δ''{{sup|13}}C in marine fossils is indicative of an increase in the abundance of vegetation.{{Citation needed|date=April 2019}}
An increase in primary productivity causes a corresponding rise in ''δ''{{sup|13}}C values as more {{sup|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 {{sup|12}}C is locked out of the system in sediments than the background ratio<!-- (because organic carbon is lighter)-->.
Oxygen levels can also impact δ<sup>13</sup>C ratios, as unusually positive δ<sup>13</sup>C ratios in the Mesozoic have been speculated to have resulted from differences in carbon isotope fractionation in resin-producing plants under the lower oxygen levels of the Mesozoic.<ref>{{Cite journal |last1=Cullen |first1=Thomas M. |last2=Longstaffe |first2=Fred J. |last3=Wortmann |first3=Ulrich G. |last4=Huang |first4=Li |last5=Evans |first5=David C. |date=13 January 2023 |title=Anomalous 13 C enrichment in Mesozoic vertebrate enamel reflects environmental conditions in a “vanished world” and not a unique dietary physiology |url=https://www.cambridge.org/core/journals/paleobiology/article/anomalous-13c-enrichment-in-mesozoic-vertebrate-enamel-reflects-environmental-conditions-in-a-vanished-world-and-not-a-unique-dietary-physiology/7B9D3E388501EDF466FE45E7DC71EC53 |journal=Paleobiology |language=en |volume=49 |issue=3 |pages=563–577 |doi=10.1017/pab.2022.43 |issn=0094-8373 |access-date=17 March 2026 |via=Cambridge Core|doi-access=free }}</ref>
== Geologic significance ==
C{{sub|3}} and C{{sub|4}} plants have different signatures, allowing the abundance of C{{sub|4}} grasses to be detected through time in the ''δ''{{sup|13}}C record.<ref name=Retallack2001>{{cite journal | author = Retallack, G.J. | year = 2001 | title = Cenozoic Expansion of Grasslands and Climatic Cooling | journal = The Journal of Geology | volume = 109 | issue = 4 | pages = 407–426 | doi = 10.1086/320791 | bibcode=2001JG....109..407R| s2cid = 15560105 }}</ref> Whereas {{c4}} plants have a ''δ''{{sup|13}}C of −16 to −10‰, {{c3}} plants have a ''δ''{{sup|13}}C of −33 to −24‰.<ref>{{Cite journal| last1 = O'Leary | first1 = M. H.| title = Carbon Isotopes in Photosynthesis| jstor = 1310735| journal = BioScience| volume = 38| issue = 5| pages = 328–336| year = 1988| doi = 10.2307/1310735}}</ref>
===Positive and negative excursions===
Positive ''δ''{{sup|13}}C 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.<ref>{{cite journal | title=Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere | first1=Donald E. | last1=Canfield| first2=Lauriss | last2=Ngombi-Pemba | first3=Emma U. | last3=Hammarlund | journal=Proceedings of the National Academy of Sciences of the United States of America | date=15 October 2013 | volume=110 | issue=42 | pages=16736–16741 | doi=10.1073/pnas.1315570110| pmid=24082125 | pmc=3801071 | bibcode=2013PNAS..11016736C | doi-access=free }}</ref> For example, the evolution of large land plants in the late Devonian led to increased organic carbon burial and consequently a rise in ''δ''{{sup|13}}C.<ref>{{cite web |url=https://www.lpi.usra.edu/meetings/impact2000/pdf/3072.pdf |title=THE LATE DEVONIAN MASS EXTINCTION – IMPACT OR EARTH-BOUND EVENT? |first1=M.M. |last1=Joachimsk |first2=W. |last2=Buggisch |website=Lunar and Planetary Institute}}</ref>
==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== * {{delta|18|O|link}} * {{delta|15|N|link}} * {{delta|34|S|link}} * Isotopic signature * Isotope analysis * Isotope geochemistry * Isotopic labeling
==References== {{Reflist}}
==Further reading== * {{cite book |first=Charles B.|last=Miller |title=Biological Oceanography |author2=Patricia A. Miller |year=2012 |orig-year=2003 |edition=2nd |location=Oxford |publisher=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.
{{DEFAULTSORT:Delta18O}} Category:Bioindicators Category:Carbon Category:Isotopes of carbon Category:Environmental isotopes Category:Geochemistry Category:Paleoclimatology Category:Isotope excursions