View source for Δ13C

{{Short description|Measure of relative carbon-13 concentration in a sample}}
{{DISPLAYTITLE:''δ''<sup>13</sup>C}}
[[Image:Benthic foraminifera.jpg|thumb|right|200px|[[Foraminifera]] samples]]

In [[geochemistry]], [[paleoclimatology]], and [[paleoceanography]] '''''δ''<sup>13</sup>C''' (pronounced "delta thirteen c") is an [[isotopic signature]], a measure of the ratio of the two [[stable isotope]]s of [[carbon]]—[[Carbon-13|<sup>13</sup>C]] and [[Carbon-12|<sup>12</sup>C]]—reported in [[parts per thousand]] (per mil, ‰).<ref>{{cite book|last=Libes|first=Susan M.|title=Introduction to Marine Biogeochemistry, 1st edition.|year=1992|publisher=Wiley|location=New York}}</ref> 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.<ref>{{cite journal |last1=Schwarcz |first1=Henry P. |last2=Schoeninger |first2=Margaret J. |title=Stable isotope analyses in human nutritional ecology |journal=American Journal of Physical Anthropology |date=1991 |volume=34 |issue=S13 |pages=283–321 |doi=10.1002/ajpa.1330340613|doi-access=free }}</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 materials for stable isotope analysis|reference material]].

''δ''<sup>13</sup>C varies in time as a function of productivity, the signature of the inorganic source, [[Oceanic carbon cycle#Burial|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%.<ref>McDermott, J.M., Seewald, J.S., German, C.R. and Sylva, S.P., 2015. [http://www.pnas.org/content/pnas/112/25/7668.full.pdf Pathways for abiotic organic synthesis at submarine hydrothermal fields]. Proceedings of the National Academy of Sciences, 112(25), pp.7668–7672.</ref>

==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 <sup>13</sup>C:<sup>12</sup>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}}</ref>), and was established as ''δ''<sup>13</sup>C value of zero.

Since the original PDB specimen is no longer available, its <sup>13</sup>C:<sup>12</sup>C ratio can be back-calculated from a widely measured carbonate standard NBS-19, which has a ''δ''<sup>13</sup>C value of +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|issn=1365-3075|hdl=11858/00-001M-0000-0023-C6D8-8|s2cid=98812517|hdl-access=free}}</ref> The <sup>13</sup>C:<sup>12</sup>C ratio of NBS-19 was reported as <math>0.011078/0.988922=0.011202</math>.<ref>{{Cite journal|last1=Meija|first1=Juris|last2=Coplen|first2=Tyler B.|last3=Berglund|first3=Michael|last4=Brand|first4=Willi A.|last5=De Bièvre|first5=Paul|last6=Gröning|first6=Manfred|last7=Holden|first7=Norman E.|last8=Irrgeher|first8=Johanna|last9=Loss|first9=Robert D.|date=2016-01-01|title=Isotopic compositions of the elements 2013 (IUPAC Technical Report)|journal=Pure and Applied Chemistry|language=en|volume=88|issue=3|pages=293–306|doi=10.1515/pac-2015-0503|issn=1365-3075|doi-access=free|hdl=11858/00-001M-0000-0029-C408-7|hdl-access=free}}</ref> Therefore, one could calculate the <sup>13</sup>C:<sup>12</sup>C ratio of PDB derived from NBS-19 as <math>0.011202 / (1.95/1000 +1)= 0.011202/1.00195=0.01118</math>.

Note that this value differs from the widely used PDB <sup>13</sup>C:<sup>12</sup>C ratio of 0.0112372 used in isotope forensics<ref>{{Cite book|last=Meier-Augenstein|first=Wolfram|url=https://www.worldcat.org/oclc/975998493|title=Stable isotope forensics : methods and forensic applications of stable isotope analysis|date=28 September 2017|isbn=978-1-119-08022-0|edition=Second|location=Hoboken, NJ|oclc=975998493}}</ref> and environmental scientists;<ref>{{Cite book|title=Stable Isotopes in Ecology and Environmental Science|date=2007-07-14|publisher=Blackwell Publishing Ltd|isbn=978-0-470-69185-4|editor-last=Michener|editor-first=Robert|location=Oxford, UK|language=en|doi=10.1002/9780470691854|editor-last2=Lajtha|editor-first2=Kate|editor-link2=Kate Lajtha}}</ref> 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 ''δ''<sup>13</sup>C.<ref name=UGa>[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.</ref> A material with a ratio of 0.010743 for example would have a ''δ''<sup>13</sup>C value of −44‰ from <math>(0.010743\div 0.01124 - 1) \times 1000</math>.

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.<ref name=BO_186>Miller & Wheeler, ''Biological Oceanography'', p.&nbsp;186.</ref>
The <sup>13</sup>C:<sup>12</sup>C ratio for VPDB, which the [[International Atomic Energy Agency]] (IAEA) defines as a ''δ''<sup>13</sup>C value of zero is 0.01123720.<ref name=IAEA>{{cite web |url=https://www-pub.iaea.org/MTCD/publications/PDF/te_825_prn.pdf |title=Reference and intercomparison materials for stable isotopes of light elements |publisher=International Atomic Energy Agency |date=1995}}</ref>

==Causes of ''δ''<sup>13</sup>C variations==
Methane has a very light ''δ''<sup>13</sup>C signature: biogenic methane of −60‰, thermogenic methane −40‰. The release of large amounts of [[methane clathrate]] can affect global ''δ''<sup>13</sup>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 (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</sup>C, and have a ''δ''<sup>13</sup>C signature of about −25‰, depending on their [[metabolic pathway]].  Therefore, an increase in ''δ''<sup>13</sup>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</sup>C values as more <sup>12</sup>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</sup>C is locked out of the system in sediments than the background ratio<!-- (because organic carbon is lighter)-->.

== Geologic significance ==

[[C3 carbon fixation|C<sub>3</sub>]] and [[C4 carbon fixation|C<sub>4</sub>]] plants have different signatures, allowing the abundance of C<sub>4</sub> grasses to be detected through time in the ''δ''<sup>13</sup>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</sup>C of −16 to −10‰, {{c3}} plants have a ''δ''<sup>13</sup>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</sup>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 plants#Evolution of trees|evolution of large land plants]] in the late [[Devonian]] led to increased organic carbon burial and consequently a rise in ''δ''<sup>13</sup>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>

Negative ''δ''<sup>13</sup>C anomalies, which are thought to represent a decrease in primary productivity and release of plant-based carbon, often mark [[mass extinction]]s.

===Lacustrine environments===
Other important applications of ''δ''<sup>13</sup>C involves understanding its signatures from soft sediments especially in lacustrine environments. This depends on the system from which it is extracted (open system, closed system, etc.). Temporal variations in {{delta|13|C}} in organic matter are influenced by diverse internal and external processes:<ref>{{Citation |last=Cohen |first=Andrew S. |title=The Geological Evolution of Lake Basins |date=2003-05-08 |url=http://dx.doi.org/10.1093/oso/9780195133530.003.0006 |work=Paleolimnology |access-date=2023-12-19 |publisher=Oxford University Press|doi=10.1093/oso/9780195133530.003.0006 |isbn=978-0-19-513353-0 }}</ref>

# Changes in the Dominant Source of Dissolved Inorganic Carbon: In stratified lakes, the accumulation of 13C-depleted carbon in deep water is common as sinking and degrading phytoplankton cells contribute to this pool. Recirculating this water to the surface can lead to a significant decrease in {{delta|13|C}}. Prolonged stratification enriches the dissolved inorganic carbon (DIC) pool in the epilimnion with 13C. Long-term variations in factors affecting upwelling intensity or depth, such as windiness, water temperature, or salinity-related stratification, manifest as shifts between more negative and positive {{delta|13|C}} values.
# Changes in Productivity/Eutrophication: Increased productivity accelerates the transfer of organic matter with negative {{delta|13|C}} values to the hypolimnion, affecting the {{delta|13|C}} of residual epilimnetic DIC. This impact, combined with mixing effects, results in variations in the {{delta|13|C}} signal.
# Changes in Metabolic Pathways for Carbon Fixation: Major changes in lake alkalinity influence benthic and planktonic primary production. Shifts in the dominant source of DIC for photosynthesis, driven by pH changes, can lead to trends toward more positive {{delta|13|C}}, particularly in lakes dominated by autochthonous organic matter and exhibiting evidence of high alkalinity.
# Changes in Availability of Dissolved {{CO2}}: Cool water can dissolve higher concentrations of {{CO2}} than warmer water, affecting {{delta|13|C}} in organic matter during cooling events. Changes in atmospheric {{CO2}} concentrations also influence {{delta|13|C}}, with lower p{{CO2}} during glacial periods causing isotopic discrimination in plants using dissolved {{CO2}}.
# Changes in Dominant Vegetation Within the Watershed: Shifts in watershed vegetation, especially transitions between C3 and C4 photosynthetic pathways, significantly alter the carbon isotopic composition in lake sediments. These changes can be indicative of broader paleoclimatic shifts.
# Diagenetic Trends: Diagenetic processes, such as the loss of reactive components like amino acids, result in sustained shifts in {{delta|13|C}} in organic matter. Marsh sediments, rich in carbon, exhibit shifts towards more negative bulk organic matter. These diagenetic trends should be considered when interpreting isotopic changes accompanying major Total Organic Carbon (TOC) changes or methanogenesis.

Understanding these processes is crucial for interpreting {{delta|13|C}} variations in lake sediments and reconstructing paleoenvironmental conditions.

==Major excursion events==
*[[Lomagundi-Jatuli event]] (2,300–2,080&nbsp;Ma) [[Paleoproterozoic]] - Positive excursion

*[[Shunga-Francevillian event]] (2,080&nbsp;Ma) [[Paleoproterozoic]] - Negative excursion

*[[Shuram excursion|Shuram-Wonoka excursion]] (570–551&nbsp;Ma) [[Neoproterozoic]] - Negative excursion

*[[Steptoean positive carbon isotope excursion]] (494.6-492&nbsp;Ma) [[Paleozoic]] - Positive excursion

*[[Ireviken event]] (433.4&nbsp;Ma) [[Paleozoic]] - Positive excursion

*[[Mulde event]] (427&nbsp;Ma) [[Paleozoic]] - Positive excursion

*[[Lau event]] (424&nbsp;Ma) [[Paleozoic]] - Positive excursion

*[[Cenomanian-Turonian boundary event]] (93.9&nbsp;Ma) [[Mesozoic]] - Positive excursion

*[[Paleocene–Eocene Thermal Maximum]] (55.5&nbsp;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 }}
* [[Wim Mook|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]]