Ingeochemistry,paleoclimatology,archaeology, andpaleoceanographyδ13C (pronounced "delta thirteen c") is a normalized ratio of the twostable isotopes ofcarbon—13C and12C—reported inparts per thousand (per mille, ‰).[1]
The definition is, inper mille:
where the standard is an establishedreference material.
Theδ13C 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 fromecosystem respiration can be differentiated from carbon dioxide formed from the combustion offossil fuels usingδ13C, as the precursor materials (modern organic matter andpetroleum, 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 standards are used for verifying the accuracy of isotope ratio measurements, which are typically performed viaisotope ratio mass spectrometry,cavity ring down spectroscopy,tunable laser absorption spectroscopy, ornuclear magnetic resonance.
The initial reference material used to standardize carbon isotope ratios "Pee Dee Belemnite" (PDB) —aCretaceous marine fossil,Belemnitella americana, originating from thePeedee Formation inSouth Carolina. This material had an anomalously high13C/12C ratio (0.0112372[2]), and was established asδ13C 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.[3]The13C/12C ratio for VPDB, which theInternational Atomic Energy Agency (IAEA) defines as aδ13C value of zero is 0.011113.[4] 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 asδ13CPDB orδ13CVPDB.
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 theNational Institute of Standards and Technology,δ13CVPDB= 1.95‰),[5] are commonly used in the laboratory setting for standardizing measurements.
Methane has a very lightδ13C signature: biogenic methane of −60‰, thermogenic methane −40‰. The release of large amounts ofmethane clathrate can affect globalδ13C values, as at thePaleocene–Eocene Thermal Maximum.[6]
More commonly, the ratio is affected by variations inprimary productivity and organic burial. Organisms preferentially take up light12C, and have aδ13C signature of about −25‰, depending on theirmetabolic pathway. Therefore, an increase inδ13C in marine fossils is indicative of an increase in the abundance of vegetation.[citation needed]
An increase in primary productivity causes a corresponding rise inδ13C values as more12C is locked up in plants. This signal is also a function of the amount of carbon burial; when organic carbon is buried, more12C is locked out of the system in sediments than the background ratio.
C3 andC4 plants have different signatures, allowing the abundance of C4 grasses to be detected through time in theδ13C record.[7] Whereas C4 plants have aδ13C of −16 to −10‰, C3 plants have aδ13C of −33 to −24‰.[8]
Positiveδ13C 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.[9] For example, theevolution of large land plants in the lateDevonian led to increased organic carbon burial and consequently a rise inδ13C.[10]