δ ¹³ C

δ ¹³ C or Delta-C-13 is in the geochemistry , palaeoclimatology and Palaeoceanography a measure for the changed isotope ratio of stable carbon - isotopes ¹³ C and ¹² C between a sample and a standard. It is defined as:

${\displaystyle \delta ^{13}\mathrm {C} ={\Biggl (}{\frac {{\bigl (}{\frac {^{13}\mathrm {C} }{^{12}\mathrm {C} }}{\bigr )}_{\text{Probe}}}{{\bigl (}{\frac {^{13}\mathrm {C} }{^{12}\mathrm {C} }}{\bigr )}_{\text{Standard}}}}-1{\Biggr )}\cdot 1000\ ^{o}\!/\!_{oo}}$

The standard, i.e. the comparison sample, is usually Pee Dee Belemnite , abbreviated PDB or VPDB. This ensures that the smallest ratio differences are correctly measured and displayed more clearly. By measuring the relative difference between the isotope ratios, mass spectrometers can measure more accurately than if this ratio itself had to be used for comparison. These are differences in the per mil range, the independent measurement of which would be too imprecise to be compared or reproduced.

The δ13C value changes over time depending on the primary production (binding of CO ₂ during the growth of plants), the deposition of organic carbon and the type of vegetation . The background is the fact that the isotope ¹² C is somewhat preferred in photosynthesis . Plant material has a δ13C value of around −25 ‰.

The determination of the δ13C value is one of several methods of isotope analysis of organic materials in order to determine their origin and age. The δ13C values ​​of the atmosphere also fluctuate seasonally and change in historical and geological periods due to anthropogenic activities and vegetation.

A shift in the δ ¹³ C ratio can already be detected in rocks that are 3.5 billion years old , which is regarded as an indication that plant life has existed on earth since this time.

Methane has a strongly depleted δ13C signature: In the case of biogenic methane (methane from anaerobic bacterial decomposition of organic material, e.g. methane from swamps) this is −60 ‰, in the case of thermogenic methane (abiotic, thermal decomposition of organic material at great depth) the value −40 ‰.

The release of large amounts of methane hydrates affects the global δ13C signature; this was observable at the Paleocene / Eocene temperature maximum .

The δ13C signature can also be used to reconstruct the concentration of carbon dioxide in the earth's atmosphere in the past. This is particularly relevant for samples that are older than the oldest available ice cores , i.e. older than 800,000 years.

See also

• δ ¹⁸ O
• δ ¹⁵ N

Individual evidence

1. ↑ https://www.esrl.noaa.gov/gmd/ccgg/isotopes/c13tellsus.html The Data: What 13C Tells Us The Global View , communication from the Global Monitoring Laboratory / US Department of Commerce, accessed on August 10, 2020
2. ↑ David Tenenbaum: When Did Life on Earth Begin? Ask a rock. In: Astrobiology Magazine. Helen Matsos, October 14, 2002; accessed January 9, 2019 (American English). 
3. ↑ K. Panchuk, A. Ridgwell, LR Kump: Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison . In: Geology . 36, No. 4, 2008, pp. 315-318. doi : 10.1130 / G24474A.1 .
4. ^ Benjamin J. Fletcher, Stuart J. Brentnall, Clive W. Anderson, Robert A. Berner, David J. Beerling: Atmospheric carbon dioxide linked with 'Mesozoic and early Cenozoic climate' change . In: Nature Geoscience . 1, No. 1, December 9, 2007, pp. 43-48. ISSN 1752-0894 . doi : 10.1038 / ngeo.2007.29 .