δ13C
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:
where the standard is an established reference material.
δ13C varies in time as a function of productivity, organic carbon burial and vegetation type.
Contents |
[edit] 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]
[edit] 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).
[edit] 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]
[edit] See also
[edit] References
- ^ 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
- ^ 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.
- ^ 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.
- ^ O'Leary, M. H. (1988). "Carbon Isotopes in Photosynthesis". BioScience 38 (5): 328–336. doi:10.2307/1310735. JSTOR 1310735.
- ^ http://www.lpi.usra.edu/meetings/impact2000/pdf/3072.pdf
