The definition is δ13C (in per mil) = 103[(Rsample/Rstandard)-1], where Rx = (13C)/(12C) is the ratio of isotopic composition of a sample compared to that of an established standard, such as ocean water.
δ13C varies in time as a function of productivity, organic carbon burial and vegetation type.
What affects δ13C?
Methane has a very light δ13C signature: biogenic methane of −60‰ thermogenic methane −40‰. The release of large amounts of clathrate can impact on global δ13C values, as at the PETM.[1]
More commonly, the ratio is affected by variations in primary productivity and organic burial. Organisms preferentially take down light 12C, 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 12C is locked up in plants. This signal is also a function of the amount of carbon burial; when organic carbon is buried, more 12C is locked out of the system in sediments than the background ratio (because organic carbon is lighter).
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.[2]
Mass extinctions are often marked by a negative δ13C anomaly thought to represent a decrease in primary productivity.
The evolution of large land plants in the late Devonian also led to increased organic carbon burial and consequently a drop in δ13C.
References
^ Panchuk, K.; Ridgwell, A.; Kump, L.R. (2008). "Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison". Geology36 (4): 315-318. doi:10.1130/G24474A.1.
