Δ¹³C

In geochemistry, paleoclimatology and paleoceanography δ¹³C is a measure of the ratio of stable isotopes ¹³C:¹²C, reported in parts per thousand (per mil, ‰).

The definition is δ¹³C (in per mil) = 10³ [(R_sample/R_standard)-1] , where R_x = (¹³C)/(¹²C) is the ratio of isotopic composition of a sample compared to that of an established standard, such as ocean water.

d13c varies in time as a function of productivity, organic carbon burial and vegetation type.

What affects d13c?

Methane has a very light d13C signature: biogenic methane of −60‰ thermogenic methane −40‰. The release of large amounts of clathrate can impact on global d13c values, as at the PETM.cite journal
author = Panchuk, K.
coauthors = Ridgwell, A.; Kump, L.R.
year = 2008
title = Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison
journal = Geology
volume = 36
issue = 4
pages = 315-318
doi = 10.1130/G24474A.1]

More commonly, the ratio is affected by variations in primary productivity and organic burial. Organisms preferentially take down light 12c, and have a d13c signature of about −25‰, depending on their metabolic pathway.

An increase in primary productivity causes a corresponding rise in d13c 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 d13c excursions

c3 and c4 plants have different signatures, allowing the importance of c4 grasses to be detected through time in the d13c record. [Retallack 2001, see Evolutionary history of plants]

Mass extinctions are often marked by a negative d13c 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 d13c.

References