δ¹³C

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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.

δ¹³C varies in time as a function of productivity, organic carbon burial and vegetation type.

[edit] What affects δ¹³C?

Methane has a very light Template:D13C 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).

[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.^[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.

[edit] References