Stable carbon isotope compositions are expressed
in parts per thousand (‰) as d13C. They are determined
by measuring the relative amounts of 13C and 12C
in a sample and expressing this ratio relative to the
stable isotope composition of PeeDee Belemnite.
d13C is calculated using the formula

d13C={(Rsample/Rstandard)-1}x1000,
where R= 13C/12C.

Samples with greater amounts of 13C have more positive
d13C values and are referred to as “isotopically enriched”.
Samples with lower 13C contents are more negative
and are “isotopically depleted”.

d13C values can be measured for anything
that contains carbon. This includes the carbon dioxide in air,
bicarbonate in the ocean, and biomass from any organism.
For biomass, the d13C values are almost always
more negative than air carbon dioxide and ocean
inorganic carbon.

An important part of knowing how much atmospheric
carbon dioxide the ocean can take up
to mitigate global warming is an understanding of
how rapidly phytoplankton are growing.
Measuring d13C values of autotrophs indirectly tells us
about factors influencing phytoplankton carbon fixation.
During rapid growth, phytoplankton generally have
more enriched d13C values. When growth is slow
due to nutrient limitation or other factors,
phytoplankton have more depleted d13C values.
Measuring d13C values is a useful way
to estimate growth rates by autotrophs that,
for one reason or another, cannot have
their growth rates measured directly.