Isotopic signature

Algal group	δ13C range
HCO3-using red algae	−22.5‰ to −9.6‰
CO2-using red algae	−34.5‰ to −29.9‰
Brown algae	−20.8‰ to −10.5‰
Green algae	−20.3‰ to −8.8‰

An isotopic signature (also isotopic fingerprint) is a ratio of non-radiogenic 'stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample material are measured by isotope ratio mass spectrometry. This process is called isotope analysis.

The atomic mass of different isotopes affect their chemical kinetic behavior, leading to natural isotope separation processes.

For example, different sources and sinks of methane have different affinity for the ¹²C and ¹³C isotopes, which allows distinguishing between different sources by the ¹³C/¹²C ratio in methane in the air. In geochemistry, paleoclimatology and paleoceanography this ratio is called δ¹³C. The ratio is calculated with respect to Pee Dee Belemnite (PDB) standard:

${\displaystyle \delta ^{13}C_{\text{Sample}}=\left({\frac {^{13}C/^{12}C_{\text{Sample}}}{^{13}C/^{12}C_{\mathrm {PDB} }}}-1\right)\cdot 1000}$

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