Foraminifera and stable carbon isotopes


Carbon stable isotopes are $^{13}C$, $^{12}C$. But they are more complex than oxygen isotopes.

Seawater DIC $\delta^{13}C$

Key: organic matter = low $\delta^{13}C$

  • Photosynthesis (prefers $^{12}C$, enriching seawater $\delta^{13}C$) & Respiration (release $\delta^{13}C$-deplete carbon)
  • Global terrestrial carbon pool (which also increases atmospheric $\delta^{13}C$) & lithosphere input ($\delta^{13}C$-deplete carbon)
  • Air-sea equilibrium (lower temperature leads to greater differences, with $\ce{CO2}$ higher than DIC in $\delta^{13}C$. But this process influence more on the atmospheric $\ce{CO2}$ because of its much smaller (1/50 times) size of inorganic carbon pool)
  • Ocean Mixing (NADW has higher $\delta^{13}C$ from surface Atlantic, AABW has lower value due to more mixing of deep waters)

Foraminifera test $\delta^{13}C$

The difference between seawater DIC and foraminiferal test is due to abiotic kinetic fractionation and vital effects. Abiotic kinetic fractionation results in a 1% enrichment relative to bicarbonate, and this enrichment is conservative. More variation of offset is from vital effects

  • Symbionts
  • Species difference, habitat depth, sesasonal variation


Isotopic fractionation is like fliuds with different densities flowing through different reserviors (namely global carbon cycle). Some stay, some go.


Ana Christina Ravelo and Claude Hillaire-Marcel, 2007, The Use of Oxygen and Carbon Isotopes of Foraminifera in Paleoceanography