The stable isotope geochemistry of foraminiferal calcite has served as one of the primary sources of paleoceanographic information for the past three decades. Numerous studies have pointed out that shell calcite is generally not formed in isotopic equilibrium as predicted from studies on inorganic precipitation. Up to date it is assumed that these so called 'vital effects' result from changes in the d13C of the ambient TCO2 due to the production of light metabolic CO2 by the foraminifera and the selective uptake of 12CO2 by their photosynthetic symbionts. Although these combined processes can explain much of the variability in the carbon isotope system, they cannot resolve observations of disequilibria in the oxygen isotope system. Experiments carried out under controlled laboratory conditions and numerical modeling suggest that calcification rate and the ambient carbonate system are the missing factors in our understanding of the stable isotope composition of biogenic calcite.
The implication of this study is that because vital effects are mediated by the carbonate system, changes in the carbonate chemistry of the ocean during climatic oscillations may have a significant impact on the stable isotope composition of foraminiferal shells. An estimated decrease in the carbonate concentration, [CO32-], of approx. 80 mmol/kg since the last glacial maximum (Sanyal & Broecker, 1995) would produce a 0.3 to 0.4 increase in the d13C of foraminiferal shells.