The study investigates the effect of seawater carbonate concentration on the carbon and oxygen isotopes of foraminifera shells. The researchers found that the δ13C and δ18O values of the shells decrease with increasing seawater [CO3^2-], which confounds the standard interpretation of glacial foraminiferal stable-isotope data. This relationship is observed in both symbiotic (Orbulina universa) and non-symbiotic (Globigerina bulloides) plankton foraminifera. The study suggests that the glacial-interglacial shift in the δ13C ratio may be due to an increase in ocean alkalinity rather than the transfer of terrestrial carbon into the ocean. The relationship between isotopic ratios and [CO3^2-] is species-specific, with G. bulloides showing twice the slope compared to O. universa. The findings also highlight the importance of considering the influence of [CO3^2-] when interpreting δ13C differences between planktic and benthic foraminifera, as well as the δ18O values, which can affect paleotemperature reconstructions. The study provides insights into the mechanisms underlying the isotope covariance in calcifying organisms and inorganic precipitates, suggesting a kinetic mechanism that may affect all calcifying organisms.The study investigates the effect of seawater carbonate concentration on the carbon and oxygen isotopes of foraminifera shells. The researchers found that the δ13C and δ18O values of the shells decrease with increasing seawater [CO3^2-], which confounds the standard interpretation of glacial foraminiferal stable-isotope data. This relationship is observed in both symbiotic (Orbulina universa) and non-symbiotic (Globigerina bulloides) plankton foraminifera. The study suggests that the glacial-interglacial shift in the δ13C ratio may be due to an increase in ocean alkalinity rather than the transfer of terrestrial carbon into the ocean. The relationship between isotopic ratios and [CO3^2-] is species-specific, with G. bulloides showing twice the slope compared to O. universa. The findings also highlight the importance of considering the influence of [CO3^2-] when interpreting δ13C differences between planktic and benthic foraminifera, as well as the δ18O values, which can affect paleotemperature reconstructions. The study provides insights into the mechanisms underlying the isotope covariance in calcifying organisms and inorganic precipitates, suggesting a kinetic mechanism that may affect all calcifying organisms.