Respired carbon dioxide is an important constituent in the carbonates of most air breathing animals but is much less important in the carbonates of most aquatic animals. This difference is illustrated using carbon isotope data from freshwater and terrestrial snails, ahermatypic corals, and chemoautotrophic and methanotrophic pelecypods. Literature data from fish otoliths and bird and mammal shell and bone carbonates are also considered.

Environmental CO₂/O₂ ratios appear to be the major controlling variable. Atmospheric CO₂/O₂ ratios are about thirty times lower than in most natural waters, hence air breathing animals absorb less environmental CO₂ in the course of obtaining 0₂. Tissue CO₂ therefore, does not isotopically equilibrate with environmental CO₂ as thoroughly in air breathers as in aquatic animals, and this is reflected in skeletal carbonates. Animals having efficient oxygen transport systems, such as vertebrates, also accumulate more respired CO₂ in their tissues.

Photosynthetic corals calcify mainly during the daytime when photosynthetic CO₂ uptake is several times faster than respiratory CO₂ release. Photosynthesis, therefore, affects skeletal δ¹³C more strongly than does respiration. Corals also illustrate how “metabolic” effects on skeletal isotopic composition can be estimated, despite the presence of much larger “kinetic” isotope effects.