Parameters affecting the stable oxygen isotope ratio in CaCO3		  
 
Theoretical consideration, chemical experiments and empirical evidence show that the stable oxygen isotope ratio 18O (18O/16O of sample divided by 18O/16O of standard) in calcium carbonate CaCO3 is determined by two factors,
- the 18O (water) of the ambient H2O, and
- the water temperature during calcium carbonate formation.

This relation is expressed by the famous empirical paleotemperature equations of Epstein et al. (1953) for calcite and of Grossman (1982) for aragonite:

18O (calcite) =
 18O (water) + ([4.38 - SQRT(19.18 - 0.40 * (16.9 - T))] / 0.20)
18O (aragonite) =
18O (water) + ([4.56 - SQRT(20.79 - 0.76 * (20.19 - T))] / 0.38)
  Symbols: SQRT = square root; T = temperature (deg C)
 
 
Seawater is assumed to have a constant 18O. This is true for deeper water in the open ocean only, as oxygen isotope ratios are affected by freshwater-seawater exchange, i.e. freshwater inflow, evaporation & precipitation, ice formation & melting. As a rule of thumb, 18O (water) will be the "heavier" the more saline the water is.

For our purposes, however, the absolute value of either 18O is less important. The decisive fact is the coupling of 18O (CaCO3) to temperature. Owing to the underlying physical rules (law of mass action) the factor of fractionation (18O/16O in carbonate to 18O/16O in water) decreases with temperature. Hence, 18O (CaCO3) will be the "lighter" the warmer the water is during precipitation and the "heavier" the colder the water is during precipitation.

Hence, every calcium carbonate structure contains a record of the water temperature during its formation. We just have to read 18O along the growth axis to obtain this record.

Therefore 18O (CaCO3) can be used for age determination in systems with distinct annual oscillations in temperature, i.e. in most temperate environments. The annual temperature oscillation is reflected in a corresponding 18O oscillation in the calcium carbonate structure.

See examples of 18O profiles.
 

Introduction
Chemistry
18O
 13C Methods