Paleotemperatures and chlorinities from stable carbon and oxygen isotopes in shell carbonate
A procedure is proposed by which meaningful water temperatures can be derived from the carbon and oxygen isotopic composition of shell carbonate, even if the habitat of the shell animals is not purely marine as in the case of estuaries or brackish coastal waters. 
 
Since these temperatures reflect the average temperature during the growth period of the organisms in question, it should also be possible to deduce reliable paleotemperatures from fossil samples of these species and others under certain conditions. 
 
Two main aspects are considered: (1) the relation between the isotopic composition of shell carbonate and that of the surrounding water; and (2) the significance of the temperatures deduced from the 13C and 18O content of shells. 
 
In a Dutch estuary the respective variations in 13C and 18O contents of the shell carbonate on the one hand and the dissolved bicarbonate and water on the other are found to correspond. If the δ13δ18 relation is extrapolated to the isotopic composition of carbonate precipitated in isotopic equilibrium with pure sea water, water temperature can be deduced. The temperature derived from such an extrapolation, is a reasonable average for the growth period of the molluscs. The δ13δ18 relations of shells from other nearby estuaries reflect, when similarly extrapolated, the same temperature. 
 
From this it is concluded that some molluscs deposit their carbonate in isotopic equilibrium with their environment, and that the difference in carbon and oxygen isotopic composition between carbonate on the one hand and dissolved bicarbonate and water on the other is determined solely by temperature. Reasonable temperatures are also derived from some coastal samples of molluscs. 
 
The procedure developed for determining the growth temperature of series of living molluscs can also be applied to fossil shells. Besides, the extrapolation procedure provides a method for estimating the brackishness of the water in which the molluscs lived.