USING MOLLUSK SHELL SHUTDOWN TEMPERATURE TO CONSTRAIN OXYGEN ISOTOPES OF WATER

It is well known that mollusk shell oxygen isotopes (δ¹⁸O) are a function of both water temperature and δ¹⁸O value of water in which they grew. If two of these parameters are known, the third can be calculated. Unfortunately, usually only the δ¹⁸O_shell value is known, especially in fossil shells. However, because many mollusks do not precipitate shell below as certain threshold temperature - the shutdown temperature - the δ¹⁸O_water value at the time of shutdown can be calculated. While this simple technique offers information on δ¹⁸O_water values, several variables must also be considered. First, while water temperature seems to be the dominant factor controlling growth, other environmental parameters such as salinity, food availability, and turbidity can influence shell growth. If these parameters are unknown, shutdown temperatures could be different than expected. Another potential problem with this method is that shut down temperatures can be somewhat variable, so errors are certainly associated with this calculation and only large changes in calculated δ¹⁸O_water can be confidently determined (such as in estuaries or freshwater streams). For example, a ± 2 °C variability in shutdown temperature results in a possible range of δ¹⁸O_water of 1‰. This shutdown temperature will also most likely change through ontogeny so only the earliest years of growth should be used. Moreover, as bivalves age, they typically reduce annual shell growth, so time averaging may become a problem in slower growing specimens. Time averaging occurs when shell growth slows and sample interval remains the same, resulting in the same sample size representing (and averaging) more time. Time averaging will thus bring the amplitude of the δ¹⁸O_Shell cycle closer to the mean and the minimum and maximum δ¹⁸O_Shell values will be under estimated. This technique is tested on new and published data from several marine and freshwater species.