The calcitic shells of sessile barnacles (Balanomorpha) taken from ocean waters ranging in temperature from -2° to 26°C have a near constant 18O enrichment compared with mollusk values assumed to be at thermodynamic equilibrium….
Killingley, J. S., and W. A. Newman. 1982. "18O fractionation in barnacle calcite: A barnacle paleotemperature equation." Journal of Marine Research 40, (3). https://elischolar.library.yale.edu/journal_of_marine_research/1611