Isotope and Seawater Temperature Cycles

Scallop resilia with growth lines.A scallop shell is composed of CaCO3, with the oxygen having 2 natural isotopes, 18O and 16O, that are incorporated differently depending on temperature. So, the ratio of 18O /16O can be used as a thermometer. Seawater temperature cycles annually, so we can compare the ring formation to temperature.