Shells constructed from seawater incorporate the 18O/16O ratio of seawater during their lifetime within their CaCO3 shell walls, providing a paleothermometer that is used to estimate the temperatures of ancient seas.

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Explanation:

Paleothermometer definition is from two words, that is "Paleo" which means something that is old and ''thermometer" which is an instrument for measuring temperature. So, if we add this up, Paleothermometer is an instrument for measuring "old" temperature, that is temperature. One of the Paleothermometer that is been used is the δ18O which is the one in the question that has isotopic ratio of 18O/16O, and it deals with the measurement of 18O to 16O. The others include Alkenones Paleothermometer, Mg/Ca Paleothermometer, Leaf physiognomy and so on.

If the values of the isotopic ratio that is 18O/16O ratio is low, then the temperature is high. To Calculate the 18O/16O ratio for ancient ocean then we will be using the equation below;

δ18O = (z - 1) × 1000. Where z= [(18O/16O)/( 18O/16O)sm. And sm= standard mean.