Question

explain why the interaction between precipitation water and limestone usually results in an increase in the O-18 of the water

Answer 1

O-18 has two neutrons more than O-16 and hence is heavier than water. Therefore additional heat is required to vaporise the O-18 molecules than O-16's. When there is an increase in temperature, the initial water vapor formed is thus enriched in O-16 and the residual liquid in O-18. Similarly when the water vapor condensed, the O-18 enters preferentially to O-16 into the initial condensing liquid. So, precipitation water contains more of the O-18 isotope of oxygen.

Now when CaCO₃ is formed by the interaction of Ca and CO₂ with this water, first the O's from CO₂ are consumed and then the O in H₂O is consumed. So the analysis of these shells provide a correct indication of the O-18 content in water.