The oxygen isotopic composition of ocean water is measured by determining the ratio of 18O to 16O, expressed as 18O/16O. The isotope 16O preferentially evaporates into the atmosphere. During periods of global cooling, the 18O/16O ratio will:

The ¹⁸O/¹⁶O ratio will decrease.

Explanation:

The  ¹⁸O/¹⁶O ratio changes as a result of temperature. When the climate is warm, the ¹⁶O evaporates first for being lighter than ¹⁸O. When the climate is cold, the ¹⁸O precipitates first for being heavier than ¹⁶O. Thus the cold regions are rich in ¹⁶O (have little ¹⁸O) and the tropics regions are rich in ¹⁸O (have little ¹⁶O). Hence, the ratio ¹⁸O/¹⁶O is high in warm areas (for the evaporation of  ¹⁶O) and lower in cold areas (for the precipitation of ¹⁸O).

Therefore, during periods of global cooling, the ¹⁸O/¹⁶O ratio will decrease.                    

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