Discussion
Assertion (A): Ocean water becomes isotopically heavier during a glaciation.
Reason (R): Water containing the lighter isotope evaporates more readily and is preferentially stored in ice sheets.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the mechanism. Water containing oxygen-16 is lighter and evaporates more readily, so the vapour reaching the poles is isotopically light and the ice built from it is lighter still. During a glaciation that light water is removed from the ocean and stored on land, leaving the remaining seawater enriched in oxygen-18. The effect is large enough that the marine record is read primarily as an ice volume signal.
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