Ocean water that is relatively rich in 18O indicates that 16O has been trapped in ice sheets. Which statement best describes this process?

• A. 18O is produced by volcanic activity
• B. 16O is trapped in ice sheets
• C. 18O is rare in seawater
• D. 16O decays to 14C

Answer: (B)

When ice forms, the lighter oxygen isotope, 16O, preferentially becomes part of the solid ice, while the seawater is left relatively enriched in the heavier isotope, 18O. So, if ocean water shows a relatively high level of 18O, it indicates that a large amount of 16O has been locked away in ice sheets. This is the basis of using the 18O/16O ratio as a paleoclimate proxy for ice-volume changes.

Volcanic activity doesn’t specifically produce a signal that makes seawater richer in 18O, and 18O is not rare in seawater—it occurs naturally alongside 16O. Also, 16O is stable and does not decay to 14C, so that decay pathway doesn’t describe the observed enrichment.