During glacial periods, the concentration of 18o in the oceans will be higher as compared to interglacial periods.
An interglacial period (or, alternatively, interglacial, interglaciation) is a geological interval of thousands of years of warmer global average temperature that separates consecutive glacial periods within an ice age.
The current Holocene interglacial began around 11,700 years ago, at the end of the Pleistocene.
During glacial periods, the concentration of 18o in the oceans will be higher as compared to interglacial periods.
Thus, the answer for blank is "higher".
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