Glacial ice is therefore made up primarily of
water with the light O16 isotope.
This leave the oceans enriched in the heavier O18, or “more positive.” During glacial periods, more O16 is trapped in glacial ice and the oceans become even more enriched in O18.
During interglacial periods, O16 melts out of ice and the oceans become less O18 rich, or “more negative” in O18