99.4一篇阅读中内容之困惑

GBFUID: 3379645

It is now understood that the ratio of oxygen isotopes in seawater closely reflects the proportion of the world’s water locked up in glaciers and ice sheets. A kind of meteorological distillation accounts for the link. Water molecules containing the heavier isotope tend to condense and fall as precipitation slightly sooner than molecules containing the lighter isotope. Hence, as water vapor evaporated from warm oceans moves away from its source, its oxygen-18 returns more quickly to the oceans than does its oxygen-16. What falls as snow on distant ice sheets and mountain glaciers is relatively depleted of oxygen-18. As the oxygen-18-poor ice builds up, the oceans become relatively enriched in the isotope. The larger the ice sheets grow, the higher the proportion of oxygen-18 becomes in seawater—and hence in the sediments.

最后3句话，怎么跟前面没啥逻辑关系啊？前面说海洋上水蒸气蒸发，O-18比O-16更快回到海洋。冰盾和高山冰川相对就没有0-18。当缺乏O-18冰聚集增大，海洋就会富集此元素。