When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behid when water evaporates from the ocean surfaces,(20) the remaining ocean water becomes progressively enriched in oxygen 18.
原文15行的内容便是, 当冰川时代开始,大陆上的冰川增多, 逐渐的海洋水蒸气减少慢慢的转变为冰层/
因为重的同位素不如轻的同位素容易挥发,所以留在海洋里的水分含有氧18的成分越来越多 | 
