In this task, we ask you to elaborate the sentence without changing its general meaning. You can do so by explaining further the input sentence, using more precise wording, adding qualifiers and auxiliary information etc.

Anoxic events ( AE ) occur when oceans become completely depleted of oxygen ( O ) below the surface levels .
Oceanic anoxic events or anoxic events ( anoxia conditions ) were intervals in the Earth 's past where portions of oceans became depleted in oxygen ( O ) at depths over a large geographic area .