For Christ also suffered once for sins, the just for the unjust, that He might bring us to God, being put to death in the flesh but made alive by the Spirit, by whom also He went and preached to the spirits in prison, who formerly were disobedient, when once the Divine longsuffering waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight souls, were saved through water.(1 Peter 3:18-20) NKJV
Anoxic means without Oxygen. All around the world we see contemporaneous evidence of a mass extinction of marine life caused by the depletion of oxygen in the water.
This period of time has been called the Cenomanian-Turonian boundary event because a world-wide anoxic extinction took place. There are a variety of indicator fossils which are used to relate this event to deposits which show similar characteristics in different places around the world. One of them is Inoceramus labiatus a marine bivalve of widespread occurrence. What could cause this world wide depletion of oxygen?
The depletion of oxygen is no mystery: in the case of Noah’s flood it is rather obvious that, since the entire world’s flora has just been buried, decomposition would cause the oxygen depletion. This is what killed the fish and other aquatic and marine life.