1. Sea Level Falls
Basically, if the sea level dropped drastically (either through Global Cooling or drowned in the middle of the ocean ridges), can reduce the area around the Continental Shelf cause all the little fish, sharks, whales and almost all life on the big blue die. If that's not bad enough, the weather can change drastically leading to mass extinctions on the land (that means us).
2. Impact Events
Impact event was the collapse of big rocks from space. Basically it sends a large stone dust and aerosols to the atmosphere, causing not only the plants die, but also lowers the temperature. Large stones can cause tsunamis and global forest fires.
3. Global Cooling
As described in "The Day after Tomorrow", for some reason the earth would be too cold too quickly causing death to many of the polar regions and are animals. In turn will lead to mass migration to the equator. Lucky to find because most of the water to ice and snow.
4. Global Warming
It's basically the other side of Global Cooling. Thawing snow and ice will make sea deeper and possibly causing anoxic event
5. Clathrate Gun
This is the formation of clathrates in the Continental Shelf. Clathrate is a methane-covered water. If this rupture releasing methane gas into the atmosphere causes the acceleration of Global Warming.
6. Anoxic Events
Anoxic events occur when the ocean has been vacant for oxygen on the surface. Although anoxic does not happen for millions of years, the geological record shows that there have been several times in the past. Anoxic may have caused mass extinctions. Black line in the picture above is from the Aptian anoxic event.
7. Hydrogen Sulfide Emissions
This can occur when a disturbance of the balance between plankton and bacteria that reduce sulfate. Imagine emissions of hydrogen sulfide suddenly rose from the sea poison everything on land and sea. And if that was not enough, it can also destroy Ozone.
