In prehistoric times volcanic eruptions caused mass extinctions of many species of marine organisms attached to the ocean floor. what is the most likely reason for this?

Volcanic eruptions spew alot of carbon dioxide and sulphur dioxide into the atmosphere. These gases when mixed with rain and run into oceans increase their pH. Additionally, these gases are greenhouse gases that increases atmospheric temperatures and consequently those of the seas and oceans.
Low pH and high temperatures kill marine life.

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