Dead Zones

Marine dead zones are regions of the ocean, usually near the mouths of rivers or waterways, which receive a large amount of nutrient (phosphorous and nitrogen) run off mostly from fertilizer.   This causes a great phytoplankton and algae bloom that takes up carbon dioxide. However, when they die they sink to the ocean floor where other aerobic bacteria break them down with such vigor that they deplete the oxygen supply leaving an anoxic zone that cannot support marine life.  The environmental movement is striving to curb fertilizer use in an attempt to mitigate these dead zones.  There are also theories that the increase in the number of these dead zones are related to global warming.

I have a heretical thought on this regard and I haven't been able to find any information on it so if anyone knows please enlighten me.  The earth's oxygen was originally created by cyanobacteria, which make up the algae that are causing the dead zones.  So, could these dead zones actually be removing and sequestering carbon dioxide?  Once the ocean bottom becomes anoxic, would the phytoplankton and algae fecal matter and remains pile upon the ocean floor and turn into fossil fuels in a few hundred million years?  I don't think we should necessarily encourage dead zones but is there any data out there that they could be mitigating global warming?  I don't want to be another one of those staunchly leftist youths going conservative in their old age so please set me straight if I'm wrong.