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On mobile so I'll let you search for it, but start at Wikipedia for anoxic oceanic event. You don't have to stop much photosynthesis for the oceans to produce huge amounts of deadly hydrogen sulfide

The wiki does not support 'huge amounts', and makes some tenous connections to some localized events and worldwide extinction event ones, as do most of the people who bring this up. It makes it hard to have the same conversation if they are conflated.

However, note the end of the wiki, where they state that the oxygenated state of the ocean currently is likely not the 'default' state of things if extrapolated beyond human history. It is certanly a concern. However there are ways to stimulate blooms, which may help serve to sequester carbon, and add oxygen in one fell swoop. Perhaps it is time to investigate the feasibility of such an action.