[curryst]

It's all hypothetical, but it's not unimaginable that there are nasty things laying under the layers of ocean snow. Either materials that could poison the ocean, or forms of ancient bacteria that have been dormant (like the anthrax in Siberia that used to be frozen under permafrost).

It would be catastrophic if we released something that could displace phytoplankton in the food chain, but didn't absorb CO2.

It's also possible that those minerals are important to some kind of natural process. I'm not a scientist, so I don't know what that might be, but it's not unfathomable that it's part of some bacterial lifecycle, and that disrupting that bacterial lifecycle could have implications for the rest of ocean life.