The phrase "dead plankton" refers to tiny organisms in the ocean that have died. Plankton are like tiny plants and animals that float in the water. When they die, they sink to the bottom, becoming "dead plankton." Full definition
When remnants of dead plankton sink to the sea floor, organic matter from their biomass is buried, as manifested by sediments exceptionally enriched in organic carbon. (phys.org)
These bacteria degrade dead plankton and fish excrement, recycling the carbon, nitrogen, phosphorus, and other elements back into the water. (discovermagazine.com)
CaCO3 tends to dissolve in the deep ocean, both because of the high pressure and because the waters have been acidified by CO2 from rotting dead plankton. (realclimate.org)