Editing The Global Carbon Cycle (section)

=== The Biological Pump ===
Marine ecosystems also actively draw down carbon. Microscopic phytoplankton living in the sunlit surface waters perform photosynthesis, incorporating dissolved CO2 into their cellular structures. When these organisms—and the predators that consume them—die, a portion of their organic matter sinks into the deep ocean. This continuous shower of organic detritus is known as '''marine snow'''. 

Additionally, many marine organisms (like corals, oysters, and certain plankton) extract dissolved carbon and calcium from the water to build calcium carbonate shells. When they die, these shells sink to the ocean floor, eventually compacting over geological time to form massive limestone deposits, transferring carbon from the fast cycle into the slow, geological cycle.