Marine ecosystem have biotic organisms like phytoplankton and photosynthetic algae that do photosynthesis and converts the atmospheric carbon dioxide into organic matter most of the carbon dioxide is released when other organisms eat phytoplankton and release the carbon dioxide through respiration.
But some of the algae and phytoplanktons die naturally get down to the bottom of the ocean where they make carbon sink. Some organisms accumulate CO₂ in their exoskeleton that is made up of calcium carbonate.
Therefore this use of carbon by living organisms in oceans brings carbon dioxide from the atmosphere to oceans. This is why the ocean plays a critical role in the carbon cycle and called carbon sink.
