6.2A - Oceanic Sequestering

Phytoplankton sequester atmospheric carbon during photosynthesis in surface ocean waters; carbonate shells/tests move into the deep ocean water through the carbon pump and through the action of the thermohaline circulation.  

Compared with its geological counterpart, biological sequestering operates on much shorter timescales, from hours to centuries. 

The oceans are the Earth's second largest carbon store. The oceanic store of carbon is 50 times greater than that of the atmosphere. Most of the oceanic crust is stored in marine algae, plants and coral. The rest occurs in dissolved form. 

There are three types of oceanic carbon pump: (circulates and stores carbon)
  1. Biological pumps
    1. These move carbon dioxide from the ocean surface to marine plants called phytoplankton through photosynthesis. Phytoplankton are microscopic plants and plant-like organisms drifting or floating in the sea/freshwater along with diatoms, protozoa and small crustaceans. 
    2. This effectively converts carbon dioxide into food for zooplantic (microscopic animals) and their predators. 
    3. Most of the carbon dioxide taken up by phytoplankton is recycled near the surface. About 30% sinks into deaper waters before being converted back into carbon dioxide by marine bacteria. 
  2. Physical pumps
    1. These move carbon compounds to different parts of the ocean in downwelling and upwelling currents
    2. Downwelling occurs in parts of the ocean where cold, denser water sinks.
    3. These currents bring dissolved bring dissolved carbon dioxide down to the deep ocean. 
    4. Once there, it moves in slow-moving deep ocean currents, staying there for hundreds of years. 
    5. Eventually, these deep ocean currents, part of the thermohaline. circulation, return to the surface by upwelling. 
    6. The cold deep ocean water warms as it rises towards the ocean surface and some of the dissolved carbon dioxide is released back into the atmosphere. 
  3. Carbonate pumps
    1. These form sediments from dead organisms that fall to the ocean floor, especially the hard outer shells and skeletons of fish, crustaceans and corals, all rich in calcium carbonate. 

The thermohaline circulation is the global system of surface and deep ocean currents driven by temperature and salinity differences between different parts of the ocean. It can be seen as a giant conveyor belt, which plays a vital part in the carbon cycle.