6.2A Phytoplankton & Sequesteration of Carbon

• Sequestration of Carbon:

  • Sequestering is the movement of carbon into carbon stores which can lower the amount of carbon dioxide in the atmosphere
  • Photosynthesis is the main process responsible for sequestering carbon from the atmosphere
• Ocean Sequestration:

  • 93% of carbon dioxide is stored in undersea algae, plants, coral and dissolved form, making oceans the largest carbon store on Earth
  • The movement of carbon within oceans is controlled:
  • Vertically by carbon cycle pumps
  • Horizontally by thermohaline circulation
  • There are three carbon cycle pumps

  -> Biological pump

  -> Carbonate pump

  -> Physical pump
• Biological pump (Sequestration):

  • The biological cycle sequesters carbon in the ocean through photosynthesis by phytoplankton and other marine animals which converts CO2 into organic matter (10GtC per year)
  • transporting carbon from the oceans' surface to the intermediate and deep ocean stores (10 GtC per year)
  • As the biological organisms die, their dead cells, shells and other parts sink into the mid and deep water
  • Oceans regulate the composition of the atmosphere by moving carbon from the ocean’s surface and storing it in the mid and deep ocean store
• Carbonate pump (Sequestration):

  • When organisms die and starts to sink, many shells dissolve before they reach the ocean floor entering the deep ocean currents
  • The solubility cycle occurs when CO2, absorbed by the oceans from the atmosphere, forms carbonic acid which in turn reacts with hydrogen ions to form bicarbonates and then further reactions form carbonates which are stored in the upper ocean
  • This process locks up carbon in the long-term carbon cycle and does not allow an easy return to the ocean surface
• Physical pump (sequestration): 

  • CO2 is absorbed by the ocean's surface through diffusion
  • Dissolved CO2 is then taken from the surface down to the intermediate and deep ocean stores through downwelling currents (96 GtC per year)
  • Cold water absorbs more CO2
  • Salinity increases at the same time, making the water denser, therefore, the water sinks (downwelling) taking CO2 from the ocean's surface to the deep ocean stores
  • Allowing more diffusion to occur at the surface and helping to regulate the carbon stored in the atmosphere
• • The thermohaline circulation is a global system of surface and deep ocean currents driven by differences in temperature and salinity
  • These ocean currents are responsible for circulating carbon
  • Warm surface waters are depleted of nutrients and carbon dioxide through evaporation but they become enriched again through the circulation of currents
  • Also, the circulation helps move carbon in the carbonate pump from the surface to deeper waters