Ocean Acidification

Increase in CO2 concentrations not only leads to warmer oceans but also to more acidic oceans.
  • As the uptake of atmospheric carbon dioxide by the ocean increases, the concentration of hydrogen ions in the ocean increases, the concentration of carbonate ions decreases, the pH of the oceans decreases and the oceans become less alkaline – this process is known as ocean acidification.
  • Carbonic acid reacts with carbonate ions in the water to form bicarbonates. Ocean Acidification will convert more carbonate ions (which are required for shell-building by marine organisms) into bicarbonates, the animals need to expend more energy to build their shells. As a result, the shells end up being thinner and more fragile.
  • In the long run, this reaction will allow the ocean to soak up excess carbon dioxide because more acidic water will dissolve more rock, release more carbonate ions, and increase the ocean’s capacity to absorb carbon dioxide
  • Deep, cold ocean waters are naturally under saturated with carbonate ions causing the shells of most calcifying organisms to dissolve.
  • Surface waters are over saturated with carbonate ions and do not readily dissolve shells of calcifying organisms.
  • The saturation horizon is the level below which calcium carbonate minerals undergo dissolution.
  • Ocean acidification causes this horizon to rise vertically in the water column so more and more calcifying organisms will be exposed to under saturated water and thus vulnerable to dissolution of their shells and skeletons.
  • The saturation horizon of calcite (relatively less soluble mineral form found in the shells of planktonic algae, some corals, echinoderms, and some mollusks) occurs at a greater ocean depth than that for aragonite (more soluble form of calcium carbonate; it is found in most corals, most mollusks)
  • The current increased rate of dissolution of atmospheric CO2 into the ocean results in an imbalance in the carbonate compensation depth (CCD), the depth at which all carbonate is dissolved.
  • As the pH of the ocean falls, it results in a shallowing of the CCD, thus exposing more of the shells trapped in the sediments to under saturated conditions causing them to dissolve, which will help buffer ocean acidification but over a long time scale of a thousand years.

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