Chemical Reactions

Ocean acidification lowers levels of carbonate ion. Credit: University of Maryland.

As carbon dioxide dissolves in seawater, carbonate ion neutralizes free hydrogen, causing carbonate levels to drop. Credit: University of Maryland.

The term “ocean acidification” is not entirely accurate; the oceans are actually becoming less alkaline. The pH of surface seawater has fallen from 8.2 to 8.1, (a pH of 7 is neutral) in a few hundred years, after remaining constant for millions of years. A decline of .1 pH units may not sound like a lot, but on the logarithmic scale of pH it translates to a 30 percent rise in acidity. Seawater pH is projected to drop another .3 to .4 units if carbon dioxide levels reach 800 ppm –one of the scenarios projected by the Intergovernmental Panel on Climate Change by 2100 –raising levels of hydrogen ion, H +, 100 to 150 percent (Orr et al., 2005). It could take “tens of thousands of years” for the chemistry of the oceans to return to pre-industrial levels, the Royal Society of Britain estimates.

Once dissolved in seawater, CO2 reacts with water, H2O, to form carbonic acid, H2CO3: CO2 + H2O ↔ H2CO3. Carbonic acid dissolves rapidly to form H+ ions (an acid) and bicarbonate, HCO3-(a base).  Seawater is naturally saturated with another base, carbonate ion (CO3−2) that acts like an antacid to neutralize the H+, forming more bicarbonate. The net reaction looks like this: CO2 + H2O + CO3−22HCO3-

As carbonate ion gets depleted, seawater becomes undersaturated with respect to two calcium carbonate minerals vital for shell-building, aragonite and calcite. Scientific models suggest that the oceans are becoming undersaturated with respect to aragonite at the poles, where the cold and dense waters most readily absorb atmospheric carbon dioxide.  The Southern Ocean is expected to become undersaturated with respect to aragonite by 2050, and the problem could extend into the subarctic Pacific Ocean by 2100 (Orr et al., 2005).

Aragonite saturation declines at higher latitudes. Credit: Kleypas et al., 2006.

A tiny species of zooplankton, the pteropod, called “sea butterflies” for the gelatinous wings they use to swim around, may be in jeopardy. In an experiment that immersed a pteropod in seawater with low aragonite levels, part of the organism’s shell was eroded in as little as two days (Orr et al., 2005).

Over hundreds of years and longer, carbonate ion in the ocean is replenished through the chemical weathering of limestone rock and dead animals, such as pteropods, that use calcium carbonate to build their shells. The formation and dissolution of calcium carbonate depends on the saturation state (Ω) of water, or the ion product of calcium and carbonate concentrations. The solubility product in the equation, Ω = Ca2+ + CO3−2/K’sp, depends on temperature, salinity, pressure and the particular mineral. Shell formation usually happens when Ω is greater than one while dissolution happens when Ω is less than one.

With enough time, calcium carbonate dissolves in large enough quantities to return the oceans’ pH to its natural state which may be why pH in the past did not fall as dramatically as the high carbon dioxide levels in the past might suggest.

There is some indication that levels of carbonate ion could grow as the oceans warm but models suggest that this would compensate for only 10 percent of the carbonate ion loss due to ocean acidification (Orr et al., 2005).

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