Revellation

There have been a number of recent interesting posts and papers on what has loosely been gathered under the rubric of ocean acidification.  The ocean acidification tent covers a number of interconnected issues which are often not explicitly dealt with.  Before sending the bunnies off into the fields, Eli thought a brief discussion would be useful.

A good beginning takes Rabett Run back to Revelle and Suess's observation that there is only so much CO2 that you can shove into the oceans.  This is usefully described by the Revelle factor

R = ∂ln[CO2(aq)]/∂ln[DIC(aq)]

where [CO2] is the concentration of dissolved carbon dioxide and [DIC] is the concentration of dissolved inorganic carbon

[DIC] = [CO2(aq)]+ [HCO3^-] + [CO3^2-]

The square brackets [] stand for concentration and (aq) labels the state of the species.   

DIC is the carbon tied up in the equilibria between CO2 and water. 

(1)  CO2(g) = CO2(aq)

which represents the solution into and bubbling out of CO2 from the water.   The aqueous CO2 can react with the water to form hydrogen carbonate (formerly called bicarbonate) ions, HCO3^-

(2)  CO2(aq)+ H2O = HCO3^-(aq) + H⁺(aq)

and finally the hydrogen carbonate can free up another H⁺ increasing the acidity

(3) HCO3^-(aq) = H⁺(aq) + CO3^2-(aq)

and that is what happens if you start with a glass of distilled water.  After a while it becomes acidic.  That is not what happens in the oceans, and the reason is that there are other sources of carbonate ions, CO3^2-, for example dissolution of limestone or shells which are mostly calcium carbonate

(4) CaCO3(s) = Ca^2-(aq) + CO3^2-(aq)

where the calcium carbonate, CaCO3(s), is a solid.  Because there is a lot of limestone, other carbonates and shells in the oceans there is a lot of CO3^2- around.   For what follows it would be clearer if we inverted (3)

(-3) H⁺(aq) + CO3^2-(aq) =  HCO3^-(aq)

(2)  CO2(aq)+ H2O = HCO3^-(aq) + H⁺(aq)<

Comparing 2 and (-3) shows that hydrogen ions (aka protons, think about it Bohr fans) produced in (2) are consumed in (-3).  If we add them up

(5) H2O + CO2(aq) + CO3^2-(aq) = 2 HCO3^-(aq)

The balance is not perfect but equation (5) expresses how the carbonate concentration of the oceans buffers changes in acidity (expressed as pH = - log10[H]⁺(aq)], much of the excess hydrogen ions produced in the forward reaction (2) are consumed in the reverse reaction (-3).  The excess increases the concentration of hydrogen ions in the water, decreasing the pH,  pH being a logarithmic scale, the observed change.  Thus the oceans will become more acid, e.g. the pH will decrease.  The observed decrease in pH is about 0.1 units which corresponds to a 30% decrease. 

But, constant readers know that there is always a but, reaction (-3) or (5) if you wish, chews up carbonate ions and if there are fewer carbonate ions then two things will happen in succession.  First, the carbonate ion concentration will decrease, then, to compensate the decrease more CaCO3(s) will dissolve, or in the case of some shells never form.

If the bunnies start with some distilled water and toss some sea shells or limestone in so that reaction (4) is in equilibrium

(4) CaCO3(s) = Ca²⁺(aq) + CO3^2-(aq)

Then, hey you in the back, you took GChem, what happens?  Well there is an equilibrium and we can calculate the concentrations of the calcium and carbonate ions

Ksp = 1 x 10^-12 = [Ca^{2 +}(aq)]  [CO3^2-(aq)]

but because of reaction (5) there is less carbonate ion in solution, the oceans are no longer saturated, it is harder to form shells and some shells may actually dissolve.

That's the basics.

The issues are:

How will the decreasing concentrations of carbonate ions forced by changes in atmospheric carbon dioxide affect shell formation.  The answer is clear.  The ability to form shells will decrease.  Besides the esthetic issue of losing corals, the economic issues of loss of income from fishing, there are system biological issues which are concerning.

Will changes in pH have other biological effects?  This is much less studied at this time.  Pre-normal science as it were with answers very organism and environment dependent.

Will changes in DIC driven by increased atmospheric carbon dioxide affect the buffering capacity of the oceans.  The science says that this will decrease the amount of atmospheric carbon dioxide that can be absorbed.  More about this later.

Will increases in DIC increase growth of sea plants and the fish/shellfish that feed on them?  Probably yes, but not necessarily all good news as it looks like algae mats will do best.

All of these are tied up with changes in temperature, pollution and other downward slope issues.

Some places to get started reading

Brad Plummer at Wonkblog
Nick Stokes at Moyhu
The NAS on what should be studied about ocean acidification and why
Norris et al in Science on what happened way back and what could happen in the future
Wittman and Poerter on the sensitivity of the fish in the sea to ocean acidification (anybunny know of an open source for this)