For many oceanography students this is the subject that makes us shudder……carbonate chemisty. While the overriding principles are relatively straight forward, when you start to apply them to the real world it becomes a little complex, convoluted and not necessarily intuitive. The following is an overview of some of the fundamental principles behind carbon dioxide in seawater (and some links that do a much better job of explaining it than I can).
Carbon dioxide dissolved in seawater occurs in three inorganic forms (or species), with the contribution of each species being dependant on pH. The species are aqueous carbon dioxide (CO2(aq))*, bicarbonate (HCO3–), or carbonate (CO32-). The different carbon species are related by the following equations:
CO2*+ H2O = HCO3– + H+
HCO3– = CO32- + H+
* which also includes the minor (<0.3%) form of carbonic acid (H2CO3).
This relationship is best illustrated by the bjerrum plot below that demonstrates that at a seawater pH of 8.2 (salinity 35 & 25C), CO2 will react with seawater and speciate to 0.5%, 89%, and 10.5% for C02 (aq)*, HCO3–, and CO32-
Here we see that most of the inorganic carbon in the ocean is found as bicarbonate. While these inorganic carbon species are collectively known as dissolved inorganic carbon (DIC) or total CO2 (TCO2)
TCO2 = [CO2] + [HCO3–] + [CO32-] ,
total alkalinity (TA) deals with seawater’s charge balance, or the number of proton acceptors over proton donors. It describes the buffering capacity of seawater, or the ‘shock absorber’ to pH as I once saw it written, and is described
TA = [HCO3–] + 2 [CO32-] + [B(OH)4–] + [OH–] – [H+] + minor compounds (for more detail click, here).
It is this buffering capacity, or ability to neutralize excess hydrogen ions, that enables the ocean to absorb CO2, which is a weak acid, without a run away change in pH. For a more detailed view of the parameters of the carbonate system and how they relate The Encyclopedia of the Earth has an excellent description, and links within, to introduce you to the delights of the behaviour of CO2 in seawater.
To learn how these processes are related to calcification the folks at Wood’s Hole Oceanographic institute have put together a marvelous cartoon of the basic principles behind this process, proving that a picture really does say a thousand words, click here.