(CO2.aq),1 bicarbonate ion (HCO3–), and carbonate ion (CO32–) (see Box 2.1 for definitions.). CO2 dissolved in seawater acts as an acid and provides hydrogen ions (H+) to any added base to form bicarbonate:
CO32– acts as a base and takes up H+ from any added acid to also form bicarbonate:
Borate [B(OH)4–] also acts as a base to take up H+ from any acid to form boric acid [B(OH)3]:
As seen in reactions 1 and 2, bicarbonate can act as an acid or a base (i.e., donate or accept hydrogen ions) depending on conditions.
Under present-day conditions, these reactions buffer the pH of surface seawater at a slightly basic value of about 8.1 (above the neutral value around 7.0). At this pH, the total dissolved inorganic carbon (DIC ~ 2 mM) consists of approximately 1% CO2, 90% HCO3–, and 9% CO32– (Figure 2.1). The total boric acid concentration (B(OH)4– + B(OH)3)) is about 1/5 that of DIC. As discussed in section 2.2, increases in CO2 will increase the H+ concentration, thus decreasing pH; the opposite occurs when CO2 decreases. We note that isotope fractionation between B(OH)3 and B(OH)4– is used for estimating past pH values (Box 2.2).
Life in the oceans modifies the amount and forms (or species) of inorganic carbon and hence the acid-base chemistry of seawater. In the sunlit surface layer, phytoplankton convert, or “fix,” CO2 into organic matter during the day—a process also known as photosynthesis or primary production. This process simultaneously decreases DIC and increases the pH. The reverse occurs at night, when a portion of this organic matter is decomposed by a variety of organisms that regenerate CO2, resulting in a daily cycle of pH in surface waters. A fraction of the particulate organic matter sinks below the surface where it is also decomposed, causing vertical variations in the concentrations of inorganic carbon species and pH. The net result is a characteristic maximum in CO2 concentration and minima in pH and CO32– concentration around 500 to 1,000 meters depth
1 The proper notation for carbon dioxide gas is CO2.g; carbon dioxide dissolved in water is CO2.aq. However, for simplicity, these notations are not carried through the report; the text provides adequate context to determine which form of CO2 is being discussed.