Lecture notes – The seawater balance of carbon dioxide - Atmospheric CO2 enters the oceans and

Lecture notes – The seawater balance of carbon dioxide

In contrast, seawater is strongly buffered by the balance of CO2, bicarbonate (HCO3–),and carbonate

(CO3–). Atmospheric CO2 enters the oceans and either is converted to organic carbon by

photosynthesis or reacts with seawater to form carbonic acid (H2CO3), which quickly dissociates to

form bicarbonate and carbonate.

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3– ↔ 2H+ + C032-

The Carbonate Equilibrium System

Atmospheric CO2 enters seawater and is converted to organic carbon or to carbonic acid (H2CO3)

that rapidly dissociates into the weak acids bicarbonate (HCO3–) and carbonate (CO3–). Calcium

carbonate (CaCO3), a solid, precipitates to the seafloor, where it helps form a carbonate ooze. This

system keeps seawater buffered at about pH 8.0.

The oceans are effectively buffered between pH 7.6 and 8.2 by this carbonate equilibrium system.

Much like the buffer one might use in a chemistry experiment, the pH of seawater is determined by

the relative concentrations of bicarbonate and carbonate.

The reactions of the carbonate equilibrium system have taken on new importance as increases in

atmospheric CO2 result in higher rates of CO2 dissolution in seawater. In fact, the ocean is already

absorbing about half the anthropogenically produced CO2. This has led to ocean acidification; the

average pH of the ocean has dropped by 0.1 unit (to 8.1) since the pre-industrial age.

Coccolithophores Respond to Seawater Acidification

(a) Emiliania huxleyi, an abundant species in the open ocean. (b) E. huxleyi coccoliths increase in size

when grown in high partial pressures of CO2. Currently atmospheric CO2 is about 380 ppm. Other

studies have shown that in addition to an increase in size, the coccolith shape can become distorted

above 400 ppm CO2•

It is expected to decline another 0.35 to 0.50 by 2100, unless effective means of limiting CO2

emissions are implemented. This is significant because as C02 increases, the carbonate equilibrium

shifts away from CaCO3, potentially limiting its availability for the formation of shells and skeletons of

marine organisms. Because the protists known as coccolithophores produce about a third of the

marine CaCO3, they have been the focus of investigations seeking to predict the effects of ocean

acidification.

While responses are species specific, it is clear that these seemingly slight shifts in pH have an impact

(figure 30.2). Ocean acidification will also have a negative impact on corals. Although it is difficult to

Lecture notes – The seawater balance of carbon dioxide