How acidic are the Earth’s Oceans?
- Seawater has a pH of about 8.1
- Seawater is made of salts, including carbonates as minor constituent that buffers the pH of the oceans
- The concentration of dissolved inorganic carbon (DIC) in the oceans is 2.3 10-3 moles per liter. This corresponds to a pH of 8.1 in average
What forms of carbonates are present in natural waters?
- Carbonates dissolved in both fresh and marine waters include carbonic acid (H2CO3), bicarbonate (HCO3-), and carbonate (CO32-)
- Carbonic acid is in equilibrium with the CO2 from the atmosphere. If CO2 in the atmosphere increases, carbonic acid in natural waters increases in concentration. If CO2 in the atmosphere decreases, carbonic acid in natural waters decreases in concentration
- The carbonate species dissolved in water are also in equilibrium with each other, via exchange of protons (H+) which is the most common form of acidity
- Increasing CO2 in the atmosphere therefore increases acidity (under the form of H+) and lowers the pH of marine waters
Role of carbonate minerals
- Carbonate minerals present in marine environments buffer the increase in carbonic acid in the oceans by dissolution, absorbing carbonic acid and releasing bicarbonate according to the following equation:
CaCO3 + H2CO3 → Ca2+ 2HCO3-
- The problem associated with this reaction, however, is that most carbonate minerals originate from marine animals (e.g., bivalve, mollusks, coccolithophores, foraminifera)
- Calcification (the reverse of the above reaction) is not spontaneous, it costs energy that depends on the saturation state of the seawater (Cohen and Holcomb, 2009). Thus, acidifying the oceans not only promotes the dissolution of carbonate minerals, it also slows down calcification by organisms
- Although not completely understood, most calcifying organisms capture carbonic acid, Mg2+, and Ca2+ from the surrounding seawater and release protons to the outside seawater to increase the supersaturation state of the solution inside their cells (site of calcification). This process costs energy under the form of ATP used to pump ions in and out of the cells