Increasing levels of CO2 in the atmosphere are slowly causing the surface of the ocean to become more acidic. This is because the ocean absorbs some of the CO2, forming a weak carbonic acid. At present, the ocean absorbs about a third of fossil fuel emissions, but this amount is likely to increase to 90% in the future. Over the last century, the average pH of the ocean has decreased, and there are hints that the current levels are beginning to impact organisms that make their shells out of the minerals aragonite and calcite (both composed of CaCO3). Coral reefs that are made of the mineral aragonite are highly vulnerable to ocean acidification. A recent study has found, for example, that the area of coral covering the Great Barrier Reef in Australia has been cut in half since 1985. However, coccolithophores and foraminifera, organisms that serve a vital role at the base of the marine food chain that are composed of calcite, are also susceptible. Moreover, the future appears to be even more bleak; some CO2 projections suggest an additional pH decrease of 0.3 by 2100, a 150% increase in the ocean’s acidity compared to preindustrial times. Here we review the chemical changes in seawater that result from increasing CO2, and then we discuss the impact on reefs and planktonic organisms in the ocean. Finally, we discuss the evidence for acidification in ancient oceans and its impact on life in the past.
The following video provides a thorough overview of the potential impact of acidification on the oceans.
Video: Acid Test: The Global Challenge of Ocean Acidification (21:34)
The ocean contains a massive reservoir of dissolved CO2, hundreds of times more than in the atmosphere, and, actually, by contrast, the amount derived from fossil fuel burning is relatively modest. Since the beginning of the industrial revolution, about 340 to 420 petagrams carbon (a petagram or Pg is 1015 grams) in the form of CO2 has been emitted to the atmosphere, with about a third of that amount absorbed by the ocean, approximately 118 Pg. Seawater today may already contain more CO2 than at any time in many millions of years.
As we discussed in Module 5 on the Carbon Cycle, the absorption of CO2 in the ocean forms weak carbonic acid (H2CO3). Some of this acid dissociates in seawater releasing H+ ions, which make the water more acidic, as well as HCO3- (bicarbonate ions) and CO32- (carbonate ions). This reaction is as follows:
CO2(aq) + H2O = H2CO3 = H+ + HCO3- = 2 H+ + CO32-
The CO2, H2CO3, HCO3- and CO32- represent the dissolved inorganic carbon reservoir of the ocean; approximately 90% of this is HCO3- and 9% is CO32-.
Going back to your elementary chemistry course, you might remember that a pH of greater than 7 is regarded as alkaline whereas a pH of less than 7 is acidic. Surface ocean waters have a pH of between about 7.9 and 8.3, which means that they are, by definition, alkaline. Anthropogenic CO2 is thought to have decreased the mean pH of the ocean by 0.1 unit since 1800. This may not sound like that much, but more ominous is the projection that if CO2 levels continue to rise unabated (ie projections based on SRES A2 “business as usual”, pH levels will drop a further 0.3 by 2100. As we will see below, in parts of the ocean, these levels would be extremely damaging to organisms that build their skeletons out of CaCO3, which is very sensitive to CO2 addition.
CaCO3 is the dominant material used by invertebrate organisms to build their skeletons. There are two different minerals made of CaCO3, known as polymorphs: calcite and aragonite. These minerals have the same composition but different crystal lattice structure and thus their properties and behavior in seawater differ, including their ability to dissolve. To understand how CaCO3 dissolves and precipitates, we need to introduce a term Ω that represents the saturation state of the water. Where waters are highly saturated with respect to CaCO3 and Ω is high, calcite and aragonite are less likely to dissolve than where these waters are less saturated or even undersaturated and Ω is low. Likewise, calcite and aragonite are more likely to precipitate under higher Ω values. The dissolution and precipitation reactions are as follows:
Dissolution reaction: CaCO3 (solid) = Ca2+ + CO32-
Precipitation reaction: Ca2+ + CO32- = CaCO3 (solid)
The solubility of calcite or aragonite in seawater represents its ability to dissolve and is a function of K'sp, the solubility product where:
[Ca2+] x [CO32-] = K'sp and Ω = [Ca2+][ CO32-] / K'sp.
Both Ω and K'sp differ between calcite and aragonite, and K'sp is also affected by the temperature and pressure of seawater in which the CaCO3 mineral is growing.
An increase in CO2 from the atmosphere presents a double whammy for skeletons formed from CaCO3, both aragonite and calcite. The H+ ions and carbonate ions (CO32-) that derive from the dissociation of carbonic acid combine to form bicarbonate ions (HCO3-). This rapid reduction in available carbonate ions decreases Ω and limits calcification by organisms with aragonite- and calcite-based skeletons. However, here we need to dispel two myths. The first myth is that the precipitation of CaCO3 is directly controlled by pH. In fact, precipitation is affected principally by the decrease in CO32, which is coincident with the addition of H+ ions, and reduction in pH. The second myth is that precipitation of CaCO3 can occur in any water that is oversaturated with respect to the particular CaCO3 mineral. In fact, both corals and coccolithophores have been shown to have difficulty calcifying in environments when waters were actually oversaturated. Different organisms can calcify at very different Ω values, but for most the decrease in saturation that results from decreasing CO32- content is a direct threat to calcification.
The following video explains the threat of ocean acidification to the calcareous plankton.
Video: The Other Carbon Dioxide Problem (3:57)
The saturation of CaCO3 in the oceans is also a function of temperature and pressure. A delicate balance exists between the production of CaCO3 via the formation of skeletons in the shallow part of the ocean and the dissolution of this aragonite and calcite in the colder and deeper realms of the ocean where waters are less saturated. In most parts of the ocean, undersaturation occurs far below the surface. However, recent increase in dissolved CO2 is leading to a shoaling of the saturation horizon of CaCO3, and, in the future, this will impact especially the organisms that live at depth or in colder waters as well as those that make their shells of the mineral aragonite, which is more soluble in seawater than calcite.